Strengthening Micromechanisms in Cold-Chamber High-Pressure Die-Cast Mg-Al Alloys
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HIGH-PRESSURE die-cast (HPDC) Mg alloys are normally not heat treated, and, for a given alloy, their microstructure, hence their mechanical properties are determined by the die geometry and the casting processing parameters.[1,2] The good thermal contact between the die wall and the liquid metal ensured by the applied pressure, combined with the relatively low heat capacity of molten magnesium result in very high solidification rates, not only with the beneficial effect of a generally finer microstructure in comparison with other forms of casting,[3] but also with strong gradients of composition and distribution of intermetallics at both the grain size and casting cross section scales.[4–14] The heterogeneity of the cast microstructure is particularly marked when a cold-chamber press is used, in which case the grain microstructure is strongly bimodal, with predominantly fine grains at or near the surface of the cross section (or skin) and large dendritic grains, termed externally solidified grains (ESGs), in the center (or core).[7,13,15,16] The ESGs, formed by partial solidification of the melt in the unheated shot sleeve and runner,[7,13,17,18] are introduced as minute crystals disKUN V. YANG, formerly with the ARC Centre of Excellence Design in Light Metals, Materials Engineering, School of Engineering, The University of Queensland, St Lucia, QLD 4072, Australia, is now Postdoctoral Research Fellow with Department of Materials Engineering, Monash University, Clayton, VIC 3168, Australia. CARLOS H. CA´CERES, Reader in Casting Technology, is with Materials Engineering, School of Engineering, The University of Queensland, QLD 4072, Australia. Contact e-mail: [email protected] MARK A. EASTON, formerly with Department of Materials Engineering, Monash University, is now Professor with School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, VIC 3083, Australia. Manuscript submitted October 14, 2013. Article published online May 20, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
persed in the liquid during the filling of the die.[3,13,19,20] The dynamics of die filling and that of solidification itself[21] concentrates the ESGs at the core, particularly for the concentrated alloys.[22] The ESGs have a lower than average solute content, enriching the liquid that fills the die cavity,[23,24] and the pronounced coring in the grains solidified inside the die leads to the formation of eutectic in proportions well in excess of what the phase diagram predicts.[2,23,25,26] Thus, even for Al contents as low as 2 mass pct, a microstructure of bMg17Al12 eutectic intermetallics delineates the dendrite and grain boundaries alike.[2,23,25,26] The net result, especially for the more concentrated alloys, is the formation of two well-differentiated regions across the thickness of the casting: the skin layer, which may be up to 500 microns thick, and the core encompassing the rest. Although visually the two regions can easily be differentiated from each other on a polished cross section, defining the bounda
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