Boron Interstitial and Boride Strengthened Grain Boundary for Toughening Refractory High-Entropy Alloys

Refractory high-entropy alloys exhibit a high mechanical strength and an excellent structural stability at elevated temperatures, and are of great potential for applications to aerospace and nuclear power components. However, the drawback of poor grain boundary cohesion and consequent early brittle interganular fracture renders their practical applications limited. According to the literature, boron interstitials (or borides) have been verified to improve the cohesion strength of grain boundaries and thus the ductility of some intermetallic compounds such as nickel aluminide(s). Introducing boron interstitials (or forming multicomponent borides) particularly at grain boundaries was hence considered in this study for enhancing grain boundary cohesion and toughening refractory high-entropy alloys. A very small amount (0.1 at.%) of boron was added in arc-melted Hf_xMo_0.5NbTa_xTiV_1.5-xZr_x refractory high-entropy alloys, and the microstructure, compositions and crystallographic orientations of the alloys were characterized. Macroscale compressive tests were conducted for measuring the stress-strain response of the alloys, and micropillar compressive tests and indentations were carried out for measuring the strength of grains and grain boundaries. Microstructure observations indicated that elemental boron was uniformly distributed in the refined single-phase, solid-solution grains, while some nanosized boride particles were dispersed at the grain boundaries. With the addition of boron, the hardness of weak grain boundaries (about 5.0 GPa) was markedly improved to the equivalent level of grain interiors (about 6.2 GPa). While the yield strength of the boron-added alloys did not change (about 1450 MPa), the ultimate compressive strength effectively increased from 1690 to 2240 MPa at a higher work hardening rate. The compressive strain increased from 20-25% to 35-40%, attributable to the inhibited boundary decohesion-caused brittleness as the distinct transition of interganular-to-transgranular fracture. Micropillar compression tests suggested that the grain boundaries of the alloys without boron early decohered at a stress of below 700 MPa, whereas those with boron did not fracture even after the adjacent grains yielded. Owing to the high charge density-enhanced grain boundary cohesion, the boron-added alloys also showed a high structural stability at elevated temperatures, with a retained strength of 1190 and 990 MPa at 600 and 800^○C, respectively.