Mechanical properties and microstructure evolution in ultrafine-grained AZ31 alloy processed by severe plastic deformation

Commercial MgAlZn alloy AZ31 was processed by two techniques of severe plastic deformation (SPD)—extrusion followed by equal channel angular pressing (EX-ECAP), and high pressure torsion (HPT). Processing by ECAP was conducted at elevated temperature of 180 °C for 1–12 passes following route B_C. HPT was applied at room temperature, and the specimens of the diameter of 19 mm with different number of turns (N = ¼ ? 15) were prepared. Mechanical properties and grain fragmentation with strain due to EX-ECAP and HPT were investigated by Vickers microhardness measurements and transmission electron microscopy, respectively. Variations in dislocation density were investigated by positron annihilation spectroscopy. Differences in microhardness, grain refinement and dislocation density evolution resulting from principal differences of straining were found in the specimens. EX-ECAP resulted in homogeneous microstructure throughout the specimen's cross section as early as after four passes. On the other hand, laterally inhomogeneous microstructure with gradual reduction of grain sizes from the centre towards the periphery of the disk was observed in specimens after HPT. This microstructure and microhardness inhomogeneities were continuously smeared out and almost homogeneous ultrafine-grained structure was observed in specimen subjected to 15 HPT turns. Variations in mechanical properties and dislocation density evolution were compared in conditions corresponding to the same equivalent strain imposed by both techniques of SPD.     

Influence of Pressing Temperature on Microstructure Evolution and Mechanical Behavior of Ultrafine‐Grained Cu Processed by Equal‐Channel Angular Pressing

Pure Cu was processed by ECAP at five different temperatures from room temperature (RT) to 523 K. The influence of pressing temperature on microstructure evolution and tensile behavior was investigated in detail. The results show that as the ECAP temperature is increased the grain size and ductility both increase whereas the dislocation density and yield strength decrease. In the case of ECAP processing in the range of RT to 473 K the mechanism governing microstructural refinement is continuous dynamic recrystallization (CDRX), whereas at 523 K the mechanism changes to discontinuous dynamic recrystallization (DDRX). At higher ECAP temperatures, the kinetics of CDRX are retarded leading to a lower fraction of equiaxed grains/high‐angle grain boundaries and a higher fraction of dislocation cell structures. At 523 K, DDRX induces a high fraction of equiaxed grains with a very low dislocation density which appears responsible for the observed high tensile ductility. The sample processed at 523 K possessed a good combination of strength and ductility, suggesting that processing by ECAP at elevated temperatures may be a suitable alternative to RT ECAP processing followed by subsequent annealing.     

Multiscale modeling of back-stress evolution in equal-channel angular pressing: from one pass to multiple passes

Fine-grained materials produced by equal-channel angular pressing (ECAP) exhibit kinematic hardening due to the existence of a back-stress. This article presents a new dislocation-based model, which is able to describe the tension/compression asymmetry of the ECAP processed commercial purity aluminum. By introducing strain relaxation, and relating the back-stress to the inhomogeneous dislocation density distribution in cell walls and in cell interiors, the model can accurately predict the evolution of the dislocation densities, the cell size, and the back-stress. Compared to the other back-stress models, it takes into account the microstructure evolution and gives a better prediction.     

Analysis of strengthening mechanisms in a severely-plastically-deformed Al–Mg–Si alloy with submicron grain size

Methods of severe plastic deformation of ductile metals and alloys offer the possibility of processing engineering materials to very high strength with good ductility. After typical amounts of processing strain, a submicrocrystalline material is obtained, with boundaries of rather low misorientation angles and grains containing a high density of dislocations. In the present study, an Al–Mg–Si alloy was severely plastically deformed by equal channel angular pressing (ECAP) to produce such a material. The material was subsequently annealed for dislocation recovery and grain growth. The strength of materials in various deformed and annealed states is examined and the respective contributions of loosely-arranged dislocations, many grain boundaries, as well as dispersed particles are deduced. It is shown that dislocation strengthening is significant in as-deformed, as well as lightly annealed materials, with grain boundary strengthening providing the major contribution thereafter.     

铝合金热变形组织演化的模拟

采用基于位错攀移及位错间交互作用的多元位错模型,对Al-Mg-Si(6005)合金热变形过程中亚晶内位错密度,亚晶界位错密度,可运位错位错密度及总的位错密度的演化进行了计算机模拟,并由此对Al-Mg-Si合金热压缩过程中的流变应力演化进行了预报,模拟结果和试验结果吻合较好,采用试验研究的方法建立了亚晶尺寸随变形条件演化的半经验关系模型,为铝合金热变形组织演化的预测和控制提供了实用模型。     

Exceptional superplasticity in an AZ61 magnesium alloy processed by extrusion and ECAP

Experiments were conducted on a commercial AZ61 alloy to evaluate the potential for achieving an ultrafine grain size and superplastic ductilities through the use of the EX-ECAP two-step processing procedure of extrusion plus equal-channel angular pressing. The results show that EX-ECAP gives excellent grain refinement with grain sizes of 0.6 and 1.3 μm after pressing at 473 and 523 K, respectively. The alloy processed by EX-ECAP exhibits exceptional superplastic properties including a maximum elongation of 1320% after pressing through four passes when testing at 473 K with an initial strain rate of 3.3 × 10⁻⁴ s⁻¹. This result compares with an elongation of 70% achieved in the extruded condition without ECAP under similar testing conditions.     

Grain boundary sliding and accommodation mechanisms during superplastic deformation of ZK40 alloy processed by ECAP

Controlling mechanism during superplastic deformation of ZK40 alloy processed by ECAP was identified. Effects of twinning and dynamic strain ageing (DSA) on superplasticity were analyzed. Amplitude in stress oscillation was correlated with solute atom concentration theoretically. Twinning can be an enhancing factor in grain boundary sliding and DSA had apparent influence on stress fluctuation; they were accommodation mechanisms for superplastic deformation through grain reorientation and interaction between solute atoms and dislocations, respectively. The interaction between mobile and forest dislocations played a dominant role for the occurrence of DSA, when dislocation density was relatively low in large grains. The effect of DSA became more active with increasing temperature, although grain boundary sliding (GBS) was the controlling mechanism throughout the whole process of superplastic deformation under elevated temperatures.     

The influence of cryoECAP on microstructure and property of commercial pure aluminum

The microstructure and mechanical property of a commercial pure aluminum (1050) processed by equal channel angular pressing (ECAP) at cryogenic temperature, to an equivalent strain up to 8, was studied by hardness testing and transmission electron microscopy (TEM). The results show that the dynamic recovery is restrained partially by ECAP at cryogenic temperature (cryoECAP). High density dislocations still can be observed inside some grains after eight cryoECAP passes, which obviously differs from the microstructure produced by ECAP at room temperature (RT-RCAP). The peak hardness of cryoECAPed pure aluminum is 28% higher than that of room temperature ECAPed pure aluminum.     

Evolution of microstructure of an iron aluminide during severe plastic deformation by heavy rolling

Severe plastic deformation is generally achieved using novel techniques such as Equal Channel Angular Pressing (ECAP) or High Pressure Torsion (HPT), but may also be achieved by more conventional methods such as very heavy rolling. Microstructure evolution is examined in an iron aluminide intermetallic rolled to strains up to 3.3 using Transmission Electron Microscopy (TEM) and orientation determinations by Kikuchi line analysis. After the highest strains the microstructure is still characterized as a recovered submicron-scale dislocation structure, with generally low angles across the various boundaries, and a high density of dislocations inside these boundaries. The structures observed show a dependence on orientation of the underlying parent grain, with [001] orientations showing poorer rearrangement to cellular structures than grains with [113–111] orientations.     

Structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm Si (111) by ammonia molecular beam epitaxy

The structural properties of GaN grown on AlGaN/AlN stress mitigating layers on 100-mm diameter Si (111) substrate by ammonia molecular beam epitaxy have been reported. High resolution X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy and secondary ion mass spectroscopy have been used to study the influence of AlN thickness and AlGaN growth temperature on the quality of GaN. GaN grown on thicker AlN showed reduced dislocation density and lesser tensile strain. Three-dimensional growth regime was observed for GaN grown at lower AlGaN growth temperature while higher AlGaN growth temperature resulted in two-dimensional growth mode. The dislocation bending and looping at the AlGaN/AlN interface was found to have significant influence on the dislocation density and strain in the GaN layer. The evolution and interaction of threading dislocations play a major role in determining the quality and the strain states of GaN.     

Impact Toughness of Ultrafine‐Grained Commercially Pure Titanium for Medical Application

This study aims at achieving the best combination of strength, ductility, and impact toughness in ultrafine‐grained (UFG) Ti Grade 4 produced by equal‐channel angular pressing via Conform scheme (ECAP‐C) with subsequent cold drawing. UFG structures with various parameters (e.g., size and shape of grains, dislocation density, conditions of boundaries) are formed by varying the treatment procedures (deformation temperature and speed at drawing, annealing temperature). The tensile and impact toughness tests were performed on samples with a V‐shaped notch and different structures of commercially pure Ti Grade 4 in the coarse‐grained and UFG states. The results demonstrated that grain refinement, higher dislocation density, and their elongated shape were obtained as a result of drawing at 200 °С, which led to a decrease in both the uniform elongation at tension and the impact toughness of Ti Grade 4. Short‐term annealing at 400–450 °C could improve the impact toughness of UFG Ti with a non‐significant decrease in strength. This short‐term annealing contributes to the dislocation density decrease without considerable grain growth as a result of the recovery and redistribution of dislocations. The dependence of impact toughness on the strain hardening ability of UFG Ti was discussed.

     

Coupling kinetic dislocation model and Monte Carlo algorithm for recrystallized microstructure modeling of severely deformed copper

By coupling a kinetic dislocation model and Monte Carlo algorithm, the recrystallized microstructure of severely deformed Oxygen Free High Conductivity Copper (OFHC) is predicted at different strains imposed by Equal-Channel-Angular-Pressing (ECAP) and annealing temperatures. From a flow field model, the strain rate distribution during the ECAP of the material in a curved die is calculated. Then using the kinetic dislocation model, the total dislocation density and correspondingly the stored energy after each ECAP pass is estimated. Utilizing the Monte Carlo algorithm and the stored energy, the recrystallized microstructure is predicted. The results show that the recrystallized grain size is decreased rapidly from the strain of first to fourth pass and then it is decreased slowly. Also, it is achieved that with increasing the annealing temperature, the grain size is increased. Moreover, a good agreement is observed between the predicted results and experimental data.     

Stability of the ultrafine-grained microstructure in silver processed by ECAP and HPT

The high-temperature thermal stability of the ultrafine-grained (UFG) microstructures in low stacking fault energy silver was studied by differential scanning calorimetry (DSC). The UFG microstructures were achieved by equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) at room temperature (RT). The defect structure in the as-processed samples was examined by electron microscopy and X-ray line profile analysis. The stored energy calculated from the defect densities was compared to the heat released during DSC. The sum of the energies stored in grain boundaries and dislocations in the ECAP-processed samples agreed with the heat released experimentally within the experimental error. The temperature of the DSC peak maximum decreased while the released heat increased with increasing numbers of ECAP passes. The released heat for the specimen processed by one revolution of HPT was much smaller than after 4–8 passes of ECAP despite the 2 times larger dislocation density measured by X-ray line profile analysis. This dichotomy was caused by the heterogeneous sandwich-like microstructure of the HPT-processed disk: about 175 μm wide surface layers on both sides of the disk exhibited a UFG microstructure while the internal part was recrystallized, thereby yielding a relatively small released heat.     

Microstructure and mechanical properties of Cu and Cu-Zn alloys produced by equal channel angular pressing

Ultrafine-grained (UFG) Cu and Cu-Zn alloy were prepared using equal-channel angular pressing (ECAP) to investigate the effects of stacking fault energy (SFE) on microstructure evolution and mechanical properties. Combining with the previous researches, the grain refinement process of ECAP is divided into three stages based on the variation of tensile strength and plasticity. According to the influences of defects on strength and ductility during plastic deformation, the three stages are discussed in detail by considering the dislocation density, grain and twin boundaries. Besides, the impact of SFE on the strength and ductility of the UFG Cu-Zn alloys are evaluated, indicating that these two mechanical properties can be improved simultaneously in the whole ECAP process either through slightly or widely adjusting the SFE. This significant effect of SFE reflects in two aspects, one is in the microstructure evolution during ECAP processing and the other is in the subsequent tensile plastic deformation, both of which can be achieved through regulating the dislocation motion via changing the SFE.     

基于应变梯度位错理论的纳晶-无定形态层状复合材料的力学性能研究

纳晶-无定形态层状复合材料具有高强度高硬度还有良好的韧性等力学性能。为了定量评估这种新型材料的力学性能,建立了基于位错密度和应变梯度的力学模型。即在塑性变形的初始阶段,位错在晶态-非晶态层的交界面处产生,之后通过滑移穿过纳晶层到达对面的晶态-非晶态的交界面处被吸收。鉴于纳晶层和无定形态层的不同特性和界面的相互作用,理论分析了纳晶层中的位错演化过程以及交界面的损伤情况。结果表明,理论计算与实验结果基本吻合,且纳晶层无定形态之间的交界面具有很好的变形协调能力而不会在界面层产生失效。     

Recent developments in modelling of microhardness saturation during SPD processing of metals and alloys

Ultrafine-grained and even nanostructured materials can be fabricated using severe plastic deformation to ultra-high strains in equal-channel angular pressing (ECAP), high-pressure torsion (HPT), machining and their combinations, such as machining of ECAP specimens, HPT of ECAP billets and HPT of machining chips. This report presents recent results of investigations of the microstructures and microtextures of pure copper, nickel and aluminium subjected to different deformation processes to ultimately high imposed strains. A comparison of the microstructure, dislocation density and microhardness developed during combinations of different strain paths is performed. All characteristics were analysed by X-ray, transmission and scanning electron microscopy, and electron backscatter diffraction (EBSD). The influence of different processing routes is discussed in terms of the accumulated strain and microstructure refinement. The saturation in grain refinement is examined with reference to the recovery taking place during ultra-high strain deformation. A phenomenological model based on the Voce equation is applied for fitting parameters based on the experimental data and this is suggested for a prediction of microhardness evolution for pure metals (Ag, Au) and Cu-based (Zn, Al) alloys.     

Diffraction Line Broadening Analysis if Broadening Is Caused by Both Dislocations and Limited Crystallite Size

The determination of dislocation distribution parameters is discussed for specimens where both strain broadening caused by dislocations and size broadening occur. If the strain broadening is well described by a model due to Wilkens, several methods are possible for the analysis of the broadening of diffraction lines. In sputter deposited nickel layers, three different methods for diffraction line broadening analysis yield identical results. The recrystallization of the nickel layers was investigated by annealing the layers at various temperatures in the range 300 K to 500 K. With increasing annealing temperature, the microstructure of the layers changed from a microstructure with small grains and high dislocation density, via a microstructure that is a mixture of small grains with high dislocation density and large grains with low dislocation density, to a microstructure with large grains and low dislocation density.     

Ultrafine-grained Ti–Nb–Ta–Zr alloy produced by ECAP at room temperature

To ascertain the influence of severe plastic deformation (SPD) on a Ti–Nb–Ta–Zr (TNTZ) alloy, we studied the room temperature mechanical behavior and microstructural evolution of an ultrafine-grained (UFG) Ti–36Nb–2Ta–3Zr (wt%) alloy prepared via equal-channel angular pressing (ECAP) of the as-hot-extruded alloy. The tensile behavior, phase composition, grain size, preferred orientation, and dislocation density of the UFG alloy, processed under different conditions, were analyzed and discussed. Compared to the as-hot-extruded alloy, the ECAP-processed TNTZ alloy (3 passes) exhibited approximately 40 and 88 % increase in average ultimate strength and yield strength, respectively. Moreover, as the number of ECAP passes increased from 3 to 6, the TNTZ alloy exhibited not only the expected increase in ultimate and yield strength values, but also a slight increase in elongation. Our results suggest that the deformation mechanisms that govern the behavior of the as-hot-extruded coarse grained (CG) TNTZ alloy during ECAP involve a combination of stress-induced martensitic transformation and dislocation activity. In the case of the ECAP-processed UFG TNTZ alloy, the deformation mechanism is proposed to involve two components: first, dislocation activity induced by the strain field imposed during ECAP; and second, the formation of α″ martensite phase during the early stages of ECAP which eventually transforms into β phase during continued deformation. We propose that the deformation mechanism governing the room temperature behavior of the TNTZ alloy strongly depends on the grain size of the β phase.     

A hybrid model on severe plastic deformation of copper

A hybrid model based on the flow function and dislocation cell structure model by considering the Taylor assumption is utilized to model the dislocation structure evolution, cell size and mechanical properties of OFHC in severe plastic deformation. Here, the ECAP is chosen as a process of severe plastic deformation. In this study, the model is modified by taking the value of cell size coefficient as a function of strain and considering two different values of dynamic recovery coefficients of cell walls and cell interiors. These modifications lead to achieve the more accurate modeling results. From the flow function the strain rate distribution are achieved and then using the model the dislocation density, cell walls thickness, walls fraction, cell size, and strength are predicted. The predicted cell size and strength are compared with the experimental data and a remarkable agreement is obtained.     

Corrosion and stress corrosion cracking behavior of equal channel angular pressed oxygen-free copper in 3.5% NaCl solution

In the present study, the corrosion and stress corrosion cracking (SCC) behavior of equal channel angular pressed (ECAPed) oxygen-free copper was examined in 3.5% NaCl solution. The specimen with two ECAP process cycles was found to have the greatest resistance to corrosion and SCC in 3.5% NaCl solution among the specimens studied, while the resistance varied in a complex manner with different number of ECAP process cycles. Micrographic observation and differential scanning calorimeter study suggested that both corrosion and SCC behavior of ECAPed copper was strongly dependent on the microstructural evolution, including the change in grain size and dislocation density, during ECAP process.