Nanocrystallization and martensitic transformation in Fe–23.4Mn–6.5Si–5.1Cr (wt.%) alloy by surface mechanical attrition treatment

Nanocrystalline grains can be obtained in the surface layer of an Fe–23.4Mn–6.5Si–5.1Cr (wt.%) alloy with low stacking-fault energy through surface mechanical attrition treatment, accompanying three kinds of strain-induced martensitic transformations. The microstructure of the surface layer was investigated using optical microscopy, X-ray diffraction and transmission electron microscopy. The results indicate the majority of α martensites can be formed directly from the original matrix (γ, fcc), instead of forming at intersections of strain-induced ɛ martensites in γ matrix grains. The nanocrystallization of grains has three approaches: both the intersection of strain-induced ɛ(hcp) martensites and the formation of strain-induced α(bcc) martensites from austenite lead to refinement of austenite grains, and the martensitic transformation from ɛ(hcp) to α(bcc) makes the grain sizes of the product α(bcc) smaller than those of ɛ(hcp). The strain-induced α(bcc) martensites formed from both austenite matrix and ɛ(hcp) martensites undergo evolution from dislocation tangles, low angle grain boundaries to large angle grain boundaries.