Analysis of the Matrix/Precipitates Interface According to the Growth Behavior of Aluminum Nitride Formed on the Subsurface of NAK80 Steel During Laser Nitriding

Laser nitriding is known as one of the methods that can easily improve corrosion properties through surface alloying or microstructural alteration by forming nitride on the surface. Therefore, studies on the reinforcement of corrosion properties through laser nitriding on the surface of various materials are being conducted, but studies on the mechanism of the interface between nitride and matrix are insufficient. In this study, we performed the laser nitriding on the surface of NAK80 steel alloy and analyzed the growth behavior of nitride by interfacial energy between matrix and interface through Gibbsian interfacial free energy calculation.<br/>During laser nitriding, Al in the alloy causes the formation of many faceted AlN precipitates in the subsurface reigon. The atom-probe tomography and transmission electron microscope analysis identified AlN facets with three different crystallographic orientations, and their relationships with respect to martensite matrix were [-2110]_AlN || [-111]_Fe, [2-111]_AlN || [010]_Fe, and [-2110]_AlN || [001]_Fe. Among these, [-2110]_AlN || [-111]_Fe exhibited a basal plane of AlN at approximately -5 degree with respect to the {110} Fe plane, suggesting that a tilted basal plane was chosen to reduce 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 [-2110]_AlN || [-111]_Fe, [2-111]_AlN || [010]_Fe, and [-2110]_AlN || [001]_Fe, respectively.<br/>Consequently, it was confirmed that the interface with a large degree of mismatch parameter had high interfacial energy. To resolve the high energy, the precipitates had a tilted basal plane or more elements were segregated in the interface between matrix and precipitates. The results of the interfacial energy analysis suggest a direction for considering the efficient formation of the precipitates to be formed in order to improve the corrosion properties.