Elucidating the role of stress-induced martensite on the tensile behaviour of metastable β Ti-10V-2Fe-3Al alloy

Abstract

Stress-induced martensitic transformation (SIMT) during loading in metastable alloy systems result in strength-ductility enhancement via transformation-induced plasticity (TRIP). The present study aims to investigate the influence of stress-induced martensites (SIM) on the tensile behaviour of metastable β Ti-10 V-2Fe-3Al (Ti-1023) alloy. The solutionized, and thermo-mechanically processed (i.e., cold rolling and annealing at 820 °C for 5 to 30 min) condition of Ti-1023, with single β phase microstructure, has been considered. The β grain size of solutionized Ti-1023 was ~625 μm, while the thermo-mechanically processed Ti-1023 was in the range of ~48 μm to ~106 μm as function of annealing time. The room temperature tensile behaviour of considered samples show SIMT, which results in strength enhancement (~7.5%), along with an increase in the trigger stress (~30%) as a function of β grain size (i.e., ~48 μm to ~625 μm). However, the % strain to failure show a significant decrease. A detailed microstructural characterization indicates the relation of SIM formation capability with β grain size. For finer β grain size, SIM displays a pronounced embrittling effect on the Ti-1023 tensile behaviour. Fracture surfaces show distinct regions with lath morphologies, which are further confirmed by surface as well as cross-sectional transmission electron microscopy (TEM) of the tensile fractured surfaces, illustrating the embrittling effects induced by SIM. In comparison, the dislocation-induced plasticity regions in the surrounding remnant β matrix result in dimple morphology. Thus, the coupling of enhanced martensite transformability, with reduced grain size, and the eventual decrease in martensitic laths spacing (i.e., from ~48 μm to ~7 μm), increase the density of martensite/matrix interfaces and the embrittling action. This results in pronounced dislocation pileups and formation of multiple martensitic variants facilitating premature failure by micro-void nucleation (at the martensite/matrix interface), growth and coalescence mechanism. The observations suggest that grain boundary engineering be optimized in order to fully exploit SIM's potential in enhancing the strength-ductility relation in Ti-1023 alloy.