Design and fabrication of ductile in situ bulk metallic glass composites

Abstract

Bulk metallic glasses (BMGs) possessing high strength and elasticity are excellent candidates for the high-demanding market of advanced materials. Inherent brittleness of the amorphous structure, however, inhibits wide use of the metallic glasses as structural materials. The problem of low plasticity and toughness can be alleviated through formation of a composite structure containing crystalline phase(s) in the high-strength amorphous matrix. In this thesis, bulk metallic glass composites have been produced in different glass-forming systems. By utilising a copper mold casting method, a series of BMG composites with high Mg concentration (>80 at. %) was fabricated in the Mg-Ni-Gd and Mg-Ni-Gd-Y systems. Zr-rich BMG composites were produced in Zr-Cu-Ni-Al alloy system by arc-melting. To promote formation of crystalline phases in Zr-based glasses, Y2O3 particles were introduced to the melt as possible inoculants. The conditions for formation of amorphous, amorphous-crystalline and completely crystalline structures were found to be dependent on alloy chemistry and sample thickness in both the Mg and Zr-based systems. Stability of the amorphous matrix and devitrification processes during heating were investigated in the Mg-based system. The precipitation of a soft primary Mg-rich phase in the monolithic glass was observed in the compositions with Mg content ≥ 84 at. %. Soft crystallites homogeneously dispersed throughout the Mg-rich glassy matrix enhanced the plasticity of the resulting composite structure with a compressive plastic strain to failure of ~9.5 %. For the Zr-Cu-Ni-Al system, the formation of a monolithic amorphous structure and composite structure was observed for a maximum Zr content of 76 and 72 at. %, respectively. The initiation or suppression of crystalline phase growth on the surface of inoculating Y2O3 particles in the Zr-based glass was dependent on the cooling rate during casting. The glass-forming ability and thermal stability of the Zr-rich composite alloys was affected by these inoculating particles. The relatively high glass transition temperatures of the Mg-based composites, their low density (2.4 - 2.6 g/cm3) and improved plastic strain to failure may prove useful for future lightweight engineering applications. Heterogeneous nucleation of crystalline phases on the surface of oxide particles introduced into amorphous high Zr-bearing matrix shows possibilities for generating new types of the composite structures.