Abstract
Ultrasonic melt processing (USP) is gaining quite an interest in recent years due to the benefits of this technology to the melt quality and structure refinement. A number of mechanisms have been identified that govern the effects of USP at different stages of melt processing. Technologically it is advantageous to apply USP to the fluid melt rather than to a mushy solidifying alloy. In this case heterogeneous nucleation on available or activated/multiplied substrates is the main mechanism. Among these substrates, primary crystals of Al3Zr phase were shown to be potent and effective. This paper gives a review of the own research into the role of Al3Zr in structure refinement in various groups of Al alloys, from solid-solution type to hypereutectic. This overview includes the evidence of a possible eutectic reaction between Al and Al3Zr in Al-rich alloys, mechanisms of Al3Zr formation and refinement under USP (that enables these primary crystals to be active substrates for Al and some other primary phases), the role of USP in facilitating primary solidification of Al3Zr in the Al-Zr system, and the additional benefits of solute Ti presence. The paper is illustrated with the data obtained over the last 15 years of research led by the author.
Highlights
It seems like a strange topic to discuss as it is well known that the Al3Zr phase reacts with liquid aluminium through a peritectic reaction L + Al3Zr = (Al) at 661 °C (934 K) [1] with the peritectic point at 0.28 wt% Zr and the maximum liquid solubility of Zr in (Al) 0.11 wt% at 661 °C [2]
Solidification reaction does not change the fact that the Al–Zr alloys are forming supersaturated solid solution during solidification, albeit the amount of Zr available for that will be the function of how much Zr has been consumed by primary and, eutectic particles
An equilibrium or nonequlibriun eutectic reaction may take place in Al-rich alloys under conventional casting conditions. This results in the formation of Al3Zr particles at the dendrite boundaries, effectively decreasing the amount of primary Al3Zr phase
Summary
It seems like a strange topic to discuss as it is well known that the Al3Zr phase reacts with liquid aluminium through a peritectic reaction L + Al3Zr = (Al) at 661 °C (934 K) [1] with the peritectic point at 0.28 wt% Zr and the maximum liquid solubility of Zr in (Al) 0.11 wt% at 661 °C [2] This assumption works well for most practical purposes and seems to fit with most of experimental data. Note that the eutectic point in the new version almost coincides with the peritectic point (maximum liquid solubility) in the currently adopted version of the Al–Zr phase diagram The question whether this eutectic solidification reaction is equilibrium or nonequilibrium remains open.
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