Calcoli di teoria del funzionale della densità in leghe a memoria di forma di Ni-Ti

Density Functional Theory calculations where conducted in the B2, B19, B19' and B33 phases of the NiTi shape memory alloy as a function of the Ni concentration on the Ni- rich side of the concentration range. In accordance to previous ab-initio results, the lattice minimizing the ground state energy in equiatomic NiTi was found to be the B33, a more inclined monoclinic structure of higher symmetry, which contradicts experimental evidence. An increase on the Ni content was found to have an effect on lowering the stability of the martensite phases, while having little effect on the stability of the B2 phase. The effect of a lower stability was observed on the density of states at the Fermi Level, which was shifted up drastically in all martensite phases with a little increase on the Ni content. By becoming less stable at 0 ºK, the transformation to the lattice of higher symmetry will take place at lower finite temperatures, which is precisely the effect observed experimentally.