Effect of Cooling Rate on the Grain Refinement of Mg-Y-Zr Alloys
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MG alloys have potential for increased application in the automotive and aerospace industries due to their high specific strength and low density.[1] During casting of Mg alloys, grain refinement can improve castability, mechanical properties, and post-formability. Recently, there has been increased understanding of the mechanisms controlling the extent of grain refinement of metallic alloys.[2–5] In particular, the Interdependence Model[4,5] showed that both nucleating particles and solute are required to achieve the formation and refinement of equiaxed grains. Nucleant particles provide heterogeneous sites for nucleation of the primary solid phase at low undercooling, while the solute elements provide constitutional supercooling (CS) to restrict grain growth and facilitate further nucleation events. To obtain effective nucleation, two key parameters are important: the potency of nucleants (defined as
MING SUN is with the School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, P.R. China. Contact e-mail: [email protected] DAVID H. STJOHN is with the Centre for Advanced Materials Processing and Manufacturing, School of Mechanical and Materials Engineering, The University of Queensland, St Lucia, QLD 4072, Australia. MARK A. EASTON is with the Centre for Additive Manufacturing (CAM), School of Engineering, RMIT University, Melbourne, VIC 3053, Australia. Contact e-mail: [email protected] KE WANG is with the National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. JIAMING NI is with the Shanghai Aerospace Precision Machinery Institute, Shanghai 201600, P.R. China. Manuscript submitted June 7, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
inverse of the undercooling required for nucleation DTn) and number density of active particles. The effectiveness of solute elements can be determined using the growth P restriction factor Q where Q = mi(ki 1)C0i for each solute element, i, present in the alloy, mi is the slope of the liquidus on the associated binary phase diagram, ki is the solute partition coefficient, and C0i is the solute content. Equation [1] is a simple analytical equation[6–8] that relates the grain size (dgs), solute content, and the potency and number density of nucleant particles. dgs ¼ a þ
b Q
½1
where a is related to the number of the activatable nucleant particles, and b is associated with the potency of nucleant particles when the casting conditions are constant. A smaller intercept ‘‘a’’ indicates a larger number of activatable nucleants, while a lower slope ‘‘b’’ corresponds to particles of higher potency.[6–8] Because commercial structural products have complex shapes with thin and thick sections, the cooling rate _ may vary significantly throughout a casting. Eq. [2] (T) has been proposed[9] based on Eq. [1] with the effect of cooling rate being taken into account. d¼aþ
b pffiffiffiffi Q T_
with cooling rate T_ c
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