PSI - Issue 71

176 S.R. Reddy et al. / Procedia Structural Integrity 71 (2025) 172–179 average stress exponent navg~2.5 is calculated from the slope of the strain rate (s -1 ) vs. peak true stress ( )(MPa) whereas the activation energy ~204 kJ/mol measured from the slope of true stress (MPa) (σ ) vs. 1/T (K -1 ) (Q= n.R.Slope (kJ/mol)). The stress exponent is in line with the super plasticity reported in the present work and the activation energy measured shows the region II corresponds to that of grain boundary diffusion (Chokshi, 2017, 2020). The activation energy increases from ~165 kJ/mol to ~200 kJ/mol as the strain rate decreases from 10 -1 to 3x10 -4 s -1 , consistent with the observed behavior in superplastic materials (Nieh et al., 1997). This indicates a transition in plastic deformation mechanisms from low to high strain rates. The EBSD phase maps (Fig.5) of the fractured samples deformed at 700 °C (Fig.5 (a-c)), 800 °C (Fig.5 (d-f)) and 900 °C (Fig.5 (g-i)) at different strain rates of 10 -1 s -1 (Fig.5 (a,d,g)) 10 -3 s -1 (Fig.5 (b,e,h)) 3x10 -4 s -1 (Fig.5(c,f,i)) respectively. Microstructural observations of the fractured samples show ultrafine grain size development at low temperatures i.e., 700 °C and high strain rate 10 -1 s -1 with grain size of ~0.5 um whereas at 900 0 C it shows a grain size of ~1.25 um as shown in Table 1. Further the table shows average grain size of the grip and fractured regions to show the effect of heating only in grip portion and heat + deformation in fractured region. With the increase in the temperature and decrease in the strain rate there is an increase in the grain size. The increased grain growth relates to the superplastic behavior with grain boundary sliding as the dominant deformation mechanism where GBS induced grain coarsening is observed(Chokshi, 2001; Prasad A.H., 2010). Further retention of equiaxed grain morphology instead of elongated grains confirms significant contribution from GBS(Panicker et al., 2009) as confirmed form the activation energy measurements as well. The transition from superplastic to non-superplastic is also consistent with the strain rate sensitivity change. The retention of equiaxed grain size in both phases can be observed from the microstructures.

Fig. 5 Phase maps (obtained from EBSD) representing gauge sections in samples deformed at (a-c) 700 °C (d-f) 800 °C (g-i) 900 °C at different strain rates of (a,d,g) 10 -1 s -1 (b,e,h) 10 -3 s -1 (c,f,i) 3x10 -4 s -1 .