Materials World Network: Deformation via the Transformation of Hierarchical Microstructures

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This Materials World Network research project between Prof. Peter Müllner, Prof. William Knowlton, and Res. Assist. Prof. Paul Lindquist at Boise State University (BSU) and Profs. Steffen Brinckmann and Alexander Hartmaier at the Ruhr University Bochum, Germany, focuses on the deformation mechanism of materials with hierarchically twinned microstructures. While such microstructures occur in many functional materials including shape-memory alloys and ferroelastic ceramics, the experimental part of this study concentrates on magnetic shape-memory alloys (MSMAs). The mechanical and magneto-mechanical properties of magnetic shape-memory alloys strongly depend on details of the twin microstructure, which can be varied via appropriate thermo-magneto-mechanical training treatments. For well-trained single crystals with a single operating twinning system, the twinning stress is very low (about 0.1 MPa), the magnetic-field-induced strain is large (6-10 %), and the fatigue life is short (about 10,000 cycles). For un-trained single crystals with self-accommodated, hierarchically twinned martensite, the twinning stress is relatively high (about 2 MPa), the magnetic-field-induced strain is small (about 0.1 %), and the fatigue life is long (exceeding 100 million cycles). The deformation mechanism of hierarchically twinned microstructures will be studied experimentally at Boise State University on all length scales with in-situ deformation experiments while numerical studies will be performed at the Ruhr University Bochum. The experiments include in-situ straining in the transmission electron microscope (microscale), in the scanning probe microscope (mesoscale), and with optical imaging in the tensile test bench (macroscale). 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. By supplying 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. A strong impact on technology is anticipated: The quantitative understanding gained with this study will allow the development of fast and high-rescission MSMA actuators with large stroke and long lifetime for applications including valves for ink-jet printers and micropumps for biomedical systems. This project will produce the computational tools required for leading MSMAs from the research laboratory to industrial technologies.