The effect of processing on the 455 °C tensile and fatigue behavior of boron-modified Ti-6Al-4V

This work investigated the effect of nominal boron (B) additions of 0.1 wt.% and 1.0 wt.% on the elevated-temperature (455 °C) fatigue deformation behavior of Ti-6Al-4V (wt.%) for maximum applied stresses between 250 and 550 MPa (R = 0.1, 5 Hz). The alloys were evaluated in the as-cast condition as well as the cast-and-extruded condition. Boron additions resulted in a dramatic refinement of the as-cast grain size, and larger boron additions resulted in larger titanium-boride (TiB) phase volume percents. For the as-cast alloys, the B-containing alloys exhibited longer average fatigue lives than those for Ti-6Al-4V, which was suggested to be related to their increased strength and stiffness due to the addition of the strong and stiff TiB phase. The longest average fatigue lives were exhibited by the Ti-6Al-4V-0.1B alloy, which also exhibited the greatest elongation-to-failure value. The extrusions, which were performed in the β-phase field, resulted in a significantly smaller grain size, a smaller α-colony size, and finer α-lath width compared to that for the as-cast B-modified alloys. The TiB whiskers were aligned in the extrusion direction and the α-phase was also strongly textured such that the basal plane was predominately oriented perpendicular to the extrusion axis. Together these microstructural features were responsible for the significantly higher 455 °C yield strength, ultimate tensile strength, and fatigue strength exhibited by the cast-and-extruded alloys compared with the as-cast alloys. In the extruded condition, B-addition did not improve the tensile or fatigue strength.