The isothermal curing kinetics of polymethacrylimide/nano-SiO2 composites were investigated using a dynamic thermomechanical analysis. The relative conversion was defined with the storage modulus. ...The Avrami model-fitting method, Friedman method and integral method were applied to analyze the curing kinetics. The storage modulus and loss modulus increased appreciably, spanning three orders of magnitude throughout the curing. The frequency correlation of the relative conversion was noticeable at 180 °C because the glass transition took place when the curing degree was not high enough. The Avrami model-fitting analysis gave good fits for the experimental data. The activation energy calculated with the Avrami equation changed from 65.46 kJ/mol to 25.28 kJ/mol at 180–190 °C, while at 190–200 °C, the activation energy changed from 107.14 kJ/mol to 63.82 kJ/mol. The model-free analysis revealed the dependence of the activation energy on the relative conversion. The activation energy increased from 104.3 kJ/mol to 130.6 kJ/mol with the use of the Friedman method when the relative conversion ranged between 0.4–0.8. Similarly, the activation energy calculated with the integral method increased from 71.5 kJ/mol to 103.4 kJ/mol. When the relative conversion exceeded 0.8, the activation energy decreased gradually. The mobility of the reactive groups was hindered and the crosslinking density of the composite was much higher. The curing kinetics became diffusion controlled. The activation energy of the PMI/SiO2 composite was greater than that of PMI, which could be attributed to the hindrance effect caused by nano-SiO2.
The hot-deformation behavior of the as-cast Mn18Cr18N high-nitrogen austenitic stainless steel, produced with the electroslag-remelting metallurgical technology, was studied using ...isothermal-compression tests in a temperature range of 1223–1473 K) and a strain-rate range of 0.001–1 s–1). The flow-stress curves of the Mn18Cr18N steel were obtained under different hot-deformation conditions. By establishing the hyperbolic sine-law Zener-Hollomon equation, the hot-deformation activation energy of the Mn18Cr18N steel was obtained. Based on the mechanism of dislocation evolution, a physically-based constitutive model was established. In addition, the expression of the dynamic-recovery coefficient of the model was modified. Compared with the model before the modification, the modified constitutive model could effectively improve the prediction accuracy of the flow stress for the as-cast Mn18Cr18N austenitic stainless steel.