Constitutive Modelling of Twinning in Magnesium Alloys

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CONSTITUTIVE MODELING OF TWINNING IN MAGNESIUM ALLOYS There has been considerable societal and government legislative pressure on automotive producers to reduce fuel consumption and exhaust gas emissions from vehicles. One of the most effective ways to achieve this is by vehicle weight reduction that can be obtained by replacing steel and aluminum with magnesium in the structure and body panels. However, magnesium alloys have a number of perceived and real technological limitations, the most significant of which is their reduced formability as compared to aluminum and steel. It is believed that this could be eliminated by the development of improved alloys, or circumvented through the use of new manufacturing methods. The forming behavior of magnesium and its alloys is poorly understood compared to that of steel and aluminum. From the experiences gained in developing other materials like aluminum and steel for similar applications, appropriate constitutive modeling is an important and foundational part of the development process. Therefore, the main purpose of this proposed research program is to develop/improve and validate constitutive models for describing large strain behavior of magnesium alloys under various deformation processes. At McMaster, a great effort has been focused on constitutive modellling of Mg alloys. In 2012 we proposed a twinning and de-twinning model (TDT) model for Mg alloys and implemented it into the finite strain Elastic-Viscoplastic Self-Consistent (EVPSC) model we developed in 2010 for polycrystalline materials. It has been shown that the EVPSC-TDT model is able to capture key features observed in large strain behaviour of Mg alloys. Although the EVPSC-TDT model represents the state-of-the-art in this research area, there remain some longstanding challenges. In particular, it is necessary to develop a more physically-based twinning model which appropriately describes twinning and interactions of twin-twin and twin-slip. The focus of the proposed program is thus on developing the best twinning model. The major impact of the proposed work will be the development of basic scientific knowledge and the training opportunities that arise while conducting this research. The results of this research will be of great value to the key industries involved in light metal fabrication processes and will ultimately lead to cost-effective fabrication processes utilizing optimum formability and performance characteristics.