Mechanical properties of Ni₃AI and nickel-base alloys with high volume fraction of γ'

Abstract

The mechanical properties of Ni3AI (γ’) are first reviewed, with particular emphasis on the flow-stress and creep properties. It is shown that the flow stress of Ni3AI has a very unusual temperature dependence: it increases with increasing temperature. This behaviour is shown to be related to the nature of the dislocation dissociation in Ni3AI, the increase in flow stress being the result of thermally activated cross-slip from {111} planes to {010} planes of ½(110) screw dislocations. This model is shown to be in agreement with the observed changes in the flow stress with changes in test temperature, sample orientation, and sense of the applied stress. The effects of compositional changes on the flow stress are also discussed. Stage II work hardening, strain-rate effects, and the creep deformation of γ’ are reviewed and shown to be only poorly understood. The flow strength of γ/y’ alloys is then discussed, and it is shown that many of the anomalies in the behaviour of pure γ’ are also found in γ/γ’ alloys. Finally, the creep properties of γ/γ’ alloys are examined. It is concluded that surprisingly little is known about the mechanisms which control the creep rate in this important class of alloys.