Effect of Ti on the microstructure and mechanical properties of equiatomic NbTaTi medium-entropy alloy

NbTa and NbTaTi alloys were processed using vacuum arc melting and both the alloys have resulted in single phase BCC structures. Heat treatment at 1473 K for 24 h did not alter the phase structure in the NbTa alloy and single phase BCC was retained. However, under similar heat treatment conditions, a phase transformation was observed in NbTaTi alloy, forming a minor HCP phase along with a major BCC phase. These experimental findings are also in good agreement with CALPHAD (Calculation of Phase Diagrams) studies. The addition of Ti reduced the overall density and grain size from ≈190 μm to ≈55 μm in the heat-treated condition. Ti addition also improved the hardness of the NbTa alloy (i.e., NbTa as-cast: 3.07 ± 0.02 GPa and NbTaTi as-cast: 4.3 ± 0.05 GPa). After heat treatment, the hardness of the NbTa alloy did not change significantly, however, the hardness of the NbTaTi alloy decreased from 4.3 ± 0.05 GPa to 2.56 ± 0.13 GPa. This reduction in hardness is attributed to decreased solid solution strengthening due to the reduction in Ti at. % within the major BCC phase after heat treatment. Another striking feature of this investigation is these novel medium-entropy alloys (MEAs) with less number of alloying elements in them exhibit excellent density-normalized hardness (=hardness/density), demonstrating their potential as high-performance refractory MEAs for advanced applications.