Directional Solidification of Mg-Al Alloys and Microsegregation Study of Mg Alloys AZ31 and AM50: Part I. Methodology
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xtent of microsegregation present in solidified alloys impacts the successful production and properties of both castings and subsequently processed material. The workability and mechanical properties are particularly affected by the presence of microporosities and embrittling secondary phases. These detrimental aspects are typically intimately linked with interdendritic segregation, which is defined as the compositional variability existing on the length scale of dendrite arms. Because most alloys solidify dendritically under commercially relevant conditions, a thorough understanding of solute partitioning and the buildup of microsegregation is vital if these defects are to be controlled effectively. In addition, the heat-treatment efficiency and final mechanical properties, such as yield strength or toughness, are strongly influenced by the extent of the as-cast segregation.[1] Various microsolidification models are available for elucidating the effects of microsegregation[2] and crystal growth morphology.[3] The models range from a simple Scheil model with the most restrictive assumptions up to the highly sophisticated approaches, such as the promising phase-field method.[3,4] The sophisticated models demand complete information about solid/liquid interface equilibria and various kinetic and thermophysical alloy parameters. Thus, their applicability is limited. This limitation is particularly valid if magnesium alloys DJORDJE MIRKOVIC´, Project Engineer, formerly with Clausthal University of Technology, Institute of Metallurgy, is with Salzgitter Mannesmann Forschung GmbH, D-38239 Salzgitter, Germany. RAINER SCHMID-FETZER, Professor, is with the Clausthal University of Technology, Institute of Metallurgy, D-38678 ClausthalZellerfeld, Germany. Contact e-mail: [email protected] Manuscript submitted July 14, 2008. Article published online February 14, 2009 958—VOLUME 40A, APRIL 2009
are treated, for which the lack of necessary data is being only slowly resolved. Only recently, Bo¨ttger et al.[5] reported on controlling the microstructure in magnesium alloys by a combination of thermodynamic, experimental, and phase-field approaches. However complex the model may be, it is obvious that the reliability of any microsolidification model needs to be validated against dedicated experimental measurements. Perhaps one of the most stringent tests for any microsegregation model is a comparison of the measured and model-predicted solute profile,[1] even if some doubt has been raised in this regard.[6] The term ‘‘solute profile’’ is used for the solute concentration (content) vs solid phase fraction during solidification. It will be shown later in this work that, more specifically, the solute profile needs to be extended to include precipitates containing solute components. The X-ray mapping over a representative region of the sample, usually performed by electron probe microanalysis (EPMA), provides experimental data for solute profile determination that are typically superior to simple line scans. Random sampling sites are commonly p
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