PSI - Issue 60

M. Suresh Kumar et al. / Procedia Structural Integrity 60 (2024) 433–443 Suresh Kumar et al., / Structural Integrity Procedia 00 (2023) 000 – 000

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Fig.6 (a) Al-Mg-Sc-Ti-Zr vertical section at 0.3Sc-0.15Ti-0.15Zr (Raghavan (2009)). The black arrow in fig.6a shows the β precipitate formation zone for this alloy; (b) DSC plot of AR, 200 HT and 300 HT samples. The endothermic peak (marked by black arrow in fig.6b) repre sents β precipitate dissolution in 200 HT sample Fig.6a shows the phase diagram of Al-Mg- 0.3%Sc alloy that indicates a possible formation of β (Al 3 Mg 2 ) precipitates in the temperature range of 200 to 250°C for the current alloy composition, i.e., 4.3% Mg (Raghavan (2009)). The β precipitates are usually of nano -size and hence cannot be detected through optical or scanning electron microscopy. Hence, in this study, the presence of β phase was examined by DSC analysis. DSC plot of AR, 200 HT, and 300 HT samples is shown in Fig.6b. Based on Krishnamurthy et al. (2023), the endothermic peak (dissolution) observed for the 200 HT sample at 260° C is indicative of formation of β phase in the material after exposure to 200 °C for 2 hours. The β precipitates are known to adversely affect the mechanical properties of 5XXX alloys depending on their morphology and content. Therefore, the observed reduction in the elongation of the alloys exposed to temperatures in the range of 200 to 280 °C may be attributed to the precipitation of β phase at these temperatures . A study on similar alloy systems also reports deterioration of elongation when subjected to post-roll annealing of sheet material at 150 and 200°C for 3 hours (Pozdnyakov et al. (2019)). Although the ductility was affected, there was a marginal reduction in strength properties. This is due to the dominating strengthening effect resulting from the interaction between dislocation and Al 3 Sc precipitates in relation to the solid solution strengthening effect (Kendig et al. (2002)). 3.3. Effect of creep forming temperature on high cycle fatigue properties Fig.7 shows the maximum stress (S) vs number of cycles to failure (N) plot for the AR, 200 HT, and 300 HT samples. As expected, at higher stress levels, the fatigue life was low. It is known that at higher stresses, cracks that initiate at microstructural discontinuities, such as inclusions, porosity, and grain boundaries, lead to lower fatigue life (Shukla and Mishra (2020)). The fatigue strength of samples at 10 6 cycles for AR, 200 HT, and 300 HT conditions was found to be approximately 250, 290, and 255 MPa, respectively. These results suggest that exposure to 200 0 C (200 HT condition) imparts relatively higher fatigue strength as compared to AR and 300 HT conditions. The fatigue stress amplitude σ a and fatigue life 2N f during high cycle fatigue can be expressed by the following Basquin equation: σ a = σ f  (2N f ) b .......................... (1)