Microstructural evolution and its influence on mechanical properties of Ti-Nb binary alloys

Abstract

Metallic biomedical implants that are currently used for orthopedic applications have shown biological and biomechanical incompatibilities. The first-generation titanium alloys that are currently commercially used are CP-Ti and Ti-6Al-4V. However, these alloys have higher elastic moduli with CP-Ti and Ti-6Al-4V 110 GPa compared to that of the human bone which ranges from 4-30 GPa. The higher elastic modulus causes the stress shielding effect which leads to bone absorption and causes implant loosening and eventually implant failure. The alloying elements aluminum and vanadium of Ti-6Al-4V have been reported to cause allergic reactions in the human body and have lasting detrimental effects to the body. Therefore, development of new biomedical metallic alloys specifically of β-titanium alloys has been underway that are biocompatible and have suitable combination of mechanical properties. This study aimed at developing a biocompatible β-Ti-Nb alloy with improved mechanical properties for different applications. The effect of the β-stabilising element Nb is specifically investigated on the microstructural evolution and the mechanical properties of the Ti-Nb alloys. The influence of heat treatment and the use of different cooling mediums was evaluated on the Ti-Nb alloys. Ti-Nb alloys with a composition range of 1-49 wt.% were fabricated using the vacuum arc remelting process using CP-Ti and Nb as starting materials. The as-cast and solution-treated Ti-Nb alloys were characterized for microstructures and phases formed with OM, SEM-EDX and XRD. The phase transformations that occurred as the as-cast alloys were heated from room temperature up to the β-phase field were determined using the DSC. The Vickers microhardness and tensile properties of the as-cast Ti-Nb alloys were determined to demonstrate the effect on Nb on the mechanical properties. The microstructures of the as-cast Ti-Nb alloys evolved from α-lamellar to a mixture of α-αʹ-and αʺ-martensite, and the metastable β-phase and then the metastable β-phase was fully retained as the Nb content increased. The microhardness, 0.2% yield strength and ultimate tensile strength of the as-cast alloys showed a general increase with increasing Nb content before it decreased for the alloys with high Nb content. However, the Ti-49Nb alloy had the highest UTS and 0.2%YS and the lowest elongation, exhibiting good strength but poor ductility behavior. The addition of the β-stabilising Nb element lowered the Young's modulus as its content was increased with Ti-35Nb alloy having the lowest elastic modulus of 65.2 GPa. The slow cooling of the Ti-Nb alloys after solution treatment led to the formation of the α-phase for low Nb content alloys and the martensitic phases for high Nb content alloys. Whereas rapidly cooling led to increase in stabilising the metastable β-phase at room temperature from Ti-13Nb alloy and produced a fully martensitic microstructure for Ti-7Nb alloy. The different cooling rates did not alter the phases obtained by Ti-1Nb alloy however the α-lamellar microstructure was refined as the cooling rate increased. The air-cooled samples showed the maximum microhardness due to the refined microstructures and martensitic phases formed. The metastable β-phase retained in water-quenched Ti-Nb alloys led to a decrease in hardness. The Ti-Nb alloys exhibited lower Young's modulus compared to the conventionally used titanium alloys and good mechanical strength. The Ti-35Nb alloy can be considered as a potential biomedical implant due to its lower Young's modulus that is comparable to that of the human bone