Formation of non-equilibrium ductile solid solutions and textures in NbCr₂ bulks produced by mechanical milling and spark plasma sintering

Bulk NbCr₂ samples with ultra-fine grains have been fabricated by mechanical milling and subsequent spark plasma sintering. The room-temperature fracture toughness of these bulk samples achieves as high as 11.3 ± 0.5 MPa m^1/2, which is remarkably improved compared to the as-cast monolithic NbCr₂ Laves phase (1.2 MPa m^1/2). The microstructural analysis indicates the presence of abundant non-equilibrium Nb and Cr solid solutions among the grain boundaries. The formation of Nb and Cr solid solutions is attributed to the applied pulsed electric current during spark plasma sintering, which can decrease the critical nucleation undercooling, accelerate the atomic diffusion and enhance the nucleation rate. Based on the microstructural characterization, the enhancement in fracture toughness is ascribed to formation of non-equilibrium Nb and Cr solid solutions toughening and grain boundary toughening mechanisms. Furthermore, strong texture components have been detected in these bulk samples. For the sample sintered with the powder milled for 30 h, the <100> direction of NbCr₂ Laves phase is perpendicular to the compressive axis while the Nb and Cr solid solutions show random orientation. When sintered with the powder milled for 60 h and 90 h, the texture component in NbCr₂ Laves phase transforms into <110> fiber texture, and the Cr solid solution also show strong <100> and <110> texture components perpendicular to the compressive axis. The evolution of texture component is discussed by considering the crystallization behaviour of the milled powders.