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 formation of 〈010〉-dislocations in the γ′-phase of superalloy single crystals during high temperature low stress creep
The cutting of γ′-particles by dislocations during shear creep deformation of CMSX 6 superalloy single crystals loaded on the macroscopic crystallographic shear system 〈110〉{111} at T = 1025°C and τ = 85 MPa is analysed using transmission electron microscopy (TEM). The formation of a 〈010〉-dislocation in the γ′-phase was observed and investigated by means of stereo microscopy, line trace andg·b-analysis. Two γ-matrix channel dislocations withdifferent Burgers vectors (b) of type α0/2〈110〉 jointly shear the γ′-particle where they form a superdislocation with a total Burgers vector of α0〈010〉. The present paper provides microstructural evidence for this high temperature cutting process. The results are discussed in relation to the evolution of the dislocation structure in the γ-channels during primary creep. Peach-Koehler forces were calculated to explain the formation of the 〈010〉-dislocation.
Internal stresses due to dislocation walls around second phase particles
Nickel base superalloys with large γ′ volume fractions exhibit a high threshold stress during a tensile test, and a large internal stress when tested in high temperature creep. Arrays of regularly spaced edge dislocations which develop during the first stages of creep have been observed. They lie in the γ-γ′ interfaces and form dipolar arrangements on opposite sides of the γ′ cuboids. The various components of the stress tensor are calculated for such a dipolar wall configuration and mapped by drawing equistress lines. The stress field resulting from such dislocation configurations has a very strong component along the tensile axis and opposes the applied stress. The shear component in the glide plane of similar dislocations is also very large between the precipitates and would tend to repel any new dislocation, in the absence of an external stress. The structural instability of these alloys under creep strain is also interpreted by this model.