Three-Dimensional Characterization of Superdislocation Interactions in the High Temperature and Low Stress Creep Regime of Ni-Base Superalloy Single Crystals
Monocrystaline Ni-base superalloys are the material of choice for first row blades in jet engine gas turbines. Using a novel visualization tool for 3D reconstruction and visualization of dislocation line segments from stereo-pairs of scanning transmission electron microscopies, the superdislocation substructures in Ni-base superalloy LEK 94 (crept to e = 26%) are characterized. Probable scenarios are discussed, how these dislocation substructures form.
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 [0 0 1] and [1 1 0] directions at 1293 K and 160 MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [1 1 0] as compared to [0 0 1] tensile loading were identified. [1 1 0] tensile creep is associated with a higher number of γ′ phase cutting events, where two dislocations with equal Burgers vectors of type 〈1 1 0〉 jointly shear the γ′ phase. The resulting 〈2 2 0〉-type superdislocation can move by glide. In contrast, during [0 0 1] tensile loading, two dislocations with different 〈1 1 0〉-type Burgers vectors must combine for γ′ phase cutting. The resulting 〈2 0 0〉-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 [1 1 0] 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 (〈2 2 0〉 glide) in the γ′ phase.
Superdislocation line directions in γ-particles after double shear creep of superalloy single crystals
Two methods are presented which facilitate the analysis of dislocation line directions in micrographs obtained by transmission electron microscopy (TEM). The first method is simple to use and gives approximate results. It is based on the projection of principal crystallographic directions into the image plane of the micrograph. In crystals with cube symmetry this method can be termed 'cube projection method' (CPM). The second analytical approach yields results of higher precision. In this case, two micrographs from different tilt positions are evaluated. For comparison, both methods are applied to investigate the line directions of γ-particle superdislocations after double shear creep of superalloy single crystals (SXs) at 1025°C. Both methods indicate that superdislocation line segments are preferentially oriented along -directions in the L1 2 lattice for the externally loaded shear system (111) [011].