Plastic Deformation Of Magnesium Alloy Research Articles

Термическая стабильность микроструктуры сплава MG-Y-ND в экструдированном состоянии

Introduction. Today, bioresorbable magnesium alloys possessing the required physical, mechanical, corrosion, and biological properties, are promising materials for orthopedic and cardiovascular surgery. The addition of rare earth elements such as yttrium, neodymium, and cerium to magnesium alloys improves its properties. Compared to widely used titanium alloys, magnesium alloys have a number of advantages. Bioresorbable materials slowly dissolve in the body, and recurrent operation to remove the implant is not needed. Biocompatible magnesium alloys have a fairly low elastic modulus (10 to 40 GPa), approaching to that of cortical bone, that reduces the contact stress in the bone-implant system. At the same time, strength properties of magnesium alloys alloyed with rare earth elements do not always meet the requirements for medical applications. Severe plastic deformation, for example, equal channel angular pressing, torsion under quasi-hydrostatic pressure, uniaxial forging, extrusion, is therefore very promising technique to gain the high level of mechanical properties of metals and alloys. Severe plastic deformation of magnesium alloys improves its structural strength by 2.5 times due to the generation of an ultrafine-grained and/or fine-grained structure. The issues related to the study of heat resistance, structure and phase composition of magnesium alloys with appropriate strength are relevant. Purpose of the work is to determine the influence of thermal effects on the microstructure of the extruded Mg-Y-Nd alloy. Methodology. The extruded Mg-2.9Y-1.3Nd alloy (95.0 wt. % Mg, 2.9 wt. % Y, 1.3 wt. % Nd,  0.2 wt. % Fe,  0 wt. % Al) is investigated in this paper. The thermal stability of the alloy microstructure is studied after annealing at 100, 300, 350, 450 and 525 °С in argon for one hour. The microstructure and phase composition are investigated using optical, transmission and scanning electron microscopes and analyzed on an X-ray diffractometer. Results and discussion. The extruded Mg-2.9Y-1.3Nd alloy has the bimodal fine-grained microstructure. It is found that along with the stable α-Mg phase, the alloy structure consists of Mg24Y5 intermetallic particles and -, -, and 1-phase precipitates. Annealing in the temperature range of 100–450 °С for one hour has no effect on the structure of the Mg-2.9Y-1.3Nd alloy, but promotes the growth in the linear dimensions of -, -, and 1-phases precipitates. In the temperature range of 300–450 °С, the morphology of -, ,- and 1-phases changes, while the average grain size of the major -phase remains unchanged. Annealing at 525 °С leads to a notable transformation of the bimodal microstructure of the alloy, which is associated with the intensive growth in the grain size of the -phase, Mg24Y5 particles, and -, -, and 1-phases precipitates. Annealing in the temperature range of 100–450 °C leads to an increase in the linear dimensions of Mg24Y5 particles, -, -, and 1-phases precipitates and bimodal microstructure of the Mg-2.9Y-1.3Nd alloy remains unchanged.

Dislocation evolution mechanism of plastic deformation process of AZ31 magnesium alloy with different grain size

The evolution of grain size and dislocation has a great influence on the plastic deformation of the material. The variation of dislocation lines in the deformation process of magnesium alloys with different grain size was studied by molecular dynamics(MD) simulation technology combined with transmission electron microscope(TEM) experimental methods. The decrease in the size of grains will lead to an increase in the dislocation density during the plastic deformation of the magnesium alloy, and the dislocation line will become tangled. This provides a new means and theoretical basis for studying the plastic deformation of AZ31 magnesium alloy.

Joining magnesium and aluminum alloy sheets by a novel hole hemming process

The use of magnesium alloys represents a novel trend in the automotive industry, being used to manufacture high performance vehicle structures. These alloys possess the lowest density among all structural metals. However, the use of joining processes based on plastic deformation of magnesium alloys is restricted by the low formability of these materials at room temperature. In this research work, a novel joining process, called hole hemming, is developed for attaching magnesium AZ31 and aluminum AA6082-T4 alloy sheets in which only one of the sheets should be sufficiently ductile and there is no need for heating or using additional elements, such as rivets. Firstly, the fracture limits of the materials were characterized under different loading states. Then, the process for joining the mentioned material combination was designed with the support of finite element analysis and the critical regions prone to fracture and their loading paths were specified. In addition, the influence of process parameters on the mechanical interlock was investigated to determine process windows. A flexible tool was then developed and, for the first time, novel hole-hemmed joints were manufactured with different process parameters to thoroughly evaluate the performance of the designed process. Finally, single-lap joints were tested under shear loads. The results of this tests show that the novel hole hemming process can suitably join the magnesium and aluminum alloy sheets, with the hole-hemmed joints supporting a maximum load of 2.9 kN with a gradual failure mechanism of hole bearing. Thus, the hole hemming process is shown to be a viable technique for joining materials with very different formability.

Study of HCP→FCC phase transformation mechanism under different hot compression rates of AZ31 magnesium alloy

At present, there are few studies on the phase transition during the thermocompression plastic deformation of magnesium alloy. In this study, the evolution model of thermal compression plastic of AZ31 magnesium alloy was constructed by molecular dynamics, and the phase transition relationship between HCP and FCC at different thermal compression rates was studied. By combining GLEEBLE thermal compression experiment with transmission electron microscopy experiment, high-resolution transmission electron microscopy images were taken to analyze the transition rules between HCP and FCC during plastic deformation at different thermal compression rates, and the accuracy of molecular dynamics analysis was verified. It is found that the slip of Shockley's incomplete dislocation produces obvious HCP →FCC phase transition at low strain rate and base plane dislocation at high strain rate, which makes the amorphous phase transition of HCP→OTHER more obvious, which provides theoretical guidance for the formulation of forming mechanism and preparation process of magnesium alloy.

Deformation mechanism finite element analysis and die geometry optimization of magnesium alloys by equal channel angular processing

Magnesium alloys are one of the highly promising structural metals. hcp structure makes it less plastic at room temperature, which greatly limits the development of magnesium alloys. The problem of poor plastic deformation of magnesium alloy can be solved by grain refinement methods, and equal channel angle pressing is one of the more effective methods in grain refinement. In this paper, two-dimensional dynamic simulation of equal channel angle pressing of magnesium alloy is carried out by using finite element software DEFORM F2™. The deformation of magnesium alloy with different of die angles and die corner angles was analyzed. The results show that in the main deformation zone, when the die angles are different, the deformation of the workpiece in the horizontal direction is very uniform. However, in the longitudinal direction of the workpiece, the larger the die angle is, the more uniform the workpiece deformation is. The die corner angle has no significant effect on the uniformity of workpiece deformation in the longitudinal direction, but it has an effect on the dead zone area and workpiece warpage. The dead zone area and workpiece warpage increase with the increase of die angle.

Modeling of plastic deformation of magnesium alloys with random texture: A challenge to constitutive model for twinning

Crystal plasticity based constitutive models have been successfully applied to plastic deformation of magnesium (Mg) alloys with strong textures. However, there has not been even one single case where crystal plasticity could accurately simulate plastic deformation of Mg alloys with random texture. This is due to the fact that a random texture of a polycrystal results in a wide range of the Schmid Factor for twinning, and twinning in the polycrystal is expected to be very gradual, which is distinct from the cases with very strong textures. Modeling of plastic deformation of a random texture is thus very challenging. In the present work, an improved EVPSC-TDT model is applied, for the first time, to successfully model the deformation behavior of a randomly textured Mg-2w.t.%Al under uniaxial tension and compression.

Mechanisms of elastic–plastic deformation in magnesium alloy studied using neutron diffraction and crystallite group method

Mechanisms of elastic–plastic deformation in magnesium alloy studied using neutron diffraction and crystallite group method

Plastic deformation of magnesium alloy with different forming parameters during ultrasonic vibration-assisted single-point incremental forming

The research of forming parameters on the ultrasonic vibration single-point incremental forming of magnesium alloy plastic deformation can provide a theoretical basis for the establishment of the forming parameters. According to the forming characteristics of magnesium alloy sheet, a new method of ultrasonic vibration-assisted single-point incremental forming was proposed. The influence of forming parameters on the plastic deformation of magnesium alloy was studied by finite element simulation and experimentation. The influence of vibration frequency, amplitude, friction coefficient, and tool head size on stress and thinning rate of magnesium alloy during ultrasonic vibration-assisted single-point asymptotic forming was studied. The results show that the vibration frequency of 20 kHz and forming tool radius of about 5 mm are beneficial for plastic deformation magnesium alloy in ultrasonic vibration-assisted single-point incremental forming. With vibration amplitude increasing, the maximum shear stress tends to decrease as a whole, but at the amplitude of 0.16 mm, the thinning rate is large and fracture occurs easily. With friction coefficient increasing, the maximum shear stress tends to increase, and there is a good linear relationship between the maximum thinning rate and the friction coefficient.

The dislocation-twin interaction and evolution of twin boundary in AZ31 Mg alloy

The reaction of lattice dislocation with twin boundary plays a crucial role in the plastic deformation of magnesium alloys. In this study, we visit the basal dislocation-twin interaction in a hot-rolled AZ31 sheet through pre-compression along rolling direction (RD) and subsequent compression along 45° of RD and normal direction (ND), with focus on the twin boundary (TB) structure evolution and nucleation structure characterization. It is found that basal dislocation slip dominates the compression deformation when the strain along 45° of RD and ND is less than 14%; when the strain reaches 14%, new deformation modes are initiated. When the strain is in the range of 5%–14%, basal-prismatic (BP/PB) boundaries are created by dislocation-twin interaction. Meantime, the number of the BP/PB boundaries increase linearly with strain, leading to an extremely incoherent TB. When the strain reaches 14%, {101¯2} twin nucleates from parent of previous {101¯2} twin and {303¯4} twin, a twinning mode not reported before, nucleates from previous {101¯2} TB. Based on the direct experimental observations, the nucleation mechanisms of the new nucleuses are proposed. Moreover, TBs of these new nucleuses present faceted structures and previous {101¯2} twin-new {303¯4} twin interaction results in a low-angle asymmetrical tilt boundary. These correlations will significantly benefit the development of crystal plasticity modeling and enhance meso-scale understanding of structural evolution in hexagonal close-packed materials.

Dislocation-dynamics-based dynamic constitutive model of magnesium alloy

Since the dynamic plastic deformation of magnesium alloy, a hexagonal close-packed metal, consists of dislocation slipping and twinning, it is assumed that the dynamic plastic deformation of magnesium alloy is controlled by the mechanism with lower resistance, i.e., dislocation slipping or twinning. So, the evolution law of dynamic plastic deformation of the alloy can be determined by that of the resistance of dislocation slipping or twinning. After the evolution equations of the deformation resistances are deduced, the dynamic constitutive model of magnesium alloy is developed in this work. The proposed model is further verified by describing the dynamic tensile and compressive deformation of AZ\(_{31}\)B magnesium alloy obtained by the split Hopkinson pressure bar device. It is observed that the simulations agree well with the experimental results.

Effect of thermal tempering on microstructure and mechanical properties of Mg-AZ31/Al-6061 diffusion bonding

The objective of this study is to investigate the effect of the types thermal tempering of aluminum alloy on microstructure and mechanical properties of AZ31-O Mg and Al 6061-T6 diffusion bonding. Using Optical Microscope (OM) and Scanning Electron Microscopes (SEM) equipped with EDS analysis and line scan the interfaces of joints were evaluated. The XRD analysis was carried out to characterize phase constitution near the interface zone. The mechanical properties of joints were measured using Vickers micro-hardness and shear strength. According to the results in bonding of AZ31-Mg/Al-6061-O, in less plastic deformation in magnesium alloy, diffusion rate of most magnesium atoms occurred to aluminum alloy and formation of diffusion zone with minimum micro-hardness (140 HV) and maximum shear strength (32MPa) compared to Al 6061-T6/Mg-AZ31 bonding. Evaluation of fracture surfaces indicates an occurrence of failure from the brittle intermetallic phases.

Microstructure and Low-Temperature Superplasticity of Fine-Grain ZK60 Magnesium Alloy Produced by Equal-Channel Angular Pressing

The microstructure and low-temperature superplasticity of ZK60 magnesium alloy processed by equal-channel angular pressing (ECAP) were investigated. Homogeneous and refined microstructure was achieved after four passes of ECAP at 200 °C. After ECAP, the ZK60 alloy showed excellent superplastic elongation of 628% at 250 °C under strain rate of 1.67 × 10−3 s−1. Low-temperature superplasticity was obtained after four passes of ECAP at 200 °C. A theoretical foundation for severe plastic deformation of magnesium alloy at low temperature is developed in this work.

Cyclic Plasticity Constitutive Model for Uniaxial Ratcheting Behavior of AZ31B Magnesium Alloy

Investigating the ratcheting behavior of magnesium alloys is significant for the structure’s reliable design. The uniaxial ratcheting behavior of AZ31B magnesium alloy is studied by the asymmetric cyclic stress-controlled experiments at room temperature. A modified kinematic hardening model is established to describe the uniaxial ratcheting behavior of the studied alloy. In the modified model, the material parameter m i is improved as an exponential function of the maximum equivalent stress. The modified model can be used to predict the ratcheting strain evolution of the studied alloy under the single-step and multi-step asymmetric stress-controlled cyclic loadings. Additionally, due to the significant effect of twinning on the plastic deformation of magnesium alloy, the relationship between the material parameter m i and the linear density of twins is discussed. It is found that there is a linear relationship between the material parameter m i and the linear density of twins induced by the cyclic loadings.

Enhanced Plastic Deformation of Magnesium Alloy Produced through Accumulative Diffusion Bonding

Plastic deformation of magnesium alloy is limited because of its HCP structure. Though the grain refined magnesium alloy exhibits superplastic deformation at elevated temperature, the strength decreases due to induced cavitation. This study focusses on developing the random texture of magnesium alloy and causes relatively high plastic deformation even at room temperature. Random texture is produced through newly proposed accumulative diffusion bonding process using commercial AZ91 magnesium alloy sheets. Tensile and V-bending tests reveal that the enhanced ductility of the produced magnesium alloy sheet.

Effect of severe plastic deformation on microstructure and mechanical properties of bulk AZ31 magnesium alloy

The equal channel angular pressing (ECAP) was used to make large plastic deformation of magnesium alloy, and ECAP processing and relevant process parameters were obtained by using finite element method. The strain distributions in the workpieces and the loads on the dies were studied. The mathematical model of accumulated deformation results, microstructure refinement and mechanical properties were established. Through the analysis, the relationships between grain refinement and mechanical properties were obtained. The refined grain size and the obtained mechanical properties are forecasted by using characterization of accumulation deformation.

In situ identification of twin-related bands near yielding in a magnesium alloy

Digital image correlation and acoustic emission were used simultaneously to investigate the near-yielding behavior of textured samples of a magnesium alloy. Distinct band evolution near yielding was monitored in situ and directly related to profuse localized twin formation, verified by ex situ electron backscatter diffraction. The presented results demonstrate for the first time quantitatively in both space and time the dominant role of twinning in the early stages of plastic deformation of magnesium alloys.

Influence of Impurities to Deformation Behavior in Magnesium Single Crystal

Generally, plastic deformation of magnesium alloys is difficult at room temperature. In order to improve formability of magnesium, impurity elements in magnesium were reduced by vapor deposition technique. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was applied to the determination of trace elements in refined magnesium. To investigate influence of impurity element to deformation behavior, high purity magnesium single crystals were prepared. When the magnesium single crystals are stretched in <11-20> direction, {11-22} <-11-23> pyramidal slips were activated just after yielding in the range of 77K to 293K. The yield stress of high purity magnesium was a half of the stress in raw magnesium.