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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The occurrence of concurrent microstructure evolution of different types, as discussed above controls the nature of flow curves. Abnormal stress-strain curves observed during the superplastic deformation of IN718 superalloy are presented in Fig. 7(a). Also, as shown in Fig. 7(b), there appears a cluster of cavities in Al-Cu eutectic alloy during superplastic deformation, which might be related to cooperative grain boundary sliding of cluster of grains. The nature of varying stress-strain curves and concurrent grain growth and cavitation are also presented in Al-Si eutectic (Chung and Cahoon, 1979) and Al-24Cu alloys (Bakshi and Kashyap, 1995). Based on the effects of grain growth and cavitation, the nature of stress-strain curve given in Fig. 7(a) can be explained as follows. The early flow hardening in the stress-strain curve may be due to the dominance of grain growth, whereas the subsequent flow softening can be ascribed to the dominant effect of cavity formation. Once the large cavity volume is developed it becomes possible that there occur inter-linkages between them to form larger cavities and ligaments between them. While externally, the sample was deformed at a constant strain rate, the strain rate in the ligament region will be much higher due to the apparently much smaller gage length being created within the macroscopic sample. The locally higher strain rates of ligaments will necessitate higher stress for deformation and more so when m is high in the present superplastic material. Therefore, after the minima in flow stress there occurs flow hardening. A systematic study was carried out to separate the effects of flow hardening due to grain growth and that caused by deformation of cavity ligament in Al-Cu eutectic alloy. It led to interesting results that the flow stress increment ( Δσ ) caused by cavitation hardening follows the H-P type relationship with inter-cavity spacing Λ as illustrated in Fig. 7(c).

Fig. 7. (a) Nature of stress-strain curve of IN718 superalloy (Kashyap and Chaturvedi, 2003); (b) micrograph of large size cluster of interconnected cavities in Al-Cu eutectic (Kashyap, 1997); and (c) variation in flow stress increment as a function of square root of inter cavity spacing in Al-Cu eutectic (Kashyap, 2003). The experimental results of high temperature deformation and concurrent microstructures, presented in the literature and further analyzed here, follow systematic interdependencies. Such relations support the summary presented in Fig. 1. However, more work needs to be done on synergized experimental and theoretical approaches to bring clarity to this aspect of high temperature deformation.