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

Abstract

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 〈100〉 crystal direction. This angle characterizes the orientation of the grain beneath the nitrided layer. Deeper diffusion is observed in grains with orientation close to 〈100〉 than in those grains with orientation close to 〈111〉. 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.