Understanding the Co-Poisoning Effect of Zr and Ti on the Grain Refinement of Cast Aluminum Alloys
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GRAIN refinement by inoculation has been widely used in the casting of aluminum and its alloys for over half a century. Inoculation promotes heterogeneous nucleation during solidification and the formation of fine, uniform, and equiaxed grains in the cast products. Since a grain-refined microstructure not only improves the mechanical properties, but also facilitates subsequent mechanical processing, such as rolling and extrusion, extensive research work has been carried out to develop better inoculants (i.e., grain refiners) and to understand the mechanism by which they work.[1–4] As a consequence of Cibula’s discovery[5] that addition of titanium, in particular, with boron, leads to a significant grain refining effect for aluminum, a series of Al-Ti-B master alloys have been developed with the best performance obtained by using Al-5Ti-1B.[6] Normally, the grain size of commercial grade Al alloys can be reduced from a few millimeters to 200 lm at an Al-5Ti1B addition level of only 1 part by weight in 1000 (1 ppt, or 0.1 wt pct). However, the grain refining efficiency of Al-5Ti-1B is dramatically weakened by some alloying elements, such as Si, Cr, and Zr, contained in Al alloys.[3] This adverse effect on grain refinement is generally called ‘‘poisoning.’’ Unlike Si and Cr, Zr is of itself a grain refiner for Al,[3,7] because not only does solute Zr have considerable segregating power,[2] but the Al3Zr phase is also an effective nucleant[8] when the Zr D. QIU, Australian Research Fellow, and M.-X. ZHANG, Associate Professor, are with the School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072, Australia, CAST Cooperative Research Centre, Brisbane, QLD 4072, Australia, and ARC Centre of Excellence for Design in Light Metals, Brisbane, QLD 4072, Australia. Contact e-mail: [email protected] J.A. TAYLOR, Principal Research Fellow, is with the School of Mechanical and Mining Engineering, The University of Queensland, and CAST Cooperative Research Centre. Manuscript submitted March 17, 2010. Article published online September 14, 2010 3412—VOLUME 41A, DECEMBER 2010
content is over 0.11 wt pct. Nevertheless, when Ti and Zr are added together in a commercial grade purity Al melt, the grain refining effect is less than that which occurs with either individual addition,[9] a phenomenon termed ‘‘co-poisoning.’’ The loss of grain refining potency through poisoning mechanisms is known to be time dependent; that is, grain size increases with holding time after additions are made.[7,10] In general, loss of refining potency by poisoning is nonrecoverable, unlike gravity-induced fading (i.e., settling of nucleant particles), which can usually be recovered by stirring. As a trace element, Zr at a concentration around 0.1 to 0.2 wt pct is sometimes present in 7xxx series alloys, which have superior strength and ductility. However, even this small amount of Zr impairs the grain refining effect of Al-5Ti-1B master alloy. As reported by Ahmady and his co-workers,[11] the grain size in Zr-free 7050 alloy refined by Al-5Ti-1B m
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