Solute Content and the Grain Microstructure of High Pressure Diecast Magnesium–Aluminium Alloys

Abstract

Seven binary alloys, with Al contents from 0.47 to 11.6 mass%, have been studied using electron back scattered diffraction. Grain size measurements were made at two opposing surface locations and at the core of the castings cross-sections. The alloy with 0.47 mass% Al showed a uniform microstructure of relatively large grains over the entire cross section, whereas higher Al content alloys showed an increasingly bimodal, non-uniform grain structure, with mostly fine grains near the surfaces and mixed fine and coarse grains at the core. The average grain size at the surfaces decreased for solute contents of up to 5.51 mass% Al. No further grain refining was observed for higher solute contents. The average size, d, of the surface grains was found to fit the relationship, d ¼ a þ b = Q , where Q is the growth restriction factor imposed by the solute and a and b are constants. The proportion of coarse grains at the core appeared to be related to the alloy's solidification range, and reached a maximum also for the alloy with 5.51 mass% Al. The degree of bimodality of the grain populations at the core and surface, portioned according to statistical techniques, was maximum for the alloy with 5.51 mass% Al.Despite the large amount of effort devoted to relate the tensile properties to the solidification structure of high pressure diecast (hpdc) magnesium (Mg) alloys, [1][2][3][4][5][6][7][8][9][10][11][12][13][14] little systematic work has been reported on how the grain microstructure is affected by the solute content. This is important for two reasons. One is whether grain nucleation and growth in hpdc is controlled by the solute concentration as in quiescent castings. The other refers to the non-uniformity of the grain structure which characterises hpdc. The small grain sizes near the surface of the castings are usually considered to dominate the strength [3,4,[6][7][8][14][15][16][17] through a strong Hall-Petch effect. However, as discussed thoroughly by Kurzydlowski and Bucki, [18] a mixture of grain sizes on the specimen cross-section may actually lead to an overall softening, and generally affect the flow behaviour. Therefore, an accurate description of the distribution of grain sizes over the casting's cross-section for different solute contents is necessary in order to both quantify the Hall-Petch contribution to the overall strengthening, and understand the way MgÀAl alloys yield plastically.The region near the surface of the casting cross-section normally contains a predominantly fine grain structure, while the centre, or 'core', includes a large fraction of coarse grains. [4,15,19] This bimodal grain structure is thought to be formed through two main processes: [10,20,21] the small grains near the surface result from the fast solidification rate inherent to the hpdc process, and are thus related to the so-called 'skin effect'; [4,6,7,15,16,19] the large grains, sitting mainly at the core, are normally ascribed to the formation of crystals through partial solidification in the shot sleeve, which are injected in...

Heat treatment of vacuum high pressure die cast magnesium alloy AZ91

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Deformation Behavior of the Percolating Intermetallic Microstructure of High Pressure Die Cast AZ91 Alloy

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Recent advances in grain refinement of light metals and alloys

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