Unusual plate-type iron-carbonitrides development in nitrocarburized Fe-4 at.% V alloy

Abstract

The influence of substitutionally dissolved V in ferritic Fe-4 at.% V alloy, on the structure and morphology of iron-carbonitrides ε-Fe3(N, C)1+x and γ'-Fe4(N, C)1-y was investigated upon salt bath nitrocarburizing. An unusual plate-type morphology of γʹ and ε carbonitrides development was evidenced during nitrocarburizing of Fe-V alloy as against to the usual layer-type growth of γʹ and ε nitrides reported for nitrided Fe-V alloys. The ε phase developed first along ferrite matrix grain boundaries, followed by its formation as plates on the plates of γʹ within the grains of ferrite matrix and finally as a layer at the surface. In contrast to ε, γʹ has developed only as plates within the ferrite matrix grains either with Kurdjumov-Sachs or Pitsch orientation relationship. Initially the inwardly diffusing N gets consumed for the development of VN precipitates in the ferrite matrix while C gets enriched at the ferrite-matrix grain boundaries leading to the development of ε carbonitride. Once all the V precipitates as VN, the development of γʹ occurs as plates within the grains and upon further increase in the N content lead to the development of ε within grains. This demonstrates that as a consequence of drastically different solubility and diffusivity of C and N in ferrite, ε and γʹ phases can lead complex microstructural evolution upon nitrocarburizing of Iron-based alloys.