On the nature of γ′ phase cutting and its effect on high temperature and low stress creep anisotropy of Ni-base single crystal superalloys

Abstract

The creep anisotropy of the single crystal superalloy LEK 94 deformed in tension along [001] and [110] directions at 1293K and 160MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [110] as compared to [001] tensile loading were identified. [110] tensile creep is associated with a higher number of γ′ phase cutting events, where two dislocations with equal Burgers vectors of type 〈110〉 jointly shear the γ′ phase. The resulting 〈220〉-type superdislocation can move by glide. In contrast, during [001] tensile loading, two dislocations with different 〈110〉-type Burgers vectors must combine for γ′ phase cutting. The resulting 〈200〉-type superdislocations can only move by a combination of glide and climb. The evolution of dislocation networks during creep determines the nature of the γ′ phase cutting events. The higher [110] creep rates at strains exceeding 2% result from a combination of a higher number of cutting events (density of mobile dislocations in γ′) and a higher superdislocation mobility (〈220〉 glide) in the γ′ phase.