Effect of hydrogen charging on tensile properties of B-modified Ti–6Al–4V alloy

Abstract

Trace addition of B to Ti and its alloys leads to a marked microstructural refinement, which in turn enhances the tensile and fatigue properties of the as-cast alloys. This can be particularly advantageous in applications wherein Ti alloys are used in the as-cast form. In some of these, the environment containing H and Ti alloy components is susceptible to embrittlement due to H uptake. Whether the addition of B to Ti–6Al–4V improves the relative mechanical performance of such cast components used in H environments is examined in this work. Cast Ti–6Al–4V–xB (0≤x≤0.55wt%) alloys were H charged at 500 and 700°C for up to 4h. Microstructures and room temperature tensile properties of the resulting alloys have been evaluated. Experimental results show that charging at 700°C for 2h 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 H. The mechanical performance of the B modified alloys was found to be relatively higher, implying that B addition not only refines the as-cast microstructure but also is beneficial in applications that involve H environment. A direct correlation between the volume fraction of TiB particles in the microstructure and the relative reduction in the strength of H-embrittled alloys suggests that the addition of B to Ti alloys, in optimum quantities, can be utilized as a strategy to design alloys that are more resistant to H embrittlement.