Effect of Mo on the microstructure and mechanical properties of (Hf0.73Ta0.27)100-XMoX (X = 0, 5, 16, 21 and 30 at.%) alloys

Abstract

Microstructure and compression properties of (Hf0.73Ta0.27)100-XMoX alloys, where X = 0, 5, 16, 21 and 30 at. %, are reported. The alloys were prepared by vacuum arc melting and hot isostatically pressed at 207 MPa for 3 h at 1400 °C. The alloys with 0, 5, 16 and 21 at. % Mo contained two phases, BCC and HCP, while the alloy with 30 at. % Mo contained three phases, BCC, HCP and cubic Laves (C15). The Hf73Ta27, Hf69Ta26Mo5 and Hf61Ta23Mo16 alloys experienced eutectoid transformation with the formation of a mixture of Hf-rich HCP and Ta-rich BCC phases. The volume fraction of the eutectoid regions decreased from 55% to 0% with increasing Mo from 0 to 21 at.%. The alloy with 30% Mo (Hf51Ta19Mo30) experienced eutectoid transformation with the formation of a mixture of the Mo-rich cubic Laves (C15) phase and Hf-rich HCP phase. The volume fraction of the eutectoidally transformed regions in Hf51Ta19Mo30 was ~20%. Among the studied alloys, Hf73Ta27 had the highest room temperature yield stress of 1738 MPa. The yield stress continuously decreased with increasing Mo concentration to 1468 MPa at 16% Mo and then increased to 1672 MPa at 30% Mo. The room temperature ductility strongly depended on the amount of Mo. The maximum true fracture strain of 0.137 was achieved in Hf69Ta26Mo5, but it rapidly decreased to 0.027 in Hf51Ta19Mo30. All the studied alloys were ductile at 1000–1400 °C and no fracture was observed after 0.7 true strain. The yield stress increased almost linearly with increasing the amount of Mo at these high temperatures.