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 thephase 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 -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 -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 -phase consisted of approximately 80% of the microstructure, and the -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 þ phases. This was followed by = interface cracking and ductile failure of the þ microstructure.

Effect of Boron on the Elevated-Temperature Tensile and Creep Behavior of Cast Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Weight Percent)

“…The volume percent of the b phase in the Ti-6Al-2Sn-4Zr-2Mo-0.1Si-xB system is approximately 1.5 times that for the Ti-6Al-4V-xB system. [8,26] This was expected based on the larger amount of b-phase stabilizers (Mo, Zr), and this also occurred when no B was present in these alloys. The grain refinement with increased B content in the Ti-6Al-2Sn-4Zr-2Mo-0.1Si-xB alloy system was similar to that for the Ti-6Al-4V-xB system; however, the starting grain size of the baseline Ti-6Al-2Sn-4Zr-2Mo-0.1Si casting was more than 3 times smaller than that for the Ti-6Al-4V casting.…”

“…However, the results of this work indicate that the TiB phase did not have as dramatic of an effect on the minimum creep rates of the Ti-6Al-2Sn-4Zr-2Mo-0.1Si-xB alloy system as it had on the Ti-6Al-4V-xB system. [7,8] Thus, it is believed that the creep resistance of the TiB phase more closely resembles that of the Ti-6Al-2Sn-4Zr-2Mo-0.1Si. This along with the lower TiB-phase volume fraction in the Ti-6Al-2Sn-4Zr-2Mo-0.1Si-xB alloy system can explain the relatively small effect that B has on the creep resistance of Ti-6Al-2Sn-4Zr-2Mo-0.1Si compared to that on Ti-6Al-4V.…”

“…This may have been the reason for the slightly lower TiB-phase volume percents for the Ti-6Al-2Sn-4Zr-2Mo-0.1Si-xB system compared to the Ti-6Al-4V-xB system. [7,8,26] That is, the addition of Mo and Zr may have impaired the growth of the TiB whiskers by slowing the diffusion kinetics during the TiB-phase formation. The overall volume fraction of the TiB phase may have been expected to decrease with increased Mo, as the addition of Mo has been shown to reduce diffusion-controlled reactions in Ti-alloy systems.…”

“…The minimum creep rates measured for the as-cast Ti-6Al-2Sn-4Zr-2Mo-0.1Si-xB system were significantly lower than those measured for the as-cast Ti-6Al-4V-xB system (Table V). [7,8] This result was expected based on the superior elevated-temperature creep and tensile strength of Ti-6Al-2Sn-4Zr-2Mo-0.1Si compared with that for Ti-6Al-4V. However, the results of this work indicate that the TiB phase did not have as dramatic of an effect on the minimum creep rates of the Ti-6Al-2Sn-4Zr-2Mo-0.1Si-xB alloy system as it had on the Ti-6Al-4V-xB system.…”

“…[2][3][4][5][6] Small amounts of boron have also significantly improved the creep resistance of as-cast Ti-6Al-4V. [7,8] In fact, conventional Ti alloys containing small additions of B have received considerable interest due to their attractive mechanical properties, including high roomtemperature specific stiffness and strength along with reasonable elongation to failure (e f ). [9][10][11][12][13][14][15][16][17] Recent work has shown that the addition of small amounts (0.1 wt pct) of B to Ti-6Al-2Sn-4Zr-2Mo-0.1Si (wt pct)* and Ti-6Al-4V decreases the as-cast grain size by approximately an order of magnitude.…”