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The alloying and magnetic disordering effects on the site occupation, elastic property, and phase stability of Co2 YGa (Y=Cr, V, and Ni) shape memory alloys are systematically investigated by using the first-principles exact muffin-tin orbitals method. It is shown that with increasing the magnetic disordering degree (y), their tetragonal shear elastic constant C' ((C 11 - C 12)/2) of the L21 phase decreases whereas the elastic anisotropy (A) increases, and upon tetragonal distortions the cubic phase gets more and more unstable. Co2CrGa and Co2VGa alloys with y ≥ 0.2 thus can show the martensitic transformation (MT) from L21 to D022 as well as Co2NiGa. In off-stoichiometric alloys, the site preference is controlled by both the alloying and magnetic effects. At the FM state, the excess Ga atom always tends to take the Y sublattice, whereas the excess Co atom favors the Y site when Y=Cr, and the excess Y atom prefers the Co site when Y=Ni. The Ga-deficient Y=V alloys can occur the MT also at the FM state by means of Co or V doping, and the MT temperature (T M ) should increase with their addition. In the corresponding FM Y=Cr alloys, nevertheless, with Co or Cr substituting for Ga, the reentrant MT (RMT) from D022 to L21 is promoted and then T M for the RMT should decrease. The alloying effect on the MT of these alloys is finally well explained by means of the Jahn-Teller effect at the paramagnetic (PM) state. At the FM state, it may originate from the competition between the austenite and martensite about their strength of the covalent banding between Co and Ga as well as Y and Ga.

The effects of local atomic and magnetic configurations on the phase stability and elastic property of the face-centered cubic (fcc) and two body-centered tetragonal [face-centered tetragonal (fctI) and fctII, with 0.9<c/a<1 and 0.71<c/a<0.9, respectively, in the fct unit cell] phases of Fe1−xPdx (0.28≤x≤0.34) shape memory alloys are systematically investigated by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. It is shown that, considering four types of atomic configurations in a fcc unit cell, the two with one random sublattice are both preferable in each x below 300 K. When T=300 K, the one with three random sublattices also changes to be stabilized for x≤0.30, whereas that with four random sublattices becomes stable in most of these alloys until T≥600K. Upon tetragonal distortions, in these fully disordered alloys, both the fctI and fctII phases are unstable. The fctI phase is found for 0.29≤x≤0.33, having only the configuration with one random sublattice on the same layer with the Pd site in the unit cell, whereas the fctII phase is obtained for x≤0.30, possessing all the configurations with one, two, and three random sublattices. These results representing the phase diagram of these alloys, their determined equilibrium lattice parameters, and elastic constants of the three phases at 0 K are in line with the experimental and theoretical data, and their estimated structural (TM) and magnetic (TC) transition temperatures are also close to the experimental data. Adding 4% magnetic disorder in Fe0.70Pd0.30, the fctII structure is effectively prevented, whereas the thermoelastic martensitic transformation of fcc–fctI can still be retained at 0 K.