Evolution and analysis of γ ′ rafting during creep of single crystal nickel base superalloy

Abstract

By means of TEM observation and finite element analysis, an investigation has been made into the directional coarsening of the γ′ phase for a single crystal nickel base superalloy with [001] orientation during creep at 1040°C. The results show that the strain energy change related to the elastic strain is to be the driving force for γ′ rafting. The extruded strain of the lattice in the cuboidal γ′ interfaces results in a supersaturation of the elements Ta and Al of larger atomic radius. The extrusion or expansion strain in the lattice of the cuboidal γ′ planes may repel or trap these atoms to promote the directional growth of the γ′ phase into a needle-like raft structure along the direction parallel to the stress axis under an applied compression stress, or into a meshlike raft structure along the direction perpendicular to stress axis under applied tensile stress. The normal direction of the expanding lattice is supposed to be the one in which the γ′ rafts grow. The rate of γ′ rafting is enhanced by increasing viscoplastic flow in the γ matrix and elastic strain in the γ′ phase. Therefore, there is a smaller rate of growth under compressive than under tensile stress as a result of the smaller expansion strain and viscoplastic flow occurring in the former.