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The coarsened grain boundaries(GBs) with vein-like morphology frequently appear in the nitrogen diffusion layer of nitrocarburized carbon steels.The electron probe X-ray microanalysis shows that such vein-like GBs are rich in nitrogen and carbon atoms.Transmission electron microscopy and scanning electron microscopy further reveal that the coarsened GBs consist ofγ’-nitrocarbide(Fe_4(C,N)) andε-nitrocarbide lamellas that are formed during nitrocarburizing due to high nitrogen concentration at GBs.It is shown that many of {111}<112> micro twins exist in the y’-phase lamellas and thin e-phase slices prefer to nucleate at their twin boundaries with the orientation relationship of {0001}_ε//{111}_γ’.Upon growing large y’-lamellas may become faceted and thin e-lamellas may grow thicker and become the main portions in the vein-like GBs.The microstructure evolution occurring in the vein-like GBs can be depicted as:α+[N]/[C]→>γ’ +[C,N]→ε-nitrocarbides.
The coarsened grain boundaries (GBs) with vein-like morphology frequently appear in the nitrogen diffusion layer of nitrocarburized carbon steels. The electron probe X-ray microanalysis shows that vein-like GBs are rich in nitrogen and carbon atoms. Transmission electron microscopy and scanning electron microscopy further reveal that the coarsened GBs consist of γ’-nitrocarbide (Fe_4 (C, N)) and ε-nitrocarbide lamellas that are formed during nitrocarburizing due to high nitrogen concentration at GBs.It shows that many of {111} <112 > micro twins exist in the y’-phase lamellas and thin e-phase slices prefer to nucleate at their twin boundaries with the orientation relationship of {0001} _ε // {111} _γ’.Upon growing large y’-lamellas may become faceted and thin e-lamellas may grow thicker and become the main portions in the vein-like GBs.The microstructure evolution occurring in the vein-like GBs can be drawn as: α + [N] / [C] →> γ ’+ [C, N] → ε-nitrocarbides.