The role of thermal activation in the strength anomaly of Ni3Al

Using two transient mechanical tests which are described (repeated load relaxations and repeated creep tests) some microscopic parameters of the deformation processes in Ni3Al compounds such as the activation volume V of the dislocation velocity and the exhaustion rate of mobile dislocations are determined. V values in the range of 80 to 350 b3 (Ni75Al25 polycrystal at 300 K) that dislocation mobility is thermally activated, while large values of the exhaustion parameter correlate well with the work-hardening coefficient. This new information about quantitative aspects of dislocation exhaustion are supported by the observation of a yield point at reloading subsequent to the relaxation transient. The variation of the amplitude of this yield point as a function of temperature is similar to the variation of the exhaustion parameter. Both curves peak at 500 K which is also the peak temperature of the work-hardening coefficient. The variation with temperature of this latter parameter is discussed in the light of the present results.