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

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10

( e f = 199%, T = 803 K, ̇ =2×10 −2 s -1 )

( e f = 402%, T = 803 K, ̇ =2×10 −4 s -1 )

Fig. 5. (a) True stress-true strain curve (T = 803 K, ̇ =1×10 −3 s -1 ) with arrows indicating different strain levels (Fan and Chaturvedi, 2000); (b) the optical micrographs along with the texture evolutions examined at the different strain levels presented in (a);(c) the change in fraction of low angle boundary (LAB) with strain (Fan et al., 2001); (d) dynamic recrystallization; and (e)the cavitation upon fracture ( e f ) in AA8090 Al-Li alloy (Kashyap and Chaturvedi, 2003).  Detailed study of deformation behavior and concurrent microstructure evolution made in fine grained materials (Suery and Baudelet, 1978; Kashyap et al., 1985; Kashyap and Tangri, 1986; Friedman and Ghosh, 1996) revealed interesting qualitative and quantitative microstructure-property relationship. Presented here is the cavitation behavior during high temperature deformation. Normally, cavity volume and cavity size increase with increasing strain and decreasing temperature but, with strain rate, the maximum cavitation occurs at intermediate strain rate, Fig. 1. The increase in cavity volume comprises both the increase in number and size distribution of cavities with increasing strain depending on test condition. Cavity nucleation is promoted by the higher stress condition, Eq. 10, (higher strain rate and lower temperature), whereas their growth will be promoted by diffusional process (giving round cavities) at lower strain rate and higher temperature or power law condition of higher strain rate and lower temperature (giving elongated cavities). At any strain level, the cavity volume is a function of number of cavities and the sum of the sizes of individual cavities, which can vary with test conditions differently [Kashyap and Tangri, 1989]. Similarly, the maximum grain growth also occurs for the condition of maximum m . Thus, the mechanism for superplastic deformation supports cavitation behavior, according to which cavitation should increase as m increases in materials where accommodation of grain boundary sliding is not sufficient, but the stress relief occurs by cavity formation. The maximum grain growth under the condition of maximum m can be attributed to the compatible grain boundary migration as an accommodation process for grain boundary sliding. The plot of m as a function of strain rate shows an increase in its magnitude to its maxima at an intermediate strain rate, beyond which it decreases with increasing strain rate. The superplastic materials that undergo cavitation accordingly show maximum cavitation in the stress-strain rate region of maximum m . However, the theoretical cavity growth rate supported by some experimental data, when plotted as a function m , is reported to exhibit decreasing cavity growth rate with increasing m (Lombard et. al., 2001) of type = 1.4( ) −0.79 R 2 = 0.72 (19) It appears thatthe maximum cavitation observed at the intermediate strain rate is not due to the damaging effect of increasing cavity growth rate but its rolein grain boundary sliding accommodation along with acting as a facilitator to grain switching and grain rotation, which keep the cavities apart and allows the maximum ductility to be attained. Thus, seemingly anomalous variations are understandable. A study on the superplastic behavior of IN744 stainless steel, in fact, revealed that m linearly increases with grain size (Kashyap and Mukherjee, 1983). As a function of strain rate ( ̇ ) the grain growth rate ( ̇ ) in Al-