Toughening a double-yielded metastable Ti-Nb-Fe-Al alloy with Ni addition

To explore the inherent mechanism of modifying strain hardening capacity and ductility, two low-Mo equivalent Ti-Nb-based alloys that involve multiple deformation mechanisms are meticulously designed. Al and Ni elements are added into Ti-17Nb-2Fe (Ti-172) alloy to manufacture a novel Ti-17Nb-2Fe-2Al (Ti-1722) and its derivative Ti-17Nb-2Fe-2Al-1Ni (Ti-17221) alloys. The Ti-1722 alloy exhibits a double-yielded phenomenon with a yield strength of 613 MPa and a strain hardening rate of approximate 4 GPa. Microstructural analysis reveals that the addition of Al into the Ti-172 alloy activates stress-induced α' martensitic transformation (SIMα′) and {332} twinning. The addition of Al suppresses the formation of SIMα″ in the Ti-172 alloy. In contrast, the Ti-17221 alloy, with incorporation of Ni, eliminates the double-yielded phenomenon and exhibits an obviously enhanced ductility of 36.8 % with a great strain hardening capacity maintained at high strains. The microstructural characterizations show that further addition of Ni element transforms the plastic deformation into sequential twinning instead of the predominant SIMα′ bands. This work advances the understanding of transforming multiple mechanisms to achieve the balance of mechanical properties in metastable alloys.