Comparison of microstructures and properties of Zr-based bulk metallic glass composites with dendritic and spherical bcc phase precipitates

In this paper, the microstructures and mechanical properties of the two BMG composites with the same composition of Zr_56.2Ti_13.8Nb_5.0Cu_6.9Ni_5.6Be_12.5, which contain in situ formed dendritic and spherical bcc β-Zr (Ti, Nb) solid solutions, are compared based on micrograph observations, XRD analysis and uniaxial compression tests at room temperature. The dendritic β phase exhibits well-developed primary dendrite axes with lengths of 20-50 μm and diameters of about 1-3 μm; the spherical β phase with the same volume fraction of about 30% as the dendritic β phase has a much larger average diameter of about 18 μm as compared to the primary dendrite axes of the β phase. Compression tests demonstrated that the yield stress, strain at the yield point, ultimate fracture strength and fracture plastic strain of the composite containing dendritic β phase are 1208 MPa, 1.78%, 1757 MPa and 8.82%, respectively. For the composites containing spherical β phase, however, a much larger fracture plastic strain of about 12% was achieved, and combined with a somewhat higher yield strength of 1350 MPa and a larger strain of 2.32% at the yield point, the ultimate fracture strength was measured to be about 1800 MPa.