Study of Mushy-Zone Development in Dendritic Microstructures with Glass-Forming Eutectic Matrices Using Electrostatic Levitation

The coarsening of dendrites in two bulk metallic glass matrix composites was investigated using noncontact techniques in a high vacuum electrostatic levitator. Progressive degrees of coarsening were observed after isothermal holds of varying duration. Specific volume and surface tension during heating and cooling were measured. Hysteresis in specific volume was observed. 1. Introduction Bulk metallic glass matrix composites (BMGMCs) are a relatively new class of materials that have demonstrated benchmark material properties [1–3], such as extensive ductility in tension [4] and ultrahigh fracture toughness. These properties are obtained by reinforcing monolithic bulk metallic glasses (BMGs) with soft, crystalline dendrites, grown in situ from the liquid, that are effective at inhibiting crack growth. Amorphous metal hardware—both monolithic and composite—has traditionally been fabricated by die-casting or suction-casting liquid into molds to form net-shapes. However, the length scale of the resulting composite microstructure is neither homogenous nor optimal for improving mechanical properties of the composite (Figure 1(b)). Recent work has explored the manufacture of net-shape composite parts using the technique of semisolid forging, with promising results [5]. However, successfully producing parts in this manner presents a series of difficult choices at each step in the process, due to the high sensitivity of these materials and techniques to processing conditions. Figure 1: Schematic of a typical processing chamber configuration (a) ESL; (b) suction casting; (c) semisolid forging, with a corresponding SEM micrograph of BMGMC microstructure obtained from each method. BMGs—the matrix material for BMGMCs—have been shown to be highly sensitive to processing conditions in general, with particular regard for levels of contamination and for thermal history. The most widely known commercial alloy, Vitreloy 1, has demonstrated widely divergent measurements of fracture toughness, which can be attributed to differing levels of dissolved oxygen, chemical purity, casting temperature, carbide impurities, and part thickness [6–9]. Recent work by Garrett et al. has demonstrated that changes in configurational enthalpy lead to a strong dependence of fracture toughness on thermal history for identically prepared alloys, even with no visible microstructural changes [10]. For BMGMCs, several studies have demonstrated a strong correlation between material properties and the morphology of the crystalline dendrites present in these alloys [11–13]. In particular, fracture