Alloying Effects On The Structure And Phase Stability Of Co₂NiGa Alloy By First Principles Study

Heusler-type ferromagnetic shape memory alloys,as new intelligent materials,have extremely rich physical properties,such as ferromagnetism,superconductivity,thermoelectric effect,large magnetostrain,high response frequency and magnetic shape memory effect,and so on.In recent years,the ferromagnetic shape memory alloys with these excellent properties have been widely used in magnetic refrigeration,actuators,and sensors.The conventional Ni₂MnGa ferromagnetic shape memory alloy with a low martensitic transformation temperature and high material brittleness has been difficult to put into practical production and development.Relevant experiments and theoretical calculations have now confirmed that,unlike the Ni₂MnGa alloy,the austenitic phase of the Co₂NiGa alloy can have an XA structure and have occurred from cubic to tetragonal martensitic transformation with the decrease of temperature.It is expected to replace the conventional Ni₂MnGa alloy as a shape memory alloy with excellent performance because of its good ductility,high Curie temperature and spin polarization rate,and super elasticity at high temperature.The site occupation and lattice structure of the Co₂NiGa alloy will be changed with the alloying composition,which will be influenced on its phase stability.However,the systematical experimental and theoretical calculations on Co₂NiGa-based ternary disordered alloys have been rarely reported.Therefore,it has been shown that mastering the alloying effect on its site occupation,structural transformation and mechanical properties is an important part of designing high performance Co₂NiGa-based ferromagnetic shape memory alloy.By using first-principles exact muffin-tin orbitals combined with coherent potential approximation,we have systematically investigated that the alloying effects on the site occupation and phase stability of Co₂NiGa alloy at 0 K and their physical mechanism.It is shown that the Co₂NiGa alloy has an XA-stable structure in the austenitic phase with mechanical instability,and the total electronic energy in the martensitic phase is relatively lower.From the energy and mechanical point of view,it is shown that the XA-Co₂NiGa alloy is prone to martensitic transformation.And the phase stability of the XA and L1₀ phases can be explained by the Jahn-Teller instability effect.Thereafter,it has further explored the effects of Co and Ni excess on the structure and site occupation,ground state properties,magnetic properties and mechanical properties of Co₂NiGa alloy,and it is hoped that the positive conditions for the tetragonal lattice transformation of the alloys can be found from them.It is analyzedthatCo_2+xNi_1-xGa,Co_2+xNiGa_1-x,Co_2-xNi_1+xGaand Co₂Ni_1+xGa_1-x（0.1≤x≤0.4）alloys have the XA-stable structure in the austenite phase.With the increase of Co and Ni content,the energy of Co_2+xNiGa_1-x and Co₂Ni_1+xGa_1-xalloys is gradually reduced and the C?is softened,which is shown that the replacement of Ga by Co and Ni,respectively,could contribute to the martensitic transformation of Co₂NiGa alloy.In this way,by revealing the regularity of the alloying effects on the properties related to the Co₂NiGa-based ternary disordered alloy,it is expected to provide some degree of data reference for the design optimization of this alloy in the experiment.