Revealing the displacive and diffusional phase transformations in a low-modulus Ti-12Nb alloy

Non-toxic and biocompatible Ti alloys with low modulus are promising biomaterials for implants. Except from the intensely studied metastable β Ti-Nb-based alloys, the α′-type Ti alloys could also present low modulus. But due to the low β stability, the microstructures of these α′ Ti alloys are sensitive to the heat-treatments especially to the cooling rates, which could lead to very different deformation mechanisms and mechanical properties. Thus, the phase transformations and the ‘microstructure–mechanical property’ correlation in the α′ Ti alloys need to be established. In this study, a novel Ti-12Nb wt% alloy was prepared and cooled from the β region temperature to obtain the water-quenched (WQ), air-cooled (AC), and furnace-cooled (FC) samples. Systematic investigations revealed the displacive β → α′ martensitic transformation in the WQ and diffusional β → β + α transformation in the AC and FC samples, in which the α′/α show evident variant selections. The inter-variant orientation relationships (ORs) among the α′/α variants and their ORs with the retained β phase, were found to be agree well with the classical Burgers OR. The high-resolution transmission electron microscope (HRTEM) studies show coherent α′/α′ interfaces in the WQ samples and α/β phase boundaries in the AC and FC samples, the abnormal β nanodomains inside the α′ martensites in WQ samples and the ω nanoparticles in the β phase of AC and FC samples were also presented. The tensile tests show increasing Young's modulus of 61.8 GPa, 77.1 GPa and 108.6GPa in the WQ, AC and FC samples, respectively. The difference in the modulus of these samples was explained by the Nb dependency of Young's modulus in the Ti-Nb alloy. The correlations between the microstructures, deformation mechanisms and mechanical properties were established. The {101¯1}α′ and {101¯2}α′ deformation twins inside the α′ martensites are believed to contribute to the good plasticity in the WQ samples, while the fine α + β structures and severely piled-up dislocations should account for the high yield strength and the poor plasticity in the AC samples. The abundant mobile dislocations in the α bands and deformation-induced ω needles in the surrounding β phase led to the best plasticity in the FC samples among all Ti-12Nb samples. This study systematically studied the displacive and diffusional transformations and the plastic deformation in a Ti-12Nb alloy, the results might pave the way for designing low-modulus biomedical α′ Ti alloy with low content of β stabilizing elements and provide insights into tailoring of the phase transformations and mechanical properties in these Ti alloys.