Engineering, 2010, 2, 237-256
doi:10.4236/eng.2010.24034 Published Online April 2010 (
Copyright © 2010 SciRes. ENG
The Stretch, Limit and Path Forward for Particle Rein-
forced Metal Matrix Composites of 7075 Al-Alloys
Rupa Dasgupta
Advanced Ma terials & Processes Research Institute [AMPRI],
Received November 30, 2009; revised February 8, 2010; accepted Febr ua r y 12, 2010
Al-based metal matrix composites [MMCs] have been the research interest of a wide spectrum of material
scientists throughout the world for some over two decades now. The present paper has chosen one alloy sys-
tem namely the 7xxx series and from an extensive literature review concluded that since the beginning of the
new millennium nothing note worthy has been added to the knowledge already gained in the last quarter of
the last century except confirm the earlier findings that MMCs if properly fabricated by choosing the proc-
essing route and with appropriate size and volume fraction of dispersoids can improve most of the mechani-
cal, corrosion and wear resistant properties of the base alloy. The author’s own research activi ties using this
alloy system for making MMCs that include attempts to improve upon the properties by making composites,
ageing and also secondary processing have been included. An attempt has been made to establish the stretch
to which improvement is possible in the alloy system by making composites and trying all other routes
known for meaningful improvement in properties. Further, the way forward for such particulate composites
has been drawn to realise the material scientists’ dream of seeing such MMCs as engineering components.
For this, the areas which now need research include mass production of composites, focus on its machining,
joining, processing as also reduction in the size of dispersoids are some of the areas that have been identified
and discussed in the paper.
Keywords: Al-based MMCs, 7xxx Series, 7075 Alloy, Tempering, Ageing, Retrogression And Reageing,
Hardness, Mechanical Properties, Wear Properties, Extrusion, Corrosion, Machining, Joining, Mass Produc-
1. Introduction
Material scientists and researchers in this area h ave been
fulfilling the demand of the engineering sector in synthe-
sizing materials to attain the demanded properties to en-
hance efficiency and cost savings in the manufacturing
sector. In fulfilling this demand a certain trend has been
followed, the materials presently been used is tried for
improvement through known methods of alloy additions,
heat treatment, grain modification and the like. Once the
limit is reached through these methods either due to
economic constraint or difficulty in mass production or
further improvement is ruled out, does a different line of
thought emerge in further improving the properties or
decreasing cost and increasing efficiency. At times, a
completely new system takes over, like was done around
three decades back when due to the limits reached in
alloy systems, metal matrix composites were thought of
and after some two decades of experimental research the
economically feasible routes, dispersoids and alloy sys-
tem that can give meaningful improvement have been
narrowed down. Alongside, basic understanding of mix-
ing between the alloy and dispersoid has also now been
reached. In the last decade though research continues in
this field yet the results obtained were a mere confirma-
tion of the previously attain ed results an d a plateau in the
improvement possible has been reached. But before the
clogging takes place, it was essential to change the direc-
tion of research once again and it is only obvious that
further improve ment could be obtain ed in any system by
adopting another route of fabrication. In this line secon-
dary processing was the obvious choice, though much
work has not been re p orted in this direction.
The present paper has made an attempt of finding the
Table 1. Chemical composition limits of 7075 alloy.
Weight Cu Mg Mn Si Fe Cr Zn Ti Others
Each Total
Minimum 1.2 2.1 - - - 0.18 5.1 - - -
Maximum 2.0 2.9 0.30 0.40 0.50 0.28 6.1 0.20 0.05 0.15
limit of enhancing the mechanical and wear resistance
properties of a commonly used Aluminium alloy, the
7075 alloy by different routs; by making (i) composites
(ii) heat treatment of the alloy and composite and also (iii)
extruding the alloy an d composite to rods. A comparison
in the properties attained by the different routes would
help find the limit up to which the properties in this alloy
system can be stretched. Further research in this area
after this would not have anything much to add and this
type of an activity to be used as a tool for ‘stretching to
the limit’ any system before they see the light of the day
as an engineering component.
2. The Al-Zn-Cu-Mg Alloy System
2.1. Alloying Additions
The main alloying element in the 7XXX series of
Al-alloys is Zinc and is known for its high specific
strength. In this series the 7075 alloy is the most com-
monly used by the engineering sector especially the
aerospace industry. The chemical composition limit of
7075 alloy is as follo ws [1-4]:
The presence of Mn in the alloy is responsible for the
uniform deformation through cross slip, for retarding
fatigue damage accumulation and enhancing fatigue life.
The addition of Cu resu lts in an increases in the stabil-
ity of Guinier-Preston zones formed at room temperature
together with a decreases in the nucleation temperature
and also results in an increase in the strength ening ability
and a lower sensitivity to th e heating rate of ageing tem-
perature, however, the higher super saturation of the Cu
bearing alloy results in a higher quench higher sen sitivity
and at slow quench rates of the ternary alloy shows a
higher strengthening potential. Again when the rein-
forcement particle size is decreased, tensile strength of
the composite increases but fracture toughness follows
the opposite trend-it increase as the particle size in-
In addition to Zn, Mn and Cu which are the two majo r
constituents of the 7075 alloys, Zr is added. Its presence
results in grain refinement and diffusion of Cu relatively
quickly to grain boundaries and due to in consequence of
non equilibrium segregation during quenching a solute
depleted layer develops near the grain bou nd a ri es
2.2. Phase Diagram and Phase Precipitation
The phase diagram for the 7075 alloy is given in
Figure 1. Phase diagram for the Al-Zn-Cu-Mg system.
Figure 1 [1].
The phase in equilibrium with an Aluminium matrix in
commercial alloys are designated MgZn2 [M-Phase],
Mg3Zn3Al2 [T-Phase] and Mg5Al3 [
-Phase]. The first
phase ranges in composition from MgZn2 to Mg4Zn7Al.
The T-Phase has a wide range of composition, from 74%
Zinc-16%Magnesium to 20%Zinc-31%Magnesium. The
-phase appears only when the magnesium content is
considerably greater than the Zn content. Such alloys ar e
strengthened primarily by Mg in solid solution. Precipi-
tation hardening of alloys with Zn in excess of Mg oc-
curs in the sequence zones through coherent precipitates
to the M-Phase. Quaternary alloys contain copper, mag-
nesium and zinc. The M-Phase composition ranges in the
quaternary system from MgZn2 to CuMgAl and may be
described as Mg[Al,Cu,Zn]2. The range of composition
for the T-Phase is from that of Al-Mg-Zn ternary to that
of Phase designated CuMg4Al6 and may be described as
Mg3[Al,Cu,Zn]5. A third phase is CuMgAl2 [S-Phase],
with a small range of composition. The CuAl2 phase ap-
pears only if copper is considerably in excess of Mg.
Several nonequilibrium invariant melting reactions are
encountered in the high strength quaternary alloys. A
Copyright © 2010 SciRes. ENG
reaction at 475°C involving M, T, and S phases is usually
encountered, but as copper content increases melting
may be encountered down to 460°C. Usually the J-Phase
has the highest solvus temperature and is slow to dissa-
pear during ingot homogenization. Precipitation harden-
ing in high strength alloys is by the sequence leading to
M-Phase. Zones and coherent precipitates are low in
copper content and precipitates increase in copper con-
tent in the overageing regime. Additions of iron, manga-
nese and silicon interact with one another and with cop-
per and magnesium. Chromium reacts with aluminium
and magnesium to form a dispersoid.
The ageing of rapidly quenched Al-Zn-Mg alloys
[5-10] from room temperature to relatively low ageing
temperatures is accompanied by the generation of GP
zones having an approximately spherical shape. With
increasing ageing time, GP zones increase in size and the
strength of the alloy increases. Extended ageing at tem-
peratures above room temperature transforms the GP
zones in alloys with relativ ely high Zn-Mg ratios in to the
transition precipitate known as  or M, the precursor of
the equilibrium MgZn2, or M phase precipitate. Ageing
time and temperatures that develop the highest strength
characteristics of the T6 temper produce zones to have an
average diameter of 2 to 3.5 nm.
The addition of higher copper contents affords greater
precipitation hardening, with some contribution of Cu
atoms to zone formation. Crystallographic arguments
indicate that copper and aluminium atoms substitute for
zinc in the MgZn2 transition and the phase MgZn2 and
MgAlCu form an isomorphous series in which an alu-
minium atom and a copper atom substitutes for two Zn
atoms. Electropotential measurement and X-ray analysis
indicate that copper atoms enter into the  phase during
ageing temperatures above about 150°C [300°F], thus
ageing these alloys above this temperature substantially
increase their resistance to stress corrosion cracking.
In cast ingot form, alloy 7075 forms one or more
variants of [Fe,Cr]3SiAl12 , Mg2Si and a psuedobinary
eutectic made up of Al and MgZn2. The latter phase
contains Al+Cu as a substitute fo r Zn and can be written
as Mg[Zn,Cu,Al]2. Subsequent heating causes the iron
rich phases to transform to Al7Cu2Fe. Mg2Si is rela-
tively insoluble and tends to somewhat spheroidize,
Mg[Zn,Cu,Al]2 rapidly begins to dissolve and at the
same time some Al2CuMg precipitates also dissolve as
this phase r equires high temperatu re and leng thy soaking
to completely dissolve. Cr is precipitated from super
saturated solution as Cr2Mg3Al18 dispersoid concen-
trated heavily in the primary dendrite regions. A well
solutionized wrought alloy contains only Al7Cu2Fe,
[Fe,Cr]3SiAl12 and Mg2Si, along with the dispersoid.
Recrystallized grains are extremely elongated or flat-
tened because of dispersoid bonding and unrecrystallized
regions are not unusual even in sheet. The unrecrystal-
lized regions are made up of very fine subgrains in which
boundari es a re decorated by hardening precipi t a t e .
2.3. Ageing Behaviour
The 7075 alloys are used both in the un-tempered condi-
tion and tempered conditio n corresponding to T6 and T7
tempering [5-10]. T6 tempering is done by solution heat
treatment at 466°C to 482°C followed by water quench-
ing followed and ageing at between 115°C to 130°C for
obtaining high strength in reasonably short ageing cycle.
An alternate tempering known as T7 or RRA [Retrogres-
sion and Reageing] technique is also used which is a two
stage treatment after T6 tempering; the samples are ret-
rogressed at 200°C for duration short enough to allow
only dissolution of precipitates. The second stage in-
volves reageing i.e. the retrogressed samples were reaged
at 120°C to fully restore the peak aged condition of the
T6 temper. In the RRA treatment the maximum static
tensile and compressive properties of T6 temper are
combined with the resistance to stress corrosion of the
T73 temper. RRA treatment results in precipitate disso-
lution and then re-precipitation growth of coarser equi-
librium phase particles in the grain boundary regions. For
short time retrogression, the hardness can be restored
after reageing but almost no restoration is observed for
prolonged retrogression time [> 50 min]. RRA is effec-
tive only for alloys con taining co herent disperso ids but is
less effective for alloys containing incoherent dispersoids.
The RRA process improves the resistance of stress cor-
rosion cracking in high strength 7xxx alloys which have
the same strength of T6 temper [11-13]. The TMT proc-
ess improved strength, fracture toughness, fatigue prop-
erty and corrosion resistance. Also stability in property,
higher strength, improv ed corrosion resistance and lower
rate of growth of fatigue cracks are obtained by the use
of elevated temperature ageing. TEM studies have shown
that 75 min retrogressed 7075-T6 Al alloy has smaller
grain boundary precipitated and a higher dislocation
concentration than the T6 stage. The optimum retrogres-
sion time changes with material thickness.
2.4. Properties of Alloys
Density is reduced by addition of magnesium and in-
creased by all other element. For commercial alloys it
ranges between 2.740 kg/m3 and 2.830 kg/m3, with most
alloys around 2.800 kg/m3 [1].
The mechanical properties of the cast and aged alloys
are shown in Table 2 [1-4,14].
These alloys achieve their strength by precipitation
through a complex sequence of Guinier Preston [GP]
zones,  nucleating on GP z o n es,  transforming into .
In addition their processing route is rather complex, in-
cluding some plastic strain followed by natural ageing
Copyright © 2010 SciRes. ENG
Table 2. Tensile properties of 7075 alloy.
Property As cast Naturally Aged Artificially Aged
Hardness [HV] 500-700 500-800 800-1200
UTS [MPa] 100-150 150-200 180-250
YS [MPa] 50-100 80-150 120-200
Elongation [%] 1-3 2-5 0-2
Shear strength [MPa] 152 MPa 331 MPa 317 MPa
Poisson Ratio 0.33 at 20°C[680F]
Elastic Modulus [GPa] Shear : 26.9 Tension: 71.0 Compression: 72.4
Fatigue Strength [MPa] 159 MPa [23 Ksi] at 5 × 108 cycles
and a multistep artificial ageing treatment. In these alloys
zinc is the major constitu ent.
The 7XXX alloys are typically used for aircraft struc-
tural parts and other highly stressed applications where
very high strength and good resistance to corrosion is
required. These alloys are widely used in aeronautics
industry and aerospace structural applications
2.5. Property Improvement through Mechanical
The alloy system has been moderately subjected to dif-
ferent types of secondary processing. The following ob-
servations have been reported on secondary processed
alloys of this series:
The grain structure of the rolled plate [15-16] was
found to be comprised of high aspect ratio grains elon-
gated in the rolling direction of the plate. The structure
consisted largely of unrecrystallized material although a
small volume fraction of recrystallized grain was present.
The elongated grains contained an internal structure of
small [1 to 5 µm] equiaxed subgrain with an average
misorientation across subgrain boundaries ~ 5 deg.
The microstructure of a forged product which is
characterized by ‘pan cake’ unrecrystalized grain mor-
phology [17]. Observation of unetched section revealed
the presence of constituent phases with particle sizes up
to 20 µm. The major constituent phase was present as
fragmented rounded gray coloured particles strung out
into longitudinal direction. Qualitative energy dispersive
x-ray analysis of these particle identified them as Al, Cu,
and Fe reached. Upon etching in Keller’s reagent, these
particles turned light brown conforming their identifica-
tion as
The grains of the 7475 alloy formed sheet were
longer, approx. 11.5 µm than in the as received sheet,
approx 9.5 µm [18]. Addition the original grain had pan
cake shape, whereas the grain in the form parts appeared
to be more equiaxed.
Forming pressure corresponding to optimum super
plastic condition and low back pressure result in a more
uniform thickness distribution in the formed part. The
average size of the originally non equiaxed grain in-
creases 20% during initial training. Grains become more
equiaxed with increasing strain while maintaining nearly
constant grain size for strain in the range 1-1.4 [19].
The compressive stress-strain response of as cast
and aged 70 75 allo ys is foun d to d ep end str on gly on bo th
the applied strain rate and the test temperature. Howev er,
the aged material is generally found to be stronger than
the as cast material. The work hardening rate is seen to
decrease with increasing strain, stain rate and tempera-
ture and its value is higher in the aged material than in
the as cast material [20].
3. AMMCs of the Series
3.1. From the Beginning, Since 1980s
Literature review of Al-Zn based composites show
that this system has been widely investigated both in
terms of understanding the mechanism of formation and
property evaluation of composites [21-53]. Different
routes for preparation of composite have been adopted,
however for practical purposes the casting, powder and
squeeze casting routes have been adopted and in some
cases these have been subjected to secondary processing
i.e. extrusion of as cast composite and consolidation fol-
lowed by extrusion for composite prepared by powder
metallurgy ones. Properties have been characterized for
cast and aged composite.
The main dispersoid that have been investigated are
SiC and Al2O3. Stronger matrix alloys tend to produce
stronger composites although th e increase in streng th due
to reinforcement tends to be lower when higher strength
matrix alloy are used and in the case of matrix alloys
with less strength reinforcement has been observed to
lead to reduction in strength; it was found that compos-
ites containing 13 µm particles possesses greater tough-
ness than those containing 5 µm particles.
The excellent flow resistance and ductility of Al 7xxx
series alloys is achieved through the formation of ex-
tremely small, uniformly dispersed particles of a second
phase within the original aluminium phase matrix, i.e.
ageing process. The enhancement in strength is highly
dependent on the type, distribution and size of precipitate
particles present.
The microstructural studies of composite in the cast
Copyright © 2010 SciRes. ENG
Table 3. Best mechanical properties attained by MMCs of 7xxx alloy system.
[Vol.%] Method of Forma-
tion Ageing Condi-
tion Hardness [HV]UTS [MPa] Reduction in Area
[%] 0.2% Proof Stress
SiC [15%]
[5 µm] Spray deposited Under aged
Peak aged
Over aged
SiC [15%] [13 µm] Spray deposited Under aged
Peak aged
Over aged
SiC [15%] [60 µm] Spray deposited Under aged
Peak aged
Over aged
SiC [15%] [2-3
SiC [5%] [2-3 µm] [490/2.07]
condition have been widely investigated. The micro-
structure is dependent on a number of parameters like
macro segregation of dispersoid, primary and secondary
dendrite spacing, solidification rate, solidification time,
dendrite ripening; the cast structure exhibits coarse den-
drite grains, however in contrast small non dendritic
structure was observed in the aged structure. In both ma-
terials the grain size decreases as the strain rate in-
The tensile properties of the composite have been in-
vestigated by several investigators with a view to assess-
ing its properties vis-à-vis conventional alloy to find
commercial application of the composite. Properties have
been evaluated in cast, ag ed and processed cond itions for
composites prepared by different route. The main obser-
vations are: 1) addition of 5 and 13 µm particles in 7075
alloy increased the 0.2% proof stress and UTS values in
composites made by spray deposition over the mono-
lithic material. The addition of 60 µm particle however,
reduced the 0.2% proof stress and UTS of the resulting
composite in the under aged stress and UTS of the re-
sulting composites in the under aged and peak aged con-
dition. There are only small differences in the 0.2% proof
stress and UTS of 5 and 13 µm SiC particulates rein-
forced composite but 60 µm particulate reinforced mate-
rial has much lower yield and fracture strength. 2) It has
been reported that the 0.2% proof stress and tensile
strength tends to increase and toughness and ductility
decreases with increasing volume fraction. For a constant
volume fraction of reinforcement tensile properties gen-
erally tend to increase with a decrease in particle size for
a larger particle composite ductility is reduced compared
with that of monolithic material. 3) The yield strength
[0.2% offset] and ultimate tensile strength increased only
marginally with an increase in strain rate. Increase in
tensile strength is accompanied by decrease in tensile
ductility. 4) Environment was found to have little influ-
ence on yield strain of the high purity alloy. However,
strength increases only marginally with an increase in
strain rate.
Literature on secondary processing of composites of
this series of alloys is limited although extensive reports
are available on the secondary processing of alloys of
7xxx series. However, these observations are not been
detailed in this p aper since it deals with compo sites only.
Some studies have been carried out on the effect of al-
loying elements in secondary processing of this compos-
ite system; it mention s that the presence of iron pr oduces
a slight decrease in strength, elongation and fracture in
wrought product. Iron together with manganese may
produce a slight increase in strength with limited in-
crease in elongation. Manganese, chromium, molybde-
num and zirconium appear to have a strong strengthening
effect with corresponding decrease in % elongation. The
effect of silicon addition varies some what depending on
Mg: Zn ratio [54,55]. The properties attained by investi-
gators adopting different processing routes for making
composites are given in Table 3 below:
Since this series of alloys are not generally used for
wear resistant purposes there is no report on the wear
properties of composites prepared from this series of
alloys. However there is a study in which the wear be-
haviour of high tensile strength aluminium alloys under
dry and lubricated conditions for 7004-T6 alloys were
compared with 2014 alloys. It has been observed that
these alloys are easily worn out. Severe wear at high
contact load have been reported. In paraffin oil, the wear
rates were approx 1/10 the wear rates under dry condi-
Copyright © 2010 SciRes. ENG
tions 7004-T6 showed a lower friction coefficient com-
pared to 2024-T4. For 7004-T6, wear cracks which
would propagate in the sliding direction occurred and
large and elongated particles were detected [48-53].
Creep re sistance of the alloys and composites both are
relatively low, however longer creep life can be achieved
if the load is intermittently relieved and the material is
allowed a period of rest. Residual stresses from working
processes may reduce creep resistance and so does pre-
cipitation during creep [56-58].
3.2. The Recent Past [Since 2000]
The recent past has not since much progress in the re-
search of this series of composites. The researchers have
more or less confirmed the findings of earlier researchers
and at times provided an understanding of the behaviour
observed. The role of work hardening characteristics of
matrix alloys in the strengthening of metal matrix com-
posites has been shown to be associated with a high dis-
location density in the matrix due to the difference in
coefficient of thermal expansion between the reinforce-
ment and the matrix. In a study where the composites
were made by a pressureless infiltration method, the
composite reinforced with SiC particles exhibited higher
strength values than the control alloy in all aging condi-
tions (underaged (UA), peak-aged (PA), and overaged
(OA)), as well as a solution treated condition. Spontane-
ous infiltration was further prompted owing to the com-
bined effect of both Mg and Zn. This may lead to an en-
hancement of wetting between the molten alloy and the
reinforcement. Consequently, strength improvement in a
composite may be attributed to good bond strength via
enhancement of wetting. The grain size of the control
alloy is greatly decreased to about 2.5 mm compared to
10 mm for the commercial alloy. In addition, the grain
size in the composite is further decreased to about 2mm.
These grain refinements contributed to strengthening of
the control alloy and the composite [59]. In another re-
cent study, the role of work hardening characteri stics has
been shown to be due to increased prismatic punching of
dislocations. This relationship of decreasing work hard-
ening rate associated with increasing prismatic punching
of dislocations for different Al-based matrices is in the
order 7075, 2014, 7010, 2024, 6061 and commercial
purity aluminium leading to increased strength incre-
ments has been noted. From the study, it is concluded
that lower the work hardening rate, higher is the
strengthening and vice versa in particulate metal–ma-
trix composites [60]. The corrosion behaviour of the
MMCs was studied by electrochemical measurements to
study the effect of the addition of silicon carbide on the
corrosion behaviour of the MMC [61]. The electro-
chemical noise result shows that the amplitude of the
potential noise of the composite is lower than that of the
spray deposited 7075 alloy. The potentiodynamic polari-
zation curves results show that both the cathodic oxygen
reduction current density and the anodic dissolution cur-
rent density of the 7075/SiCp MMC are less than those
of the 7075 alloy. Thus, the addition of SiC particles
increases the corrosion resistance of the MMC. This may
be due to that the microstructure of the spray deposited
MMC is compact and SiC particles are nonmetallic ma-
terial, the addition of it minimizes the real corrosion area
of the alloy. In another study [62], an attempt has been
made to fabricate Al-SiCp and Al-Al2O3 compos ite mate-
rials by powder metallurgy technique at different volume
percentage of reinforcement (5, 10, 15, 20, and 25%).
The corrosion behavior of the composites was analyzed
using AC Gill potentiostat with 3.5 wt% NaCl medium.
Four factors, five level, central composite, rotatable de-
sign matrix is used to optimize the required number of
experiments. The mathematical models were developed
by the response surface method (RSM). The developed
models have been checked for their adequacy and sig-
nificance by the F-test and t-test, respectively. The re-
sults obtained from the mathematical models have been
optimized and also tested using conformity test runs that
closely match the experimental results. A mathematical
model has been successfully developed [63] to predict
the wear rate of AA7075 aluminium/SiCp composite
material fabricated by powder metallurgy technique at
95% confid en ce leve l with in rang e of inv estig atio n b ased
on the results obtained after studying the effect of rein-
forcement particle size, applied load and slid ing sp eed on
the dry sliding wear behaviour of AA7075 alumin-
ium/SiCp composites fabricated by powder metallurgy
technique with 5–2 5 vol.% SiCp with an average particle
size of about 40–150µm using the conventional powder
metallurgy (P/M) process. The effect of selected process
variables on the porosity of 7075 Al alloy 10% SiC
composite and subsequent optimal settings of the vari-
ables have been obtained using Taguchi method [64].
The results indicate that the holding time, holding tem-
perature and stirring speed are the significant variables.
Stirring time is insignificant variable. As the holding
time, holding temperature and stirring time increases the
porosity decreases. When the stirring speed increases,
porosity increases. The pr edicted optimal valu e of poros-
ity is 4.71%.
Although very limited studies have been reported on
processing of composites, a particular study reports in
detail the effect of extrusion and rolling on a SiCp/7075
aluminum alloy composite fabricated by squeeze casting
using a vortex method [65]. The composites were hot
rolled to the total rolling reduction of about 94% at tem-
peratures between 573 K and 773 K and at a rolling
strain per pass of 0.10. Superplastic characteristics such
as microstructure and apparent activation energy were
compared with these of the composites made by other
processes in order to clarify the sup erplastic deformation
mechanism. Fine grain size of about 1 μm was attained
Copyright © 2010 SciRes. ENG
in the composite rolled at a temperature of 573 K and at
the strain per pass of 0.1. In the case of rolling at 573 K,
the composite obtained exhibited high strain rate super-
plasticity with a maximum elongation of about 230% at
strain rate of 7 × 10-1 s 1 and at 798 K. Plots between ε1
and σ showed linear relation when exponent of n = 2 was
assumed. Threshold stresses obtained from the linear
relations were largely dependent on the testing tempera-
ture. Apparent activation energy determined from rela-
tionship between strain rate and testing temperatures was
244 kJ/mol, which was smaller than that for the pow-
der metallurgical and mechanically alloyed Alumin-
ium composite. It seems that there is no substantial dif-
ference of high strain rate superplastic mechanism be-
tween the composite fabricated by a vortex method and
powder metallurgical aluminum alloy composites.
3.3. What Has Seldom Been Tried
Inspite of the enormous amount of R&D that has gone
into Al-based MMCs of every possible alloy with dif-
ferent dispersoids establishing beyond doubt the useful-
ness of making composites that can be competitive on a
production scale to Al-alloys and steels in some cases,
and although some of these have already been in com-
mercial use now, a few areas still remained to be ex-
plored. These areas have seldom been researched on or
reported in the literature though all researchers would
agree that these areas needs to be addressed to before
Al-base composites are freely available in the market as
an alternative to commercial alloys for engineering ap-
plications. The areas in which very little or no work has
been done/reported include the following:
Machining of Composites: Particulate MMCs are
invariably harder than the base alloy and more abrasive
due to the incorporation of the second phase. Hence ma-
chining of these poses difficulty as tools used for
Al-based alloys are generally used even for the MMCs;
the tools wear out fast and needs frequent replacement
that adds to the cost of processing of these materials.
Most of the times when conventional tools used for Al
do not work properly or are blunted tools for machining
harder materials like diamond tip tools are used which
are costly. But a systematic study has not been done in
trying to fabricate/design tools especially for MMCs.
Joining of Composites: This aspect has not at all
been reported though it is a very important area as mak-
ing components will require joining of two pieces quite
frequently. Welding could be the most apt method of
joining but welding rods for the purpose needs to be de-
veloped. The other kn own methods of joinin g similar and
dissimilar metals needs to be assessed and researched on.
Primary/Secondary Processing: The literature avail-
able on primary processing including forging, extrusion
and rolling is very limited and scanty. Most of the work
is on extrusion of only MMCs made from powder met-
allurgy route in which small samples have been extruded.
But before thinking of engineering applications for
MMCs, the difficulties in processing these materials
needs to be addressed to and parameters optimised. The
present paper reports some of the authors’ experience in
this field.
Equi Channel Angular Pressing: Equi Channel An-
gular Pressing [ECAP] is emerging as a competitive
route of processing Al-based alloys wherein the grain
size is drastically decreased by passing billets through a
die containing two channels, equal in cross section, in-
tersecting at a certain angle introducing large shear strain
without any reduc tion in the cr oss section of th e material.
Since the process is applicable to large samples, it ap-
pears to have the potential for significantly changing the
material properties by effective grain size reduction even
in a bulk form. This area has never been reported to have
been tried for MMCs. It is felt that Al-based MMCs will
respond positively to ECAP adding to the benefits of
composite making.
Ultrafine Dispersoids: Dispersoid size, shape, vol-
ume fraction, wettability and distribution play the most
important role in the properties attained in MMCs. A lot
of research has centered on optimizing these parameters
and based on the results property designing of compos-
ites is possible. However, all the experimentation has
stopped at decreasing the dispersoid level below 10µm,
possibly due to the d ifficulty in dispersing uniformly and
without coagulation finer particles in the matrix espe-
cially when the liquid metallurgy route is adopted for
making the composites. If the coagulation problem can-
not be addressed on decreasing the particle size below
10 µm, alternate methods of fabrication have to be at-
tempted at like in-situ composites, which has been a
modern trend of research for making MMCs. Again, in
the present age of nano materials methods of dispersion
or making composites with nano sized dispersoids holds
a lot of potential for commercial exploitation in the fu-
Bulk Production: One of the causes could be the
most important cause as to why Al-based MMCs inspite
of all its advantages are yet to see the light of the day
extensively as engineering components is the level of
production . Researchers are content with the h igh quality
yet small quantities that they can repetitively produce in
their laboratories, but the engineering sector is not con-
vinced that the same benefits can be replicated on up
scaling. Either party is not ready to bridge the gap by
producing the material in bulk due to a number of rea-
sons different for either party. But unless the fear in the
mind of the users is removed as to the property im-
provement even when produced in bulk quantity by the
researchers, their dream to see their research product in
the market will be difficult to realise. So also, the manu-
facturing units should be entrepreneurial enough to take
some risk in this matter, as certain changes might be
Copyright © 2010 SciRes. ENG
needed in their production line up when alloys are re-
placed by composites. Somewhere by some groups are
needed to take up the challenge of bridging the gap if
Al-MMCs are to see the light of the day in the near fu-
4. Our Results
The present paper has made an attempt of finding the
limit of enhancing the mechanical and wear resistance
properties of a commonly used Aluminium alloy, the
7075 alloy by different routs; by making 1) composites 2)
heat treatment of the alloy and composite and also 3)
extruding the alloy an d composite to rods. A comparison
in the properties attained by the different routes would
help find the limit up to which the properties in this alloy
system can be stretched. The experimental setups and
results obtained are detailed below.
4.1. Experimental Details
4.1.1. Making of Allo y and Composite
The Al-based alloy confirming to the 7075 composition
with 1.6 Cu, 2.5 Mg, 5.6 Zn, 0.3 Cr was selected for the
present study. The chemical composition of the alloy was
analysed using SPARKMET optical spectrometer
[Model: SPECTRA] for confirmation .
For preparing composites from these alloys; 10 vol-
ume % of dispersoids of SiC of were used with size be-
tween 20-40 m. The composites were prepared by the
liquid metallurgy technique. For preparing the compos-
ites, the alloys were melted in a gas fired furnace in a
graphite crucible. The melted alloy was fluxed with
Coveral 11 and degassed with dry nitrogen gas. The pre
weighed quantity and preheated dispersoids were poured
into the melt after passing through a sieve. During inser-
tion of the dispersoid the melt was stirred constantly by
means of a stirrer placed in the melt operated by a motor.
The stirrer speed was controlled as required. After com-
plete insertion of the dispersoids in the molten alloy, the
alloy was simultaneously stirred and heated for some
time for uniform mixing and temperature. The melt with
the dispersoids were then poured into preheated perma-
nent moulds of required size and shape.
4.1.2. Ageing and Tempering
The alloys and composites were aged under T6 temper-
ing and adopting retrogression and re-ageing method
[RRA]. In the RRA method, the samples were quenched
and aged as per the fo llowing ste ps:
The alloys and composites were aged at 490°C for 8
hours and water quenched. The quenched samples were
heat treated at 120°C for 24 hours and were furnace
cooled. This corresponds to T6 ageing.
The T6 aged samples were retrogressed at 200°C for
45 minutes.
The retrogressed samples were re-aged at 120°C for
24 hours.
4.1.3. Extrusion
A 400 tonne hydraulic press with extrusion facility was
used for extrusion experimentation. The alloy and com-
posites were sized to a diameter of 50mm diameter and ~
60mm length for extrusion to rods at an extrusion ratio of
10:1. The billets were homogenised under at 480ºC for
12 hours. A pressing speed of 0.4mm/sec was used and
billets were soaked at 350ºC for two hours before extru-
sion. The container, liner and billet were maintained at
the extrusion temperature by externally heating the com-
plete setup, to prevent heat loss during extrusion. The
pressure required for successful extrusion was compared
for optimizing the extrusion process.
4.1.4. Laboratory Tests
For microstructural studies, samples from the cast and
aged alloys and composites of size approximately 20 mm
diameters were metallographically polished and etched
in Kellar’s reagent. The microstructure of the sample was
observed in the optical microscope at lower magnifica-
tions upto X500 and in the JEOL scanning electron mi-
croscope operating at 20KV at higher magnifications for
a better understanding of the constituent phases in the
microstructures. The secondary mode of electron emis-
sion was initially used for the microstructural investiga-
tions. However, when the precipitates expected to be
precipitated at the grain boundaries was not properly
visible with this mode, the back scattered electron emis-
sion mode of the scanning electron microscope was used
for observi n g t he precipitation of phases.
Bulk hardness was determined using the EQUOTIP
SN 716-1159, Vers-1.16 hardness tester in which the
indentation was produced using the Tungsten carbide
ball indenter of 20mm diameter with a test tip of 3mm
The tensile properties i.e. ultimate tensile strength
[UTS] and % elongation were determined in the Shima-
dzu make Universal Testing Machine [UTM] of capacity
100 tonne. Tests were carried out at a load of 4 tonne
with load increment of 0.4 tonne. Fractographic analysis
of the fractured tensile samples was carried out in which
the fractured surface was observed in the scanning elec-
tron microscope in ord er to understand the mechanism of
material failure and changes in the mode of failure, if an y,
as a result of adding particulates to the alloy and extrud-
ing them.
The specimens were subjected to dry sliding wear tests
under dry conditions using a pin-on-disc wear testing
[Figure] [Cameron Plint make]. The test specimen in this
case were in the form of cylindrical pins of length 53mm
and 8mm in diameter and the disc counterpart was of
AISI 304 grade stainless steel [Fe 0.08-C 2.0-Mn 1.0- Si
8.0-Ni 18.0 Cr] having a hardness of 194 HV. The
specimen was held against the rotating steel disc and this
Copyright © 2010 SciRes. ENG
Copyright © 2010 SciRes. ENG
sliding motion results in wear of the pin. The weight of
the specimen was taken after every 500 m, sliding dis-
tance. The weight loss was calculated from the difference
in weight between the initial weight and weight after a
specified number of rotations. Dividing the weight loss
by the density of the material [calculated by Archimedes
principle] gives the volume loss. This volume loss was
taken for comparing the wear behaviour. Further, the
wear rate was calculated by dividing the volume loss by
the total distance traversed. Experiments were conducted
in the present stud y at two speeds of rotation, one low at
200 rpm and the other high at 750 rpm corresponding to
linear speeds of 1.04 m/s and 3.92 m/s respectively.
Tests were carried out under three applied pressures, i.e.
1, 3 and 5 MPa; wear loss was measured at distance cor-
responding to a sliding distance of 2500 m at intervals of
500 m and the corresponding volume loss calculated.
Temperature rise of the specimen was not noted under
the present study. The worn surface was studied under
the scanning electron microscope [SEM] to study the
nature of material removal and surface damage and to
compare between material removal mechanism between
the cast, aged and extruded conditions and between the
alloy and composites under identical conditions.
4.2. Results and Discussion
4.2.1. Microstructural Observations
The microstructure of the cast, aged under T6 and RRA
conditions and extrud ed rods for the alloy and compo site
is shown in Figure 2.
In the as cast state, the 7075 alloy shows heterogene-
ous precipitation along the grain boundaries as well as
inside the grain boundaries [Figure 2(a)]. The alloy ex-
hibits a black filigree of Mg Zn [18]. Und er T6 con dition s ,
there is heavy precipitation in the grains although the
grain boundaries are not very clearly defined [Figure
2(b)], and under RRA conditions dissolution of grain
boundaries was observed [Figure 2(c)] and shows a
granular structure with grains of varying size and distri-
bution of very fine phases in the grains in the Al-matrix
resulting from non uniform solidification rates during
casting [Figure 2(a)]. On extrusion directionality is seen
along the longitudinal direction and the grain structure is
completely broken down [Figure 2(d)]. The transverse
section exhibits a granular structure [Figure 2(e)] but the
phases are not much resolved. The cast composite exhib-
its [Figure 2(f)] a near uniform distribution of the par-
ticulates within the matrix and clear dendrites are seen in
the matrix of Aluminium; grain boundaries are clearly
defined with some precipitates in the grains. On age-
ing the 7075 composite, dissolution of grain bounda-
ries was observed together with coagulation of pre-
cipitates around the SiC dispersoids [Figure 2(g)].
Again on extrusion directionality [Figure 2(h)] is seen
and distribution of particulates uniformly is also ob-
served. In the transverse section of the extruded compos-
ite, studs of the particle heads can be seen uniformly dis-
tributed in th e matrix [Figure 2(i)]. A few particles [dis-
persoids] are seen to have been conglomerated in the
extruded condition.
4.2.2. Hardness
The bulk hardness [HB] was measured on at least ten
locations on the polished and etched sample and the av-
erage of the values with standard deviation are tabulated
in Table 4.
It is seen from the above table that making composites
drastically increases the hardness of the alloy by nearly
40% and extruding them further increases the value by
some 25% [Table 2]; the latter improvement is due to
strain induced strengthening during the processes in-
4.2.3. Tensile Properties and Fractographic Studies
The stress-strain graph for the extruded alloy and com-
posite is shown in Figure 3(a) and (b) respectively.
From the nature of the graph it is seen that the alloy
exhibits a ductile behaviour but the composite shows a
brittle failure. This is possibly due to the incorpor ation of
the SiC particles that are uniformly distributed in the
matrix giving the composite a brittle nature. The fracto-
graphic studies of the alloy and composite in the cast
condition [Figure 4(a) and 4(b) respectively] confirm
the loss of ductility on forming composites as the former
shows cup and cone type of fracture relating to ductile
nature and the latter shows cleavage facets, voids with
inclusions [marked by *] and micro cracks [marked by ar-
row] which are characteristic features of brittle nature of
failure. The deep voids with inclusions are the possible
cause of fract ure in t his case .
The Ultimate Tensile strength [UTS] of the cast, ho-
mogenised and extruded alloys and composites are shown
in Table 5 and plotted in Figure 5 for comparison. It is seen
that homogenisation increases the UTS for both the alloy
and composite resulting possibly from uniformity in the
distribution of phases. Extrusion further improves the UTS
in the case of alloy though marginally; however for com-
posites a slight decrease has been recorded. The reported
results are an average of three sam ples tested in each case.
The fractographs of the tested samples reveals the nature
of failure and the effect of homogenisation and/or ex trusion
on the nature of the sample if any. The fractograph of
the homogenised alloy [Figure 6(a)] is very similar to
the cast alloy [Figure 6(a)] exhibiting uniform dim-
Table 4. Hardness of alloy and composite under dif-
ferent conditions.
System As Cast RRA Extruded
Alloy .22 7+
.0650 3+.071 3+.696
Composite .35 8+
.679 3+.172 4+.2101
a: Cast Alloy b: Aged Alloy –T6 c: Aged Alloy - RRA
d: Extruded Alloy- longitudinal d irection e: Extruded Alloy- transverse direction f: Cast Composite
g: Aged Composite –T6 h: Extruded Composite-longitudi nal direction i: Extruded Com posite- tra ns v er s e directio n
Figure 2. Microstruct ur e under different conditions.
0 2 46 810121416
Stress, MPa
-0.0500.05 0.10.15 0.20.25 0.3
Stress, MPa
Figure 3. (a) Stress-Strain gr aph for e x tr ude d alloy; (b) Stress-Strain graph for extruded composite.
ples [marked A], with tear r idges indicative of cup and cone
type of fracture [marked B] and large number of voids
[marked C] indicating ductile fracture [66]. The smooth
and fibrous region with proper neck formation at lower
magnifications [Figure 6(b) & (c)] confirms a pure ductile
mode of fracture in both the homogenised and extruded
samples. The broken surface of the extruded alloy also
exhibit similar structure [Figure 6(d)] with a
Copyright © 2010 SciRes. ENG
Table 5. UTS of Samples.
Sample UTS, MPa
Cast Alloy 87.57 + 5.23
Homogenised alloy 159.09 + 1.88
Extruded alloy 165.59 + 2.08
Cast Composite 81.85 + 6.77
Homogenised Composite 112.08 + 1.48
Extruded Compo s it e 103.10 + 3.15
Figure 4. Fractured surface (a) ductile failure for alloy (b)
brittle failure of composites.
high density of equiaxed dimples that is found mainly in
ductile materials. Fracture by micro void coalescence [6 7]
is the only mode of fracture to be confirmed. No other
physical parameter can be seen in the fractographs of the
In the case of composites, the fractured surface exhib-
its a slight modification on homogenisation in which
voids are seen along with the cleavage in a very scattered
manner [Figure 7(a)] unlike the cast condition where
only cleavage facets indicative of brittleness is observed
[Figure 7(b)]. The river patterns on the facets can also
be seen [Figure 7(b)] a clear indication of the brittle
mode of the fracture in the homogenised samples [68].
Extrusion however increases the number of voids in the
fractographs to a considerable extent; certain portions
reveal a smoother surface of fracture, revealing numer-
ous equiaxed dimples, suggestive of ductile tensile frac-
ture [Figure 7(c)]; at times incomplete material removal
processes are also seen [marked by arrow]. Higher mag-
nification fractographs [Figure 7(d)] shows dimples with
an inclusion, which could have been the cause of fracture
along with some stretched cleavage facets and majority
of shallow dimples. This indicates a mixed mode of
fracture i.e. ductile and brittle fracture in the extruded
4.2.4. Slidi ng Wear Behaviour and Worn Surf ace
Sliding wear tests were carried out on the cast, homoge-
nised and extruded alloy and composite under two
speeds of rotation 200 and 750rpm and under three pres-
sures corresponding to 1, 3 and 5 MPa and the volume
loss against sliding distance plo tted. It was found that the
alloy in all the conditions seized at 5 MPa pressure at
both the speeds of rotation, whereas composites sus-
tained the pressure at the lower speed of rotation. Again,
alloys seized even at 3MPa pressure at the higher speed
of rotation whereas the composites could sustain the
conditions upto the sliding distance tested i.e. 2500 m.
Seizure of the material can be understood from the ma-
chine stopping and also from the surface being tested
which shows material movement; the scanning electron
micrographs of seized samples [Figure 8] shows com-
plete overlapping of material and material movement
cutting across wear tracks [Figure 8(a)]; the seized ma-
terial in turn do not show any structure and looks like a
lump of mass on the surface adhered to the surface and
material from the inner surface ar e also exposed [Figure
In order to compare the behaviour between the alloy
and composite for a particular experimental condition,
the volume loss against sliding distance is plotted as in
Figure 9, shown below. It is seen that under all the
experimental conditions, the composite exhibits lesser
volume loss as compared to the alloy through the
duration of the test. Moreover, homogenisation of alloy
or composite doesnot have any significant bearing in
reducing the volume loss and this is probably due to
softening of the matrix as a result o f homogenisation and
volume loss is directly dependant on the hardness and
stiffness of the material. The volume loss for composites
is significantly lower than the alloy under a particular
experimental condition, especially under the same
sample condition. Extrusion has a significant effect in
decreasing the volume loss for both alloy and
compositeand under a pressure of 3 MPa and 200 rpm,
Copyright © 2010 SciRes. ENG
159.09 165.59
112.08 103.1
C ast Alloy
Figure 5. Comparative plot of UTS of the samples.
(a) )b(
(c) (d)
Figure 6. Fractographs of alloys under different conditions. (a) Homogenised alloy; (b) neck formation at lower magnifica-
ions for homogenised and extruded alloy s; (d) e x tr ude d alloy. t
the extruded alloy even exhibits less volume loss i.e.
better wear resistance than the cast and homogenised composite. It is interesting to note that higher speeds of
rotation results in lesser volume loss which is probably
Copyright © 2010 SciRes. ENG
(a) )b(
(c) (d)
Figure 7. Fractographs of composites under different conditions.(a) Homogenised composite; (b) homogenised composite
showing river patterns on the cleavage facets; (c) extruded composite showing incomplete material removal processes marked
by arrow; (d) magnified view of ‘c’ showing stretched cleavage facets and shallow dimples.
(a) )b(
Figure 8. Micrographs of seized samples.
due to less time that the surfaces get to slid e against each
other due to the enhanced speed; the material damage is
also less in such cases as seen from comparing its worn
surfaces at a particular load at different speeds [Figure10].
The effect of load on volume loss is more p redomin ant in
the cast condition that in the extru d ed conditio n as can be
seen on comparing Figures 9(a) and (b).
Unlike alloys, which have seized even under a pres-
sure of 3 MPa at higher speeds of rotation, composites
have not experienced seizure under a pressure of 3 MPa
Copyright © 2010 SciRes. ENG
05001000 1500 2000 2500 3000
Sli ding Distan ce , m
Volume Loss, mm3
Alloy CastAlloy Homogenised
Alloy Ext rudedComposite Cast
Composite HomogenisedComposite Extruded
1 MPa, 200 rpm
05001000 1500 20002500 3000
Sliding Dist ance, m
Volume Loss, mm3
Alloy CastAlloy Homogenised
Alloy ExtrudedCo mpo s i te Cast
Composite HomogenisedCom po site Extruded
1 MPa, 750 rpm
05001000 1500 20002500 3000
Sli ding Distance, m
Volume Loss, mm3
Alloy CastAlloy Homogenised
Alloy Ext rudedCompos i te Ca st
Composite HomogenisedComposite E xtr uded
3MPa, 200 rpm
Figure 9. Comparison of volume loss with sliding distance
between alloy and composite under different conditions.
under any condition; however the cast and homogenised
composites have seized after traversing a minimal dis-
tance when the pressure was increased to 5MPa, but even
in this condition at both the speeds of rotation they could
complete the duration of the test in the extruded condi-
tion without any signs of seizure on its worn surface
[Figure 11]. The advantage of making composites even
Figure10. Worn surface showing effect of speed at a
pressure of 1 MPa. (a)alloy at 200 rpm; (b) alloy at 750 rpm;
(c) composite at 200 rpm; (d) composite at 750 rpm.
Copyright © 2010 SciRes. ENG
Copyright © 2010 SciRes. ENG
In order to compare the effect of extrusion on wear
behaviour of composites, the variation for composites
under experimental conditions where alloys have seized
is plotted in Figure 12. In the Figure, the nature of lines
represents the pressure applied like dashed lines repre-
sents a pressure of 3 MPa and a filled line a pr essure of 5
MPa; again filled symbols represents a sliding speed of
200 rpm and unfilled symbols a speed of rotation of
700 rpm; likewise the letters A, D and G is the cast, B, E
and H the homogenised and C, F and I the extruded con-
ditions. From the graph it is seen th at the (i) under condi-
tions where the cast and homogenised composites seize
at 5 MPa pr essure when th e speed of rotation is 750 rpm
but the extruded composites do not seize (ii) homogeni-
sation or heat treatment softens the material leading to
more volume loss under the same experimental condi-
tions even over the cast condition (iii) extruded compos-
ites exhibits better wear resistance [as measured from
inverse of volume loss] than the cast condition at high
pressures; however at lower pressures the cast condition
performs marginally better or it can be said that there is
no beneficial effect of extrusion when the load is com-
paratively less, that is to say that extrusion makes the
composites more versatile to withstand harsher condi-
tions of wear re l at e d dam a ge.
5. Achievements in a Nutshell
Al-Zn based composites prepared through different
routes like casting, powder and squeeze casting routes
have been extensively reported and have been exten-
sively been characterised on a laboratory scale and in
very rare cases have these been subjected to secondary
processing. However any engineering component from
the composites has still no t been reported. The main dis-
persoid that have been investigated are SiC and Al2O3.
The main findings and best results obtained as a result of
forming MMCs are as follows:
Stronger matrix alloys tend to produce stronger
composites although the increase in strength due to rein-
forcement tends to be lower when higher strength matrix
alloy; it was found that composites containing 13 µm
particles possesses greater toughness than those contain-
ing 5 µm particles.
The microstructural studies of composite in the cast
condition is dependent on a number of parameters like
macro segregation of dispersoid, primary and secondary
dendrite spacing, solidification rate, solidification time,
dendrite ripening; the cast structure exhibits coarse den-
drite grains, however in contrast small non dendritic
structure was observed in the aged structure. In both ma-
terials the grain size decreases as the strain rate in-
Figure 11. Worn surface of composites at 5MPa pressure. (a)
Cast at 200 rpm; (b) Extruded at 200 rpm; (c) Extruded at
750 rpm.
from a non conventional wear resistant alloy like the one
being discussed is thus established from the above stud-
ies and it brings home the point that extrusion improves
the sliding wear resistance properties in both the alloy
and composite.
The tensile properties of the composite have been
investigated by several investigators with a view to as-
sessing its properties vis-à-vis conventional alloy to find
05001000 15002000 2500 3000 3500
Sliding Distance, m
Volume Loss, m m3
A: Cast, 3MPa, 750 rpm; B: Homogenised, 3MPa, 750 rpm; C: Extruded, 3MPa, 750 r pm
D: Cast, 5MPa, 200 rpm; E: Homogenised, 5MPa, 200 rpm; F: Extruded, 5MPa, 200 rpm
G: Cast, 5MPa, 750 rpm; H: Homogenised, 5MPa, 750 rpm; I: Extruded, 5MPa, 750 rpm
Figure 12. Comparison of wear behaviour under different conditions for composites.
commercial application of the composite. It was found
that the properties are dependant on several factors like
particle size and volume fraction and there is a peak in
the properties attained limiting the size and volume of
dispersoids possible for better properties over the mono-
lithic material.
The best mechanical properties attained in MMCs
of this class are made through powder metallurgy routes
though this route is not practical for large components
and is not economical.
Strength improvement in a composite may be at-
tributed to good bond strength via enhancement of wet-
ting; also presence of the combination of Mg and Zn
helps in wetting in this alloy system. This again is de-
pendant on the forming technique and spontaneous infil-
tration has been found to give the best properties due to
an enhancement of wetting between the molten alloy and
the reinforcement.
Effect of alloying elements has an effect even on
the properties even after secondary processing of this
composite system; the presence of iron produces a slight
decrease in strength, elongation and fracture in wrought
product. Iron together with manganese may produce a
slight increase in strength with limited increase in elon-
gation. Manganese, chromium, molybdenum and zirco-
nium appear to have a strong strengthening effect with
corresponding decrease in % elongation. The effect of
silicon addition varies some what depending on Mg: Zn
It has been observed that these alloys are easily worn
out. Severe wear at high con tact lo ad have been reported. In
paraffin o il, the wear ra tes were approx 1/10 the w ear rates
under dry conditions; however 7004-T6 showed a lower
friction coefficient compared to 2024-T4.
Creep resistance of the alloys is relatively low, es-
pecially for high strength alloys. However, such studies
have not been reported for composites of these alloys.
The role of work hardening characteristics has been
shown to be associated with a high dislocation density in
the matrix due to the difference in coefficient of thermal
expansion between the reinforcement and the matrix.
The corrosion behaviour of the MMCs was studied
by electrochemical measurements to study the effect of
the addition of silicon carbide on th e corrosion behaviour
of the MMC. The corrosion behaviour of the metal ma-
trix composite show that both the cathodic oxygen re-
duction current density and the anodic dissolution current
density are less than those of the allo y. Thus , th e addition
of SiC particles increases the corrosion resistance of the
Very limited studies have been reported on proc-
essing of composites. In a study, the composites were hot
rolled to the total rolling reduction of about 94% at tem-
peratures between 573 K and 773 K and at a rolling
strain per pass of 0.10.
Inspite of the enormous amount of R&D that has
Copyright © 2010 SciRes. ENG
gone into Al-based MMCs of this alloy establishing be-
yond doubt the usefulness of making composites they are
yet to produced at competitive rates on a production
scale. Until this is done the possibility of making com-
ponents from the composites inspite of all its advantages
will remain a dream.
Again, before components are thought of some ar-
eas needs address. Till now conventional methods/tools
as used for the alloys have been used for the purpose but
further development and exclusive research on compos-
ites per se needs to be addressed to. These include:
Machining of C omposites
Joining of Composites
Primary/Se condary Pr o cessi n g
Equi Channel An g ul a r Pressing
Composites from Ultrafine Dispersoids
6. The Stretch, Limit and The Future
The contributions made towards increasing the state of
knowledge in enhancing the physical, mechanical, wear
and corrosion resistance has established the efficacy of
making composites with better properties than the alloy;
the conditions under which the improvements can be best
realised have also been established by different research-
ers throughout the world mainly th rough laboratory scale
studies. It is seen that the properties attained has been
stretched to the maximum possible and there is no much
scope of further improvement in the properties as the
methods and materials used have all been optimised; and
since the last decade no further improvement in terms of
properties have been recorded. This in a way has put a
limiting factor for further improvem ent.
The next step would be to mass produce these com-
posites adopting the techniques and adhering to parame-
ters that have been established to give the best results
and establish feasibility of making composites on a mass
scale retaining the improved prop erties. Until this is done
the ultimate aim of making meaningful engineering
components from the composites will never be realised.
Together with mass production and making compo-
nents for the engineering sector, certain aspects specifi-
cally for composites need to be ad dressed like machining,
joining, primary and secondary processing.
Further some basic research is also needed like de-
creasing the size of the dispersoid further to nano- or
micro-size and to establish the parameters for uniform
distribution; this definitely will further enhance the
properties. Also computer simulation and mathematical
modeling of composite fabrication is an area where some
scattered work has been done; this area would also need
to be addressed to that would cut down on actual ex-
perimentation and prediction of properties attainable if
the nature, shape, size, volume fraction and processing
techniques are known.
Some twenty years of research has gone into making
composites and with that we till date know so much. An-
other ten to fifteen years of research in certain areas still
not addressed to as mentioned above would help see the
composites in the market readily available. It is time in-
stead of reinventing the wheel and treading known paths
of property evaluation and improvement that are now
more or less guaranteed, the critical issues as mentioned
above are taken up for research that would help in taking
the laboratory scale composites a step further and a step
closer to the engineering sector.
. References
[1] J. E. Hatch, “Aluminium, Properties, Physical Metal-
lurgy and Phase Diagrams,” American Society for Met-
als, Metals Park, Ohio, 1971.
[2] I .J. Polmear, “Light Metal Age, Metallurgy of the Light
Metals,” Edword Arnold Publishers Ltd, London, 1981,
[3] I. J. Polmear, “Light Metal Age, Metallurgy of the Light
Metals,” Edword Arnold Publishers Ltd, London, 1981,
[4] L. F. Mondolfo, “Grain Refinement in Aluminum Al-
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