Micromechanical modelling of microstress fields around carbide precipitates in alloy 600

Elastic stresses in the grain boundary regions caused by the mismatch between the elastic properties of a carbide precipitate and a nickel alloy matrix were studied by finite-element modelling. The effects of the metal matrix elastic anisotropy, the precipitate elastic anisotropy and the external stress state on the microstress fields around the precipitate were analysed. An intragranular carbide precipitate is not as effective a stress raiser as a carbide precipitated at a grain boundary triple point due to the contributing effect of strain incompatibilities between the grains in the latter case. The precipitate, which has an effective elastic modulus that is higher than that for the surrounding grains, experiences more stress than the matrix, i.e. the stresses inside the precipitate are higher than in the alloy grains. The mismatch between the elastic moduli of the alloy and the precipitate has a pronounced effect on the stress distribution; the higher the precipitate modulus compared with the matrix modulus, the higher is the local stress concentration in the grains of the alloy matrix. The shape of the inclusion plays a role in the stress distribution in the grain boundary regions; sharp cowers raise stresses more effectively than rounded edges of oblong-shaped precipitates. Shear anisotropy in the precipitates affects the distribution of the von Mises (shear) stresses, and the compression anisotropy affects the state of the hydrostatic pressure: the higher the elastic anisotropy, the higher are the corresponding stress concentrations. The selection of the external stress state (shear versus hydrostatic) can reduce or enhance the effects of the precipitate elastic anisotropy. Equi-biaxial loading provides for a more uniform distribution of stresses than does uniaxial loading. The high stress fields are confined to the vicinity of the precipitate (their range is much smaller than the size of the precipitate). When the spacing between carbides at the grain boundary becomes large, their elastic interactions become insignificant.