A Novel Method to Achieve Grain Refinement in Aluminum
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roach to control the scale of the microstructure, either before or after solidification, is of significant practical importance for cast metals, in that castings with fine-equiaxed structure usually exhibit better castability and mechanical properties than those with large columnar structure.[1,2] In comparison with the thermal-mechanical treatment at solid state, for which high energy and cost are required to achieve fine-equiaxed grains,[3,4] grain refinement by inoculation is more commonly applied in industrial practice due to its technical and economical advantages.[5,6] As a simple and reliable technique for melt treatment, inoculation involves the addition of alloying elements or/and potent nucleating particles, which can promote the formation of a fine-equiaxed grain structure upon solidification. Currently, it is generally acknowledged that effective grain refinement requires both potent nucleating particles and adequate solute elements.[6–8] The nucleating particles can act as substrates for heterogeneous nucleation of the solid phase, and their potency can be
KUI WANG, Ph.D. Candidate, HAIYAN JIANG and QUDONG WANG, Professors, BING YE, Research Assistant, and WENJIANG DING, Academician, are with the National Engineering Research Center of Light Alloys, Net Forming, Shanghai Jiao Tong University, 200240, Shanghai, P.R. China, and also with the State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University. Contact email: [email protected] Manuscript submitted April 12, 2016. Article published online July 26, 2016 4788—VOLUME 47A, OCTOBER 2016
assessed by the lattice disregistry between the solid and the substrate.[9,10] As the nucleation potency is an inherent property of a nucleating system composed of the substrate, the solid, and the liquid composition, it is fixed for a given nucleating system, although the nucleation efficiency can be changed by modifying the specific physical characteristics of a given nucleating particle and solidification conditions, such as number density, size, and size distribution, as well as cooling rate. Moreover, the presence of solute elements can generate constitutional supercooling (CS) not only to provide additional driving force for new nucleation in the CS zone, but also to restrict the growth of an already nucleated grain.[11] The effect of solute on grain refinement is quantified by growth restriction factor @ ðDTCS Þ , where DTCS and fs are the constituQ¼ @fs fs!0
tional supercooling and the fraction of solid, respectively.[12,13] The higher the value of Q is, the higher the grain refining efficiency. However, given that Q is dominated by the physical and chemical characteristics of an alloy system, including the alloy composition, the phase diagram, and chemical interactions between solutes,[14,15] it may set the limitation to refine the grains through solute additions. With an aim to overcome these limitations associated with inoculation, it is of great necessity to develop a new method or strategy for grain refinement. Over the past few decades, numerous studi
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