The Elevated-Temperature Creep Behavior of Boron-Modified Ti-6Al-4V Alloys

Abstract

This work investigated the effect of nominal boron (B) additions of 0.1 mass% and 1 mass% on the elevated-temperature (673–728 K) tensile-creep deformation behavior of a Ti-6Al-4V(mass%) alloy for applied stresses between 400–600 MPa. The alloys were evaluated in the as-cast and cast-then-extruded conditions. Boron additions resulted in a dramatic refinement of the as-cast grain size and TiB whisker volume percents of approximately 0.6 and 6.0 for the Ti-6Al-4V-0.1B and Ti-6Al-4V-1B alloys, respectively. The extrusions were performed in the β-phase field and resulted in the TiB-phase whiskers aligning in the extrusion direction. The creep resistance of the as-cast alloys significantly improved with increased B concentration, where around an order of magnitude decrease in the secondary creep rate was observed between the Ti-6Al-4V-1B and Ti-6Al-4V as-cast alloys. Grain refinement due to the B addition did not deleteriously affect the creep resistance in the temperature and stress ranges considered, where dislocation creep was suggested to be the dominant secondary-creep mechanism. The enhanced creep resistance was attributed to load sharing by the TiB whiskers. For the same nominal B contents, the cast-then-extruded alloys exhibited significantly greater creep resistance and tensile strength than the as-cast alloys. This was explained to be an effect of the α-phase texture and the decreased lath spacing in the cast-then-extruded alloys compared with the as-cast alloys. The cast-then-extruded alloys exhibited four times lower lath widths than the as-cast alloys, and the α-phase was strongly textured such that the basal plane was predominately oriented perpendicular to the extrusion axis. Comparing the cast-then-extruded alloys, the Ti-6Al-4V alloy exhibited the greatest creep resistance. Overall the α-phase consisted of approximately 80% of the microstructure, and the α-phase texture appeared to be more dominant to the creep resistance and tensile strength than the small volume percent of TiB-phase in the microstructure. Although B is not necessary to optimize the elevated-temperature creep performance of the Ti-6Al-4V alloy, when boron was present, greater boron additions increased the creep resistance. In-situ creep observations of the surface indicated that the TiB whisker cracking occurred prior to slip and void formation in the α+β phases. This was followed by α⁄β interface cracking and ductile failure of the α+β microstructure.