Microstructural Evolution by Cooperative Motion of Nano-grains

The characteristics of microstructural evolution by the cooperative motion of nanocrystalline ceramics were investigated in order to study the mechanical principles which are common to the deformation by cooperative motion of cell structure aggregate, for example, foams, cells and nanocrystalline ceramics. We studied the possibility for the prediction and the control of the microstructural evolution through constructing a three-dimensional topological model for computer. The results are summarized as follows, 1)The deformations of 3 mol%Y_2O_3-stabilized tetragonal ZrO_2 polycrystals(Y-TZP)and single crystal of ZrO_2 were studied as a model material for analyzing the characteristics of microstructural evolution by motion of nanocrystalline ceramics. The yield stress(240 MPa)of tetragonal single crystal was much larger than the flow stress of Y-TZP(20 MPa). It is concluded that the dislocation motion does not play an important role in superplasticity of Y-TZP. 2)Three dimensional simulation was conducted to study the dynamic topological transformation in superplasticity. The simulation clearly demonstrated that the deformation wad induced by grain boundary sliding, in which the grain switching is processes of formation of contact and separation of grains. 3)The topology of grain boundary network is modified by the motion of grain boundary which is drived by curvature of grain boundary. The three-dimensional simulation showed that the boundary network approached a steady structure. We demonstrated the topological correlations on the shapes of neighboring cells, the Aboav-Weaire law and the Neumann-Mullins law on the kinetics of grain growth. Furthermore, we proposed a new concept of grain growth based on the statistics of disappearing grains.