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

Abstract

This work investigated the effect of nominal boron additions of 0.1, 0.4, and 1 wt pct on the intermediate-temperature (455 °C to 565 °C) tensile and tensile-creep deformation behavior of as-cast Ti-6Al-2Sn-4Zr-2Mo-0.1Si (wt pct) for applied stresses between 138 and 600 MPa. A 0.1 wt pct boron addition resulted in a refinement of the as-cast grain size from 550 to 75 μm. Additional boron additions resulted in a less dramatic refinement of the as-cast grain size. Boron additions stabilized the orthorhombic TiB phase where the average TiB-phase volume percents were 0.7, 2.3, and 5.4 for the Ti-6Al-2Sn-4Zr-2Mo-0.1Si-0.1B (wt pct), Ti-6Al-2Sn-4Zr-2Mo-0.1Si-0.4B (wt pct), and Ti-6Al-2Sn-4Zr-2Mo-0.1Si-1B (wt pct) alloys, respectively. Overall, the boron additions did not have a dramatic effect on the creep behavior of Ti-6Al-2Sn-4Zr-2Mo-0.1Si, though the Ti-6Al-2Sn-4Zr-2Mo-0.1Si-1B (wt pct) alloy exhibited lower minimum creep rates than the baseline Ti-6Al-2Sn-4Zr-2Mo-0.1Si (wt pct) alloy. The sequence of surface deformation events during the elevated-temperature tensile deformation was characterized using in-situ experiments performed inside a scanning electron microscope. The TiB whisker microcracking occurred at stresses well below the global yield stress. Multiple and extensive TiB cracking occurred after global yielding. The α + β phase slip occurred after TiB whisker cracking.