新的单参数动态再结晶动力学建模及晶粒尺寸预测

A NEW ONE-PARAMETER KINETICS MODEL OF DYNAMIC RECRYSTALLIZATION AND GRAIN SIZE PREDICATION

通过引入动态再结晶的演化速率,分析了基于Avrami方程的经典动态再结晶动力学模型的不足.提出了一种新的具有单参数的动态再结晶动力学模型,反映了动态再结晶过程缓慢快速缓慢的特点.采用Gleeble-1500热模拟试验机,对典型的具有动态再结晶特性的材料镁合金AZ31B进行了热压缩实验,通过进行参数回归得到了其动态再结晶动力学模型,并与实验结果相对比,验证了该模型的正确性.进一步将稳态变形条件下获得的微观组织演化模型改写成分步叠加形式.与动态再结晶晶粒尺寸模型相结合,应用到非隐态条件的晶粒预测,模拟与实验的对比表明计算结果和定量金相法所获得的结果基本一致,说明了非稳态变形过晶粒的叠加预测方法的合理性.

Dynamic recrystallization （DRX） is considered as one of the most important mi- crostructural evolution mechanisms to obtain fine metallurgical structures, eliminate defects and im- prove mechanical properties of products. Although the DRX kinetics models proposed by researchers have some differences in parameters and forms, they are all based on the Avrami function describing the relationship between dynamically recrystallized volume fraction and strain or time. Avrami equation is in the form of exponential function and the kinetics curve of DRX exhibits different when the exponent is assumed to be different （between 1 and 2）. Under these conditions, however, the exponential function cannot exactly describe the ＂slow-rapid-slow＂ property of the development speed of DRX process. By introducing the velocity of development of DRX process, which is referred to as the variation of the dynamically recrystallized volume fraction with strain, namely, the first partial derivative of the dynamically recrystallized volume fraction to strain, a new kinetics model of DRX was proposed in comparison with the classical kinetics model of DRX. The new model represents the characteristics of DRX： the dynamically recrystallized volume fraction equals zero when the strain is smaller than the critical strain, and the maximum of the dynamically recrystallized volume fraction equals 1; once the strain exceeds the critical strain, the dynamically recrystallized volume fraction slowly increases first, and then rapidly increases, at last slowly increases. Consequently, the new kinetics model is in agreement with the development law of DRX process and includes few parameters which have clearphysical meaning and are easy to determine. By conducting Gleeble-1500 thermomechanical simulation compression tests at the temperatures ranging from 523 to 673 K and at the strain rates 0.001, 0.01, 0.1 and 1 s-1, the kinetics model for Mg alloy AZ31B characterized by DRX for instance was built and parameters were determined. Microscopic examination shows that the experimental results are in good agreement with the predicted values, which validates the accuracy of the new kinetics model. Then combined with grain size of DRX model, the kinetic model built under steady state conditions was rewritten as superimposed step form to apply in the prediction of grain size under unsteady state conditions. The simulated data accord with the experimental results by means of quantitative metallography, which verified the rationality of the superimposed prediction method.