Grain Coarsening of Cast Magnesium Alloys at High Cooling Rate: A New Observation
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GRAIN refinement of cast metals has been widely accepted as an effective approach to produce alloys with improved mechanical properties, good castability, and better formability.[1,2] Generally, grain refinement of cast metals can be achieved through stirring during solidification, such as magnetic stirring[3–5] and supersonic stirring,[6–8] melt treatment by inoculation,[2,9,10] and fast cooling.[11–13] Inoculation treatment is more practical and widely used in industry due to its low cost and high reproducibility. The process is generally associated with addition of solutes, which is believed to restrict the grain growth, and adding nucleants that promote heterogeneous nucleation. The grain growth restriction effect of solutes can be described by the growth restriction factor, which is quantified as Q-value.[14,15] In binary systems, the Q-value is calculated by linking the liquids gradient (m) with the alloy composition (C0) and the partitioning coefficient (K) using the equation Q ¼ mC0 ðK 1Þ: This value represents the capacity of the solute to create a constitutional undercooling zone at the front of the solid/liquid (S/L) interface as a result of the solute segregation, which facilitates the heterogeneous nucleation.[16] The effective nucleant refers to solid particles in
YAHIA ALI, Ph.D. Student, and MING-XING ZHANG, Professor, are with the School of Mechanical and Mining Engineering, The University of Queensland, St. Lucia, QLD 4072, Australia. Contact mail: [email protected] GUOQIANG YOU, A/ Professor, and FUSHENG PAN, Professor, are with the College of Materials Science and Engineering, Chongqing University, Chongqing, 400030, China. Manuscript submitted August 13, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
the melt that are crystallographically, chemically, and thermodynamically compatible with the primary solid to be formed[17,18] and are big enough[19] to promote heterogeneous nucleation. The nucleants can be either in situ formed, such as the Al3Ti particles for refining the Al alloys through subsequent addition of the Al-5Ti-1B master alloy and the Al2Y particles formed in the Mg-Y system after adding Al,[20] or externally added to the melt as foreign particles, such as the Zr particles,[21] the ZnO, and the CaO[22,23] for refining the non-Al-containing Mg alloys. In most cases, the highest grain refining efficiency can be achieved when both solutes with high Q-values and effective nucleants co-exist in the melt.[2,22,23] The high grain refining efficiency of Zr particles for non-Al/Si/Mn-containing Mg alloys is attributed to both the very high Q-vlaue of Zr in Mg melt and the Zr particles that are effective nucleants.[2,21,24,25] Recent work[22] also indicated that CaO can effectively refine the pure Mg because CaO particle itself can be an effective nucleant, and Ca, which formed through partial reduction of the CaO by Mg, is associated with high Q-value in the Mg melt. Another parameter that highly affects the final grain size is cooling rate.[26] It is commonly accepted that fast cooling leads to gr
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