Effect of grain size on the uniform ductility of a bulk ultrafine-grained alloy

A bulk metastable austenitic alloy with different ultrafine grain sizes (0.3, 0.6, and 2 μm) was fabricated by a repetitive thermomechanical process consisting of conventional cold rolling and annealing. The ultrafine-grained specimens showed not only high tensile strength (∼900 MPa) but also large elongation (∼40%) because of the strain hardening enhanced by the strain-induced martensitic transformation during tensile test. The uniform elongation of the ultrafine-grained specimens was divided into two different regions: the lower strain-hardened region (Region I) governed by dislocation plasticity in austenite, and the higher strain-hardened region (Region II) dominated by strain-induced martensitic transformation. The finer grain size, the wider the lower strain-hardened region the narrower the higher strain-hardened region and total uniform elongation of ultrafine-grained austenitic alloy.