Microstructure and Mechanical Properties of CoWB Based Composites Produced by Crystallization of Ni-Co-Zr-Ta-W-B Bulk Metallic Glass

CoWB based composites were produced by annealing Ni33.6CO23.2Zr0.5Ta4W23.7B15 bulk metallic glass above the crystallization temperature. The effect of annealing on the microstructure of the composites was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Microhardness (Hv) and indentation fracture toughness (K-C) of the composites were also measured. Microstructural examinations revealed that the first precipitated phase as a result of annealing is a nickel solid solution. As the annealing time is increased, CoWB and the nickel solid solution begins to precipitate. If the annealing time is increased further, the CoWB remains stable, while the nickel solid solution transforms into Ni3Ta. The microhardness of the as-cast alloy was determined to be 1190 HV. As a result of annealing, composites with a microhardness of around 1400 HV were obtained due to the precipitation of CoWB. K-C of the composite with the maximum hardness was determined to be similar to 3 MPa.m(1/2). The effect of microstructure on the mechanical properties of the composites is discussed.