The Effect of Solute Elements on the Grain Refinement of Cast Zn
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ries/models developed in the three light metals can be directly used to cast Zn alloys. Addition of solutes is regarded as an effective approach to refine the grains of cast metals and has been widely studied by a number of researchers in light metals.[12–19] It is generally considered that the formation of the constitutional undercooling zone at the front of the solid/liquid interface during solidification promotes nucleation and therefore results in smaller grains. Youdelis[20] believed that adding solute increases the nucleation entropy and therefore increases nucleation rate. The effect of solutes on grain size evolution can also be characterized using three undercooling parameters, i.e., the growth restriction factor (Q), undercooling parameter (P),[18,21,22] and the freezing range (DT),[15] as shown in Figure 1. These parameters are related to the degree of grain refinement. The calculation of Q and P is given by Eq. [1]. Q ¼ mCo ðk 1Þ;
ZHILIN LIU and FENG WANG, Ph.D. Candidates, DONG QIU, Australian Research Fellow, JOHN A. TAYLOR and MINGXING ZHANG, Associate Professors, are with the School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. Contact e-mail: [email protected] Manuscript submitted April 23, 2013. Article published online July 9, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
½1
where m is the liquidus gradient, Co is the solute concentration, and k is the equilibrium partition coefficient.[21] Equations [2] to [3] and [4] to [5] are used to approximate DT for binary peritectic and eutectic alloys: DTpe ¼ P;
Because of the good corrosion resistance, sound damping properties, low melting temperature, and good casting dimensional tolerance,[1–3] zinc and its alloys are normally used as cast in transportation, electronic, and electrical industries. However, cast pure zinc is associated with low strength and very coarse grains. The latter leads to low ductility and brittle fracture of components during service.[2,4] Hence, pure zinc is normally alloyed with Al, Mg, and Cu to refine the microstructure and improve the strength. It has been predicted that the wellrefined zinc alloys have the potential to partially replace cast Al or Cu alloys as structural and pressure-tight components[4] and to replace bronze to be used in mining engineering.[5] However, how the addition of alloying elements will affect the grain size in the cast zinc alloys has never been comprehensively studied, and the actual grain refinement mechanism is also beyond understanding. Up to date, most studies on grain refinement of cast metals are focused on aluminum,[6–8] magnesium,[9,10] and titanium alloys.[11] Unlike Al, Mg, and their alloys, in which reliable grain refiners are well developed, no practical grain refiners are available for cast zinc and its alloys. The question is whether the grain
P ¼ mCo ðk 1Þ=k;
Co Cm
DTpe ¼ Tpe Tm Cpe Co = Cpe ;
½2 Cm
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