In-situ development of MB₂ and their effect on microstructure and mechanical properties of refractory Hf_0.5Mo_0.5NbTiZr high entropy alloy matrix composites

To improve the room temperature comprehensive mechanical properties of the refractory high entropy alloy Hf_0.5Mo_0.5NbTiZr, boron element is added to form the boride phase as the strengthening phase. The Hf_0.5Mo_0.5NbTiZrB_x (x = 0.1, 0.3, 0.5, 0.7, and 0.9) were produced by vacuum arc melting and effect of boron element on the microstructure and room temperature mechanical properties of Hf_0.5Mo_0.5NbTiZr alloy was investigated. The boride MB₂ phase form and the volume fraction of boride phase increases with the addition of boron element. The microstructure exhibits the evolution from the hypoeutectic structure to the eutectic structure, and then to the hypereutectic structure with the increase of boron element. Because of the difference of the formation path, the morphology and chemical compositions of MB₂ phase is different. The lamellar or needlelike MB₂ phase forms through the eutectic reaction and is rich in Zr element but poor in Mo and Nb elements. The agglomerated or block sexangle MB₂ phase forms as the proeutectic phase and is enriched with Nb element and are depleted with Mo and Zr elements. When the content of the boron is x = 0.1 and 0.3, both the strength and ductility are improved due to the refined structure, the clean interface between the BCC solid solution phase and MB₂ phase and the precipitated strengthening of boride phase.