Ultrasonic and atomic force studies of the martensitic transformation induced by temperature and uniaxial stress in NiAl alloys

The single-crystal elastic constants were measured in NiAl alloys with nickel compositions between 49.4 and 63 at. %. For Ni compositions in excess of 61 at. %, the crystals undergo a martensitic transformation. The transformation was studied as a function of temperature and uniaxial stress using ultrasonic, optical, and atomic force microscopy techniques. Uniaxial stress applied along the [001] direction on Ni₆₃Al₃₇ crystals induced a transformation and yielded a value of dM_S/dσ=0.53K/MPa. Atomic force microscopy measurements yield quantitative information of the surface relief associated with the transformation and provide clear evidence of a surface transformation occurring approximately 5 K prior to the bulk transformation as determined by ultrasonic measurements. The results are compared with earlier studies of the elastic constants and with neutron-diffraction studies of the temperature and stress dependence of the slow transverse-acoustic branch in NiAl.