Papers by Keyword: Dynamic Recrystallization

Abstract: The Harmonic Structure (HS) design was implemented in a high-entropy CrMnFeCoNi alloy compact to study its deformation characteristics at elevated temperatures, with particular emphasis on comparison with the homogeneous (Homo) compacts. The HS compact was prepared by powder metallurgy, employing a mechanical grinding process with a planetary ball mill in an argon atmosphere. The rotational speed was set at 150 rpm, and the milling time was either 180 or 360 ks. The resulting powders were then exposed to spark plasma sintering at 1223 K for 1.8 ks under 50 MPa. Subsequently, the compacts were subjected to high-temperature compression tests at 1073 K or 1173 K, at varying initial strain rates over a range of temperatures. These tests were conducted after the sintering process was completed. Homo exhibited a work hardening at the initial stage of deformation, followed by a slight decrease in flow stress, which then remained nearly constant. In contrast, HS exhibited a distinctive softening in flow stress following initial work hardening. A thorough examination of the microstructure during the softening process revealed that adjacent Shell/Core units caused grain boundary sliding in the Shell region. Furthermore, each Core exhibited a rotation of approximately 2.3 degrees and a lateral displacement of 1.5 μm. Observation of the softening phenomenon during high-temperature deformation was confirmed through TEM analysis, revealing that this softening resulted from dynamic recrystallization within the Shell region. Consequently, dynamic recrystallization in the Shell was postulated, followed by rotation of the Shell-Core unit through grain boundary sliding of the UFG structure.

Abstract: Ultra-fine-grained (UFG) Al alloys have excellent mechanical properties such as high tensile strength without remarkable loss of elongation. Severe plastic deformation (SPD) process is an effective method for obtaining UFG microstructure. SPD-processed Al alloys has extremely high strength than the extrapolated from Hall-Petch relationship due to their microstructure with residual excess strain after dynamic recrystallization. Especially, on account of Al alloys have high stacking fault energy, the dislocation rearrangement in the dynamically recrystallized grain is difficult to form high angle grain boundary. As a result, there are substantial dislocation wall and low-angle grain boundary after SPD processing. These dislocations remain in the grain after recrystallization and partially form low-angle grain boundaries and subgrain boundaries. Consequently, the strength increases from Hall-Petch relationship, which is the degree of extra-hardening, was measured up to 200 MPa in as-SPD processed Al-3%Mg alloy. The authors previously reported that the low-angle grain boundaries distributed in the microstructure after the repetitive equal-channel angular extrusion processing. The strength difference calculated by Bailey-Hirsch equations was not in accord with measured extra-hardened strength. In this study, the effect of grain boundary distributions on the extra-hardening was investigated by changing SPD-processing and subsequent annealing conditions.

Abstract: Magnesium alloys have spurred a strong interest in automobile and aerospace industries owing to their high specific strength and stiffness, with magnesium being the lightest structural metal. Alloying with rare earth improves tensile properties considerably. However, the availability of rare-earth elements is a concern. Therefore, attempts are being made to prepare alloys without rare-earth elements. One Mg-1Ca alloy was prepared with different amounts of Sn to study the impact on the high-temperature strength after dynamic recrystallization by hot rolling. Optical and electron microscopy analyzed the change in the microstructure. XRD and EDS were used to identify phases and composition of different microconstituent particles and high-temperature strength was measured at 250°C, 300°C, and 350 °C under 2 x 10^-4 s ^-1 strain rate and at 3000C, 3500C, 4000C, and 4500C temperature under 5 x 10 ^-4 s ^-1 strain rate. According to XRD investigation, the alloys essentially comprise the Ca-containing phase and Mg₂Sn particles. The Mg-1Ca-1Sn alloy exhibited maximum high-temperature strength at 250°C, attributed to the maximum amounts of MgCaSn particles. It was also found that dynamic recrystallization was accelerated by particle-stimulated nucleation and maximum refinement was found at 1% Sn-containing alloys.

Abstract: The combination of advection and migration of grain boundaries is analyzed on the basis of a simple mesoscale model, where parallelepipedic grains are considered under uniaxial compression straining. Strain hardening and dynamic recovery are described by the classical Yoshie-Laasraoui-Jonas equation. Grain-boundary migration is driven by the difference in dislocation densities between one representative grain and the average over the material. Finally, nucleation is assumed to occur at grain boundaries. Special attention is paid to the aspect ratio, which starts from unity (infinitely small cubic nucleus) and tends to zero when the grain disappears. In spite of the role of migration, the average shape of the grains is determined as a first approximation by their lifetimes.

Abstract: Magnesium alloys are highly desirable for weight critical applications owing to their high weight to strangth ratio. However, their poor formability at room temperature limits their widespread use in industrial applications. In this study, we invstigate the hot deformation behaviour of AZ31 and AZ31-0.7% Ca magnesium alloys and explore their microstructural and thermal properties. Our findings reveal that dynamic recrystallization during hot deformation leads to successful grain refinement in the AZ31 alloy, resulting in a normal grain size distribution. In contrast, the AZ31-0.7% Ca alloy shows bimodal grain size distribution due to the addition of calcium. Additionally, the number and size of β-Mg₁₇Al₁₂ particles were found to increase with the addition of a small amount of calcium. These particles are responsible for the discontinuous precipitation phenomenon, which strongly influences microstructural changes during hot rolling. Our study provides valuable insights into the dynamic recrystallization and discontinuous precipitation phenomena of magnesium alloys, which can aid in the development of novel alloys with improved formability and mechanical properties.

Abstract: Today environmental aspects are of great importance in the sustainability of the planet, in this aspect anti-corrosive treatments facilitate the durability of metal structures. Among the most widely used anticorrosive metals is Zinc and its alloys. In the deep galvanizing process of large steel structures, tanks containing Zinc in a molten state at a temperature of 460 °C are necessary. Then, to protect elements that are too large or that need to be treated "in situ", metallization is used, which consists of projecting molten zinc wire on the metal surface that has previously been subjected to a process sandblasting (mechanical abrasion). The two main methods of metalizing are electric arc and flame. In the present work an industrial wiredrawing draft has been studied, determining the drawing force and the power required in each stage. For this purpose, linear strain hardening model vs non-linear strain hardening model that takes strain rate hardening into account has been proposed for its implementation in the analytical model of the process and finite element model (FEM) has been developed too. The use of Hall Petch equation has been allowed to get a prediction of the evolution of the grain size during the wiredrawing sequence.

Abstract: A novel type of titanium alloy was investigated in this article. The microstructure and mechanical properties of Ti-3573 and Ti-3873 titanium alloys were observed and measured after 20% hot deformation, respectively. The results shown that both titanium alloys occurred dynamic recrystallization (DR) during hot deformation. The tensile strength of Ti-3873 titanium alloy was better than that of Ti-3573 titanium alloy. Both titanium alloys have fine secondary α-phase which appeared granular or acicular near the β-grain boundaries or within the β-grain after hot deformation. Better tensile strength (847 MPa) of Ti-3873 titanium alloy is contributed to the higher content of β-phase stable element (Mo). The improvement plasticity (12 %) of Ti-3573 titanium alloy is due to the occurrence of dynamic recrystallization during hot deformation.

Abstract: ECAP is a continuous multi-pass extrusion process that enables the specimen to obtain considerable cumulative deformation to refine the grain. In this paper, ECAP was used to deform AS41 magnesium alloy at 350°C, and the microstructure was observed and analyzed. The results show that the ECAP process has excellent effect on grain refinement and uniform microstructure. The grain size of AS41 decreases from 200μm to 20μm, and the microstructure is more uniform than that of as-cast sample. The reason is that the original grain is broken and refined under the action of shear force, and dynamic recrystallization occurs at the same time, resulting in small recrystallized grains. The Mg₂Si particles were redistributed during ECAP and uniformly distributed in the crystal in rod shape.

Abstract: In this paper, the high temperature flow behaviors of 6061 Al alloy was studied by thermal compression experiments. The effects of temperature, strain rate and strain on the microstructure evolution and flow behavior of the alloy were investigated by experiments. The results show that the flow stress of the alloy increases with the increase of strain rate and it decreases with the increase of deformation temperature. The flow curve reaches the dynamic equilibrium under the interaction of work hardening and dynamic softening mechanism. The uprising deformation temperature promotes thermal excitation dynamic recrystallization of deformed microstructure. With the increase of strain, the microstructure of the alloy is transformed from equiaxed crystal morphology to fibrous structure and strain-induced dynamic recrystallization occurs. As strain rate increases, the action time of dynamic softening mechanism for the studied alloy is reduced, resulting in the fraction of dynamic recrystallized structure is reduced and the flow stress increases.

Abstract: Some feature of discontinuous dynamic recrystallization (DRX) in an Fe-0.4%C-18%Mn austenitic steel during isothermal compression tests at temperatures of 973-1373 K and strain rates of 10^-3-10^-1 s^-1 were studied. The DRX microstructures consisted of various grains, i.e., DRX nuclei, growing DRX grains, and work-hardened DRX grains, which differentiated with the grain orientation spread (GOS). DRX was commonly promoted by a decrease in temperature-compensated strain rate, i.e., Zener-Hollomon parameter (Z), corresponding to an increase in deformation temperature and/or a decrease in strain rate. In contrast, the GOS distribution varied non-monotonously with Z. The large area fraction of DRX grains with small GOS below 1° appeared at definite temperature/strain rate conditions. The large fraction above 0.6 of DRX grains with small GOS was observed in DRX microstructures with a large ratio of CSL Σ3 boundary fraction to low-angle subboundary fraction. The GOS distribution in the DRX microstructures is discussed in terms of the DRX grain nucleation and growth rates.