cylindrical specimens of dimensions, 10 mm in diameter and 10 mm in Height were machined from the extruded rods. The hot compression tests  were performed on a 1OT servo controlled universal testing machine for different strains (0.1 - 0.5), strain rates (0.001 s–1 - 1.0 s–1) and temperatures (300˚C - 500˚C). Temperature of the specimen was monitored with the aid of a chromel/alumel thermocouple embedded in a 0.5 mm hole drilled half the height of the specimen as stated by Yi Liu et al. . The thermocouple was also used for measuring the adiabatic temperature rise in the specimen during deformation. The specimens were effectively lubricated with graphite and deformed to a true strain of 0.5. After compression testing, the specimens were immediately quenched in water and the cross section was examined for microstructure. Specimens were deformed to half of the original height. Deformed specimens were sectioned parallel to the compression axis and the cut surface was prepared for metallographic examination. Specimens were etched with Keller’s solution. The microstructure of the specimens was obtained through Versamet 2.0 optical microscope with Clemex vision Image Analyser and mechanism of deformation was studied. Using the flow stress data, power dissipation efficiency and flow instability were evaluated for different strain rates, temperatures at a constant strain of 0.5. The processing maps were developed for 0.5 strains for 7075/Al/15%SiCp composites.
3. Results and Discussion
3.1. Flow Curves
The flow stress data was generated covering the temperature range of 300˚C - 500˚C and strain rate range 0.001 - 1.0 s–1 from compression testing of solid cylinders of size 10 mm in diameter and 10 mm in height using servo hydraulic testing machine capable of imposing constant true strain rates on the specimen. Adiabatic temperature rise during high strain rate testing was measured and the flow stress was corrected for the temperature rise. The specimens were compressed to 50% of their initial height and the load-stroke curves obtained in the hot compression were converted in to true stress-true plastic strain curves by subtracting the elastic portion of the strain and using the standard equations for the true stress and true strain calculations. The values of the efficiency parameter (η) and the strain rate sensitivity parameter (m) to use the instability condition are determined from the flow stress test data of the material.
Typical true stress-true strain curves obtained for different strain rates and constant temperature are given in Figure 1. At 0.1 s–1 strain rate, the flow stress reaches a peak at a critical strain and then sharply decreases to a steady value at the temperature of 450˚C. The characteristics of this flow stress curves demonstrate the happening of DRX. Generally, DRX is a beneficial process in hot deformation since it not only gives stable flow and good workability to the material by simultaneously softening it but also reconstitutes the microstructure .
Therefore, it is observed that the flow stress is in direct correlation with the strain rates. The variation of stress is related to a variation in dislocation density as well as the formation and development of sub grain boundaries, which result from working hardening and softening mechanism. At lower strain rates, the deformation is isothermal but at high strain rates it is adiabatic . This phenomenon at higher strain rate deformation is the consequence of the inadequate heat dissipation. Further, this leads to an increase in work hardening rate at higher strain rate deformation. However, at lower strain rate, the strain hardening is almost compensated by softening due to higher temperature. Hence, near steady state flow curve is observed.
Typical true stress vs true strain curves at different temperatures with strain rate of 0.1 s–1 are shown in Figure 2. It can be seen that the flow stress decreases with increasing temperature and decreasing strain rate, thereby, exhibiting typical behaviour of metal deformed under hot working conditions. True stress vs true strain curve at 450˚C exhibits work hardening (Figure 2) whereas at higher temperature the flow softening is observed after a critical strain. The curves are characterized by an initial sharp increase of flow stress with strain up to a certain value that is followed by slope of flow decreasing and very limited flow softening . From the mentioned flow stress-strain character, it can be concluded that the work-hardening effect is pronounced at low temperatures. Generally, during the deformation the matrix around the SiC particles presents much higher dislocation density than that of normal alloy. The high dislocation density regions restrict the plastic flow and contribute to a competing behavior between dynamic softening and the work hardening.
3.2. Processing Map
Processing maps were developed and shown in Figure 3 by superimposing contour plots of instability parameter
Figure 1. The flow curves of 7075 Al/20%SiCp (63 μm) for different strain rates at constant temperature of 450˚C.
Figure 2. The flow curves of 7075 Al/20%SiCp (63 μm) for different temperatures at constant strain rate of 0.1 s–1.
Figure 3. Processing map at 0.5 strains.
on those of power dissipation efficiencies. The approach of processing map has been introduced to study the deformation mechanisms and to optimize hot forming process for this material. These maps show the stable and unstable deformation in the processing space on the axes of temperatures and strain rates. It is helpful to understand the interaction effects between strain rates and temperatures. Consequently, the components with controlled microstructure and properties can be manufactured without macro or micro defects. In the process maps, the contour numbers represent efficiency of power dissipation and shaded region corresponds to flow instability .
The contour numbers represent the valves of the instability parameter ξ (έ) in the instability maps. It can be noted that the values of parameter ξ (έ) are negative at the lower deformation temperature and higher strain rate; with increasing strain, the region of negative contour becomes larger. The region in which the value of parameter ξ (έ) becomes negative suggests the possibility of unstable flow. The greater the negative magnitude of ξ (έ), the higher the possibility of unstable flow.
Figure 3 illustrates the contour maps of strain rate sensitivity and efficiency of power dissipation calculated at strain 0.5. If the values of m and η are negative, the material presents some types of unstable flows. Furthermore, it is generally considered that higher values of m and η reduce the tendency for flow localization and result in the maximum ductility. The contour maps for strain rate sensitivity parameter and efficiency of power dissipation, corresponding to strains of 0.5 exhibit interesting domain. The domain of η maps occurs in the temperature range of 445˚C to 485˚C and strain rate range of 0.018 s–1 to 0.16 s–1. The maximum values of η for strains of 0.5 in this domain have increased up to 34% at the deformation temperature of 450˚C with strain rate of 0.1 s–1.
3.3. Microstructure Examinations
3.3.1. Instability Zones
Dynamic recrystallization (DRX) observed at 450˚C with strain rate of 0.1 s–1 are presented in Figure 4. The flow curves in this domain represent oscillations in the flow curves before reaching steady state. The microstructure has higher density dislocation domains and dislocation cell structures. And the more complex dislocation structures were formed around SiC particles. The high dislocation density regions restrict the plastic flow and contribute to the strengthening and strain hardening. The flow curves exhibit work hardening at 450˚C with strain rate of 0.1 s–1. The flow deformation behavior mainly presents working hardening under this condition. As the temperature increases, the strengthening effect of sic is considerably diminished, so that the material shows a similar softening behavior of the pure metals. In Figure 4, it can be seen that uniform and fine grains were formed, which can be attributed to the occurrence of dynamic recrystallization (DRX). DRX is a beneficial process in hot deformation as it gives good intrinsic workability by simultaneous softening and reconstituting the microstructure. Therefore, DRX is a chosen domain for hot workability optimization and good microstructure control in the safe zone. So, the optimum processing pa-
Figure 4. Dynamic recrystallization at 450˚C at a strain rate of 0.1 s–1.
rameters attained by the maps are the temperature and strain rate ranges of 445˚C - 485˚C and 0.018 s–1 - 0.16 s–1, respectively. Traditionally, DRX is associated with hot working of low stacking fault energy (SFE) metals which exhibit flow softening after reaching a critical strain. Aluminium is a high stacking fault energy material and is characterized by easy dislocation climb and cross slip during deformation. However, the added SiC particles affect the density and distribution of dislocations in the matrix, and may influence the driving force for recrystallization. In addition, the accumulation of dislocations in the vicinity of SiC particles may lead to the fact that those regions become sites for recrystallization nucleation.
Dynamic recrystallization thus becomes easier to occur. Hence, the domain can be interpreted to represent the region of possible dynamic recrystallization [5,27,28]. Elongation obtained on the specimens deformed at temperature of 450◦C and strain rate of 0.1 s−1 are shown in Figure 5.
3.3.2. Instability Zones
Figure 6 shows the particles alignment along the shear direction at 300˚C with the strain rate of 1.0 s–1 for strain of 0.5. At higher strain rates, since the time is short,
Figure 5. Grain elongation at 450˚C and at a strain rate of 0.01 s–1.
Figure 6. Shear band formation at 300˚C and at a strain rate of 1.0 s–1.
conducted away to the colder parts of the body, a drop in the flow stress occurs locally and therefore slip becomes localized . This is called adiabatic shear bands which exhibit cracking, recrystallization along macroscopic shear planes and this generally occurs sharply at 45˚ with respect to the principal stress axis . The SiC present in the aluminium matrix is disturbed due to the flow of matrix in the shear direction. So, the particles alignment along the shear directions is observed. The flow localization is due to the interface cracking or the SiC particle itself breaking and adiabatic shear bands present in the metal matrix, which are the manifestations of flow instability predicted by the instability maps. Therefore, those flow instability regions must be avoided.
The interface cracking observed at 500˚C with the strain rate of 1.0 s–1 is shown in Figure 7. The presence of SiC particles in aluminum matrix during deformation causes the interface to crack since the matrix undergoes plastic flow while the particles do not deform . As a result, the accumulated stress will emerge at the interface between the matrix and the particles. When the accumulated stress becomes large, the interface may separate at relatively lower temperature and higher strain rates. The
Figure 7. SEM image of interface cracking at 500˚C and a strain rate of 1.0 s–1.
particle breakage is due to accumulated stress at the interface exceeding the yield stress of the particle; hence, this results in particle breakage .
It is concluded that with increasing strain, the flow instability manifestation of composites becomes more significant at higher stain rate, owing to increased defects such as particle fracture, debonding and adiabatic shear band. The stable deformation mechanisms include elongation and DRX. The occurrence of DRX at higher temperatures and moderate strain rate deformation results in flow stress decreasing in those deformation ranges.
1) Hot compression tests were performed on Al/20% SiC composites produced through Stir casting technique. The flow stress was evaluated for a temperature range of 300˚C - 500˚C and a strain rate range of 0.001 - 1.0 s–1. The power dissipation efficiency and instability parameters were evaluated and processing maps were constructed for 0.5 strains. The optimum domains and instability zone were obtained for the material.
2) The optimum processing parameters attained by the maps are the temperature and strain rate ranges of 445˚C - 485˚C and 0.018 s–1 - 0.16 s–1, respectively. Dynamic recrystallization (DRX) observed at 450˚C with strain rate of 0.1 s–1.
3) The processing maps present the instability zones at higher strain rate; and with increasing strain, the instability zones enlarge. The microstructural examination shows that composites exhibit flow localization as bands of flow localizations, the interface cracking and particle breakage.
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The authors are grateful to The Department of Manufacturing Engineering, Annamalai University, Tamilnadu, India for the support rendered for the fabrication and testing of Composites