Control of individual screw and superdislocation structures on thickness-dependence of creep rate in a Ni-based superalloy during tertiary creep

Abstract

During the creep deformation, damage mainly occurs in the tertiary creep stage. Thus, creep life is essentially controlled by the tertiary stage. To find out the impact of the dislocation microstructure on the creep rate in this stage, and in particular its thickness-dependence, creep tests of Ni-based single crystal superalloy are carried out at 1100 °C/130 MPa with sheet samples of varying thickness. The creep rate in tertiary creep clearly increases and rises faster in thinner sample sheets. The observed microstructures demonstrate that the γ/γ’ phase distribution shows no topological inversion. It is found that the variety of dislocations in γ’ rafts decrease with decreasing sample thickness. The γ’ rafts in thin samples are sheared by only two types of superdislocation involving individual screw superdislocations and double Lomer-Cottrell structures, while the γ’ rafts in thick samples are sheared by three types of superdislocations, again individual screw superdislocations and double Lomer-Cottrell structures, but also Kear-Wilsdorf structures. Also the density of disclocations varies significantly. A higher density of individual screw superdislocations and of mobile dislocation pairs, but lower density of locked non-planar structures promotes the creep of thinner samples.