Specific heat of shape memory alloys with soft elastic moduli

Abstract

The crystal lattice of shape memory alloys is unstable toward shears along the certain crystallographic directions. This gives rise to a martensitic transformation accompanied by an anomalous reduction in the shear elastic moduli in a wide proximity to the transformation temperature. In the present paper, we report a considerable influence of the reduction in the elastic moduli on the temperature dependence of heat capacity of shape memory alloys. The Debye formula for the heat capacity has been modified to take into account the abnormally low values of the shear elastic moduli. The modified formula was used to achieve a good fit of the experimental values for a number of Ni–Mn–Ga ferromagnetic shape memory alloys. The fitting procedure allows the separation of the electron and phonon contributions to the heat capacity of these alloys. As a result, the Debye temperatures, the elastic moduli, and the average velocities of sound waves were evaluated.