Dilatational Wave Velocities and Dynamic Elastic Moduli of Ni-Mn Alloys

Abstract

Dilatational wave velocities in Ni-Mn alloys are measured at 25°C in the composition range 0% to 80% manganese using ultrasonic pulse techniques. Using a value of ⅓ for Poisson's ratio in the alloys, the observed values for the velocities are used to compute the corresponding values of the dynamic elastic moduli E. The dilatational wave velocities, ranging from 5638 in nickel to 3792 m/sec in the alloy with 80% Mn, and the correspondlng elastic moduli, ranging from 19350 to 7570 kg/mm2, are found, almost linearly, to decrease with increasing manganese content over the composition range 0% to 40% Mn; to increase over the range 40% to 50% Mn; and then to decrease again over the range 50% to 80% Mn. The increase in the elastic modulus with % Mn over the range 40% to 50% Mn is associated with the formation of the highly ordered NiMn(δ) phase with a bcc crystal structure. The variation of V and E with % Mn in the range of 60% to 80% Mn is found to be an extension of their variation in the range 0% to 40% Mn. This variation where the Young modulus of nickel decreases at an average rate of 125 kg/mm2 per atomic percent manganese added, is characteristic of the solid solution γ phase. At the tests' temperature, 25°C, the alloys with up to 25% Mn are in the ferromagnetic state whereas the alloys over the composition range 25% to 80% Mn are in the paramagnetic state.