Ab Initio Calculations and CALPHAD Assisted Design of L1 <sub>2</sub>-Hardened CoNi-Based Superalloys Exhibiting Low Mass Density, Large Misfit and High Yield Stress

Abstract

Frequently, CoNi-based superalloys contain a high fraction of W and/or Mo, which stabilize the precipitate phase cause a high density. In this work, new L12-hardened, low-density CoNi-based superalloys were designed combining ab initio and CALPHAD calculations. The alloys exhibit a very high misfit, good phase stability and excellent mechanical strength. From the ab initio calculations it was possible to draw conclusions how different Ni contents and Ti/Al ratios could stabilize the L12 structure of the γ′ phase. Both, Ni and Ti additions lead to alloys that exhibit a higher formation energy of the new type of (Co,Ni)3(Al,Ti) L12 phase compared with the Co3(Al,W) phase. By CALPHAD calculations screening for promising alloy compositions, it was also found that Ni and Cr can increase the solvus temperature and volume fraction of the γ′ phase. Several alloys were studied experimentally and their microstructures, elemental distributions, lattice parameters and the lattice misfit between the γ and γ′ phases at room temperature were evaluated using scanning electron microscopy, transmission electron microscopy, atom probe tomography and high energy X-ray diffraction. It was found that a two-step heat treatment can improve the strength of the new alloys as reflected by hardness test. The new alloys have low mass density and high yield stress compared with conventional Co-based superalloys, Co-Al-W based superalloys and Co-Ti based superalloys, as well as Ni-based superalloys, i.e. Udimet 720Li. The new alloy compositions also overcome the problem of a low lattice misfit associated with high Ni and Cr contents in L1 2 -phase hardened Co-based superalloys and show cuboidal shaped precipitates similar as Co-Al-W and Co-Al-Mo based superalloys.