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The dynamic recrystallization (DRX) behavior of magnesium alloy AZ31B with large initial grain size was investigated. Two methods were applied to establish the DRX kinetics model. One is based on the values of DRX volume fraction evaluated by true stress-strain curves (Model 1). The other is based on the values of DRX volume fraction evaluated using optical micrographs (Model 2). It is found that the predicted DRX volume fraction by Model 1 is significantly larger than that by Model 2. It indicates that the value of DRX volume fraction (Xdrx) cannot be calculated by the true stress-strain curves for the magnesium alloy AZ31B with large initial grain size. To understand the phenomenon, the reason was analyzed and discussed. We found that the main reason can be attributed to the strong fine-grain strengthening (FGS) effect of magnesium alloy AZ31B. For the studied material, the flow behavior will be mainly affected by two factors during DRX: 1) the decrease of average dislocation density which gives rise to the decrease of flow stress; 2) the decrease of average grain size which results in the increase of flow stress. The latter causes that the DRX volume fraction evaluated by the true stress-strain curves is obviously larger than the real one. The research results will be good references for the evaluation of DRX volume fraction of other FGS materials with large initial grain sizes.

It is usually one of important goals to obtain the homogeneous microstructure for forgings by hot working, such as hot die forging. However, there is always a large inhomogeneous deformation in die forgings during hot deformation. It often results in an inhomogeneous microstructure of forging due to the different dynamic recrystallization (DRX) fractions and grain sizes in different strain regions. In this study, a method was proposed to improve the homogeneity of deformed microstructure by annealing treatment. The microstructural evolution of a typical nickel-based superalloy during annealing treatment has been investigated. The Optical Microscope (OM), Electron Backscatter Diffraction (EBSD) and Scanning Electron Microscope (SEM) technologies were applied to observe microstructures. It is found that adopting an annealing treatment after deformation can effectively improve the homogeneity of microstructure due to the static recrystallization. There is an incubation period which is not less than 5 min for the occurrence of static recrystallization. The initial heterogeneous grains become homogeneous firstly, and then heterogeneous again when the annealing time is increased from 5 to 30 min. The deformation accelerates the dissolution of delta phase in annealing treatment. The fraction of delta phase decreases at the early stage of annealing whether the temperature exceeds the solution temperature. Based on a comprehensive consideration of all factors, the optimum annealing parameters, which are the temperature of 980 °C and the time of 10 min, have been obtained.

The metadynamic recrystallization (MDRX) behaviors of a solid-solution GH4169 superalloy were studied by two-pass hot compression tests. The effects of pre-strain (deformation degree of the first pass) on MDRX behaviors were analyzed. Results show that the MDRX behaviors intensely depend on the pre-strain. The MDRX fraction first increases and then drops with the increased pre-strain. This is because a relatively large pre-strain can facilitate the nucleation of dynamic recrystallization (DRX) grains, which promotes the subsequent MDRX behaviors. However, due to the decreased stored deformation energy at relatively high pre-strain, the number of DRX grains cannot continuously increases. Thus, the MDRX fraction is reduced with the further increase of pre-strain. The segmented MDRX kinetics models were proposed and validated to describe the significant strain dependence of MDRX behaviors.

The dynamic recrystallization (DRX) behavior of 42CrMo steel was studied by hot compressive tests and cellular automaton (CA) simulation. The experimental results showed that initial grain size besides deformation parameters has an obvious influence on dynamic recrystallization (DRX) behavior. It is found that the initial grain size not only influences the number of DRX nucleation site, but also the work-hardening (WH) coefficient, dynamic recovery (DRV) coefficient, DRX nucleation rate and grain boundary (GB) mobility. Moreover, the relationships between the WH coefficient, DRV coefficient, DRX nucleation rate, GB mobility and initial grain size can be well described by the power function. Based on the findings, the traditional CA model was extended to describe the effect of initial grain size on DRX behavior. The comparison between experimental and simulated results showed that the microstructural evolution of 42CrMo steel with different initial grain sizes during hot deformation can be accurately simulated by the extended CA model.

In this study, a 2D Cellular automaton (CA) model was developed to visually and quantitatively predict the microstructural evolution of magnesium alloy AZ31B during hot deformation. Hot compressive and metallographic tests were carried out to evaluate the CA model parameters including the work hardening (WH) coefficient, dynamic recovery (DRV) coefficient, nucleation rate and grain boundary mobility. Firstly, the values of CA model parameters under tested conditions were identified based on the experimental true stress-strain curves. Then, the functional relationships between the model parameters and deformation conditions (deformation temperature and strain rate) were established. Using the developed CA model, the flow behaviors and microstructural evolution of magnesium alloy AZ31B during hot deformation were simulated and discussed. The agreement between CA simulated and experimental results indicates that the developed CA model can accurately predict the microstructural evolution of magnesium alloy AZ31B. In addition, a new method to calculate the CA simulated average grain size was proposed. The new proposed method can effectively decrease the effects of DRX nuclei on the calculated value of average grain size.

The microstructural evolution of an aged Ni-based superalloy under two-stage hot compressive deformation with different strain rates is investigated. In terms of strain rate, the hot compressive tests include two types: constant and stepped strain rates. For the first type, the strain rate remains constant during the whole hot compression, while for the second type, the hot compression process consists of two stages: I and II. Moreover, the strain rate of stage I (0.01, 0.1, and 1 s−1) is larger than that of stage II (0.001 s−1). It is found that the strain rate and strain of stage I greatly affect the microstructural evolution for the hot compressive deformation with stepped strain rates. In the tested strain rate and strain, both the dissolution rate of δ phase and the dynamic recrystallization (DRX) volume fraction increase with the decrease of strain rate or the increase of strain in stage I. Compared with the constant strain rate hot deformation, the stepped strain rates can obviously promote DRX behaviors and the dissolution of δ phase if the appropriate strain rate and strain of stage I are chosen. These findings can be directly applied in the practical industrial production of superalloy components. Keywords: Hot deformation, Alloy, Dynamic recrystallization, Microstructural evolution

The classical dynamic recrystallization (DRX) kinetics models, such as Avrami equation, are often used to describe the DRX behaviors of alloys. However, it is found that the classical DRX kinetics models cannot be directly applied to evaluate DRX volume fractions under inconstant deformation conditions, such as at fluctuant deformation temperature and strain rate. It obviously limits their application in the practical industrial production. Therefore, an improved DRX kinetics model is proposed based on the hypothesis that the derivatives of DRX volume fraction with respect to strain only depends on the current deformation temperature, strain rate, and DRX volume fraction. To verify the improved DRX kinetics model, the hot compressive tests in which the strain rate is inconstant are carried out on a solution-treated Ni-based superalloy. Experimental results indicate that the improved DRX kinetics model can well predict the DRX behavior under inconstant deformation conditions.