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 conditio ns at 1144 K, were found to exhibit very different rupture lives and c reep ductilities. A case study was conducted on two samples with creep rupture lives of 343.6 and 37.0 hours (with corresponding creep ducti lities of 12.3 and 39.9 pet, respectively) in order to find the causes of such a large variation in creep properties. Detailed microstructur al analyses using optical microscopy, scanning electron microscopy (SE M), and transmission electron microscopy (TEM) indicated that the samp le with higher rupture life and lower ductility had deformed uniformly along the length of the gage section, whereas the sample with lower r upture life and higher ductility (sample L) deformed by localized plas tic 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 local ization and a premature failure in sample L, has been described in det ail.