Grain refinement effect of ZrB2 in laser additive manufactured metastable β-titanium alloy with enhanced mechanical properties

Significant grain refinement of beta grains in metastable beta TB2 titanium alloy (Ti-8Cr-3Al-5Mo-5V) was achieved through the minor addition of zirconium diboride (ZrB2) during laser melting deposition (LMD). The size of beta grains decreased from-150 mu m in the as-deposited TB2 alloy to-70 mu m in TB2+0.5ZrB2 (TB2 with 0.5 wt% ZrB2 addition) alloy. This study demonstrates that TB2+0.5ZrB2 exhibits good combination of ultimate tensile strength (UTS E896 MPa) and ductility (EL E15%) in comparison to the as-deposited TB2 (UTS E811 MPa and EL E18%). At similar strength (UTSE 900 MPa), TB2+0.5ZrB2 shows superior ductility than TB2+0.4Zr+0.1B and TB2+0.1B because of the formation of fine TiB particles (length of-0.6 mu m) by uniformly introducing boron (B) into TB2 and lower diffusion rate of B. The excellent mechanical properties of TB2+0.5ZrB2 are attributed to the synergistic effect of strengthening from refined beta grains and fine-scale TiB precipitation. The mechanism associated with refinement of beta grains is elucidated and discussed in the context of relevant phase diagrams.