An ultrasonic study of the martensitic phase transformation in NiAl alloys

The single-crystal elastic constants and their temperature derivatives were measured in β-phase NiAl alloys. Crystals with Ni compositions between 49.4 and 63 at.% were studied. For compositions in excess of 61 at.% Ni the crystals undergo a martensitic phase transformation which exhibits a shape memory effect. The elastic constant C´ associated with the slow transverse mode softens as the temperature is lowered for Ni compositions in excess of 55 at.%. Near M_s, the temperature dependence becomes nonlinear for the three independent elastic constants and an increase in the ultrasonic attenuation is also observed. The results are compared with earlier studies of the elastic constants and recent inelastic neutron diffraction studies on NiAl