The present authors have studied the plasma-nitriding behavior of Fe-high Cr alloys and reported that ferrite grains are newly formed by holding the strict σ9 coincide site lattice (CSL) relation with respect to the original ferrite grain [G. Miyamoto et al., Acta Mater. 54 (2006), 4771]. In this study, effects of pre-exsiting grain boundaries on the growth behavior of the newly formed ferrite grains in the plasma-nitriding of an Fe-18mass%Cr alloy is investigated by means of electron backscatter diffraction analysis. The growth of newly formed ferrite grains is mostly stopped at pre-existing boundaries misoriented larger than 5 degrees because of losing the σ9 CSL relation at the boundaries. Instead, ferrite grains nucleate at such boundaries by holding the σ9 CSL relationships with respect to the ferrite grain in front. By two-step nitriding, it is confirmed that ferrite grains can nucleate even inside of grains without a presence of grain boundaries or the free specimen surface.
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