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A series of hot compression tests of medium carbon Cr–Ni–Mo-alloyed steel, 34CrNiMo steel, were conducted on a Gleeble-1500 thermal mechanical simulator, in a wide temperature range of 1173–1423 K and at a strain rate range of 0.002–5 s−1. Three constitutive models, namely the Johnson–Cook (JC) model, strain compensated Arrhenius model, and the physically based constitutive model, were established to describe the hot deformation of 34CrNiMo steel. A comparative study of the three models was investigated by comparing the accuracy of prediction of flow stress behavior. The results imply that the JC model is not able to adequately represent the high-temperature flow behavior with the existance of recovery and recrystallization. The Arrhenius-type model based on mathematics has a good prediction in the flow stress behavior in all strain ranges during the hot deformation. The physically based constitutive model gives a better prediction accuracy of the deformation behavior in both flow stress and deformation mechanism.
To investigate the recrystallization behavior of large sized Nb–V microalloyed steel rods during thermomechanical controlled processing (TMCP), a series of isothermal hot compression tests were conducted on a Gleeble 1500 thermomechanical simulator. The kinetics and microstructure evolution models of dynamic recrystallization, static recrystallization, metadynamic recrystallization and the grain growth model of the tested steel were developed. Based on the developed models, a finite element (FE) model coupled with the recrystallization behavior of large sized Nb–V microalloyed steel rods during TMCP was established. Then, the distributions and evolutions of recrystallization fraction and grain size during the whole deformation process are obtained and analyzed. Finally, the predicted results were compared with experimental ones, and they show good agreement. This illustrates that the recrystallization models of the tested steel are valid and the developed FE model of large sized Nb–V microalloyed steel rods during TMCP is effective.
In this work, the morphology of BiFeO3 was successfully modulated from microsphere to microcube by using a polyanion, poly (methyl vinyl ether-alt-maleic acid) (PMVEMA), in a microwave assisted hydrothermal route. A simple ultrasonic purification method has been developed to obtain pure phase BiFeO3 from the crude products without using any chemicals. X-ray diffraction results confirmed the capability of this purification method. When increasing the amount of PMVEMA, the morphology of BiFeO3 gradually changed from microsphere to microcube as illustrated by scanning electron microscopy. A mechanism was suggested for the morphology evolution of BiFeO3. After the formation of the small BiFeO3 single crystal, PMVEMA preferentially absorbed on one side of the crystals through specific and/or noncovalent interactions, resulting in the preferential integration of these crystals to form microcubes. The magnetic properties of these microcrystals were also investigated and the magnetization of the microcubes increased with the decrease of temperature.
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