The microstructure of the diffusion zone of a gaseously nitrided Fe–1·5 wt-%Cr–1·5 wt-%Al alloy

Abstract

Gaseous nitriding experiments of an Fe–1·5 wt-%Cr–1·5 wt-%Al (i.e. Fe–1·6 at.-%Cr–3·1 at.-%Al) alloy were carried out as a function of time at 853 K. The microstructure of the diffusion zone was characterised by microhardness, electron probe microanalysis (EPMA), X-ray diffraction analysis (XRD), scanning transmission electron microscopy (STEM) in combination with energy dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy (AES). Chromium and aluminium precipitate together as a mixed Cr_1−xAl_xN phase in the diffusion zone. The size of the (semi)coherent precipitates and the amount of excess nitrogen have a strong influence on the microstructure of the diffusion zone. Crack formation occurs after a certain nitriding time starting from the specimen surface and propagating along grain boundaries more or less perpendicularly to the surface towards larger depth. The grain boundary brittleness could be ascribed to the precipitation of excess nitrogen as nitrogen gas at the grain boundaries and to the segregation of Al at grain boundaries promoting precipitation of AlN at the grain boundaries. The residual stress–depth profile was determined and a simple model was proposed to explain the surprising initially occurrence of tensile stress parallel to the surface in the diffusion zone.