Dynamic phase transformation during superplastic deformation of Nb/Nb3Al in-situ composite

Abstract

Nbss/Nb3Al in-situ composite with the nominal composition of Nb-16 mol pct Al-1 mol pct B, consisting of bcc niobium solid solution (Nbss) and A15 ordered Nb3Al, was synthesized by arc melting, homogenization annealing, and isothermal forging, and their superplastic deformation behavior was investigated by tensile tests and microstructure observations. Maximum superplastic elongation over 750 pct was obtained at 1573 K and at a strain rate of 1.6 × 10−4 s−1 for as-forged specimens. Phase transformation from Nbss to Nb3Al was observed to occur during superplastic deformation. Dynamic phase transformation during superplastic deformation progresses more quickly than static phase transformation during annealing without applied stress. Dynamic phase transformation is accompanied by phase-boundary migration, which operates as an accommodation process of grain-boundary sliding. Dislocation creep dominates deformation and grain-boundary sliding is inhibited at a high strain rate, while grain-boundary sliding and cavity formation are promoted at a low strain rate because of insufficient accommodation of grain-boundary sliding arising from sluggish dynamic phase transformation. It is concluded that there exists an optimum strain rate that guarantees the grain-boundary sliding and the rapid dynamic phase transformation to achieve maximum superplastic elongation.