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Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.7, pp.651-659, 2011
jmmce.org Printed in the USA. All rights reserved
Development Processes of Globular Microstructure
Department of Materials Science and Engineering, Obafemi Awolowo University, Ile-Ife,
Department of Metallurgical Engineering, Kwara State Polytechnic, PMB 1375, Ilorin,
Semi solid metallurgy offers distinct advantages over other near-net-shape manufacturing
processes. By this process, components are produced from slurry kept at a temperature
between the solidus and the liquidus isotherms, resulting in breakdown of the dendritic
structure. A new structure in which the morphology of the crystals of the primary phase is
globular evolves. In this present paper, the importance of globular structure is identified. The
theories of evolution of globular crystals in thixo – processing are identified and discussed.
Keywords: Semi solid metallurgy, globular crystals, thixo-processing
Semi solid metallurgy (SSM) is of growing industrial significance particularly for the low
melting alloys . Although SSM is already a viable manufacturing method, it is still under
intensive development and critical breakthroughs are still much expected . The production
of SSM feedstock is of considerable interest and current research effort is concentrated on
production of feedstock alloys where the primary phase in the microstructure consists of
globularized particles [3-6]. This is one of the needed requirements for semi solid metallurgy
feedstock [2, 3, 7] .
The belief that the utilization of SSM as a manufacturing process follows from the generally
accepted advantages of hardware performance and energy economy is apparently a
simplification. Also, the influence of globular structure on the component integrity is
complex. In general, microstructure affects components properties [8-10]. For the semi solid
652 A.V. Adedayo Vol.10, No.7
components, product integrity is affected through a reduction in porosity. The turbulent flow
of a liquid into a mould can result in the entrapment of air and mould gases into the melt,
which in turn may translate into micro- and macro-porosity. Smooth flow of the semi-solid
slurry minimizes these defects. Similarly, segregation and shrinkage porosity are affected. All
these: flow behaviour, segregation and solidification shrinkage depend on the nature of the
globular microstructure of the SSM component.
To assess morphological details of a globular structure, shape factor and sphericity are
parameters which are very prominent and stick out. For single crystal, the size of which is
defined by some length parameter or equivalent diameter, d, and density, ρ the following
relationships can be applied:
The constants f
may be called volume and surface shape factors respectively . For
spherical (diameter = d) and cubic (length of side = d) particles
f (Sphere) and 1 (cube) (4)
(Sphere) and 6 (cube) (5)
The shape factors are readily calculated for other regular geometrical solids. The other
prominent quantity that has been used to characterize phase morphology in microstructures is
the sphericity, ψ, defined as the ratio of the surface area of a sphere having the same volume
as the phase to the apparent estimated surface area of the phase . This can be rewritten as
For isometric shapes ψ is close to 1 while for needles or platelets its value is much lower.
Evaluation of ψ is useful for checking the values of f
since 0< ψ<1.
In general, the influence of globular microstructure on the direction of changes of alloy
properties is not universally positive and should be evaluated for individual alloy chemistry.
The understanding of the theories of development of globular crystal is vital and useful for
SSM process designs and other engineering applications. It will also provide insight on
fundamentals for achieving the much expected breakthroughs in SSM.
2. METHODS FOR PRODUCING GLOBULAR CRYSTALS
Under conventional solidification conditions, dendritic microstrutres result . This process
is well discussed in literatures [12, 13]. The process of producing globular crystals in metallic
Vol.10, No.7 Development Processes of Globular Microstructure 653
alloys which is suitable for SSM was commonly believed to rely upon the fragmentation of
the secondary dendrite arms during solidification [3, 14]. A number of technologies have
been developed over the last three decades, mainly in a laboratory environment, to take
advantage of the unique behavior of semi-solid slurries to produce globular crystals. All the
technologies can be divided into two fundamentally different basic routes: thixo-processing
and rheo-processing. There are also hybrids that combine features of both routes.
The thixo-route involves two stages: first, billet preparation and, second, billet re-heating and
isothermal holding within the mushy zone of the alloy. The crystals which form in the
process of conventional solidification of a metal have structures which are dendritic, lamellar
or fibrous , needle type (acicular) or globular, depending on the rate of cooling and the type
and amount of admixtures or impurities (intermetallics) in the melt . Perfect crystals of
proper external shape can be obtained only if crystallization develops under condition when
the degree of super cooling is very slight and the metal has a very high purity . In great
majority of cases, branched or tree-like crystals are obtained which are called dendrites.
Figure 1 shows steps in the formation of a dendritic crystal.
(a) (b) (c)
Figure 1: Formation of dendritic crystals [1, 13] .
654 A.V. Adedayo Vol.10, No.7
A crystal nucleus forms as shown in (a) and then proceeds to send out shoots or axis of
solidification as shown in (b), (c), (d) forming the skeleton of a crystal. Atoms then attach
themselves to the axes of the growing crystal from the melt in progressive layers (layers 1, 2,
3, 4 and 5 as shown in Fig. 1e), finally filling up these axes, and thus forming a completed
However, during billet re-heating thixo-processing of the metallic alloy there is partial
melting of the structures in the alloy. These may be any of the various compounds found in
the alloy. Constituents with low melting points melt into liquid. Also, as the temperature
rises, there is break-up of the primary and secondary axis into smaller unit , and random
re-orientation of the break-up network. Due to high diffusion rates in the mushy zone, atoms
of the melted constituents diffuse from the liquid phase and joined up with the existing solid
crystals. This phenomenon leads to evolution of new structure which is globular. This process
is shown in Fig. 2.
(a) (b) (c)
Figure 2: Development of globular structure (a) dendritic structure formed during
solidification in a casting (b) breakdown of dendritic network to form new nuclei during semi
solid isothermal heating, (c), (d) and (e) the process of emergence of globular structure .
Vol.10, No.7 Development Processes of Globular Microstructure 655
3. TEMPERATURE AND GLOBULAR MORPHOLOGY
In conventional castings, the material in the interior of the ingot cools more slowly, and
solidification takes place at a higher temperature. Usually, some of the grains near the surface
simply grow inwards as heat flows outwards. The resulting structure is columnar (illustrated
in Fig. 3). The columnar grains are not randomly oriented but rather have their directions of
most rapid growth normal to the mould walls, which is the direction of heat withdrawal. In
general, crystals grow in certain directions depending on some factors such as direction of
heat flow and presence of impurities.
Figure 3: Sketch of chill zone and columnar structure in conventional casting
The direction of heat flow is a function of the temperature field. In mathematical physics, the
temperature field is the totality of temperature values at a given point in time for all points of
the space considered in which heat transfer process takes place. During solidification, the
temperature at various points change with time and heat propagates from places at a higher
temperature to places at lower temperature. It follows that during solidification, there is
variation of temperature both in space and time. If the temperature of a body is a function of
space coordinates and time, the temperature field is referred to as transient kind. i.e.
where x, y, z are point coordinates, t is the time. If the temperature of a body is a function of
space coordinates only and does not vary with time, the temperature field is referred to as a
steady state i.e.
656 A.V. Adedayo Vol.10, No.7
The nature of the temperature field also determines the temperature gradient.
For alloy materials, when the grain boundary grooving occurs such that the boundary
intersects the liquid-solid interface, the curvature in the neighborhood of the groove is
determined by the requirement that :
rmm TTXGTT ∆−=∆−=
Where T* is the liquid-solid interface temperature, G is the thermal gradient and
X is the
distance back from the isotherm at T
, the equilibrium melting point of the alloy material (the
liquidus temperature ).
srss VTSG 2
are the changes in free energies of liquid and solid, respectively. Vs is
the volume of the solid,
is the surface curvature in the groove neighborhood, T
decrease in equilibrium melting point,
is the surface energy of the interface. Assuming that
is isotropic and does not change as surface area changes, at equilibrium :
It follows that:
srsrL VTSTS 2
But the curvature
at a point is described as the limit (provided it exists) of the average
Vol.10, No.7 Development Processes of Globular Microstructure 657
of an arc as the terminal point of the arc tends to its initial point . For arc
, as the terminal point of the arc M
tends to its initial point M
Figure 4: Geometric description of relationship between r,
and curvature (
(ϕ / ΜΜ)
is the angle of contingence of the arc (in radians).
(ϕ / ΜΜ)
ϕ / ϕ = 1/r r
= 1/r (25)
The thermal gradient G will depend on the thermal stability of the furnace used , and in
general the temperature field during thixo-processing. If the temperature field of the system is
transient the thermal gradient G will depend on time and space coordinate. The value of r will
vary with time. Transient temperature fields are experienced during heating and cooling of a
system. However, for isothermal heating, the temperature field is steady. Temperature
gradient G will be essentially constant and independent of time. This gives an essentially
constant value for r. This results in a spheriodal morphology. This shows that the ability of
the heat treatment furnace to maintain an efficient steady-state will affect the morphology of
the evolving crystal. Also, value of G (whether G < 0 or G > 0) will affect the concavity or
convexity of the resulting crystal.
658 A.V. Adedayo Vol.10, No.7
The study revealed that globular microstructure evolves as a result of breakdown of dendritic
structure during thixo-processing of the conventionally cast materials. The effect of
temperature field and thermal gradient of the heating furnace has also been shown to have
influence on the morphology of the evolved microstructure.
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