Review of Grain Refinement of Cast Metals Through Inoculation: Theories and Developments
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ely used in research and industry to achieve uniformly distributed equiaxed (or near-equiaxed) grain structures.[1,2] Not only does grain refinement have positive influences on microstructural refinement and castability (i.e., the columnar-to-equiaxed transition), but it also improves the mechanical properties (i.e., ductility and strength) of cast/wrought metallic materials.[3] Although some other technologies, like alloying manipulation and work hardening, can improve strength to some extent, two major safety parameters for engineering alloys, toughness and ductility, usually have to be partially sacrificed. Research on grain refinement of cast metals has been conducted for over sixty years.[4] Grain refinement can
ZHILIN LIU is with the College of Mechanical and Electrical Engineering, Central South University, Changsha, 410083, P.R. China, and with the IMDEA Materials Institute, C/Eric Kandel 2, 28906, Getafe, Madrid, Spain, and also with the School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD, 4072, Australia, Contact e-mail: [email protected] Manuscript submitted September 19, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
be achieved through controlling solidification and/or solid-state processes. The two most common methods to refine the grains of cast metals are dynamic nucleation[5] and inoculation.[1] Through fast cooling and localized convection, the former can produce numerous secondary nuclei. The latter, which is extensively practiced in industry, achieves grain refinement through adding efficient grain refiners into the metal melt. When a critical undercooling is achieved, the potential nucleant particles will induce grain refinement by enhanced heterogeneous nucleation.[6–8] These nucleant particles may be released from grain refiners or form in situ during solidification.[9–12] Easton and StJohn[13] reviewed the mechanisms of grain refinement. They divided the theoretical and experimental reports before 1999 into two categories, the ‘‘nucleant paradigm’’ and ‘‘solute paradigm.’’ Based on their review, they concluded that both effective nucleants and solute elements are required for grain refinement of Al alloys, followed by experimental validation. However, their review paper only focused on cast Al alloys. Over the last two decades, breakthroughs have been realized in the grain refinement of Al, Mg, Fe, Ti, Cu, Zn, and their alloys. The latest theories and developments need to be evaluated by reviewing recently published studies.
The inoculation method of grain refinement is critically reviewed here. First, the characteristics of effective grain refiners are summarized in Section II from four aspects: wetting configuration, segregating elements, crystallographic matching, and geometrical features. Then, the fundamental knowledge of nucleation and growth during grain refinement is briefly categorized in Section III. Section IV summarizes the current theories of grain refinement of cast metals, including the peritectic-related, hypernucleation, inert nucleant, and constitutional
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