Strength-ductility synergy of Al-bearing low-density refractory TiNbMo_0.5Al_X medium entropy alloys via precipitation

A series of TiNbMo_0.5Al_X (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 (Al_0.125) to BCC matrix + FCC-Ti dual-phase characteristics. Meanwhile, most of these alloys exhibited typical dendrite morphology, except for equiaxed Al_0.225. 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. Al_0.225, 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.