Strength-ductility synergy of Al-bearing low-density refractory TiNbMo0.5AlX medium entropy alloys via precipitation

A series of TiNbMo0.5AlX (x = 0.125, 0.15, 0.175, 0.2, 0.225, 0.25) refractory medium entropy alloys with low density (6.82–7.01 g/cm3) was developed in this work, which should synchronously meet the requirement of mechanical properties at both ambient and high temperature conditions. We found excess Al-addition was conducive to the formation of precipitates with FCC structure; with increasing Al content, the microstructure of alloys evolved from single BCC phase (Al0.125) to BCC matrix + FCC-Ti dual-phase characteristics. Meanwhile, most of these alloys exhibited typical dendrite morphology, except for equiaxed Al0.225. Controlled by elastic strain energy, the precipitates were mainly distributed in interdendrite as clusters of flake-like. All developed alloys presented excellent comprehensive mechanical properties, such as 1022 MPa yield strength and 31% fracture strain could be realized in room-temperature. Al0.225, which also retained 710 MPa strength and 39% strain at 800 °C. The dislocation-dominated deformation mechanism plays an important role in achieving excellent properties. The dislocation pile-up effectively increases compressive strength, and the magnificent compressive ductility is attributed to the dislocation-slip mechanism. The typical Kurdjumov-Sachs (K–S) coherent relationship between BCC matrix and FCC-Ti was investigated, which effectively possessed a significant enhancement in the strength-ductility synergy. In addition, no apparent softening was observed in the stress-strain curves, which made it possible for the alloy to work stably at 800 °C. This work opens new perspectives for the design and application of RMEAs with high strength-ductility synergy. •As-cast TiNbMo0.5AlX medium entropy alloy with low density (6.82–7.01 g/cm3) showed nano-size leaf-like shaped second phases (FCC–Ti) distributed in matrix (BCC).•All alloys exhibited more than 1351 MPa compressive strengths and 25% fracture strains at RT. The Al0.225 alloy showed a yield strength of 710 MPa with an unusual compressive strain of 39% at 800 °C.•The dislocation pile-up effectively increases compressive strength, and the magnificent compressive ductility is attributed to the dislocation-slip mechanism.