The Sequence of Intermetallics Formation during the Solidification of an Al-Mg-Si Alloy Containing La
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INTRODUCTION
WROUGHT aluminum alloys belonging to the 6xxx series possessing a high strength to density ratio and a good corrosion resistance are considered as a replacement for steel in the production of automotive body panels. Although the 6xxx series alloys are habitually named Al-Mg-Si alloys, they contain other components. While some of them (e.g., Cu, Mn, and Cr) are deliberately added, Fe is a component that is neither wanted nor avoidable. The presence of iron worsens the formability of the aluminum alloys, thus hindering their usage in the auto industry. The poor formability caused by the formation of various brittle iron-containing intermetallic compounds induces damage and premature failure during forming and bending operations.[1–3] Since some of the intermetallics are less detrimental than others, different approaches and techniques were tried to modify the Fe-bearing intermetallics, i.e., to alter their type, shape, size, and distribution in wrought aluminum alloys.[4–11] In particular, the present authors investigated the modification effect of small additions of Ce and La on Fe-containing intermetallics forming in a typical 6xxx series alloy.[11] It was discovered that 0.2 wt pct La reduced the fraction of detrimental platelike b-AlFeSi particles and increased the fraction of less harmful a-AlFeSi particles with the so-called Chinese script morphology. To explain this phenomenon, it was hypothesized that La behaved similarly to Sr;[5] i.e., during solidification, lanthanum covered the surface of the Chinese script particles. That La-rich layer inhibited MEHDI HOSSEINIFAR, Postdoctoral Fellow, and DMITRI V. MALAKHOV, Professor, are with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON L8S4L7, Canada. Contact e-mail: [email protected] Manuscript submitted March 3, 2010. Article published online November 12, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
the diffusion of Si from the liquid phase into the particles, thus preventing their transformation into platelike particles via a peritectic reaction:[12] L þ a-AlFeSi ! Al þ b-AlFeSi A natural way to validate this hypothesis is to examine a sequence of intermetallics formation during solidification. Such an investigation was undertaken by Samuel et al.[7] who calorimetrically studied the solidification path of the Al-0.5 wt pct Si-0.15 wt pct Fe alloy with and without Sr. They showed that in the presence of strontium, the heat effect associated with the peritectic reaction is less pronounced. In the present investigation, a similar technique is used to establish solidification paths in a reference Al-SiMg-Fe-Cu alloy and the same alloy containing 0.2 wt pct La. An analysis of differences between the two paths augmented by a modeling of solidification yields a better understanding of the mechanism by which La modifies the Fe-bearing intermetallics. II.
EXPERIMENTAL PROCEDURE
The composition of the reference alloy, which contained 0.7Si, 0.7Mg, 0.45Fe, and 0.2Cu (all concentrations are in weight percent), is representative
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