Fracture behavior of NiNb and NiNbP bulk metallic glasses

While it is known that phosphorous additions enhance the hardness, strength, and glass forming ability of NiNb(P) bulk metallic glasses, nothing is known about the trade-offs regarding the fracture characteristics. The fracture toughness and bending deformation behavior of Ni 62 Nb 38 and Ni 59.2 Nb 38.8 P 2 BMGs were studied, both of which have been reported to have very high compressive yield strength near 3 GPa. The binary NiNb BMG exhibited ~4% plastic bending strain and a precracked fracture toughness of K Q = 50 MPa√m which gives an excellent combination of strength and toughness relative to other engineering materials. With the addition of 2 at.% phosphorus, the glass forming ability was maximized, but the BMG was hardened and embrittled, fatigue precracking was no longer possible, and micronotched toughness values, which should overestimate the precracked fracture toughness, were in the range of K Q = 19 − 26 MPa√m. Additionally, a progressive hardening and bending embrittlement was observed for intermediate P additions of 1 and 1.5 at.% phosphorus. Transmission electron microscopy revealed larger medium range order clusters in the relatively softer NiNb BMG which were thought to contribute to easier shear transformations and plastic deformation by shear banding. • NiNb BMG exhibits precracked fracture toughness of K Q = 50 MPa√m with ~3 GPa strength • Progressive hardening and embrittlement occur for phosphorous additions up to 2 at% • While 2 at% P maximizes glass forming ability, the BMG became too brittle for precracking • Phosphorous embrittlement is associated with smaller medium range order clusters