CRYSTAL-STRUCTURE AND MORPHOLOGY OF ALN PRECIPITATING ON NITRIDING OF AN FE-2 AT PERCENT AL-ALLOY

The crystallography of aluminium nitride precipitating on nitriding of an Fe-2 at.% Al alloy has been investigated by electron microscopy and by X-ray diffraction. The precipitation of AIN can be described as a classical nucleation and growth process; no evidence has been obtained for clustering of alloying element atoms or other pre-precipitation phenomena. Two modes of AIN precipitation have been observed depending on the initial deformation state of the specimens: (i) in cold-rolled specimens, precipitation of cubic (rock salt) AIN particles with platelet-type morphology exhibiting a Bain orientation relationship with the alpha-Fe matrix, and (ii) in recrystallized specimens, precipitation of hexagonal (wurtzite) AIN particles without specific morphology and, for a significant part exhibiting a Pitsch-Schrader orientation relationship with the alpha-Fe matrix. A small fraction of cubic (rock salt) AIN also occurs in the recrystallized specimens. Although the hexagonal (wurtzite) crystal structure of AIN is thermodynamically the most stable one, the cubic (rock salt) AIN precipitates in the cold-rolled specimens have not been observed to convert to hexagonal (wurtzite) ones even after long-time nitriding (60 h at 833 K).