Brittle-to-ductile transition of a multiphase intermetallic alloy based on NiAl

The microstructure and mechanical behaviors of a multiphase beta + gamma + gamma' + alpha-Cr Ni-Al-Cr alloy have been investigated. This alloy exhibits only 1% tensile ductility despite the presence of the continuous ductile phases. It is noted that both fractograph and BDTT are typically dependent on strain rate, with a two orders of magnitude increase in strain rate resulting in approximately a 150 K increase in BDTT. An asymmetry between the compressive and tensile yield strength was also observed at low temperature. SEM observation of the side surface of the fractured specimen indicated that the phase boundary seemed to be weaker than the fracture strength of the constituent phases at temperature below the BDTT; the random orientation relationship between the beta phase and the gamma/gamma' phases may result in difficulty in strain transfer from soft phase to hard phase, which, in turn, contributes to crack initiation at phase boundary. (C) 2000 Elsevier Science Ltd. All rights reserved.