Effect of zigzag grain boundary on creep and fracture behaviours of wrought γ′ strengthened superalloy

Abstract

Straight and zigzag grain boundaries (SGBs and ZGBs) have been obtained in a Ni–15Cr–6W–3Mo–2Al–2Ti superalloy by using conventional and isothermal heat treatment, respectively. However, the morphology, mean size, and volume fraction of the γ′ in the heat treated alloy were maintained at almost the same values. The role of ZGBs during creep and fracture has been studied at 800°C over a stress range of 196–490 MN m⁻². The results show that there is no marked difference in the deformation behaviours during the secondary creep and the earlier stage of tertiary creep for the two grain boundary conditions (GBCs). It is found that in the earlier stage of tertiary creep cavities nucleated extensively along both ZGBs and SGBs. Some significant differences in the creep and fracture behaviours were observed between the two GBCs during the later stage of tertiary creep. The alloy with ZGBs exhibited a lower rate of creep strain and crack development than the alloy with SGBs. While intergranular r-type cracks developed in the alloy with ZGBs, w-type cracks of one grain boundary facet in length formed in the alloy with SGBs. The final fracture resulting from catastrophic propagation of the longest surface cracks is discussed in terms of a creep fracture criterion.

MST/764