Title

Design, microstructure and mechanical properties of new Ti-based alloys

Date of Award

2016

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

First Advisor

Associate Professor Lai-Chang Zhang

Abstract

In this PhD study, using the DV-Xα molecular orbital method three new series of TixFe- yTa, Ti-Fe-xNb and Ti-Nb-xFe alloys are designed for the first time. The alloys are then produced by cold crucible levitation technique. The influence of Ta, Fe and Nb contents on the phase transformation, β phase stability and microstructural evolution of the alloys are investigated. Additionally, the resulting mechanical properties of the alloys are evaluated and compared with those of the widely used biomaterials, i.e. CP-Ti and Ti-6Al-4V to ascertain their suitability for orthopaedic application.

The microstructural studies revealed that Ti-xFe-yTa (x= 8, 9, 10 wt% and y= 0, 2, 5, 8, 9, 10 wt%) alloys substantially consist of α″ and β phases. However, addition of more Fe and Ta β-stabilizers to Ti–8Fe alloy reduced the likelihood to form α″ martensite during quenching, hence decreasing the volume fraction of α″ phase and finally reaching 3% in Ti- 10Fe-10Ta alloy. It is noteworthy noting that all alloys possess higher yield strength (1008- 1560) MPa and hardness (343-468) HV5 than those of CP-Ti and Ti-6Al-4V. However, among them, Ti-10Fe-10Ta alloy with dominant β phase structure exhibits the lowest elastic modulus (92 GPa) and highest plasticity (40%).

In Ti-7Fe-xNb (x= 0, 1, 4, 6, 9, 11 wt%), it was observed that only Ti-7Fe-11Nb alloy shows single β phase microstructure, while others are comprised of β and α″ phases. Moreover, Nb addition up to 11 wt% enhances the stability and volume fraction of β phase in the microstructure, hence reducing the propensity of the alloy system to form α″ phase during quenching. Yield strength, hardness and wear rate of the alloys are (985-1847) MPa, (325- 520) HV5 and (3×10-15-1×10-13) m3/m respectively. Among the alloys, Ti-7Fe-11Nb possesses the lowest elastic modulus (84 GPa) and highest deformability (42% strain) due to sence of shear bands indicates excessive deformation as a result of strain localization ADDIN EN.CITE Haghighi2

The microstructure of Ti-11Nb-xFe (x= 0.5, 3.5, 6, 9 wt.%) alloys shows that with the exception of β-type Ti-11Nb-9Fe, all alloys are comprised of β and α/α″ phases. Moreover, Fe addition is effective in suppressing α and α″ formation, hence making β more stable. Yield strength and hardness of the alloys are (796-1137) MPa and (278-357) HV5 respectively. It is noteworthy that upon increasing Fe content, the elastic modulus of the alloys decreases, while their plastic strain and elastic energy are enhanced due to retention of more β phase. The ii lowest elastic modulus (82 GPa) and highest plastic strain (38%) are observed in full β Ti- 11Nb-9Fe alloy. Moreover, Ti-11Nb-9Fe presents higher elastic energy (7.08 MJ/m3) than that of some commercial Ti-based biomaterials.

The results suggest that among the designed alloys, Ti-10Fe-10Ta, Ti-7Fe-11Nb and Ti-11Nb-9Fe present the best combination of mechanical properties making them more desirable than the commonly used CP-Ti and Ti–6Al–4V materials for implant application. Therefore, this research demonstrates that through proper alloy design it is possible to design new Ti alloys with favourable properties better than CP-Ti and Ti-6Al-4V alloys for orthopaedic application.

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