Characterization of phase transformations, long range order and thermal properties of Ni₂MnGa alloys

Martensitic and magnetic transformation temperatures were investigated for Ni₂MnGa alloys as a function of the degree of long-range order (LRO) associated with L2₁ structure. The degree of order was changed by changing the heat treatment temperature. Besides, thermal properties such as thermal expansion (CTE), specific heat (C_p) and thermal conductivity (λ) were measured up to 1273 K. It was found that with increasing the heat treatment temperature, the degree of second neighbor ordering lowered, but that the degree of first neighbor ordering did not show good correlations with the heat treatment temperatures. Martensitic transformation temperature (M_s) and Curie temperature (T_c) vary with the square of the degree of LRO. C_p and λ at room temperature are 29.5 Jmol^-1K^-1 and 23.2 Wm^-1K^-1. CTE is ranged between 15 × 10^-6K^-1 and 34 × 10^-6K^-1. Temperature dependence of these thermal properties is described.