Anisotropy in flow behaviour of directionally solidified Ni-based superalloy under compression at an elevated temperature

Abstract

Directionally solidified (DS) superalloys consist of coarse columnar grains with <001> preferred orientation parallel to the maximum stress axis; but, grain orientations in the transverse direction are not controlled. However, during service, substantial loading could occur in the transverse direction as well, leading to early intergranular failure of components such as DS turbine blades. The present study investigates the effect of crystallographic orientation as well as specimen orientation (transverse and longitudinal with respect to grain growth direction) on the mechanical response of DS Ni-based superalloy in compression. Compression tests were performed at 700 ºC on transverse (with predominantly <101> or <001> oriented grains) and longitudinal (with <001> oriented grain) samples. Though there were negligible changes in yield stress, variations in hardening behaviour and ductility were observed in all the tested samples. Unlike transverse samples, longitudinal samples showed significant strain hardening with no failure even up to 0.3 true strain. Deformation mechanisms that led to these differences are identified using advance characterization techniques. γ' shearing with antiphase boundary formation was observed in the sample containing <101> grains, whereas γ' shearing with the formation of superlattice intrinsic stacking faults occurred in the sample having <001> grains. In addition, substantial lattice rotation and deformation twinning also contribute to strain hardening, especially in longitudinal samples. The effect of orientation is explained in terms of effective stacking fault energy. The study suggests that grains with <001> orientation undergo significant strain hardening and thereby accommodate large strain as compared to <101> oriented grains in DS alloys.