New strategy in light-weight and ultrastrong Ti40Nb15Mo30(NbC)15 refractory complex concentrated alloy

Traditional refractory high-entropy alloys (RHEAs) generally exhibit a trade-off between high-temperature strength and light weight. In present work, a novel design strategy based on tailoring element distribution is proposed to achieve excellent high-temperature strength at a density lower than 7 g·cm-3. Specifically, a Ti40Nb15Mo30(NbC)15 composite was designed and prepared by powder metallurgy. The composite is found to be composed of two ultrafine-grained (UFG) phases including a body-centered cubic (bcc) solid-solution phase and a face-centered cubic (fcc) ceramic reinforcement phase (Ti, Nb)C. The as-sintered composite shows a uniform and UFG microstructure where two phases are interconnected. Due to this unique microstructure, the Ti40Nb15Mo30(NbC)15 composite displays superb specific yield strengths among surveyed RHEAs, complex concentrated alloys, and metal-matrix composites from 800°C (243 MPa·g-1·cm3) to 1000°C (127 MPa·g-1·cm3). The outstanding high-temperature compressive strength was found to be associated with high resistance to dislocation motion and strong dislocation interactions in both the bcc and fcc phases. The phase interface after hot compression remained semi-coherent, vindicating its high stability. The high-density of stable phase interfaces not only retards the dislocation motion due to the large image force near the phase boundary but also induces a high value of activation energy for diffusion. The high activation energy can further achieve significant microstructure stability even after a long-term annealing (36 h) at 1000°C. This work provides new perspectives for the design and application of light and ultrastrong refractory complex concentrated alloys (RCCAs) by comparison to the insufficient strength of many traditional and light RCCAs.