Elastic properties and phase stability of shape memory alloys from first-principles theory

•  Ni-Mn-Ga and In-Tl are two examples of shape memory alloys. Their shape memory effect is controlled by the martensitic transformation from high temperature face-centered-cubic (fcc) phase to the low temperature face-centered-tetragonal (fct) phase. Experimentally, it was found that the martensitic transformation is related to the elastic properties. In order to better understand the phase transition and facilitate the design of new materials with improved shape memory properties, the atomic scale description of the thermophysical properties of these alloys is needed. In the present thesis, the elastic properties and phase stability of Ni-Mn-Ga and In-Tl shape memory alloys are investigated by the use of first-principles exact muffin-tin orbital method in combination with coherent-potential approximation.It is shown that the theoretical lattice parameters and elastic constants of stoichiometric Ni2MnGa and pure In agree well with the available theoretical and experimental data, indicating that the employed theoretical approach is suitable to study the elastic properties of both cubic and tetragonal crystals. For most of the off-stoichiometric Ni2MnGa, the excess atoms of the rich component prefer to occupy the sublattice of the deficient one, except for the Ga-rich alloys, where the excess Ga atoms have strong tendency to take the Mn sublattice irrespective of the Mn occupation. With increasing e/a ratio (the number of valence electrons per atom), it is found that the theoretical bulk modulus B and the shear constant C44 increase but the tetragonal elastic constant C′ decreases. Except for Mn-rich Ga-deficient alloys, C′ is generally inversely proportional and the energy difference between parent and martensitic phases is directly proportional to the martensitic tansformation temperature TM. For In1-xTlx alloys, the tetragonal lattice parameter c/a and the shear modulus C′ in the fct phase and the total energy difference between the fcc and fct phases decrease with Tl addition, whereas the negative C′ of the fcc phase increases with x turning positive around x=0.35. All of these composition dependent thermophysical properties can be understood by investigating the electronic structure of In and In-Tl alloys and they are in line with the experimentally observed lowering of TM with addition of Tl.

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