Enhanced mechanical properties of lightweight refractory high-entropy alloys at elevated temperatures via Si addition

A new lightweight refractory high-entropy alloys were prepared by introducing Si element into AlMo_0.5NbTiV alloy. The effects of Si content on microstructural evolution, density and high temperature mechanical properties were analyzed. The results show that the addition of Si leads to the formation of hard and brittle M₅Si₃-type (M = Ti and Nb) phase at the grain boundary of the matrix, which gradually transforms the alloys from a single body centered cubic (BCC) structure to a dual-phase structure. While the density of AlMo_0.5NbTiVSi_x alloys decreases from 6.01 to 5.59 g/cm³ with the increase of Si content, and the mechanical properties are also significantly improved, AlMo_0.5NbTiVSi_0.5 alloy has the best yield strength of 1428 and 390 MPa at 1073 K and 1273 K, respectively. Among them, AlMo_0.5NbTiVSi_0.1 alloy has the best compression ductility strain at 1073 K, up to 22.2 %, indicating that the addition of appropriate Si can improve the compression ductility of the alloy, which is also caused by the combination of grain refinement and the appearance of eutectic structure. On the one hand, the segregation of Si elements at grain boundaries results in slow growth of the primary crystal, which inhibits the growth of grains. On the other hand, the eutectic structure composed of M₅Si₃ and BCC is distributed at the grain boundary, which is not conducive to the growth of grains, and the combined effect of the two eventually leads to the grain refinement. The strengthening mechanisms of the alloys can be attributed to the second phase strengthening, solid solution strengthening and grain refinement strengthening. In addition, the softening of the alloys at 1273 K is caused by dynamic recovery and dynamic recrystallization, and the dynamic recovery is the main softening mechanism.