Brittle-to-ductile transition in NiAl single crystal

The brittle-to-ductile transition (BDT) of NiAl single crystals was studied as a function of displacement rate and prestraining using double-notched tensile specimens loaded along a [110] orientation with the crack front parallel to a [001] direction. The tensile properties were also evaluated as a function of strain rate and test temperature. For specimens tested, the BDT coincided with the onset of. net-section yielding. It has been found that decreasing the applied displacement rate reduced the BDT temperature and strain hardening rate without affecting the low temperature toughness and yield strength significantly. Prestraining improved the low temperature toughness level but increased the BDT temperature. It has been shown that fracture in NiAl single crystal occurs by development of microcracks on slip bands and their subsequent instability. The effects of displacement rare and prestraining are discussed in terms of the plastic flow mechanisms and their effects on crack initiation and propagation processes. It has been suggested that the cross-slip of screw dislocations at elevated temperatures and low strain rates retards plastic strain localization,and hence, toughness is improved. (C) 1998 Acta Metallurgica Inc.