Competitive Heterogeneous Nucleation Between Zr and MgO Particles in Commercial Purity Magnesium
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increasing demand for automotive light weighting to improve fuel economy and to reduce CO2 emission, Mg alloys have attracted considerable attention as the next generation of structural materials due to their low density, high specific strength, and excellent castability.[1] High volume of near net-shaped components can be produced at relatively low cost via casting processes for engineering applications without the necessity of further processing. This makes a refined microstructure in the as-cast components particularly important. The benefits of grain refinement are many folds, such as uniform microstructure, reduced propensity for defect formation and chemical segregation, improved surface finish, and mechanic performance.[2] Grain refinement in practice is usually achieved through enhanced heterogeneous nucleation by either chemical inoculation[3–6] or physical grain refinement.[7–10]
G.S. PENG is with the School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan, 243002, P.R. China. Y. WANG and Z. FAN are with the BCAST, Brunel University, Uxbridge, Middlesex, UB8 3PH, UK. Contact e-mail: [email protected] Manuscript submitted October 18, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Chemical inoculation is achieved by the addition of a specially selected chemical substance (grain refiner) to provide exogenous crystalline particles to the alloy melt prior to solidification processing. Such exogenous particles will act as heterogeneous nucleation sites to increase the number of nucleation events leading to a refined as-cast microstructure. Currently, two major alloy systems that benefit from the excellent grain refining ability of inoculants are Al-free Mg alloys grain refined by Zr[11] and commercial Al alloys inoculated with Al-5Ti-B master alloys.[3] For Mg alloys, Zr has the same crystal structure (hcp), similar lattice parameters,[12] and the highest growth restriction factor,[13] and is therefore commonly used for grain refinement of Al-free Mg alloys. However, it suffers from some drawbacks, such as high material cost and low alloying efficiency. For instance, when a Zirmax master alloy (Mg-33.3Zr, all compositions are in wt pct in this paper) is used, an addition of 7 pct of the master alloy (equivalent to addition of 2.33 pct Zr) is necessary to ensure effective grain refinement.[11] This inefficiency not only increases the processing cost, but also affects the as-cast microstructure. To improve the grain refining efficiency of Zr, tremendous efforts have been made to understand the grain refining mechanism.[11,14–22] To date, the exact mechanism of grain refinement is still being debated. Early work reported in the 1940s
suggests that only Zr-rich particles forming near the peritectic temperature can result in nucleation by peritectic reaction with a-Mg.[14] They believe that Zr-rich particles survived at temperatures well above the peritectic temperature grow to a size being too large to initiate crystallization or became too contaminated on the surface through deposition of other el
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