The effect of zirconium addition on the mechanical properties of high temperature titanium alloy

Abstract

Titanium and its alloy have been widely used for aerospace application because of its high specific strength, good corrosion resistance, good fatigue, creep strength and etc. The development of high temperature titanium alloys have contributed significantly to the spectacular progress in thrust-to-weight ratio in aircraft gas turbines. Titanium alloys have comparatively low modulus, good fatigue strength, low density, high strength to weight ratio, formability, machinability, corrosion resistance, and biocompatibility. This thesis discusses an overview on the development of high-temperature titanium alloys for use in gas turbines.Ti-Al-Zr-Sn-Mo-Si based near alpha titanium alloys are widely used in jet engine applications in the temperature range of 450◦C – 600◦C. To increase the operating temperature and to increase the high temperature strength of these alloys, numerous number of research works have been carried out. Alloying addition is one of the fundamental and effective methods to enhance the mechanical properties of titanium. Zirconium (Zr) is one of the effective solid solution strengtheners in titanium alloys. The effect of increase in zirconium content on high temperature strength of titanium alloys have not yet been extensively studied.In the present study, the effect of increase in zirconium content has been studied on the properties of Ti-6Al-xZr-2Sn-0.5Mo-0.5Nb-0.3Si (x = 2, 4, 6, 8) based high temperature titanium alloy at room temperature and at 500 ºC. The alloys were prepared in vacuum arc furnace under an argon atmosphere. The alloys were solution treated followed by ageing. The titanium alloys prepared were subjected to various characterization techniques like optical microscopy, tensile testing, hardness testing, wear and wear surface analysis, DSC and XRD. The microstructure revealed lamellae structures with the lath size decreasing with increasing Zr content. Tensile test was performed to find out the yield strength and UTS of each alloy, and their hardness were measured usingRockwell hardness test. The rate of wear were analysed for each alloy, and the wear surfaces were examined using optical microscopy and roughness test. DSC analysis was used to check the transformation temperature of the alloys. XRD analysis was used to identify the phases formed in the alloys. The results showed that with an increase in zirconium content, the tensile strength at room temperature and at 500 ºC, and hardness at room temperature increased significantly. The rate of wear was found to decrease with increase in Zr content in the alloys. The results obtained are discussed based on microstructure analysis using optical microscope. It is inferred that zirconium strengthens the Ti-6Al-xZr-2Sn-0.5Mo-0.5Nb-0.3Si at both room temperature and at high temperature (500 ºC).The results obtained from optical microscopy, tensile test, hardness test, wear and wear surface analyses, DSC and XRD would be helpful for the future development of new high temperature titanium alloys, which would be useful for reducing weight and better fuel consumption.