A strategy to regulate the yield ratio of a metastable high Zr-containing β titanium alloy: Synergistic effects of the β domain, β stability and β/α interfaces by varying the α phase content

To meet the demands of both processing and serving, to the best of our knowledge, this is the first report exhibiting large range control of the yield ratio from 0.31 to 0.96 with decent elongations over 10% in the same alloy with low-cost thermal treatments. The yield ratio of the metastable Ti-30Zr-5Mo alloy was regulated via adjusting trigger stress of the stress-induced phase transformation and work-hardening ability through changing the α phase content. Materials with acicular α phase of different contents were successfully prepared via low-cost thermal treatment. The effects of the α phase content on the stress-induced α' martensite phase transformation and work hardening behavior were then investigated. In the Ti-30Zr-5Mo alloy with dual phases, due to the crystal difference and element partitioning, the hardness of the α phase is higher than that of the β matrix, and the hardness difference between the phases increases with increasing α phase. In addition to Mo, Zr plays an important role in stabilizing the β phase in high-Zr-containing alloys. Deformation initiates in the β phase of both single-phase and duplex-phase alloys. The deformation mechanism of the β phase is dependent on both the β domain and β stability. Due to the low trigger stress and excellent work hardening ability, stress-induced α' martensite phase transformation is helpful to lower the yield ratio. As the α phase content increases, the trigger stress increases, and when the α phase content increases to 40%, dislocation slip dominates rather than stress-induced α' martensite phase transformation, and a high yield strength of 944 MPa is obtained. The α/β phase interfaces act as effective obstacles to hinder dislocation movement and provide working hardening, and the obstruction effect is more significant with an increase in the hardness difference between the α and β phases. The stress-induced α' phase transformation and/or the deformation coordination between the α and β phases guarantee decent elongations of no less than 10% in the large control of the yield ratio from 0.31 to 0.96 with a yield strength from 254 to 1013 MPa. It paves the way to develop “Unititaniam” alloys for wide possible applications.