High resolution electron back-scatter diffraction analysis of thermally and mechanically induced strains near carbide inclusions in a superalloy

Abstract

Cross-correlation-based analysis of electron back-scatter diffraction (EBSD) patterns has been used to obtain high angular resolution maps of lattice rotations and elastic strains near carbides in a directionally solidified superalloy MAR-M-002. Lattice curvatures were determined from the EBSD measurements and used to estimate the distribution of geometrically necessary dislocations (GNDs) induced by the deformation. Significant strains were induced by thermal treatment due to the lower thermal expansion coefficient of the carbide inclusions compared to that of the matrix. In addition to elastic strains the mismatch was sufficient to have induced localized plastic deformation in the matrix leading to a GND density of 3 × 10 13 m –2 in regions around the carbide. Three-point bending was then used to impose strain levels within the range ±12% across the height of the bend bar. EBSD lattice curvature measurements were then made at both carbide-containing and carbide-free regions at different heights across the bar. The average GND density increases with the magnitude of the imposed strain (both in tension and compression), and is markedly higher near the carbides particles. The higher GND densities near the carbides (order of 10 14 m –2) are generated by the large strain gradients produced around the plastically rigid inclusion during mechanical deformation with some minor contribution from the pre-existing residual deformation caused by the thermal mismatch between carbide and nickel matrix.