Cyclic plasticity induced transformation of austenitic stainless steels

Abstract

A complete understanding of ‘cyclic plasticity induced transformation' of austenitic stainless steels is documented in this article through different experiments, their critical analysis and comprehensive literature review. The formation and nucleation micro–mechanisms of deformation induced martensites (i.e., ϵ (hcp) and α/ (bcc)) have been thoroughly examined by analytical transmission electron microscope after cyclic plastic deformation of AISI 304LN austenitic stainless steel at various total strain amplitudes (Δϵt) under ambient environment. Magnetic measurements are performed for all fatigue failed specimens at different strain amplitudes. The measured characteristic of magnetization is sensitive to employed strain amplitude, corresponding deformed microstructural features and cyclic plastic response of the material. As strain amplitude increases, the extent of α/ (bcc) martensite increases. It has also been found that deformation induced martensites can nucleate at a number of sites through multiplicity of mechanisms with different transformation sequences, i.e., γ(fcc)→ϵ(hcp),γ(fcc)→ϵ(hcp)→α/(bcc),γ(fcc)→ deformation twin →α/(bcc) and γ(fcc)→α/(bcc). It is also investigated that these martensites can nucleate at a number of sites, such as: shear band intersections, isolated shear bands, grain boundary triple junctions, shear band–grain boundary intersections, dislocation piled–up etc., wherever total interaction energy favours to form. Deformation induced dislocation sub–structures are also characterized to show that cyclic plasticity towards higher strain amplitude loading results in the organization of more uniform dislocation cell structure. Dislocation sub–structure evolution (i.e., arrays, planar, veins, patches, ladders, wall, labyrinth, maze and cell) is also used to explain the shape of hardening/softening curves of the material for all strain amplitudes. The proposed study also quantitatively connects the strain amplitude dependency of the formation of deformation induced martensite and corresponding organizations, configurations and patterning of dislocation sub–structures. Two-dimensional striation spacing and crack density quantified from the fatigued fracture surfaces are also observed to predict the nature of variation in cyclic plastic response of the material as a function of strain amplitudes.