Precipitation of Intermetallic Phases from Rapidly Solidifying Aluminum Alloys

Abstract

<p>Despite all efforts during the last 30-40 years, the formation of metastable Fe-rich and Si-rich intermetallics in dilute aluminum alloys is still an unsolved mystery. Based on the equilibrium Al-Fe-Si phase diagram, in dilute aluminum alloys only one equilibrium intermetallic, namely Al₁₃Fe₄, is expected. It is known, however, that a rapid solidification, i.e. solidification at a high cooling rate, results in dozens of metastable phases seen in the as-cast alloys. It is firmly established that the greater the cooling rate (i.e. the rate of heat extraction), the greater the supercooling (supersaturation) achieved in the course of solidification.</p> <p>Understanding the nature and a sequence of formation of these intermetallic phases precipitating from the supersaturated melt is at the centre of this research. In fact, this endeavor was launched to answer the following fundamental question: "What governs the formation of intermetallic c phases from a rapidly solidifying alloys, in general, and from aluminum alloys, in particular?" Prior to starting this investigation, it was believed that the concept of the driving forces for the beginning of precipitation originated by Miroshnichenko, Cahn and Hillert, could be used to explain experimental findings. Was that belief justified? Although a definite answer to this question has not been found, there are strong indications that the concept is likely operative, although it has to be refined by taking into account the surface energies.</p> <p>To evaluate applicability of this concept to the formation of Fe-rich and Si-rich intermetallics in aluminum alloys, in this research an array of experimental information related to microstructures of as-cast alloys having different compositions are obtained. Then, the collected experimental results are interpreted using the concept of the driving forces for the beginning of precipitation.</p>