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Predicting Susceptibility of Binary Mg Alloys to Cracking During Solidification

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Document pages: 33 pages

Abstract: Extensive testing has been conducted recently on Mg alloys to evaluate their susceptibility to cracking during solidification as a result of the increasing use of Mg alloys to reduce the vehicle weight. A tool is needed to predict, based on the phase diagram, the most crack-susceptible solute content and the maximum crack susceptibility. The tool can be best tested by a wide variety of binary Mg alloy systems, whose phase diagrams are readily available. In the present study the crack susceptibility was calculated for eight binary Mg alloy systems Mg-Al, Mg-Ca, Mg-Ce, Mg-Gd, Mg-La, Mg-Nd, Mg-Y and Mg-Zn using the maximum │dT d(fS)1 2│ as the crack susceptibility index, where T is temperature and fS the fraction of solid. The calculated results agreed with existing data of crack susceptibility testing, thus confirming the index can be used for Mg alloys. It was shown that the crack susceptibility is highest when TE and kE are both low, e.g., Mg-Zn, where TE is the eutectic temperature and kE is the equilibrium partition ratio k at TE. It was found that the liquidus and solidus lines of binary Mg phase diagrams are often curved instead of straight and this makes k TE, but k = kE has been assumed in all predictions so far. When the liquidus and solidus lines are both significantly curved, predictions based on k = kE can significantly: 1. overestimate the most crack-susceptible solute content, e.g., Mg-Gd and Mg-Y, 2. underestimate a high maximum crack susceptibility, e.g., Mg-Al, Mg-Nd and Mg-Zn, and 3. overestimate the effect of solute back diffusion on the most crack-susceptible solute content, e.g., Mg-Al, Mg-Gd, Mg-Nd and Mg-Y.

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