Stress rupture embrittlement in cast Ni-based superalloy 625

Abstract

Microstructural investigations and stress rupture tests were performed on specimens extracted from the columnar grain sections of large blocks of cast alloy 625. Metallographic examination revealed that the casting contained distinct columnar and equiaxed zones. In each zone, the nature of distribution of precipitates, defects, and micro-constituents such as Nb and Mo, were analysed. A temperature limit for the desired service duration of cast alloy 625 material was estimated by analysing the results obtained from stress-rupture tests. Rupture embrittlement was observed in the material subjected to stress-rupture tests. Detailed post-mortem analysis was carried out to understand root cause for this embrittlement. Results indicate that δ phase precipitation occurred during stress-rupture tests at elevated temperatures. The δ phase content also increases with increase in rupture duration. Mechanisms and preferential locations of δ phase precipitation in the microstructure are being reported in the present work. Correlation of rupture ductility with microstructural features conclude that the formation of δ phase is responsible for stress-rupture embrittlement of alloy 625.