There is a current interest in Co-based superalloys hardened by a L1₂-γ' phase because alloys based on Co could possess some particular advantages. The melting point of Co is slightly higher than that of Ni. Also pure Co is more resistant against sulfidation attack due to the higher melting point of Co-sulfides and higher eutectic point in the Co-S system. Unfortunately, the first system of such γ'-hardened Co-based superalloys, the Co-Al-W system, exhibited a number of drawbacks. The γ' phase is not stable at high temperature, the density is very high exceeding 9 g/cm³ and the oxidation resistance is insufficient. Due to this, there is an ongoing interest to develop γ'-hardened Co-based superalloys based on other systems than Co-Al-W, such as Co₃Ti. Here first principles calculations are presented investigating the properties of the γ'-((1-x)CoxNi)3((1-y)AlyTi) phase. To investigate the stability of the different γ' phase compositions x and y were varied between 0 and 1.

All first principles calculations were performed using the DFT software package VASP and the different chemical compositions of the γ' phase were modelled using so called special quasi random structures. A γ' phase of the composition (Co,Ni)₃(Al,Ti), i.e. equal amounts of Co and Ni on one and equal amounts of Al and Ti on the other sub lattice, exhibits a lower energy of formation than Co₃Ti and Co₃(Al,W). Nevertheless, Ni₃(Al,Ti) has an even lower energy of formation being the lowest of all L1₂-phases investigated. This finding is supported by evaluations of the phase stability using different concepts as for example energy of mixing. The results also correspond with own experimental results as well as data published in literature that in overall Co-rich alloy compositions a Ni-rich γ' phase forms.
In addition to the stability of the γ' phase in dependence on Ni and Ti content also other properties of the most stable L1₂-structures like elastic properties and electronic structures were calculated.