The temperature and orientation dependence of tensile deformation and fracture in NiAl single crystals

Abstract

The tensile deformation and fracture behaviours of stoichiometric NiAl single crystals were investigated as functions of temperature and crystal orientation. Nominal stress-strain curves largely depended on temperature and crystal orientation. The flow strength was generally high for single crystals with orientation [001] and low for single crystals with orientations [ 1 11] and [ 1 22]. The tensile elongations remained low at low temperatures increased rapidly with increasing temperature, gave sharp and very high peaks at intermediate temperatures and then decreased rapidly with increasing temperature. The ductile-brittle transition temperature (DBTT) defined by the reduction in area (RA) occurred at a slightly higher temperature than the DBTT defined by the elongation and then the values of RA reached 100% at sufficiently high temperatures. At low temperatures, cleavage fracture with river patterns was observed and their preferential fracture planes were determined to be (011). At a temperature where the elongation peak was observed, the sample surfaces became corrugated and macroscopic fracture with river patterns proceeded along a plane perpendicular to stress axis without showing apparent necking. At high temperatures where the elongation decreased with increasing temperature, fracture was established by marked necking and exhibited knife-edge or chisel point fracturing, depending on the crystal orientation. On the basis of these results, a discussion of the plastic deformation and fracture mechanisms was attempted.