Deformation via the Transformation of Hierarchical Microstructures

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The microstructure of shape-memory alloys consists of hierarchically arranged twins, where twins are formed within twins within twins and so on. Upon mechanical loading, the twins collectively rearrange on all hierarchical levels to accommodate strain. Twin-boundary motion is controlled by local stress fields, which in turn depend on the organized rearrangement of twins on all length scales. While twinning was recognized as the dominating deformation mechanism in shape-memory alloys, the mechanics of self-organization and coordinated rearrangement received little attention due to large driving forces (exceeding the twinning stress) in thermal shape-memory alloys. The discovery of magnetic shape-memory alloys (MSMA) has sparked significant interest regarding the organized rearrangement of twins because the twinning stress varies over two orders of magnitude for a given single crystal, and the magnetic driving forces are comparatively small, and hence, the magneto-mechanical properties are highly sensitive to the twin microstructure.Intellectual merit: The deformation mechanics of hierarchical twin microstructures will be studied experimentally and numerically. During deformation experiments, the rearrangement of twins will be observed in-situ with three methods – transmission electron microscopy, atomic force microscopy, and optical microscopy – covering seven orders of length-scale: from nanometer to centimeter. Numerical simulations will provide theoretical insight into the hierarchical twinning mechanics. The defect content of the twin microstructure will be obtained from the experimental study. Disclination dipoles, i.e. mesoscopic line defects with a shear displacement field, represent the displacement fields of twins on all hierarchical levels. A disclination dynamics code will be developed and applied to a large scale numerical study, which will yield a correlation between microstructure and mechanical behavior. By feeding the numerical study with experimental data, and by systematically varying experimental and numerical parameters, a quantitative fundamental understanding of the mechanical (and magneto-mechanical) properties of (magnetic) shape-memory alloys with complex hierarchical twin-microstructures will be generated.Broader impact: One of the advantages of MSMA based actuators is the gigantic stroke of 10%. Development of such actuators is hindered due to failure during cyclic actuation. MSMA with densely twinned microstructures achieve a long lifetime, however, with significant reduction of stroke. The quantitative understanding gained with this study will allow developing MSMA actuators with large stroke and long lifetime. This project will produce the computational tools required for leading MSMA from the research laboratory to industrial technologies and disseminate them via the nanoHUB.org. Graduate and undergraduate students will be trained in cutting edge characterization methods relevant for the nanotechnology and microelectronic industries. The US students, who will perform the experimental studies at Boise State University, will visit the international partner at Ruhr University Bochum, Germany for one to three months each summer to study the computational methods. They will gain international experience and acquire experimental and numerical, technology-relevant expertise. This program will prepare the students to successfully contribute to and compete in the demanding and challenging global technology market. Students will participate in K-6 school visits to introduce children to the fascinating world of modern technology. Successful recruiting approaches are in place for actively recruiting underprivileged students and students of underrepresented groups. These recruiting approaches include recruiting from the McNair program and recruiting from introductory courses in engineering.