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 beta Ti-Nb-based alloys, the alpha '-type Ti alloys could also present low modulus. But due to the low beta stability, the microstructures of these alpha ' 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 alpha ' Ti alloys need to be established. In this study, a novel Ti-12Nb wt% alloy was prepared and cooled from the beta region temperature to obtain the water-quenched (WQ), air-cooled (AC), and furnace-cooled (FC) samples. Systematic investigations revealed the displacive beta -> alpha ' martensitic transformation in the WQ and diffusional beta -> beta + alpha transformation in the AC and FC samples, in which the alpha '/alpha show evident variant selections. The inter-variant orientation relationships (ORs) among the alpha '/alpha variants and their ORs with the retained beta phase, were found to be agree well with the classical Burgers OR. The high-resolution transmission electron microscope (HRTEM) studies show coherent alpha '/alpha ' interfaces in the WQ samples and alpha/beta phase boundaries in the AC and FC samples, the abnormal beta nanodomains inside the alpha ' martensites in WQ samples and the omega nanoparticles in the beta 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<overline>1}(alpha ') and {101<overline>2}(alpha ') deformation twins inside the alpha ' martensites are believed to contribute to the good plasticity in the WQ samples, while the fine alpha + beta 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 alpha bands and deformation-induced omega needles in the surrounding beta 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 alpha ' Ti alloy with low content of beta stabilizing elements and provide insights into tailoring of the phase transformations and mechanical properties in these Ti alloys.