PNNL

Abstract

Single phase body-centered cubic (bcc) refractory medium or high-entropy alloys represent a new paradigm for high-temperature structural materials. Although these alloys can retain compressive strength at elevated temperatures, they invariably suffer from extremely low tensile ductility and fracture toughness. Here, we examine the strength and fracture toughness of a bcc refractory medium-entropy alloy comprising NbMoTaHf from cryogenic to ultrahigh temperatures (77–1473 K). We find that this alloy’s behavior starkly differs from comparable systems that retain high-temperature strength; it displays resistance to fracture across a wide range of temperatures that is unprecedented for bcc alloys. We attribute the extraordinary toughness of this material to a dynamic competition between the roles of screw and edge dislocations in controlling the plasticity at a crack tip. While the glide and intersection of predominantly screw and mixed dislocations promotes strain hardening and thus controls the uniform deformation, the coordinated slip of edge dislocations with {110} and {112} glide planes prolongs non-uniform strain through formation of kink bands. These bands suppress strain hardening by reorienting microscale bands of the crystal along directions of higher critical resolved shear stress and continually nucleate to accommodate localized strain and distribute damage away from a crack tip. Ultimately, kink bands elevate the low-temperature fracture toughness to over 253 MPavm, making this material one of the toughest bcc refractory alloys on record.