Crystal Structure and Morphology of Mixed Cr1–x Al x N Nitride Precipitates: Gaseous Nitriding of a Fe-1.5 Wt Pct Cr-1.5 Wt Pct Al Alloy

摘要

The crystal structure and morphology of nitride precipitates developing in the matrix of a Fe-1.5 wt pct Cr-1.5 wt pct Al (Fe-1.6 at. pct Cr-3.1 at. pct Al) alloy upon gas nitriding were investigated. To this end, the nitrided zone was studied using metallographic methods, X-ray diffractometry (XRD), electron probe microanalysis (EPMA), microhardness measurement, and transmission electron microscopy (TEM), including high-resolution TEM (HRTEM), and scanning TEM (STEM). Furthermore, a nitrogen-absorption isotherm was determined, for use in characterizing the nature of the nitride-precipitation process. It could be shown that the expected equilibrium nitrides, cubic CrN and hexagonal AlN, do not develop. Instead, mixed Cr1–x Al x N nitride precipitates of the cubic, rock-salt structure type develop upon nitriding the ternary alloy. These precipitates obey a Bain-type orientation relationship (OR) with the ferrite matrix and are associated with a considerable uptake of excess nitrogen and a very pronounced hardness increase.