Experimental investigation on martensitic transformation and fracture morphologies of austenitic stainless steel

Formation and nucleation mechanisms (i.e. γ → ε, γ → α′, γ → deformation twins → ε → α′ and γ → ε → α′) of deformation- induced martensite (DIM) have been studied through analytical transmission electron microscopy (TEM) after tensile deformation of AISI 304LN stainless steel at various strain rates (SR) at room temperature (RT). Quantitative metallography has been employed extensively to assess martensitic transformation (MT) as function of strain and SR. It has been observed that the enhancement of SR during tensile deformation promotes the early formation of DIM, while suppressing its saturation value at fracture. Fracture surface morphologies and dimple geometries (i.e. dimple density, dimple diameter and dimple size distribution) have been quantified through image processing (IP) of tensile fractographs. It is noted that at lower SR, dimple density is high while dimple diameter is smaller, and vice versa. Concomitantly, the strength is noted to be low and ductility is high at lower SR, and vice versa. DIM has been found to be responsible for high dimple density at low SR. At high SR, MT is suppressed and hence low dimple density. The variation in SR dependent MT accounts for the variation in dimple metrics vis-à-vis tensile properties.