Author Identifiers

Chirag Dhirajlal Rabadia
ORCID: 0000-0002-3490-8493

Date of Award

2020

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

First Advisor

Professor Lai-Chang Zhang

Second Advisor

Professor Hongqi Sun

Field of Research Code

0913, 0912

Abstract

Current biomaterials such as stainless steel, Co-Cr alloys, commercially pure titanium and Ti-6Al- 4V either possess poor mechanical compatibility and/or produce toxic effects in the human body after several years of usage. Consequently, there is an enormous demand for long-lasting biomaterials which provide a better combination of mechanical, corrosion and biological properties. In addition to this, alloys used in high-strength applications possess either high-strength or large plasticity. However, a high-strength alloy should possess a better blend of both strength and plasticity when used in high-strength applications. Metastable β-titanium alloys are the best suited alloys for biomedical and high-strength applications because they demonstrate a wide range of superior mechanical, corrosion and biological properties.

In this PhD study, the Ti-27Nb-7Fe-xCr (x = 0, 2, 4, 6, 8 wt%) alloys using inexpensive elements (Fe, Mn, Cr etc.) have been designed to check their suitability for biomedical applications, whereas the Ti-33Zr-xFe-yCr (x = 3, 5, 7 and y = 2, 4 wt%), Ti-35Zr-5Fe-xMn (x = 0, 2, 4, 6, 8 wt%) and Ti-xZr-7Fe-ySn (x = 25, 30, 35 and y = 2, 4 wt%) alloys have been designed to check their suitability for high-strength applications. Later, all the investigated alloys have been cast using a cold crucible levitation melting technique.

In the Ti-27Nb-7Fe-xCr alloys, only 2 wt% quantity of Cr is enough to retain a single β phase. Young’s moduli of the Ti-27Nb-7Fe-xCr alloys decrease from 116 GPa (in Ti-27Nb-7Fe) to 72 GPa (in Ti-27Nb-7Fe-8Cr) as the β stability improves. The Ti-33Zr-xFe-yCr alloys, except Ti- 33Zr-3Fe-2Cr alloy, demonstrate a C15 type Laves phase and a dominating β phase. Moreover, the Ti-35Zr-5Fe-xMn and Ti-xZr-7Fe-ySn alloys show C14 type Laves and β phases. It is quite interesting to investigate the deformation and strength characteristics of hexagonal close-packed C14 and face-centered cubic C15 type Laves phases in the soft β matrix. Therefore, the deformation and strength characteristics of C14 phase in Ti-35Zr-5Fe-6Mn and C15 phase in Ti- 33Zr-7Fe-4Cr, considering the same volume fraction of Laves phase (~7.0%) have been evaluated and compared using a micro-indentation method. Remarkably, dislocation activity and plastic deformation features are evident in the C15 phase, whereas the C14 phase strongly blocks dislocation motion.

The Ti-33Zr-xFe-yCr, Ti-35Zr-5Fe-xMn and Ti-xZr-7Fe-ySn alloys, designed for high-strength applications, demonstrate yield strength from 1048 to 1580 MPa, ultimate compressive strength from 1498 to 2140 MPa and plastic strain from 2.6 to 33.6%. Further, the appropriate variation in the volume fraction of Laves phase helps in achieving an improved trade-off between strength and plasticity. Moreover, fracture analyses have also been executed for the Ti-33Zr-xFe-yCr, Ti-35Zr- 5Fe-xMn and Ti-xZr-7Fe-ySn alloys. It has been found that the crack propagates along the corresponding Laves phase present in these alloys. The results of the investigated alloys suggest that Ti-27Nb-7Fe-8Cr is suitable for biomedical applications, whereas Ti-33Zr-7Fe-4Cr, Ti-35Zr- 5Fe-8Mn and Ti-35Zr-7Fe-2Sn are suitable for high-strength structural applications. This research is useful to understand the microstructure, mechanical and fracture behavior of titanium alloys used in industries such as biomedical, aerospace, automobile etc.

Access Note

Chapters 5,6,7,8 and 9 are not included in this version of the thesis. See the list of Related publications.

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