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In the present paper, the microstructural changes associated with phase transformation and the straining behavior in polycrystalline structures dueing transformation superplastic deformation are investigated. In-situ observations of microstructural changes during Ac3 transformation on rapid heating in pure iron have been carried out with a high temperature optical microscope and a dark field reflection high temperature microscope. Both of hot-stage microscopes were specially designed for this study. The distribution of superplastic strain has been examined by microscopic strain analyses by using a micro-grid pattern with 12.7 pm intervals. The main results are as follows: (1) Ac3 transformation process at a heating rate under 50 K/s is predominantly nucleation of austenite grains at the prior ferrite grain boundaries and triple points. A subsequent growth of grains into the prior ferrite matrices is observed. The growth of austenite grains is not always isotropic under a tensile stress. (2) In the initial stage of transformation, superplastic strain is induced by the sliding at rya transformation interface along the prior ferrite grain boundaries. The observed superplastic strain is also associated with the grain rotation, corresponding to the growth of austenite grains which surround ferrite grains. (3) In the intermediate stage of transformation, the sliding deformation is generated at the migrating transformation interface associated with the growth of austenite grains. Accumulated strain by the sliding is left within the previously transformed region. (4) These observations suggest that the sliding mechanism at the migrating transformation interface is a principal mechanism of transformation superplasticity.

The concept of a novel approach to make a compact SMES unit composed of a stack of Si wafers using a well-established MEMS process was proposed. The concept was backed up by pilot estimations for energy storage capacity and mechanical strength to endure electromagnetic stress. The estimated volume density of the storable energy is comparable to that of rechargeable batteries and the mechanical strength of Si wafer endures the electromagnetic stress imposed on it. These estimations support the feasibility of this novel concept, although there needs to be more detailed design of the system for its practical realization. Furthermore, there are a lot of challenges to overcome. The first step of the experimental proof of this new concept was successfully performed through several repeated test fabrications. In one of these test fabrications, the theoretically estimated upper limit value of the energy storage corresponding to a pilot design of a spiral superconducting NbN coil in the spiral trench formed on a Si wafer 10.15 cm in diameter was attained.

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