J.Z. Zhao

Kinetics of Cottrell atmosphere formation during strain aging of ultra-low carbon steels

A model has been developed based on Cottrells theory to describe the formation of Cottrell atmosphere by taking into account the effect of the decrease in the concentration of carbon atoms in solution, the saturation of the dislocations and the segregation of carbon atoms to grain boundaries. The strain aging of an ultra-low carbon bake hardening steel has been investigated both theoretically with this model and experimentally. It is indicated that the model describes the formation of Cottrell atmosphere much more reasonably than Cottrells formula does.

Quantitative analysis of the dilatation during an isothermal decomposition of austenites

The dilatation of hypoeutectoid steels during an isothermal transformation has been analysed. A model accounting for the effect of carbon redistribution has been developed which can be applied not only to the calculation of the dilatation during an isothermal transformation but also to the extraction of the phase change kinetics from a dilatation curve which may correspond to a multistep phase transformation. The model is validated by comparing it with the experimental results for an interstitial free steel, The dilatation of Fe-C steels during an isothermal transformation has been calculated and discussed. It is indicated that the isothermal formation of ferrite causes a different linear dependence of the relative length change on the volume fraction of the transformed austenite compared with the pearlite type decomposition. Isothermal transformation experiments have been carried out with an experimental bainitic grade steel. The model was successfully applied to extract the phase transformation kinetics from the dilatation curves of this steel

Microstructure evolution in immiscible alloys during rapid directional solidification

A model has been developed by taking into account the common action of the nucleation, the diffusional growth, the collisions and coagulations of the minority phase droplets and the spatial phase segregation to describe the microstructure evolution in a rapidly directionally solidified immiscible alloy. Numerical solutions have been performed to investigate the microstructure evolution in a directionally solidified hypermonotectic Al - Pb alloy. It has been shown that an undercooled region appears in front of the solid/liquid interface and the liquid-liquid decomposition takes place there. If the solidification velocity is high enough, a steady state of solidification can be established and a fine dispersion of minority phase particles in a hypermonotectic alloy can be obtained.

Model for extracting phase transformation kinetics from dilatometry measurements for multistep transformations

Abstract A detailed analysis has been made of the length change of a hypoeutectoid steel during a continuous cooling or an isothermal transformation. A model has been developed in which the transient dilatation is calculated based on the fraction of the phases present. The model accounts for the carbon partitioning effects and is applicable to the extraction of the phase change kinetics from a dilatation curve for a multistep phase transformation. It has been validated by comparing the model results with the experimental results of an interstitial free steel. Isothermal transformations and continuous cooling experiments have also been carried out with a bainitic steel. The model was used to extract the phase transformation kinetics from the dilatation curves of this steel. Excellent agreement between the calculated results and the experimental ones has been obtained.