Superheat treatment of Al-7Si-0.55Mg melt and its influences on the solidification structures and the mechanical propert
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TRODUCTION
IT has been widely observed that heterogeneous solid particles and clusters in melts, especially of multicomponent alloys, play an important role in the solidification process, and in turn, influence the solidification structures both on micro- and macroscales. For example, superheat treatment of an aluminum melt after the addition of a AlTi6 grain refiner will eliminate the heterogeneous nucleation effects of the refiner even though the Ti content is not changed. [1] Similar results were also observed in nickel-based superalloys containing carbide particles.[2] Superheat treatment of hypereutectic Al-Si alloys will refine the primary Si particles.[3] The influence of the solid particles and clusters that survived during the melting process on the resolidified structures were classified as a heredity phenomenon in the material. Six international conferences have been organized in the former Soviet Union and continued in Russia[4] on this topic. In superheat treatment experimental research on Al1.4 pct Hf alloy,[5] it was found that the solidification microstructure of the alloy depends on the superheating temperature, as well as the cooling rate during solidification. The superheating effect becomes stronger when the cooling rate is higher. The variation of the viscosity with respect to temperature also shows an abnormal change when the temperature is over a critical value. It is believed that when the melt is above the critical temperature, it becomes a socalled real solution, where no clusters or crystalline particles exist. But, at temperatures lower than the critical temperature, the clusters and crystalline particles will survive the melting process and influence the solidified structure. WANQI JIE, Professor, and ZHONGWEI CHEN, Postdoctoral Student, are with the College of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China. Contact e-mail: [email protected] W. REIF, Professor, and K. MÜLLER, Research Assistant, are with the Institute of Metals, Technic University of Berlin, D-10623, Berlin, Germany. Manuscript submitted November 28, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
Once these clusters and crystalline particles dissolve during superheat treatment, they do not form again before the melt is undercooled. For this reason, the variation of the viscosity with respect to temperature during cooling does not coincide with the heating process. Similar results were reported in Fe-based alloys, Ni-based alloys, and Al-Zr alloys and reviewed by Popel and Sidorov.[6] In a study of the mechanical properties of amorphous ribbons of Fe73.5Nb3Cu1Si13.5B9, Fe81Si4B13C2, Fe71Cr10Si7B12, and Co60Fe5Ni10Si10B15 alloys prepared by rapid quenching of the melts, it was found that both the room-temperature strength and the strength at 77 K were improved tremendously if the melts were superheated before quenching.[7] Other experimental research[8] also showed that superheat treatment of liquid metals could change the growth morphology of nickel-base
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