Microstructure and mechanical properties of Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V influence of H, O and B

Abstract: Titanium and its alloys are used in a wide range of applications from aerospace, marine, biomedical implants and consumer goods, due to their superior specific strength, excellent corrosion resistance, and biocompatibility. In aerospace applications, these alloys are predominately used as components in the aero/rocket engines because of their high strength-to-density ratio compared to other metallic materials. Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) and Ti-6Al-4V (Ti-64) are the two most commonly used alloys in aeroengines where the temperature reaches up to 300-450°C. Ti-6242 is preferred for higher temperature applications i.e. up to 450°C, owing to their excellent fatigue and creep resistance at elevated temperature. Ti-64 is used up to 300°C because of its good tensile and fatigue strength. In titanium alloys, the mechanical properties are dependent on variables such as, alloy chemistry, manufacturing methods and environmental conditions during the service. These variables greatly influence the microstructure, whichinherently affects their properties. The focus of the present research work is to understand the influence of specific elements such as hydrogen, oxygen and boron on the mechanical properties of Ti-6242 and Ti-64 alloys. In the first study, Ti-64 alloy has been exposed to gaseous hydrogen (15 MPa). Here the tensile, low cycle fatigue (LCF) and fatigue crack growth (FCG) properties were explored. Studies showed that in gaseous hydrogen, the LCF life and the FCG resistance were significantly reduced in comparison to those properties measured in ambient air. However, it was observed that there was no significant influence of gaseous hydrogen on the ductility. The influence of hydrogen on the mechanical properties seems to be dependent on the microstructure of the alloy. It was noted that the yield strength (YS), ultimate tensile strength (UTS) and LCF life in gaseous hydrogen were higher for Ti-64 alloy with smaller prior beta grains and smaller alpha colonies than compared to the coarse microstructure. Similar observation was also noted for the FCG resistance. The results in the study indicate that hydrogen mainly influences the crack growth properties, as it changes the mode of fracture from ductile to brittle at a critical stress intensity value. Secondly, Ti-6242 alloy was isothermally heat-treated in ambient air at the temperatures 500, 593 and 700°C up to 500 hours. At these temperatures and times, it was noted that a brittle layer that is enriched with oxygen was formed. This layer is termed “alpha-case”. The thickness of this layer increased with temperature and exposure time. To investigate the effect of this layer on the mechanical properties, LCF testing at strain amplitudes 0.3 and 0.4% was performed for different alpha-case thicknesses. It was noted that the LCF life reduced about 50% with 2 μm thick alpha-case and about 90% with 10 μm thickness at strain amplitudes 0.4%. The study also indicated that the life for fatigue crack initiation is affected rather than the fatigue crack propagation,and the reduction in LCF life is because of the layer enriched with oxygen. Finally, the influence of boron on the compression, tensile and LCF properties of Ti-64 alloy were investigated. Here small amount of boron (i.e. 0.06 and 0.11 wt.%) was added during casting of Ti-64 alloy. It was noted that the boron refined the coarse “as cast” microstructure by precipitating TiB precipitates along the grain boundaries. The refined microstructure increased the compressive strength, YS, UTS and ductility at room temperature. The LCF life of cast Ti-64 alloy with boron up to 0.11 wt.% was increased at strain amplitudes ≤ 0.75%. At higher strain amplitudes (1%), the LCF life was reduced . It is because of cracking of the TiB precipitates, which can easily initiate the cracks. Beside this, it was noted that the effect of grain refinement is diminishing at the temperatures above 500°C. The study showed that the increase in mechanical properties of Ti-64 alloys with boron is a result of reduction in both the prior beta grain and alpha colony dimensions.