Ramin Ebrahimi

Objectives. The aim of this work is to investigate the corrosion resistance of ultrafine-grained magnesium alloy Mg–1%Zn–0.2%Ca processed by severe plastic deformation (SPD) methods, namely equal channel angular pressing combined with extrusion (ECAP-Ex) and circular simple shear extrusion (CSSE). In order to use such materials in the development of biocompatible implants, it becomes necessary to control their degradation rate. Methods. To form an ultrafine-grained (UFG) structure, equal-channel angular pressing combined with extrusion (ECAP+Ex) and circular shear extrusion (CSSE) methods were used. Structural studies were conducted using X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In order to confirm the reliability of this material for use in biodegradable medical implants, a gravimetric corrosion resistance study was carried out at 36.6°C over a long period (month). Results. The use of ECAP-Ex processing led to the formation of an ultrafine-grained (UFG) structure in the alloy having a grain size of about 1 μm and the appearance of nanotwins, as well as the formation of highly dispersed Ca ₂ Mg ₆ Zn ₃ nanoparticles 10 nm in a size with a volume fraction of 1%. The corrosion rate of the UFG samples was 3.8 mm/year. Following additional annealing at 175°C, the UFG alloy demonstrated a decrease in the corrosion rate to 0.9 mm/year as compared with coarse grained homogenized sample (4.8 mm/year). By way of comparison, the CSSE processing resulted in grain size reduction to 1 μm and a corrosion rate of 1.3 mm/year. Conclusions. The study demonstrates that combining equal-channel angular pressing ECAP+ Ex and subsequent annealing at 175°C is an effective method for improving corrosion properties of the Mg–1%Zn–0.2%Ca alloy. By reducing the corrosion rate to 0.9 mm/year, the alloy becomes promising for medical applications such as biodegradable implants.

Discontinuous dynamic recrystallization is a critical microstructural evolution mechanism during high-temperature deformation, influencing material properties significantly. This study develops a two-dimensional phase-field model to predict steady-state creep rates in the AZ31 magnesium alloy, focusing on DRX during creep. To enhance simulation accuracy, initial microstructures are generated from optical microscopy data, enabling simulations at larger scales with higher representativeness. A novel nucleation methodology is implemented, eliminating the need for nuclei order parameter adaptation, improving computational efficiency. Finite element analysis (FEA) is integrated to capture initial instantaneous deformation. The Kocks–Mecking model is employed to describe the evolution of average dislocation density, accounting for work hardening and dynamic recovery within the initial polycrystalline microstructure. Instead of conventional creep testing, impression creep, a cost-effective alternative, is used for validation. This method provides constant stress and steady penetration velocity, simulating creep conditions effectively. The model accurately predicts recrystallization kinetics and microstructural evolution, exhibiting a strong correlation with experimental results, with an error of approximately 5%. This research provides a robust and efficient approach for predicting creep behavior in high-temperature applications, vital for optimizing material selection and predicting component lifespan in industries. The methodology offers a significant advancement in understanding and predicting DRX-driven creep behavior.

Knowledge of alloy recrystallization is key to optimizing microstructures and achieving superior material properties. Computational models predicting microstructural evolution during recrystallization significantly enhance control of microstructure formation during manufacturing. Accurate prediction of microstructural parameters, including recrystallization fraction and grain size, is highly desirable. However, developing robust recrystallization models under various processing conditions remains an active research area. Herein, using interstitial free‐steel for simulations and experiments, plastic deformation of polycrystalline material is simulated using a physics‐based crystal plasticity model. A real microstructure serves as the initial configuration. The resulting inhomogeneous dislocation density distribution and deformed grain topology are used in a multi‐phase‐field simulation of recrystallization. In primary recrystallization, nucleation strongly influences kinetics and the final microstructure. In the model, the dislocation density distribution predicts both the number and positions of nuclei. Comparing simulations—one considering the dislocation density distribution in both nucleation and evolution and the other assuming constant dislocation density and random seed positioning—demonstrates the importance of heterogeneous dislocation distribution. Results confirm that static recrystallization simulations, accurately reflecting plastic deformation and utilizing the dislocation density distribution as the driving force for grain growth and nucleation, can be successfully performed using the proposed model.

In this investigation, the previously proposed phenomenological model in the literature is generalized both in terms of the mathematical form of the model and the yield function used to describe the plasticity of the material. Al 1050 is chosen as the model material, where the sheets made from this material are first annealed and then subjected to a tensile test and Erichsen cupping test to obtain tensile properties as well as the FLC of the material. The constants of the generalized model are first obtained by curve fitting, whereby in this approach least overall error is expected as a single equation is used to predict the FLC of material. Further, it is shown that it would be possible to enhance the accuracy of the model at the cost of losing the applicability of a single mathematical expression for both branches of FLC. In this approach, the generalized model would be calibrated for the right branch based on Swift’s model and for the left branch based on Hill’s model. Finally, the effect of the yield criterion used to describe the plasticity of the material on the predictions of the generalized model is investigated, and it is shown that using the Hosford yield criterion yields better results compared to using the von Mises yield criterion.