Phase Separating Bulk Metallic Glass: A Hierarchical Composite

Phase separating systems present a unique opportunity for designing composites with hierarchical microstructure at different length scales. We report here our success in synthesizing phase separating metallic glasses exhibiting the entire spectrum of microstructural possibilities expected from a phase separating system. In particular, we report novel core shell and hierarchical structures of spherical glassy droplets, resulting from critical wetting behavior and limited diffusion. We also report synthesis of a bulk phase separating glass in a metallic glass system. The combination of unique core shell and hierarchical structures in metallic glass systems opens a new avenue for the microstructure design of metallic glasses.

Figure 1

Bright field transmission electron micrographs obtained from as-melt-spun (a) ${\mathrm{Ti}}_{45}{\mathrm{Y}}_{11}{\mathrm{Al}}_{24}{\mathrm{Co}}_{20}$, (c) ${\mathrm{Ti}}_{28}{\mathrm{Y}}_{28}{\mathrm{Al}}_{24}{\mathrm{Co}}_{20}$, (d) ${\mathrm{Ti}}_{36}{\mathrm{Y}}_{20}{\mathrm{Al}}_{24}{\mathrm{Co}}_{20}$, and alloys. (b) shows an enlarged view of one of the droplets shown in (a) to highlight the unique core shell structure of the droplets. The corresponding selected area diffraction patterns and digitized intensity profile are included in (a), (c), and (d).

Figure 2

Pseudobinary phase diagram of the liquid phases in a Ti-Y-Al-Co system calculated using a subregular solution model at a temperature range of 400–1200 K for a constant aluminum and cobalt composition of 24 and 20 at. %, respectively. The diagram includes the spinodal boundary (dashed-dotted line) and the glass transition temperature ($T_{\mathrm{g}}$) (dotted line).

Figure 3

Phase field modeling showing the evolution of a microstructure with the wetted layer around the primary nucleated phase in the liquid with the composition corresponding to the left side of the critical point.

Figure 4

Bright field transmission electron micrographs obtained from an as-injection-cast ${\mathrm{Zr}}_{28}{\mathrm{Y}}_{28}{\mathrm{Al}}_{22}{\mathrm{Co}}_{22}$ alloy (diameter: 3 mm). The corresponding selected area diffraction pattern is included in the figure.