Results and Discussion
4.5 Generation of processing maps for hot workability
observed at strain rate = 0.001 s–1 and temperature > 480 ºC and (iii) contour plots are widely spaced when efficiency of power dissipation, η > 15% for which strain rate sensitivity m > 0.05. From these observations, it is evident that the efficiency of power dissipation, η is higher for regimes having higher strain rate sensitivity, m values.
(a) (b)
(c) (d)
Figure 4.40 (a) Contour plot of strain rate sensitivity (m), (b) contour plot of efficiency of power dissipation (η), (c) instability regime and (d) deformation processing map for Alloy A at a strain of 0.1
The instability regime for Alloy-A at a strain of 0.1 is shown by the shaded area in Figure 4.40 (c). The processing map, obtained by superimposing instability map on the efficiency of power dissipation map, has been shown in Figure 4.40 (d). The region of instability is found to be at temperature < 340 ºC and 0.04 s–1 >strain rate <
~0.8 s–1.
The contour plots of strain rate sensitivity, m values and processing maps for Alloy-A at strains of 0.2, 0.4 and 0.6 are depicted in Figure 4.41. From the figure it is evident that efficiency of power dissipation (η) increases with increase in strain. As the strain is increased, the instability region gradually spreads towards the region of higher temperatures and strain rates. The m value and efficiency of power dissipation (η) increases with increase in strain at all process conditions (i.e. combination of temperatures and strain rates). At strain value of 0.6, the regime of instability is identified to be at temperatures < 380 °C and strain rate > 0.1 s–1.
The contour plots for the strain rate sensitivity (m) values for the remaining alloys deformed to a strain of 0.6 are shown in Figure 4.42 (a) to (d). The strain rate sensitivity (m) value for all alloys is highest at regimes of high temperature and low strain rates. The maximum strain rate sensitivity (m) value decreases continuously from 0.25 for the alloy with 0 wt.% silver to 0.14 for the alloy with 0.1 wt.% silver, at a strain value of 0.6.
The processing maps for Alloy-B, Alloy-C, Alloy-D and Alloy-E at strain of 0.6 are shown in Figure 4.43 (a) to Figure 4.43(d), respectively. It is evident that the efficiency of power dissipation (η) decreases with increase in silver content in the alloy. As silver content increases from 0 wt.% to 0.1 wt.%, efficiency of power dissipation (η) value decreases continuously from 54% to 34%. With increase in silver content, the unstable region increases and spreads towards the region of higher temperatures. Instability regime for the alloy containing 0.03 wt.% silver (Alloy-B), after a strain of 0.6, is observed to be at strain rate > 0.125 s–1 and temperatures in the range 300–380 °C. For alloy with 0.1 wt.% silver (Alloy-E), instability regime is at strain rate > 0.125 s–1 and temperatures 300–480 °C.
The stable flow regime (safe process regime) for Alloy-E at a strain value of 0.6 is found for the low strain rate regions (i.e. <0.125 s–1) for the entire range of investigated temperatures. Maximum values of strain rate sensitivity (m), efficiency of power dissipation (η) and regimes of stable flow for alloys are shown in Table 4.18.
(a) (b)
(c) (d)
(e) (f)
Figure 4.41 Contour plots of strain rate sensitivity (m) value and processing map for Alloy-A, respectively, at strains (a) & (b) 0.2, (c) & (d) 0.4 and (e) & (f) 0.6
(a) (b)
(c) (d)
Figure 4.42 Contour plots of strain rate sensitivity (m) at strain 0.6 for (a) Alloy-B, (b) Alloy-C, (c) Alloy-D and (d) Alloy-E
Table 4.18 Process parameters for maximum power dissipation efficiency values of the alloys
Sample ID
Maximum strain rate sensitivity
factor (m)
Strain-rate ranges (s–1)
Temperature ranges (°C)
Maximum power dissipation efficiency, η (%)
Alloy-A 0.25 0.001–0.01 470–500 54
Alloy-B 0.20 0.001–0.01 450–450 45
Alloy-C 0.17 0.001–0.01 420–500 42
Alloy-D 0.16 0.001–0.01 450–500 36
Alloy-E 0.14 0.001–0.01 470–500 32
(a) (b)
(c) (d)
Figure 4.43 Contour plots of deformation processing map at strain 0.6 for (a) Alloy-B, (b) Alloy-C, (c) Alloy-D and (d) Alloy-E
Microstructure of the specimen after hot deformation was observed using optical microscope to investigate the dissipative microstructures formed during the hot deformation. The microstructure of Alloy-A before hot deformation has been shown in Figure 4.44 (a). Figure reveals the following; (i) almost equiaxed grains (ii) presence of second phase (CuAl2) particles and (iii) small amount of low melting point phases at the grain boundary regions. Figure 4.44 (b) & (c) reveals features typical of inter-granular cracking along the grain boundary regions. These micrographs correspond to the samples which were deformed at 300 ºC and strain rates of 10 s–1 and 1 s–1, respectively. Referring to Figure 4.41 these process
(a) (b)
(c) (d)
Figure 4.44 Optical micrograph of Alloy-A (a) before hot deformation (X10 magnification), (b) inter-granular cracking (X20 magnification) along the grain boundary regions at strain rate 10 s–1 at 300 °C, (c) inter–
granular cracking (X10 magnification) along the grain boundary regions at strain rate 1 s–1 at 300 °C and (d) Dynamic recrystallized grains (X10 magnification) at strain rate at strain rate 0.01 s–1 and temperature 500 °C
corresponds to the regimes of plastic instability (shown shaded). It has been reported that inter-granular cracking can occur in materials having low melting points when subjected to plastic deformation at intermediate temperatures and high strain rates [29]. Presence of CuAl2 and small amounts of low melting point phases at grain boundary regions results in the plastic instability during hot deformation by inter- crystalline cracking. Figure 4.44 (d) shows the microstructure of Alloy-A deformed at temperature 500 ºC and strain rate of 0.01 s–1. Referring to Figure 4.41 this process
condition corresponds to the regime of stable plastic flow (i.e. safe region). The micrograph reveals fine recrystallized grains nucleated at the grain boundaries indicating dynamic recrystallization.
Dynamic recrystallization is beneficial [94] since softens the material and provides a stable flow during hot deformation [95]. From the observations made from Figure 4.41 Figure 4.44 one can arrive at the fact that the safe process regimes for hot deformation of Alloy-A are at; (i) strain rates in the range 0.001 s–1 – 0.1 s–1 for all temperatures and (ii) temperature in the range 380–500 °C for all strain rates.
Photomicrographs of Alloy-B before and after deformation at strain rate 1.0 s–1 and 300 °C has been shown in Figure 4.45 (a) and Figure 4.45 (b), respectively. Figure 4.45(b) reveals void formation at triple junctions. The void formation results in stress relieving during deformation. The photomicrographs of Alloy-D deformed at temperature 300 °C and strain rate 1.0 s–1 has been shown in Figure 4.46 (a). The figure reveals void formation at grain boundary regions when deformed at low temperature and high strain rate. The microstructure of Alloy-D after deformation at temperature 500 °C and strain rate 0.001 s–1 has been shown in Figure 4.46 (b). The features represent dynamic recrystallization when deformed at high temperature and low strain rates. This region is considered to be a safe process zone. From Figure 4.43(c) and 4.46 the safe process regime for hot deformation of Alloy-D identified as;
(i) low strain rates in the range 0.001–0.1 s–1 for all temperatures and (ii) at temperatures in the range 380–500 °C for all strain rates.
Optical micrographs of Alloy-E deformed at (a) 300 ºC temperature and strain rate 10 s–1 and (b) 500 °C temperature and strain rate 0.001 s–1 has been shown in Figure 4.47 (a) and Figure (b), respectively. Figure 4.47(a) reveals void formation during hot deformation whereas Figure 4.47 (b) reveals dynamic recrystallization along with small amount of flow localization. The silver content in Alloy-E is 0.1 wt.% silver.
Figure 4.43 (d) reveals lower efficiency of power dissipation, η (~23%) value at 500 ºC temperature and strain rate 0.001 s–1 compared to other alloys. Though dominant microstructural feature at this condition is dynamic recrystallization, the formation of flow localization reduces the efficiency of power dissipation (η) for this alloy.
(a) (b)
Figure 4.45 Optical micrograph of Alloy-B (a) before hot deformation (X10 magnification) and (b) void formation at triple junctions (X20 magnification) at strain rate 1 s–1 and temperature 300 °C
(a) (b)
Figure 4.46 Optical micrograph of Alloy-D (a) void formation at grain boundary (X50 magnification) strain at rate 1 s–1 and temperature 300 °C and (b) dynamic recrystallization (X10 magnification) at strain rate 0.001 s–1 and temperature 500 °C
The regimes of flow instability during hot working of the investigated alloys have been identified. The instability regimes of all alloys are presented in Table 4.19.
From the comparison of the deformation processing maps and microstructural investigations the findings can be summarized as follows:
a. The instability regimes in the alloys are at regions of high strain rates and low temperatures.
b. Stable regime for hot deformation decreases with increase in silver content.
c. Void nucleation and intercrystalline cracking are the major mechanisms of flow instability in these alloys. Flow localization is observed in alloy containing 0.1 wt.% silver resulting in reduction in the efficiency of power dissipation.
d. High efficiency of power dissipation corresponds to regimes of stable plastic flow.
e. The safe plastic flow regimes during hot deformation are characterized by dynamic recrystallization.
(a) (b)
Figure 4.47 Optical micrograph of Alloy-E (a) void formation (X20 magnification) at strain rate 10 s–1 and 300 °C and (b) dynamic recrystallization (X5 magnification) along with small amount of flow localization at strain rate 0.001 s–1 and 500 °C
Table 4.19. Metallurgical interpretation of Alloy
Sample ID Manifestation Strain-rate ranges (s–1) Temperature (°C) Alloy-A Inter-granular cracking 0.125–10.00 300–380
Dynamic recrystallization 0.001–0.01 480–500 Alloy-B
Void formation at triple
junctions 0.125–1.00 300–370
Dynamic recrystallization 0.001–0.01 460–500
Alloy-D Void formation 0.316–1.00 300–390
Dynamic recrystallization 0.001–0.01 400–500
Alloy-E Void formation 0.125–10.00 300–480
Dynamic recrystallization 0.010–0.10 480–500