Mechanisms of elevated-temperature deformation in the B2 aluminides NiAl and CoAl

A strain rate change technique, developed previously for distinguishing between pure-metal and alloy-type creep behaviour, was used to study the elevated-temperature deformation behaviour of the intermetallic compounds NiAl and CoAl. Tests on NiAl were conducted at temperatures between 1100 and 1300 K while tests on CoAl were performed at temperatures ranging from 1200 to 1400 K. NiAl exhibits pure-metal type behaviour over the entire temperature range studied. CoAl, however, undergoes a transition from pure-metal to alloy-type deformation behaviour as the temperature is decreased from 1400 to 1200 K. Slip appears to be inherently more difficult in CoAl than in NiAl, with lattice friction effects limiting the mobility of dislocations at a much higher temperature in CoAl than in NiAl. The superior strength of CoAl at elevated temperatures may therefore be related to a greater lattice friction strengthening effect in CoAl than in NiAl.