Effect of NbC Particles on γ→ε Martensitic Transformation in Fe-22 mass%Mn Alloys

In Fe-22 mass%Mn alloys containing niobium (Nb) and carbon (C), the effects of austenite (γ) grain size and NbC particles on γ→ epsilon (ε) martensitic transformation have been investigated by means of optical microscopy, transmission electron microscopy and X-ray diffractometry. Nb and C were added to give the volume fractions of 0.5 and 1% as NbC particles. Austenite grain size was altered from 3 to 30 μm through the recrystallization of deformed γ. Martensitic transformation from γ to ε was significantly suppressed by the finely dispersed NbC particles. The amount of ε martensite depends on the mean free path (m.f.p.) of NbC particles, particularly when it was reduced to 0.2 μm or less, the γ→ε martensitic transformation was completely suppressed. Such a marked suppressive effect by a small quantity of precipitates is caused by the elastic stress field around NbC particles, which has been formed to keep the coherency with the γ matrix. It was suggested that all the γ matrix is coverd with the elastic stress field when the m.f.p. of NbC particles is reduced to 0.2 μm or less. The suppression of γ→ε martensitic transformation by NbC particles is explained in termes of interaction of partial dislocations with NbC particles and their elastic stress field: The movement of partial dislocations is indispensable for the nucleation and growth of the ε martensite, but it is stopped by the finely dispersed NbC particles themselves or their elastic stress field. It is concluded that the complete suppression of the γ→ε martensitic transformation in the region of less than 0.2 μm in m.f.p. is not due to grain refining of austenite, but due to the suppression of nucleation of the ε martensite by such an interaction between partial dislocations and NbC particles.