Materials Sciences and Applications, 2011, 2, 1627-1630
doi:10.4236/msa.2011.211216 Published Online November 2011 (http://www.SciRP.org/journal/msa)
1627
Deformation-Induced Large Ductility of Super
Saturated Solid Solution Fe-Cu Alloy
Licai Fu*, Jun Yang, Q i nling Bi, Weim in Liu
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China.
Received September 25th, 2011; revised October 29th, 2011; accepted November 7th, 2011.
ABSTRACT
The mechanical properties of super saturated solid solution Fe60Cu40 alloy has been investigated using compression test.
The results show that the grain precipitation and phase transformation occurs during compressive deformation result-
ing in large work-hardening ability, high strength and large ductility. Our results demonstrate that this novel architec-
ture offers a design pathway towards a new generation of strong materials with large ductility.
Keywords: Fe-Cu Alloy, Super Solid Solution, Precipitation, Phase Transform, Ductility
1. Introduction
Strength and ductility are of the most important me-
chanical properties of structural materials. However, they
are often difficult to obtain the high strong and large duc-
tility simultaneously [1,2]. Generally, the low ductility of
materials is attributed to the lack of work hardening
caused by their inability to accumulate dislocations [3].
Therefore, the basic idea to improve the ductility of ma-
terials is to regain the work hardening (dislocation ac-
cumulation capability), which is often accompanied with
sacrifice of strength. Classical methods for strengthening
materials contain solid solution, dislocation, grain boun-
dary and so on [4]. The solid solution strength metals are
to alloy them with elements that are dissolved in the
crystal lattice and form a solid solution. Such atoms elas-
tically distort the crystal and can thus interact with the
stress field of a dislocation and impede its movement,
which results in high strength [5]. On the other hand,
large numbers of reports indicated precipitate and phase
transformation are also beneficial to improve the ductility
during deformation of alloys. Kim showed that certain
size of precipitate can improve the hardness and work
hardening of the materials [6]. Their excellent mechani-
cal properties result from the martensitic transformation
of metastable retained austenite, induced by thermome-
chanical loading [7,8].
Dendrite composite immiscible Fe60Cu40 alloy has been
prepared successfully by combustion synthesis technique
(CS) [9]. By controlling the applied pressure, the different
super saturated solid solution Fe60Cu40 alloys have been
obtained. Especially, there are not large composite seg-
regation both in dendrite and matrix. In this paper, we
examined the mechanical properties of this special alloy
under compression test. The results show that the nano-
scale precipitation and phase transformation occurs dur-
ing compressive deformation resulting in large work- har-
dening ability and high strength. Our results demonstrate
that this novel architecture offers a design pathway to-
wards both strong and ductility materials.
2. Experimental
The Fe60Cu40 alloy has been prepared successfully by CS
[9]. In this paper, the Fe60Cu40 alloys with different mi-
crostructure have been produced by CS under different
applied pressure with 8, 6 and 4 MPa of argon gas, which
indicated as FC8, FC6 and FC4, respectively. Morpholo-
gies and compositions of the Fe60Cu40 alloys were exam-
ined using a JSM-5600LV scanning electron microscope
(SEM) equipped with an energy dispersive X-ray spec-
troscope (EDS, Kevex, USA). Cylindrical compressive
specimens with length of 4.5 mm and a diameter of 2.8
mm were cut using an electro-discharging machine, and
both ends of the compressive specimens were polished to
mirror surfaces, and coated with graphite before tests to
reduce the interfacial friction. Quai-static uniaxial com-
pression test were performed at room temperature using a
testing machine with a crosshead speed of 3.5 × 10–3 s–1.
The before and after compression test samples were in-

Deformation-Induced Large Ductility of Super Saturated Solid Solution Fe-Cu Alloy
1628
vestigated with X-ray diffractometry (XRD, Philips X’pert)
using CuKα radiation. The cross sections of fractured
Fe60Cu40 alloys were also analyzed by the SEM.
3. Results
The typical compressive engineering stress-strain curves
of the Fe60Cu40 alloys are compared in Figure 1. The
yield strength σy, ultimate fracture strength σmax , fractural
strain εf are also given in table inset of Figure 1. The
yield strengths of the FC8, FC6, and FC4 samples are 520
MPa, 900 MPa, and 790 MPa, respectively, which are
higher than that of commercial crystalline Cu-Fe alloys
[10]. The highest yield strength is obtained with applied
pressure 6 MPa. As shown, the uniform elongation of the
FC8 and FC4 are only 4% to 5%. In contrast, the uniform
elongation of the FC6 reaches about 20%. It is more than
fourfold that of the nanostructured sample and above the
critical ductility required for many structural applications.
The ductility of the FC6 enhances because of an improved
work-hardening rate. The strong working hardening allows
uniform deformation, and leads to a fast climbing curve in
the compression test, which is different with most nanos-
tructured metals and alloys that show plunging curves
peaking very early in plastic deformation [1]. So, the strain
to failure reaches 30% in the compression.
Figure 1. Compressive enginee ring stress-strain cures of the
Fe60Cu40 alloy. Table of inset is the yield strength σy, ulti-
mate fracture strength σmax, fractured strain εf of the
Fe60Cu40 alloys.
Table 1. Composition of the Fe60Cu40 alloys.
FC8 FC6 FC4
dendrite matrix dendrite matrix dendritematrix
Cu 15.4 75.5 16.1 64.4 20.0 56.6
Fe 84.6 24.5 83.9 35.6 80.0 43.4
The SEM secondary electron images of the FC8 and
FC6 and FC4 are shown in Figure 2. All of the dendrites
of the FeCu alloys are uniformly embedded into the ma-
trix. It is maybe brought from the polishing because of
the soft matrix (Cu solid solution). The matrix is com-
posed of equiaxed ultrafine grains [9]. For FC8 the pri-
mary dendrite axes have radii of about 2 - 5 µm, regular
patterns of secondary dendrite arms with spacing 1 - 3
µm are observe, having radii of about 2 µm, which is
smaller than the primary axis (Figure 2(a)). The primary
and second dendrite axes become short and coarse as the
applied pressure decreases (Figure 2(b)). The primary
dendrite axes break and second dendrite axes get to
rarely when the applied pressure decreases to the 4 MPa
(Figure 2(c)). The composite of the dendrite and matrix
are presented in Table 1 . The results shows that the den-
drite and matrix are Fe(Cu) solid solution and Cu(Fe)
solid solution, respectively, for all the FeCu alloys. It
should be noted that all the FeCu alloys are only, com-
posed of elements Fe and Cu. It confirms the small black
holes in the matrix are not impurity (Figure 2). The den-
drite and matrix are Fe(Cu) solid solution (15.4 at% Cu)
and Cu(Fe) solid solution (24.5 at% Fe), respectively, for
the FC8. However, the solid solubility of the Fe(Cu) and
Cu(Fe) increases to the 20 at% and 43.4 at%, respec-
tively for the FC4. It means that both solid solubility of
the Fe(Cu) and Cu(Fe) increase as the applied pressure
decreases. Impressively, the solid solubility of the Cu(Fe)
with applied 4 MPa is twice as with applied 8 MPa.
(a) (b)
(c)
Figure 2. SEM images of the Fe60Cu40 alloys, (a) 8 MPa, (b)
6 MPa, (c) 4 MPa.

Deformation-Induced Large Ductility of Super Saturated Solid Solution Fe-Cu Alloy1629
Figure 3 shows the XRD pattern of Fe60Cu40 alloys
before and after compressive deformation. All the FC8,
FC6, and FC4 before and after compressive deformation
contain the
 -Cu-rich and the α-Fe-rich phases. The dif-
fraction peak γ-Fe-rich phase changes weak as the ap-
plied pressure decreases, and the γ-Fe-rich phase has not
been examined using XRD when the applied pressure
decreases to 4 MPa. However, the γ-Fe structure occurs
after FC4 deformation. Another impressively characteri-
zation is that the diffraction peaks of the
 -Cu-rich struc-
ture shift towards higher angles and γ-Fe-rich towards
lower angle after Fe60Cu40 alloys deformation. It means
that the solid solution of the
 -Cu-rich and γ-Fe-rich in
Fe60Cu40 alloys decreases after the compressive deforma-
tion. It is to be noted that FC6 has the highest ratio of
 -Cu-rich to γ-Fe-rich (defined as ratio of diffraction
peaks).
The fractography of the Fe60Cu40 alloys is shown in
Figure 4. The intergranular fracture feature has been ob-
served in the fractured surface of the FC6 (Figure 4(a)).
The crack occurs along the dendrite axis because of the
stress concentration in soft matrix in the compressive
deformation. Some of the dendrite axis cracked when the
stress concentration increases to the critical value, such
as the zone of Ⅲ. The Figure 4(b), (c) and (d) are mag-
nification of the Ⅰ, Ⅱ and Ⅲ of the Figure 4(a), re-
spectively. Far from the fractured dendrite axis, large
numbers of small grains precipitate from the dendrite and
matrix (Figure 4(b)). Near the fractured dendrite axis
(Ⅱ), more precipitations occur, and lots of dimples has
been observed with grain size of ultrafine scale compar-
ing with the precipitation. The visible melt traces (Figure
4(d)) are regarded as matrix (melt pointing is about 1000˚C)
because the local temperature maybe is higher 1000˚C
the local stress concentration as the sample fractures. The
40 50 60 70 80 90100
2Thet
a
8 MPa
4 MPa
Deformed before
Deformed after
-Fe(Cu)
-Cu(Fe)
-Fe(Cu)
6 MPa
Figure 3. XRD patterns of the deformed before and after
Fe60Cu40 alloys.
Figure 4. Fractography of the Fe60Cu40 alloys (a), (b), (c), (d)
FC6; (e) FC4; (f) FC8.
dimples indicate good working har- den of the FC6. The
super saturated solid solution of the FC4 with Cu(Fe) and
Fe(Cu) is close to the FC6. Thus, the yield strength rea-
ches 790 MPa, is near the yield strength of the FC6 (900
MPa). However, the local stress concentration of the FC4
is smaller than that of the FC6, because the dendrite axis
of the FC4 is coarse and short compared with the FC6. So,
the working harden is decreases even the negative value.
For the FC8, owing to the smaller dendrite axis and sma-
ller solid solution, the strength and ductility are smaller
than that of the FC6.
4. Conclusions
In this paper, the bulk immiscible Fe60Cu 40 alloy is suc-
cessfully prepared by a combustion synthesis technique.
The Fe60Cu40 alloy is composed of Fe-rich dendritic em-
bedded in Cu-rich matrix uniformly. The content of the
dendrite and matrix are 34.5% and 65.5%, respectively.
The grain size of matrix is about 30 nm. Owing to the
very high superheating (4700˚C) of combustion synthesis,
the deeply undercooling is obtained, resulting there no
large-scaled phase separation into Cu-rich and Fe-rich
phases.

Deformation-Induced Large Ductility of Super Saturated Solid Solution Fe-Cu Alloy
1630
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