Interplay between temperature and composition effects on the martensitic transformation in Ni2+xMn1-xGa alloys.

Martensitic transformation in Ni2+xMn1-xGa alloys is known to be controlled by the soft tetragonal elastic constant C′ of the high-temperature austenitic phase. The temperature (T) and composition (x) dependence of C′(T, x) are calculated using the first-principles exact muffin-tin orbitals method. We show that the temperature factor of C′ is dominated by the phonon-smearing term. The competition between the negative alloying effect (∂C′/∂x < 0) and the positive temperature effect (∂C′/∂T > 0) is found to lead to nearly constant C′(TM(x), x) at the critical temperature TM(x). We demonstrate that a proper account of the temperature and composition dependence of C′(T, x) is indispensable for reasonable theoretical TM(x) values. [ABSTRACT FROM AUTHOR]