Stress rupture embrittlement in cast Ni-based superalloy 625

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2020-08, Vol.793, p.139811, Article 139811
• Main Authors: Pavan, A.H.V., Narayan, R.L., Swamy, M., Singh, Kulvir, Ramamurty, U.
• Format: Article
• Language: English
• Subjects: Acicular morphology; Advanced ultra super critical power plants; Alloy 625 casting; Casting alloys; Crystal defects; Embrittlement; High temperature; Inter-dendritic precipitates; Microstructure; Molybdenum; Nickel base alloys; Precipitates; Rupture embrittlement; Rupture tests; Superalloys; δ phase
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2020.139811

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. •Large scale castings of alloy 625 were characterized.•Stress-rupture tests confirm that alloy 625 is a candidate material for AUSC casings.•Embrittlement was observed at all stress-rupture test temperatures.•Critical locations of δ phase precipitation were identified during stress-rupture testing.•δ phase was responsible for reduction of stress-rupture ductility of alloy 625 castings.