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

As a novel high strength metastable beta 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 alpha phase (alpha(p)) decreases, and a platelets (alpha(s)) increases during subsequent aging treatment. Aging temperature directly determines the size and volume fraction of a, which significantly influences the strength of the alloy. Low temperature aging (510 degrees C) treatment produces a high density nano -scale distribution of alpha(s) precipitates which induces an extremely high ultimate tensile strength (UTS similar to 1630 MPa) together with considerable total elongation (El similar to 6%). As the ageing temperature increases to 630 degrees C, the UTS reduces to similar to 1208 MPa but the El dramatically improves to similar to 13%. The alloy exhibits a good strength -ductility combination with UTS similar to 1390 MPa and El similar to 10% after ST at 800 degrees C followed by aging at 570 degrees C. A Hall-Petch like equation is used to calculate the strengthening effect of alpha(s) which shows a good agreement with the experimental results. It's concluded that the alpha(s) spacing length (X) dominates the free slip length in beta matrix, and determines the alloy strength. The ductility evolution is explained by work hardening behavior. Limited elongation of samples aged at 510 degrees 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 degrees C results from the slightly dynamic recovery which could effectively postpone the occurrence of necking and increase the alloy's ductility.