Although it has been widely observed that deformation twinning plays a dominant role in the deformation response of Mg alloys at room and elevated temperatures, its precise role on the ductility of these alloys is not yet understood. Establishing the physics of the underlying deformation mechanisms in Mg alloys has been complicated by many factors, including (i) the dramatic morphological and strain hardening rate differences between the two families of deformation twins observed in Mg alloys, (ii) the strong influence of grain size and temperature on the extent of deformation twinning, (iii) the activation of dynamic recrystallization during plastic deformation at elevated temperature, (iv) the activation of rotational dynamic recrystallization in some coarse-grained Mg alloys in certain temperature ranges, and (v) the potentially different influences of extension and contraction twins on the recrystallization processes. It is proposed to undertake a detailed experimental and modeling study to develop quantitative insights into the underlying deformation mechanisms in thermo-mechanical processing of two specific Mg alloys: AZ31 and Mg-0.2wt% Ce. Experimental investigations will include room and high temperature simple compression tests which will be interrupted for detailed microstructure investigations using orientation image microscopy and measurements of the local stored energy at the grain scale in plastically deformed samples. It is also proposed to develop and validate new physics-based elastic-viscoplastic crystal plasticity models to predict the anisotropic stress-strain response and the evolution of the microstructure in thermo-mechanical deformation of these alloys. These models will be subsequently employed to develop novel processing routes for cost-effective manufacture of structural automotive parts made from Mg alloys.
NON-TECHNICAL SUMMARY: Strong but light magnesium (Mg) alloys offer tremendous potential for dramatic increases in the fuel efficiency of automobiles, with corresponding reductions in automotive CO2 emissions. The primary impediment to widespread application of these alloys is their very limited room temperature ductility, which prevents successful manufacture of the desired automotive structural components by standard inexpensive wrought processing methods. This proposal aims to produce the fundamental physical data sets and computational models of Mg alloy structure which are needed to find ways to improve the room temperature ductility of these alloys. The proposed interdisciplinary collaboration between researchers at Drexel University and at the General Motors Global R&D Center will result in the development of better Mg alloys for the automotive industry and may also have implications for the processing of other metals with similar crystalline structures. This project will produce two PhDs skilled in interdisciplinary research involving novel material characterization techniques, advanced computational mechanics, and applied mathematics. The project will expose numerous domestic undergraduate and graduate students, especially members of underrepresented groups in science and engineering, to cutting edge research methodologies and equipment.
Mg alloys with their high specific strengths exhibit tremendous potential for realizing dramatic increases in the fuel efficiency of automobiles with the concomitant reductions in CO2emissions. The main impediment to the introduction of Mg alloys in structural automotive applications is their very limited room temperature ductility, which prevents the successful manufacture of the desired automotive structural components by wrought processing. This project, conducted in collaboration with General Motors (GM) Global R&D Center, has produced a number of new insights into the main factors responisble for the limited room temperature ductility of these alloys. The main technical outcomes of the project include: (i) development and first demonstration of a new technique for multimodal characterization of the mesoscale material response using a combination of in-situ indentation in the scanning electron microscope (InSEM), electron back-scattered diffraction technique (EBSD), and digital image correlations (DIC); (ii) a detailed physical understanding of the contribution of extension and contraction twins to the recrystallized microstructure after room temperature deformation of AZ31 (a Mg alloy); (iii) a detailed understanding of the role of grain size on the strain hardening response of AZ31; and (iv) a critical evaluation and understanding of conditions needed for successful roller hemming of AZ31 sheets for automotive applications. The broader impact outcomes of the project include: (i) three archival journal papers already published/submitted with at least two more in the process of preparation for submission; (ii) about ten presentations at international conferences to disseminate the results of this work; (iii) training of two PhD students, one of whom is a female US citizen, in cutting-edge technological areas of strategic national interest (see the Materials Genome Initiative recently announced by the Administration); (iv) involvement of about six undergraduate students in this research over the project award period; and (v) transfer of the research results and technical knowledge developed in this project to industry (through three summer internships of graduate students at General Motors) and to national laboratories (the graduate PhD student is now employed full-time at Naval Research Laboratory, Washington DC).
