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Effects of Nb and Mo content on mechanical properties of Ti-Nb-Mo-Ta alloys by two-step pressure-free and hot isostatic pressing sintering

The Ti-(15-30) Nb-(2-11) Mo-3.5Ta (wt.%) alloys with the composition interval of 5.0 wt.% for Nb and 3.0 wt.% for Mo were prepared by a two-step sintering method from blended elemental powders. After pressure-free sintering and hot isostatic pressing, fully dense Ti-Nb-Mo-Ta alloys with favourable compressive properties were obtained, rendering the alloys suitable to be used as structural or biomedical materials. The phase constituent, microstructure, and compressive properties were investigated for these Ti-Nb-Mo-Ta alloys. With the increasing Nb or Mo content, the volume fraction of the α phase and the grain size of the β phase decreased. The yield strength of the Ti-Nb-Mo-Ta alloys first declined and then increased with the increase of Nb or Mo content. As for the strain at failure, it increased monotonously for the Ti-xNb-8Mo- 3.5Ta alloys with the addition of Nb, and it first increased and then decreased for the Ti-25Nb-yMo-3.5Ta alloys with the addition of Mo. After analysing and comparing the variation trend of volume fraction of α phase, grain size, yield strength and strain at failure, the dominant strengthening mechanisms of the Ti-xNb-yMo-3.5Ta alloys were interpreted. The dominating mechanism was second-phase strengthening for the alloys with lower alloying elements content, and it changed to solid solution strengthening with the increase of alloying elements content.