Evolution of local misorientations in the γ/γ'-microstructure of single crystal superalloys during creep studied with the rotation vector baseline (RVB) EBSD method.

Abstract

The present work uses the rotation vector baseline electron back scatter orientation imaging method (RVB-EBSD) to study the evolution of small misorientations between the γ- and γ'-phase in Ni-base single crystal superalloys (SXs) during creep. For this purpose, two material states of the SX ERBO1 (CMSX4 type) were characterized after creep deformation at 850°C and 600 MPa to final strains of 1% and 2%. Obtaining reliable phase boundary misorientation (PBM), kernel average misorientation (KAM) and orientation spread (OS) data represents a challenge for electron backscatter diffraction (EBSD), not only because the method operates at its limits of lateral and angular resolution, but also because it is difficult to differentiate between the two phases merely based on Kikuchi diffraction. The two phases differ in chemical composition which gives rise to different EBSD background intensities. These can be exploited to differentiate between the two phases. In the present work, crystallographic and chemical information are combined to demonstrate that orientation imaging can be used to document the formation of dislocation networks at γ/γ'-interfaces and the filling of γ-channels by dislocations. These findings are in good agreement with reference results from diffraction contrast scanning transmission electron microscopy. It is also shown that misorientations evolve between small groups of equally oriented γ/γ'-neighborhoods, on a size scale above characteristic γ/γ'-dimensions (>0.5 μm) and below distances associated with dendritic mosaicity (<200 μm). The methodological aspects as well as the new material specific results are discussed in the light of previous work published in the literature. RESEARCH HIGHLIGHTS: Microstructure evolution during [001] tensile creep of Ni-based single-crystalline alloy. Application of RVB-EBSD technique, focused on angular misorientations between γ/γ' phases, with accuracy of 0.01°. Separation of γ/γ' phases using experimental post-processing of raw EBSD data.