Enhanced mechanical performance of HoB2-based compounds without sacrificing cryogenic magnetocaloric effects

Cryogenic magnetic cooling based on magnetocaloric effect is a promising technology for green and efficient hydrogen gas liquefaction. One of the major challenges for practical application is the lack of magnetocaloric materials that exhibit excellent cyclic performance in both magnetocaloric properties and fracture toughness. HoB₂, an intermetallic compound, exhibits a giant magnetocaloric effect, but its brittleness under mechanical stress hinders practical applications. In this work, we report a 12-fold increase in the fracture toughness by doping 10 at. % Cu into the HoB₂ system, from 2.6 to 35.1 MPa·m^1/2. Vickers indentation tests followed by the microstructural evaluation revealed that the formation of (Ho, Cu)-rich intergranular phases with a fine distribution in the microstructure, encircling HoB₂, suppresses the propagation of initiated cracks in HoB₂-based compounds, thereby improving their fracture toughness. In the 10 at. % Cu-doped sample, a (Ho, Cu)-rich intergranular phase was formed with an areal fraction of ∼18 %, while the content of the secondary HoB₄ phase, commonly observed in the Cu-free sample, was reduced. Notably, no Cu dissolution was found in the matrix phase. The fracture toughness improved significantly with minimal impact on magnetocaloric performance: the isothermal field-induced entropy change (ΔS_T) was marginally reduced from 20.1 to 18.2 J kg⁻¹ K⁻¹_, and the adiabatic temperature change (∆T_ad) from 5.1 to 4.7 K in a 2 T magnetic field. The physical property analysis revealed that the sample containing the (Ho, Cu)-rich intergranular phase has lower magnetostriction, which can extend the life of the material in practical applications by reducing internal stress.