Some Mechanical Properties of Ti-6Al-4V-B Alloys

Abstract

The alloys of Ti are extensively used in a number of industries with the α+β alloy Ti-6Al-4V (referred to as Ti64 hereafter) being the most popular. Recently, it was demonstrated that the addition of a small amount of B – as small as 0.04 wt.% – results in an order-of-magnitude reduction in the as-cast grain size. Consequently, there is considerable current interest in understanding the mechanical behaviour of B-modified alloys, with particular emphasis on correlating the microstructural changes with the property variations and the deleterious effects – if any – of TiB particles especially in the context of fatigue. Prior studies have indicated that the addition of 0.1 wt.% B to Ti64 yields the most optimum combination of room temperature properties. The research reported in the current thesis builds further on it, with the objective of exploring the utility of Ti64-B alloys in the engineering applications context. Towards this end, mechanical behaviour of cast and wrought Ti64-B alloys at cryogenic and high temperatures, the possible effect of hydrogenation on the tensile properties, and strain-controlled low cycle fatigue was experimentally evaluated as detailed below. While extensive work is reported on as-cast alloys, the mechanical properties of wrought alloys have not been examined hitherto. Keeping this in view, room temperature tensile and fatigue properties of wrought Ti64-B alloys were investigated. Microstructures of wrought alloys show kinking of the  lamellae and alignment of TiB particles along the flow direction. Marginal enhancement in tensile and fatigue properties upon forging is noted. Decrease in fatigue strength of wrought Ti64-0.04B is observed due to increase in volume fraction of the grain boundary α phase with B addition, which acts as a crack nucleation site. No significant effect of TiB particles on tensile and fatigue properties is observed. Next, strain-controlled fatigue behaviour was investigated. Results show significant softening when the strain amplitudes, ΔεT/2, are ≥0.75%. B addition was found to improve the fatigue life for ΔεT/2 ≤ 0.75% as it corresponds to the elastic regime and hence strength dominated. At ΔεT/2 = 1%, in contrast, the base alloy exhibits higher life as TiB particle cracking due to strain incompatibility renders easy crack nucleation in the B-modified alloys. To examined whether the addition of B to Ti64 is beneficial in enhancing its high temperature mechanical behavior, tensile and creep tests are carried out in the temperature range of 475-550 °C. Experimental results show that the B addition enhances both elevated temperature strength and creep properties of Ti64, especially at the lower end of the temperatures investigated. The steady state creep rate in the B-modified alloys were lower than that in the base alloy, and both the strain at failure as well as the time for rupture increases with the B content. These marked improvements in the creep resistance due to B addition to Ti64 were attributed primarily to the increased number of inter-phase interfaces – a direct consequence of the microstructural refinement that occurs with the B addition – that provide resistance to dislocation motion. Titanium alloys are widely used in various ambient and high temperature applications. However, in some instances these alloys are exposed to hydrogen and low operating temperature environments. Ti64 alloy shows poor ductility in hydrogen and cryogenic environments. Whether the microstructural refinement that occurs with the B addition also improves its relative mechanical performance in such environments is examined. For this purpose, alloys were H charged at 500 and 700 °C for up to 4 h. Microstructures and room temperature tensile properties of the resulting alloys have been evaluated. Experimental results show that charging at 700 °C for 2 h leads to the formation of titanium hydride in the microstructure, which in turn causes severe embrittlement. For shorter durations of charging, a marginal increase in strength was noted, which is attributed to the solid solution strengthening by hydrogen. The mechanical performance of the B modified alloys was found to be relatively better, implying that B addition is beneficial in applications that involve H environment. Finally, the utility of B-modified Ti64 for cryogenic applications is examined through notched and unnotched tensile tests at 77 and 20 K. While the addition of B up to 0.3 wt.% increases the strength at both 77 and 20 K. However, the ductility of the alloys decreases drastically with decrease in temperature. The tensile stress-strain responses of Ti64-B alloys exhibit serrations beyond yielding at 20 K. The extent of serrations were found to be maximum in coarse grained B-free Ti64 alloy, while only one serration could be identified in B-containing alloys. Activation of deformation twinning at 20 K results in the formation of serrations. Three twinning modes were identified in coarse grained B-free Ti64 alloy- {10 ̅2}, {11 ̅1} and {5 ̅1 ̅} while only{10 ̅2}twinning mode was activated in B-containing alloys. Extensive deformation through twinning results in higher ductility of B-free Ti64 alloy at 20 K in comparison to B containing alloys.