Composition and temperature dependence of phase stability and elastic properties of Co_2Cr(Ga_(1-x)Si_x) shape memory alloys from first principles

摘要

The composition and temperature dependence of the phase stability and elastic properties of Co2Cr(Ga1-xSix) (0.0 <= x <= 1.0) alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. The calculated equilibrium properties and Curie temperature of the ground FM L2(1) phase are in good agreement with the available experimental and theoretical results. Separating the thermal effects into volume expansion, phonon-smearing, and magnetic terms, we find that dominated by the magnetic excitations in each FM alloy upon cooling, the free-energy difference (Delta F) between the D0(22) and L2(1) phases increases from negative to become positive, and the tetragonal shear elastic constant (C') of the L2(1) phase stiffens, whereas that (C-s) of the D0(22) phase softens even to become negative, resulting in the reentrant martensitic transformation (RMT) from D0(22) to L2(1). Nevertheless, with contributions from all three thermal effects, the contrary trends of Delta F similar to T, C' similar to T, and C-s similar to T are found in each PM alloy, promoting the normal martensitic transformation (MT) from L2(1) to D0(22) upon cooling. From both the energetic and mechanical points of view, the evaluated critical temperature (T-M) of the RMT decreases whereas that of the MT is supposed to increase with increasing x. The MT and RMT are further attributed to the Jahn-Teller instability of the PM L2(1) and FM D0(22) phases, respectively, caused by the relatively weak covalent bond between Co and Ga (or Si) in them.