Effect of grain orientation on the nitriding rate of a nickel base alloy studied by electron backscatter diffraction

Electron backscatter diffraction is a rather new and powerful technique that provides local orientation. In this paper we present an investigation on cross-sections of a nickel base alloy (Inconel 690) treated by low temperature plasma assisted nitriding. The studied alloy presents non-uniform nitrided layer thickness from grain to grain. A linear relationship is found between the thickness of the nitrided layer within a surface grain and the minimum angle between nitriding direction and the 〈1 0 0〉 crystal direction. This angle characterizes the orientation of the grain beneath the nitrided layer. Deeper diffusion is observed in grains with orientation close to 〈1 0 0〉 than in those grains with orientation close to 〈1 1 1〉. An anisotropic dependence of the stress on the strain is proposed to explain these phenomena. The consequences of the interpretation of X-ray diffraction pattern and nitrogen depth profiles of the nitrided layer are discussed.