Date of Award
Doctor of Philosophy
School of Engineering
Professor Lai-Chang Zhang
Professor Hongqi Sun
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.
Chapters 5,6,7,8 and 9 are not included in this version of the thesis. See the list of Related publications.
Rabadia, C. D. (2020). Microstructure and mechanical behavior of metastable beta type titanium alloys. https://ro.ecu.edu.au/theses/2278