Design, microstructure and properties of metastable beta-type biomedical titanium alloys

Many existing implant biomaterials including cobalt-chromium alloy, stainless steel, Ti-6Al-4V and commercially pure titanium have all been shown to demonstrate mechanical incompatibility, poor osseointegration and/or cause cytotoxic effects on the human body after some years of application, leading to revision surgery in most cases. Consequently, there is an immediate need for an enduring biomaterial that displays good mechanical properties and possesses biocompatibility and corrosion resistance, in order to reduce rates of revision surgeries. In this PhD work, based on the ̅̅̅̅- ̅̅̅̅̅, / ̅̅̅̅̅- ̅̅̅ and BF-d-electron superelastic theoretical relationships four new series of quaternary Ti-25Nb-8Zr-xCr, Ti-25Nb-xSn-yCr, Ti26Nb-xMn-yZr and Ti-25Nb-xMn-ySn alloys have been designed for the first time. These designed alloys were produced using the cold crucible levitation melting method, where the effect of balanced combination of β-isomorphous (Nb), β-eutectic (Cr, Mn) and neutral (Zr, Sn) elements on phase transformation, β-phase stability and mechanical properties of the alloys are investigated.Microstructural investigations of Ti-25Nb-8Zr-xCr (x = 0, 2, 4, 6, 8) demonstrate a single β phase, with the exception of Ti-25Nb-8Zr-0Cr which shows dual α" and β phases. Furthermore, the addition of Cr is shown to be effective in achieving a single β phase where suppressing the formation of α" phase. As the content of Cr increases, the yield strength (382-773 MPa) and hardness (1.91-2.63 GPa) also increase in Ti-25Nb-8Zr-xCr alloys. Notably, all the investigated alloys demonstrated significant strain hardening rates.The Ti-25Nb-xSn-yCr (x = 1, 3, 5 wt% and y= 2, 4 wt%) alloys demonstrated only β phase in their microstructures. It is of note that all Ti-25Nb-xSn-yCr alloys displayed large plasticity of ~80% without failure during mechanical testing. Yield strength, hardness and elastic modulus were (314-463) MPa, (2.36-1.93) GPa and (66-78) GPa, respectively. Ti-25Nb-1Sn-2Cr possessed the higher values of wear resistance indices (i.e. H/E and H ) as compared to commercially pure titanium and Ti-6Al-4V.The Microstructural features of Ti-26Nb-xZr-yMn (x = 4, 7, 10 wt% and y = 3, 5 wt%) alloys revealed a monolithic β phase. Notably, none of the alloys displayed failure and demonstrated substantial true plasticity of ~160% during mechanical compression testing. Yield strength, hardness and dislocation density were (609-451) MPa, (242-207) HV and (2.45×10 ) m 15 15 -0.4×10 -2 , respectively. Additionally, Ti-26Nb-4Zr-5Mn demonstrates good strain hardening ability and electrochemical kinetics in terms of high strain hardening indices (0.42 and 0.09) and small corrosion current density (0.839 nA/cm 2 ), respectively.In Ti-25Nb-xMn-ySn (x = 2, 4 wt% and y = 1, 5 wt%) alloys, it was found that only Ti-25Nb2Mn-1Sn displayed dual β and α" phases while others showed a monolithic β phase. Yield strength, hardness and superelastic recovery ratio were (710-563) MPa, (244-207) HV and (9080) %, respectively. It is of noteworthy; Ti-25Nb-4Mn-1Sn displays the low elastic modulus and high energy absorption.The results demonstrate that among the investigated alloys Ti-25Nb-8Zr-4Cr, Ti-25Nb-1Sn2Cr, Ti-26Nb-4Zr-5Mn and Ti-25Nb-4Mn-1Sn display superior combination of mechanical properties making them suitable materials for implant applications.