Effect of section thickness on creep and stress rupture behaviour of DS CM247 nickel base superalloy

Abstract

The effect of section thickness on creep and stress rupture behaviour of DS CM247 nickel base superalloy at three temperatures viz. 760, 982 and 1038°C, has been investigated in an effort to identify the factors contributing to the observed creep/stress rupture behaviour. Irrespective of section thickness, the shape of creep curves at the lowest temperature of 760°C differs from the shape of those at the higher temperatures, showing significantly higher primary creep strain (PCS). At higher temperatures the creep curves exhibit limited primary and secondary regimes, but predominant tertiary regime. Such a difference in shapes of the creep curves is related to the difference in dislocation bypass mechanisms operating at these temperatures. The variation of minimum creep rate (MCR) and time to rupture (tf) with section size indicates a progressive creep strengthening at 760 and 982°C, while a progressive creep weakening at 1038°C with decreasing specimen thickness. Such a behaviour can be explained in terms of relative extents of the surface oxide layer (contributing to creep strengthening) and γ′ depleted zone (causing creep weakening) formed at these temperatures. Further, the creep strengthening at 760 and 982°C as well as weakening at 1038°C is more pronounced for the thinner specimens.