Analysis of reverse transformation behavior in iron-based alloys based on quantitative microstructure information by neutron diffraction

Relationship between phase transformation and dislocation evolution of Fe-Mn-Si-Cr shape memory alloy upon tensile deformation and subsequent annealing treatment was investigated. Neutron diffraction and electron backscatter diffraction (EBSD) measurements were carried out to evaluate dislocation density and phase transformation. Reasonable phase fraction of martensite was evaluated by Rietveld-texture analysis via neutron diffraction. It was confirmed that EBSD tends to underestimate the phase fraction of martensite. Kernel average misorientation (KAM) analysis was carried out by the EBSD to analyze geometrically necessary (GN) dislocation density. The KAM values of austenitic and martensitic phases increased linearly with nominal strain and did not vary despite the annealing treatment for reverse transformation. On the other hand, dislocation density of austenitic phase, which was estimated by neutron diffraction line-profile analysis, decreased with the annealing treatment. The dislocation density evaluated by neutron diffraction was one digit higher than GN dislocation density estimated by KAM values. This is because neutron diffraction evaluates total dislocation density of not only GN type but also statistically stored (SS) type. Thus, it was indicated that SS dislocations annihilated by recovery whereas GN dislocation remained during the annealing treatment. Interestingly, the total dislocation density of martensitic phase was almost constant irrespective of nominal strains and increased with the annealing treatment. These dislocation evolution behaviors and the effects of dislocations on the reverse transformation were discussed.