Plastic instability during creep deformation of a NiAl-Hf single-crystal alloy—A case study

Tensile samples from NiAl-Hf single crystals, having the same nominal composition and heat treated and creep tested under identical conditions at 1144 K, were found to exhibit very different rupture lives and creep ductilities. A case study was conducted on two samples with creep rupture lives of 343.6 and 37.0 hours (with corresponding creep ductilities of 12.3 and 39.9 pct, respectively) in order to find the causes of such a large variation in creep properties. Detailed microstructural analyses using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) indicated that the sample with higher rupture life and lower ductility had deformed uniformly along the length of the gage section, whereas the sample with lower rupture life and higher ductility (sample L) deformed by localized plastic deformation resulting in shear failure. This shear failure was due to a plastic instability in sample L which was caused by the presence of a high density of large Hf-rich interdendritic particles that were formed during casting of the single-crystal ingot but did not go into solution during the homogenization heat treatment. The role of these particles in causing nonuniform deformation, which led to strain localization and a premature failure in sample L, has been described in detail.