Alloying and magnetic disordering effects on phase stability of Co2 Y Ga (Y = Cr, V, and Ni) alloys: A first-principles study

Abstract

The alloying and magnetic disordering effects on site occupation, elastic property, and phase stability of Co2 YGa (Y = Cr, V, and Ni) shape memory alloys are systematically investigated using the first-principles exact muffin-tin orbitals method. It is shown that with the increasing magnetic disordering degree y, their tetragonal shear elastic constant C′ (i.e., (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 ferromagnetism state, the excessive Ga atoms always tend to take the Y sublattices, whereas the excessive Co atom favor the Y sites when Y = Cr, and the excessive Y atoms prefer the Co sites when Y = Ni. The Ga-deficient Y = V alloys can also occur the MT at the ferromagnetism state by means of Co or V doping, and the MT temperature T M should increase with their addition. In the corresponding ferromagnetism 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 state. At the ferromagnetism 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.