Nanostructure control

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The purpose of the present research project is to establish various methods of nanostructure control and to make clear the mechanism of superplasticity in various materials such as metals and alloys, intermetallics and ceramics. The main results obtained are as follows.(1) New types of heat treatments for appearance of superplasticity in duplex stainless steel and high carbon steel were developed. Microduplex structure with a grain size of 0.5〜1.0μm can be obtained by simple treatments such as heavy cold-rolling of as-received hot-rolled plates in duplex stainless steel and the quenching from (γ + θ) temperature in high carbon steel.(2) Effect of liquid phase at grain boundaries on superplasticity was investigated in particle-dispersed aluminum alloys. A large elongation was obtained at temperatures just below the solidus one for a 7475 Al alloy. The thickness of liquid phase in the test condition corresponding to the peak elongation was estimated to be 10〜60nm for the 7475 alloy.(3) Mechanism of superplastic deformation in monolithic intermetallics was found to depend on ordered crystal structure. Grain boundary sliding plays an important role in fcc-derivative intermetallics exhibiting fine-grained superplasticity, while dynamic recrystallization and/or dynamic recovery is responsible for coarse-grained superplasticity in bcc-derivative intermetallics.(4) The effect of Mg, Ca, Sr and Ba additions on the grain size refinement of Y-TZP and Ce-TZP was studied. It was found that Ca is the most effective element for the grain refinement. The addition of small amount of CuO in Y-TZP was found to be effective in reducing the sintering temperature, grain size and the stress rcquired for superplastic deformation.