Inducing strain hardening in a Zr-based bulk metallic glass via cobalt mediated phase separations

Novel Zr63.78Cu14.72Ni10-xCoxAl10Nb1.5 bulk metallic glasses (BMGs) were prepared with progressively increasing amounts of Co substituted for Ni. Critical casting thicknesses were in the range of 4-5 mm, indicating a good glass forming ability. Transmission electron microscopy (TEM) of the as-cast amorphous structure revealed primary glassy phase separated regions a few dozens of nanometers in diameter. High yield strength (similar to 1495 MPa), fracture toughness (similar to 55 MPa root m), and strong strain hardening behavior were observed. Deformation induced secondary phase separation (<4 nm diameter) was found in a 100-300 nm wide region around a shear band. The large positive enthalpy of mixing for Co and Cu (+6 kJ/mol) are believed to contribute to the special two phase separations, which further favored the excellent strain hardening behavior. (C) 2017 Elsevier B.V. All rights reserved.