Effects of zinc, lithium, and indium on the grain size of magnesium
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M. Pekguleryuza) McGill University, Montreal H3A2B2, Quebec, Canada (Received 6 October 2008; accepted 5 December 2008)
The grain size of magnesium solid-solution alloys with lithium, indium, and/or zinc has been determined. Lithium, indium, and zinc additions decreased the grain size, D, of magnesium solid-solution alloys cast in a copper mold. The most effective grain refinement was obtained by zinc. In binary Mg–Zn alloys, grain size is related to the growth restriction factor, Q as D = 94 + 312/Q. In Mg–Li and Mg–In binary alloys, grain size versus growth relationships described as D = a + b/Q indicated that these alloys have lower numbers of nucleants but with higher potency than the Mg–Zn binary system. For Mg–Li and especially Mg–In, grain size could be related to growth restriction as D = 383Qn with higher R2. Ternary and quaternary alloys based on Mg–Zn with Li and/or In additions also follow the D = a + b/Q relationship with the parameters indicating a larger number of lower potency nucleants than the Mg–Zn binary alloys. Electron probe microanalysis showed that Mg–Zn alloys exhibit pronounced and persistent grain-boundary enrichment of Zn, pointing toward Scheil solidification. I. INTRODUCTION
The interest of automotive companies in weight reduction has led to new research in wrought magnesium sheet alloys.1–4 It has been determined that grain size and structure influence deformation behavior of magnesium.5–7 The effect of fine grain size on the ductility of magnesium at room temperature has been observed since the 1960s.8–12 Observations on coarse-grained material demonstrate an abrupt change in ductility at around 230 C (Fig. 1), while fine-grained structure smoothes out this inflexion. Very fine grain size (2 mm) demonstrates unusual room-temperature ductility.8 Currently the interest has been focusing on the effect of novel processing techniques, such as equal channel angular extrusion to significantly refine the grain size. However, these techniques are not always technologically easy to implement. At the same time, an intrinsic “grain-refining ability” is desired for the alloy to ensure fine grain size in subsequent processing and to avoid extensive grain growth. This work focuses on the effects of Li, In, and Zn additions on the grain size of magnesium. II. BACKGROUND
Grain refining can be classified as (i) native grain size determined by alloy composition and solidification
characteristics13; (ii) inoculated (refined) grain size where grains are refined by nucleating agents either added or created in situ (e.g., due to superheating of the melt)13–15; and (iii) processed grain size where plastic deformation, e.g., equal-channel angular processing (ECAP), results in a recrystallized fine grain size.5–7 The two important factors in native and inoculated grain refining are (i) the nucleation potency defined as the reciprocal of nucleation undercooling (DTn) and (ii) the degree of solute segregation defined as growth restriction factor, Q.16 StJohn et al.17 have related the two factors to grain
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