PSI - Issue 60

B.P. Kashyap et al. / Procedia Structural Integrity 60 (2024) 494–509 B.P. Kashyap et al. / Structural Integrity Procedia 00 (2023) 000 – 000 7

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explosive forming, respectively. These Microstructures are known to change during high temperature deformation and cause non-steady flow with stress varying differently with strain.

Fig. 2. (a) As-cast dendritic microstructure of eutectic Al-12Si alloy; (b) equiaxed microstructure of hot extruded hypereutectic Al-30Si alloy; (c) elongated microstructure of thermo-mechanically processed AA8090 Al-Li alloy and (d) banded microstructure of eutectic Pb 62Sn alloy after explosive forming. The common observations are that the fine grains undergo grain growth, whereas the large grains may become smaller by dynamic recrystallization under suitable conditions. Elongated and banded microstructures change to equiaxed grains with homogeneous distribution of phases, as-cast structure may get broken to give homogeneous structure. Cavitation occurs in some materials when favored by grain boundary sliding without its accommodation to remove stress concentration. These concurrent changes in microstructures result in varying effects on the nature of stress-strain curves. For example, grain refinement, the change from elongated or banded structure to equiaxed grains and cavity formation lead to flow softening whereas grain coarsening causes flow hardening. Sometimes the microstructural changes of two opposite naturebut equal effects of flow hardening and flow softening can lead to pseudo-steady state, where microstructures do change but not the flow stress with strain. Presented below are some typical results of microstructural evolution and the flow behavior during high temperature deformation.  Type 316L stainless steel deformed at high temperatures of 973, 1123, and 1173 K show flow hardening at 973 K but, at higher test temperatures, the stress follows steady state (Kashyap et al., 1988), Fig. 3(a). The occurrence of steadystate is attributed to the dynamic recovery type effect of low but continuous dislocation generation, which rearranges slowly to form the subgrains of larger size even at smaller strains (ε~0.01) and then continues to divide into smaller subgrains at larger strains. However, at 973 K, the dislocations generated are more. They remain straight in grain interior and some of which go to form extrinsic grain boundary dislocations. With the generation of large number of dislocations with increasing strain the dislocation entangled structure is formed, which is followed by their conversion into fuzzy dislocation cell wall structure.At this temperature, the occurrence of flow hardening is attributed to conventional work hardening.The strength coefficient K L (Eq. 3) was studied as a function of grain size (Kashyap and Tangri, 1995) and the same is plotted in Fig. 3(b) giving a relationship of the form = 274.3( ) −0.09 R 2 = 0.82 (15)