Remarkable improvement in the mechanical properties of TiAlCrNbVNi medium-entropy alloy through minor boron doping and thermomechanical treatment

This study investigated the effect of minor boron doping on the grain refinement and mechanical properties of the homogenized alloys. The alloy with the optimal yield strength–ductility combination was then processed using thermomechanical treatment to enhance its mechanical properties. X-ray diffraction revealed that as the amount of B doping was increased, the diffraction peaks tended to shift to the right due to B having a much smaller atomic radius than the other elements. The hardness increased from 335 Hv for the base alloy to 365 Hv for the alloy with 0.3 at% B content. B doping improved the post-homogenization yield strength from 963 MPa for the base alloy to 1022 and 1122 MPa for the alloys with 0.05 and 0.3 at% B content, respectively; by contrast, the ductility decreased drastically from 26.8 % to 11.9 %, respectively, due to the formation of plate-like TiB precipitates along the grain boundaries because of the excessive B. Of all the homogenized samples, the (Ti₆₅(AlCrNbV)₃₄Ni₁)_99.95B_0.05 alloy was found to achieve the optimal combination of 1022 MPa tensile yield strength and 26.8 % elongation, values approximately 7 % higher than and similar to those of the base alloy, respectively. The (Ti₆₅(AlCrNbV)₃₄Ni₁)_99.95B_0.05 alloy then underwent cold rolling to reduce its thickness by 70 %, and then rapid annealing up to 900 °C (15 °C/s heating rate). The optimal mechanical properties of (Ti₆₅(AlCrNbV)₃₄Ni₁)_99.95B_0.05, which achieved recrystallization annealing to 817 °C, were a yield strength of 1299 MPa, an ultimate tensile strength of 1491 MPa, and a ductility of 15.4 %.