Rapid solidification and metallic glass formation – Experimental and theoretical limits

Most of the metallic glasses which have been identified to date require rapid solidification processing (RSP) in order to obtain the high cooling rates required to avoid the thermally activated decomposition reactions that result in the formation of crystalline phases. While the free-jet melt spinning technique is commonly used for the investigation of amorphous alloy formation, many of the key process parameters remain poorly understood. In the work presented here, we utilize high speed digital imaging to investigate various melt-pool characteristics and their influence on the melt-spun ribbon. With detailed measurements of melt-pool geometry and surface temperature, we show that the length of this molten zone establishes a critical length scale which characterizes the ribbon thickness and cooling rate. Within certain limits, the ribbon thickness is shown to correlate with a simple 2D laminar flow boundary layer formed at the base of the melt-pool. Estimates of initial quench rate, based on surface temperature measurements, clearly reveal the importance of two distinct cooling periods. The first is the wheel-contact period, where the ribbon remains in intimate contact with the quench wheel and cooling is primarily governed by the heat conduction across the ribbon-wheel boundary. The second is the free-flight period, where the ribbon separates from the wheel, and cooling is governed by radiation and convection into the chamber.