Papers by Keyword: Grain Growth Model

Abstract: The effect of precipitates on austenite grain growth behavior in a Nb-V microalloyed steel was investigated. The precipitates were identified by selected area electron diffraction (SAED) and energy dispersive spectrometer (EDS) analysis. Because of pinning effect of NbC and/or VC on austenite grain boundaries, grains grew slowly at 850oC-1000oC. However, when temperature reached 1050 oC, abnormal grain growth was observed, which was attributed to dissolution of NbC particles. The NbC precipitates dissolved significantly at 1150 oC. However, grain sizes were still very small. Thus, austenite grains grew rapidly at 1050-1150 oC. The fully dissolution temperature of this steel was 1150-1250oC. Finally, the relationship between grain coarsening temperature (T_GC) and fully dissolution temperature (T_DISS) could be illustrated as follows: 100 oC≤T_DISS -T_GC≤200 oC. When heating temperatures were 850-1050 oC and 1050-1250 oC, grain growth activation energies (Q) were 59945 J/mol and 135813 J/mol, respectively. The different grain growth models were mainly caused by the gradual dissolution of NbC particles.

Abstract: Mechanical properties of ceramic materials are closely related to the grain size, and control the ceramic material grain size is the key to increase mechanical properties of materials. Study on the theory of ceramic sintering grain growth model, such as solid phase sintering, liquid phase sintering and master sintering curve (MSC) and so on. Grain growth model not only can be used to guide the actual production design, but also can be used as a computer simulation tools. It is suggest that before use any grain growth model must modify their express from traditional model, and check the equation is correct that it is agree with the result of re-experiment.

Abstract: This paper reviews our recent investigations on grain growth in ceramics. Grain growth behavior has been found to be governed by the grain boundary structure: normal growth with a stationary relative grain size distribution for rough boundaries and non-normal (nonstationary) growth for faceted boundaries. Based on the concept of nonlinear migration of faceted boundaries, the mixed control model of grain growth is introduced and the principle of microstructural evolution is deduced. This principle states that various types of grain growth behavior are predicted as a result of the coupling effect between the maximum driving force for growth and the critical driving force for appreciable migration of the boundary. A wealth of experimental results supports the theoretical predictions of grain growth behavior, showing the generality of the suggested principle of microstructural evolution. Application of this principle is also demonstrated for the fabrication of single crystals as well as polycrystals with desired microstructures.

Abstract: Austenite grain size is an important microstructure parameter when processing steels as it provides the initial condition for the austenite decomposition that determines the final microstructure and thus properties of the steel. In low-carbon steels it is frequently difficult if not impossible to quantify the austenite grain size using conventional metallographic techniques. Laser-ultrasonics provides an attractive alternative to quantify the grain size in-situ during thermo-mechanical processing of a steel sample. The attenuation of the laser generated ultrasound wave is a function of the grain size. The present paper gives an overview on the state-of-the-art of this novel measurement technique. Using isothermal and non-isothermal grain growth tests in low-carbon steels the advantages and limitations of laser-ultrasonic measurements will be demonstrated. Further, their application for deformed samples will be presented to quantify austenite grain sizes during and after recrystallization. The in-situ measurements provide significantly new insights into the austenite microstructure evolution during thermo-mechanical processing of low-carbon steels. The implications on expediting the development of improved process models will be discussed.

Abstract: Aluminum casting is widely used in aeronautical, automobile and other industries nowadays. The Cellular Automaton (CA) method was modified to simulate the microstructure evolution of Al alloy casting. Simulated program code was developed and applied into Al casting production. A nucleation model was investigated based upon the experimental data. The solute diffusion in the liquid and solid phases was also considered in developing a grain growth model. With the developed models, not only grain structure but also dendritic microstructure can be predicted during the solidification process. The microstructure simulation of the Al alloy turbine wheel was studied in detail.