Growth behaviors of aluminum nitride in steel during laser nitriding: Insights into interfacial energy and segregation throughout its growth habit

We present a study on laser nitriding on NAK80 steel alloy. The alloy contains Al, which results in many faceted AlN precipitates formed in the subsurface region. The AlN facets have three different crystallographic orientations, and their relationships with respect to martensite matrix were [2¯110]_AlN || [1¯11]_Fe, [21¯11]_AlN || [010]_Fe, and [2¯110]_AlN || [001]_Fe. Among these, [2¯110]_AlN || [1¯11]_Fe exhibited a basal plane of AlN at approximately −5° with respect to the {110}_Fe plane, suggesting that rotation reduced the interfacial energy between AlN and the matrix. The mismatch parameters of each facet direction were calculated as 6.19%, 4.03%, and 0.86% for [2¯110]_AlN || [1¯11]_Fe, [21¯11]_AlN || [010]_Fe, and [2¯110]_AlN || [001]_Fe, respectively. Consequently, a greater degree of mismatch requires more elemental segregation to reduce the interfacial energy. The analysis results of the interfacial energy between the matrix and precipitate makes it possible to consider the efficient formation of precipitates.