Microstructural dependence of strength and ductility in a novel high strength β titanium alloy with Bi-modal structure

As a novel high strength metastable β alloy, Ti–5Al–4Zr–8Mo–7V, it is essential to establish the correlation between its microstructure and mechanical property. In this paper, the microstructure evolution, mechanical properties and work hardening behavior with Bi-modal structure were studied in detail. With the increase of solution treatment (ST) temperature, the volume fraction of primary α phase (α_p) decreases, and α platelets (α_s) increases during subsequent aging treatment. Aging temperature directly determines the size and volume fraction of α_s which significantly influences the strength of the alloy. Low temperature aging (510 °C) treatment produces a high density nano-scale distribution of α_s precipitates which induces an extremely high ultimate tensile strength (UTS~1630 MPa) together with considerable total elongation (El~6%). As the ageing temperature increases to 630 °C, the UTS reduces to ~1208 MPa but the El dramatically improves to ~13%. The alloy exhibits a good strength-ductility combination with UTS~1390 MPa and El~10% after ST at 800 °C followed by aging at 570 °C. A Hall-Petch like equation is used to calculate the strengthening effect of α_s, which shows a good agreement with the experimental results. It's concluded that the α_s spacing length (λ) dominates the free slip length in β matrix, and determines the alloy strength. The ductility evolution is explained by work hardening behavior. Limited elongation of samples aged at 510 °C is attributed to the rapid decrease of work hardening rate after yielding. A slow decrease of working hardening rate of the samples aged at 630 °C results from the slightly dynamic recovery which could effectively postpone the occurrence of necking and increase the alloy's ductility.