Microstructural Evolution by Cooperative Motion of Nano-grains

研究了纳米晶陶瓷协同运动下的微观组织演化特征，以探讨泡沫、胞和纳米晶陶瓷等胞状结构集合体协同运动变形的共同力学原理。研究了通过建立三维拓扑模型来预测和控制显微组织演化的可能性。1）研究了3mol%Y_2O_3稳定的四元ZrO_2多晶（Y-TZP）和ZrO_2单晶的形变，作为分析纳米晶陶瓷运动过程中微观结构演变特征的模型材料。TetraZrO 2单晶的屈服应力（240 MPa）远大于Y-TZP的流变应力（20 MPa）。结果表明，位错运动对Y-TZP合金超塑性的影响不大。2）采用三维数值模拟方法研究了Y-TZP合金超塑性过程中的动态拓扑转变。模拟结果表明，变形是由晶界滑移引起的，其中晶粒转变是晶粒接触和分离的形成过程。3）晶界曲率驱动晶界运动，改变晶界网络拓扑结构。三维模拟结果表明，边界网络接近稳定结构。我们证明了相邻胞形状的拓扑相关性，晶粒生长动力学的Neumann-Mullins定律和Neumann-Weaire定律。在此基础上，提出了基于消失晶粒统计的晶粒长大新概念。

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.