Ab-Initio Lattice Instability Analysis on Ni and Ni₃Al Single Crystals

Abstract

The purpose of the present study is to elucidate the “ideal strength” of the Ni and Ni₃Al single crystals, the main compositions of Ni-based superalloy, from the viewpoint of the lattice stability. The unit lattices of Ni and Ni₃Al, fcc and L1₂ ordered alloy, are subjected to the [001] uniaxial tension/compression and hydrostatic tension/compression by using the Vienna Ab-initio Simulation Package (VASP) with the generalized gradient approximation (GGA) and ultrasoft pseudopotential. The elastic stiffness matrix is numerically evaluated at each point in the applied deformation pass, then the lattice stability is discussed based on the positiveness of the matrix. Both Ni and Ni₃Al reach the Born’s stability criteria against the bifurcation to the anisotropic Poisson’s contraction in the [001] uniaxial tension, while they do the spinodal criteria against the structural transformation in the [001] uniaxial compression and hydrostatic tension. The hydrostatic compression increases the stability and shows no limit, however, it is also suggested that the spinodal instability appears when the ideal isotropy was broken. The “ideal strength” is evaluated with these stability limits and indicated as “yield curve” on the normal strain-lateral strain or normal stress-lateral stress planes.