Plastic Deformation Of Metallic Materials Research Articles

A discrete dislocation dynamics model of creep in polycrystals

The dislocation creep represents high temperature plastic deformation of metallic materials. However, the subject is usually approached with constitutive and phenomenological formulations. The notable exceptions are the recent use of numerical modeling. For example, single-crystal discrete dislocation dynamics (DDD) has been adopted to involve dislocation glide and climb together with diffusion of vacancies. This naturally provides a physics-based framework for dislocation creep. However, the existing model(s) are limited to single crystals and hence are limited in scope. This study extended a single crystal dislocation creep model into polycrystalline ensemble. This was then used to simulate steady state creep rate (ε˙) with respect to creep stress (σ), temperature (T) and grain size (d) in polycrystalline aluminum. Our virtual experiments showed an Arrhenius relationship between ε˙ and T, and a power law scaling with σ. Further, our simulations also revealed a power law increase in ε˙ with d. This is striking, as available experimental data indicate both grain size dependence as well as independence. The answer to this apparent contradiction also emerged from our numerical simulations. It was shown that ε˙ is controlled by interactions of dislocations with static obstacles and grain boundaries. Dominance of static obstacles, in particular, significantly diminished the grain size effect. The role of dislocation mean free path on the pinning and dislocation creep behavior of polycrystalline metallic material was thus mechanistically established.

A continuum model for dislocation climb

Dislocation climb plays an important role in understanding plastic deformation of metallic materials at high temperature. In this paper, we present a continuum formulation for dislocation climb velocity based on densities of dislocations. The obtained continuum formulation is an accurate approximation of the Green’s function-based discrete dislocation dynamics method (Gu et al., 2015). The continuum dislocation climb formulation has the advantage of accounting for both the long-range effect of vacancy bulk diffusion and that of the Peach–Koehler climb force, and the two long-range effects are canceled into a short-range effect (integral with fast-decaying kernel) and in some special cases, a completely local effect. This significantly simplifies the calculation in the Green’s function-based discrete dislocation dynamics method, in which a linear system has to be solved over the entire system for the long-range effect of vacancy diffusion and the long-range Peach–Koehler climb force has to be calculated. This obtained continuum dislocation climb velocity can be applied in any available continuum dislocation dynamics frameworks. We also present numerical validations for this continuum climb velocity and simulation examples for implementation in continuum dislocation dynamics frameworks.

CRSS determination combining ab-initio framework and Surrogate Neural Networks

Critical Resolved Shear Stress (CRSS), fundamentally linked to the dislocation glide stress, is a crucial measure in dictating plastic deformation in metallic materials. A recent ab-initio predictive model for dislocation glide stress in Face-Centered Cubic (FCC) materials is developed which accurately predicts available experimental data, considering the anisotropic continuum energy, the atomistic misfit energy, and the minimum energy path for the intermittent motion of Shockley partials. The CRSS of a material is predominantly controlled by six parameters, namely, lattice constant, unstable/stable stacking-fault energies, and three anisotropic elastic constants for cubic materials, which are inputs to the predictive model. In this work, a large material dataset is produced incorporating properties of real materials and generating hypothetical combinations, subsequently calculating the CRSS for each combination using the predictive model. The hypothetical combinations of properties are employed to train a machine learning-based Surrogate Neural Network (SNN), and the ones of real materials are utilized to validate the SNN model yielding a 94% accuracy for 1,033 materials. The generated dataset is used to unravel the sensitivity of each material parameter to the predicted CRSS establishing a general trend for the FCC materials for the first time guiding the field in achieving superior mechanical properties.

Strain Hardening Exponent and Strain Rate Sensitivity Exponent of Cast AZ31B Magnesium Alloy

The flow curves of as-cast AZ31B magnesium alloy during high temperature deformation were obtained with a thermal compression test, and the effects of deformation amount, grain size, strain rate, and deformation temperature on the flow stress, strain rate sensitivity index, and strain hardening index were analyzed. The results showed that deformation and grain size were negatively correlated with both the strain rate sensitivity index and strain hardening index. The increase in strain rate increased the strain hardening index but made the strain rate sensitivity index show an opposite trend. Increasing temperature reduced the strain rate sensitivity index and strain hardening index but, when the temperature exceeded 700 K, the strain rate sensitivity index was no longer affected by temperature. Since the strain rate sensitivity index m and strain hardening index n are important parameters for measuring the plastic deformation of metal materials, this study has great significance for guiding the selection of process parameters in the plastic processing of as-cast AZ31 magnesium alloy.

Enhanced plastic deformability of copper single crystals under low-frequency vibrational loading

Tensile tests on copper single crystal ([110]) foils using low-frequency vibrational loading of frequencies ranging from 0 to 50 Hz were performed at room temperature to explore the mechanisms of vibration-facilitated plastic deformation in metallic materials. It was found that the total elongation of copper single crystal foils under vibrational loading increased gradually with increasing vibration frequency, indicating enhanced plastic deformability. This behavior could be rationalized with the formation of equiaxial dislocation cells (EDC) in copper single crystal ([110]) foils under vibrational loading, which reflects the activation of multiple codirectional slip systems induced by the approximate biaxial tensile stress state during the unloading stage of vibrational loading. Additionally, the formation of Lüders bands under vibrational loading at high frequencies validates their dislocation-proliferation-mediated nucleation mechanism. This work would help to understand the vibration-lowered forming resistance in metallic materials of FCC lattice structures.

МНОГОУРОВНЕВОЕ МАСШТАБНОЕ КОМПЛЕКСНОЕ ИССЛЕДОВАНИЕ ДЕФОРМИРОВАНИЯ МЕТАЛЛИЧЕСКИХ МАТЕРИАЛОВ

The paper considers the theoretical and experimental questions connected with researches of elastic and plastic deformation of metal materials. The article shows that joint multilevel large-scale study performed by physics-mechanics and material engineers of the patterns connecting characteristics of stress-strain state with parameters of a structural condition of a metal material will allow to develop modern physical and mathematical models of a material. For this purpose in-depth study of the shift deformations forming plastic properties of a material and processes formation of defects and a new free surface is necessary. The paper presents the examples of such a research, and provides methods of analysis of structure and physical and mechanical characteristics at different structural levels of a polycrystalline metal material.

Size effect on the shear damage under low stress triaxiality in micro-scaled plastic deformation of metallic materials

Micro-forming is one of the major micro-manufacturing methods with promising application potentials, in which the damage response and fracture behavior need to be insightfully addressed. Among all the damage criteria to predict fracture, GTN model is a widely-used one and able to predict void-dominated fracture in micro-scale deformation. However, it is not very applicable under low stress triaxiality and shear-dominated condition. An in-depth understanding of shear damage and its potential size effect are crucial to explore the micro-scaled damage and fracture mechanisms. This research characterizes the size effect on flow stress via employing a combined constitutive model, and an approach for applying a phenomenological shear damage evolution law to the GTN-Thomason model via considering the size effect is developed. The prediction of micro-scaled fracture in a wide stress triaxiality range is thus enabled. Through simulation and experiment, the proposed model is validated and verified. In addition, stress state parameters including stress triaxiality, Lode parameter, and weight function, are also discussed, and the two damage parameters are analyzed quantificationally to reveal different fracture mechanisms occurring in different stress states and grain sizes. The research thus facilitates the physical insight and in-depth understanding of the size effect on damage evolution and fracture formation in micro-scaled plastic deformation of materials.

Energy absorption and failure pattern of hybrid composite tubes under quasi-static axial compression

Novel fiber-metal hybrid tubes with overlapped fiber reinforced plastics (FRP) and aluminum layers were proposed in this paper. By combining progressive failure behavior of composites and large plastic deformation of metal materials, the structure was hybridized at mesoscopic scale. To this end, circular fiber/aluminum hybrid tubes with various inner diameters (40, 60, and 80 mm) combined with parent materials (Al-1060, carbon fiber, and glass fiber) were fabricated using vacuum bag molding process. Quasi-static uniaxial compressive tests were conducted to comprehensively explore the effects of geometric factors and failure patterns on energy absorption capability. The experimental results revealed that the pristine FRP and CF/Al (carbon fiber and aluminum) hybrid tubes collapsed in a progressive failure mode and generated abundant intra- and inter-laminar cracks during crushing. The GF/Al (glass fiber and aluminum) hybrid tubes with large diameters collapsed in an unstable and inefficient mode due to local buckling and delamination. Compared with bare aluminum tubes (Al 6061-T6), the specific energy absorption (SEA) and crushing force efficiency (CFE) of CF/Al hybrid structures were improved significantly by respectively 54.3% and 40.4% for tubes with 40 mm inner diameter. Furthermore, the CFE of CF/Al hybrid structures improved by more than 40% when compared to their bare CFRP counterparts, though the SEA reduced by more than 5.5%. In sum, the proposed hybrid design efficiently reduced the peak crushing force with desirable SEA, which has great potential for low-cost and lightweight energy absorber applications.

Experimental verification of a crystal plasticity-based simulation framework for predicting microstructure and geometric shape changes: Application to bending and Taylor impact testing of Zr

This paper is concerned with experimental verification of a recently developed multi-scale simulation framework for plastic deformation of metallic materials from quasi-static to impact deformation conditions. The framework is a visco-plastic self-consistent (VPSC) polycrystalline model embedded in an implicit finite element method (FE-VPSC) to provide a microstructure-sensitive constitutive response at each material point. Each material point of the FEM model is a polycrystalline aggregate with crystallographic deformation mechanisms operating at the single crystal scale with their evolving activity based on a dislocation density-based hardening law and texture. Four beams and three cylinders machined in different orientations from a textured plate of high-purity zirconium are tested quasi-statically in 4-point bending and at speeds of 100 m/s, 170 m/s and 243 m/s during Taylor impact tests, respectively. The variation in dimensional changes resulting from different sample orientations in the plate with respect to loading directions is measured for each sample. Moreover, texture and twinning characterization is performed using electron backscattered diffraction (EBSD). The deformation processes and underlying evolution of microstructure are successfully simulated using the FE-VPSC framework. In doing so, the model parameters are optimized and validated across a broad range of strain rates and temperatures. Simulation results in terms of geometrical changes and microstructural evolution are compared with the experimental measurements. The model predicts anisotropic material flow resulting from the hard-to-deform crystallographic directions, the development of gradients in texture and twinning through the geometries, tension–compression asymmetry, as well as the extent of plasticity under impact.

A New Experimental Method for Determining the Thickness of Thin Surface Layers of Intensive Plastic Deformation Using Electron Backscatter Diffraction Data

It is, in general, essential to investigate correlations between the microstructure and properties of materials. Plastic deformation often localizes within thin layers. As a result, many material properties within such layers are very different from the properties in bulk. The present paper proposes a new method for determining the thickness of a thin surface layer of intensive plastic deformation in metallic materials. For various types of materials, such layers are often generated near frictional interfaces. The method is based on data obtained by Electron Backscatter Diffraction. The results obtained are compared with those obtained by an alternative method based on microhardness measurements. The new method allows for determining the layer thickness of several microns in specimens after grinding. In contrast, the measurement of microhardness does not reveal the presence of this layer. The grain-based and kernel-based types of algorithms are also adopted for determining the thickness of the layer. Data processed by the strain contouring and kernel average misorientation algorithms are given to illustrate this method. It is shown that these algorithms do not clearly detect the boundary between the layer of intensive plastic deformation and the bulk. As a result, these algorithms are unable to determine the thickness of the layer with high accuracy.

Effect of cryogenic temperature equal channel angular pressing on microstructure, bulk texture and tensile properties of AA1050

Plastic deformation of metallic materials at cryogenic temperature is believed to be able to suppress the dynamic recrystallization and further refine their microstructures compared to that at room temperature. This paper investigates the microstructure, bulk texture as well as the tensile properties of commercially pure aluminum AA1050 processed by equal channel angular pressing (ECAP) at both cryogenic temperature (CT-ECAP) and room temperature (RT-ECAP). The experimental results suggest that CT-ECAP process is more effective than RT-ECAP in grain refinement and strength enhancement for the studied material. As for the bulk texture, a significant difference is observed between the 1-pass CT-ECAP and RT-ECAP samples, which is attributed to the suppression of texture development at cryogenic temperature. After 2 or more passes CT-ECAP, the texture obtained is similar to RT-ECAP and can be interpreted by the ideal ECAP texture for FCC metals. It can be concluded that CT-ECAP suppresses the dislocation sliding and dynamic recovery as well as texture evolution, thus it has the capability to fabricate materials with higher strength. The CT-ECAP process induced higher strengths than RT-ECAP can be explained by two main mechanisms, namely the grain boundary strengthening and dislocation strengthening. It has also been found that the grain boundary strengthening mechanism plays the dominant role.

Aspects on methods of assessing prelucrability through cold plastic deformation of metallic materials

In the field of cold pressing, the notion of machinability refers to the behavior of the material during deformation, the technological equipment and the machine. In this paper we presented the methods of estimating the behavior of the semi-finished material during processing.

Effects of High Strain Rate and Self-heating on Plastic Deformation of Metal Materials Under Fast Compression Loading

The results of dynamic tests for compression of four metals after annealing (aluminum alloy, copper, brass, stainless steel) in the range of strain rates 100–10,000 s–1 are presented. To obtain dynamic true stress-true plastic strain curves a specific dynamic upsetting computation method was used starting from time variation of force pulse. The high-speed loading of specimens with a height to diameter ratio equal to 1.5 was carried out using three experimental devices: a drop-hammer with a free-falling mass, a ram impact machine with rubber accelerators and an impact machine with powder acceleration. A technique for direct measuring the specimen temperature during plastic deformation using an “artificial specimen” thermocouple (specimen-thermocouple) was proposed, in which the deformable metal itself is the “hot” junction, while the “cold” junction is connected to the measuring instrument. A thermoelectrode made of an alloy of noble metals tungsten and rhenium with a diameter of 0.1 mm was welded to the specimen by radiation from a pulsed YAG:Nd3+ laser. As a result of the tests, true dynamic stress–strain diagrams and flow stress versus strain rate were obtained. Experimental estimation concerning self-heating temperature variation corresponding to high-speed plastic deformation at room temperature is also developed. Experiments for determining the temperature effect of plastic deformation of the samples after annealing during cold settling showed that at the high speed of ~ 50 m s−1 and high strain-rates of ~ 10,000 s−1 the increase in temperature of the specimens compressed to a degree of deformation of 0.6 reached 55 °C for an aluminum alloy, 60 °C for copper, 80 °C for brass and 140 °C for stainless steel.

Linking local and heterogeneous deformation behavior to global deformation of materials by in-situ experimental techniques

Plastic deformation of metallic materials is essentially heterogeneous and localized. In the case of dislocation slip, for example, slip of dislocations occurs on particular crystallographic planes and in particular directions, and dislocations are multiplied from dislocation sources located on limited slip planes. Heterogeneous deformation is also induced by the constraint of adjacent grains in polycrystalline materials and appears in different length scales in the form of different slip patterns within each grain, deformation bands, and so on. To achieve higher strengths, the microstructures of structural metallic materials have become more and more complicated and are often composed of different phases (or domains) having different strength levels. In such complicated microstructures, deformation becomes even more heterogeneous. Therefore, it is important, especially in high strength metals, to link local and heterogeneous deformation aspects to the global deformation behavior, in order to fundamentally understand the nature of deformation and then to design advanced materials for performing desired mechanical properties. Conventional material testing, such as tensile tests can determine the global deformation behavior of materials, but the obtained stress-strain curves show only the average material response, in which local information is hidden. Local and heterogeneous characteristics of deformation have been observed by various kinds of microscopy, but it is typically difficult to correlate such local and heterogeneous features with the global and average materials response, especially during deformation. Recently, significant progress has been made in various kinds of in-situ experimental techniques, and a breakthrough in deepening the fundamental understanding of deformation of materials is expected. Such techniques include in-situ neutron/synchrotron diffraction during deformation, in-situ deformation observations in scanning and transmission electron microscopy, nano-indentation, and digital image correlation (DIC) during deformation. In the current paper, some recent results obtained in the author’s group linking local and heterogeneous characteristics to the global deformation response for several metallic materials are introduced, illustrating the use of state-of-the-art techniques.

Optimization of Crystallographic Texture for Sheet-forming Applications Using Taylor-based Models

Plastic deformation of metallic materials is an inherently anisotropic process as a result of the presence of preferential orientations in their crystallographic texture. Crystal plasticity modeling, which allows simulating the response of polycrystal aggregates taking into account their texture and other microstructural parameters, has been extensively used to predict this behavior. In this work, crystal plasticity models are used to deal with the opposite problem: given a desired behavior, determine how to modify a texture to approximate this behavior in the most efficient way. This goal can be expressed as an optimization problem, in which the objective is to find the texture with the best formability properties among all the possible ones. An incremental optimization method, based on the gradient descent algorithm, has been developed and applied to different initial textures corresponding to typical steel and aluminum sheet products. According to expectations, the textures found present a stronger gamma fiber component. Moreover, the method sets the basis for the development of more complicated optimization schemes directed toward optimizing specific materials and forming processes.

Plastic deformation of metallic materials during dynamic events

Recently, fully implicit computational capabilities have been developed to predict the plastic behavior of metallic materials (i.e. Ti, Mo) during dynamic events. It is to be noted that within this formulation framework, the equilibrium equations are solved for each time increment. The couplings of the numerical framework to the Cazacu et al. [1] plasticity model that accounts for all the key features of the plastic behavior of airframe materials, i.e. the tension-compression asymmetry and the orthotropic behavior, results in high fidelity prediction of the mechanical behavior during dynamic events. The improved predictive capabilities have been assessed for different strain rate conditions and different metallic materials. Furthermore, validation of the models and FE formulation for Taylor impact conditions through comparisons of experimental deformed profiles of Taylor specimens for Ti and Mo has been done. It is worth noting that for the first time, the extent of the zone of plastic deformation, change in geometry and the transition from transient to quasi-steady plastic wave propagation was captured with great fidelity. Furthermore, the model was used to gain understanding of the dynamic deformation process in terms of time evolution of the pressure, the extent of the plastically deformed zone, distribution of the local plastic strain rates, and when the transition to quasi-steady deformation occurs for different dynamic events. It was thus shown that this model has the potential to be used for virtual testing of complex systems.

A numerical solution for a closed die forging process

One of the manufacturing processes that can be permanent improved is plastic deformation of metallic materials, as incorporating reserves on the manufacture of products with reduced material and energy consumptions. This paper presents finite element analysis for a closed die forging process, showing the stresses, strains and temperature into deformed part and stresses in the working tools. The analysis of obtained results for different flash dimensions of the working tools has enabled optimization of the forging process studied. To be able to validate the numerical results obtained, experimental tests were conducted. The compared data series show a good agreement between the numerical and experimental data.

Effect of initial grain size on the microstructure and mechanical properties of high-pressure torsion processed twinning-induced plasticity steels

The initial grain size of TWIP steel can affect the deformation mechanism during the HPT processes and the mechanical properties of severely deformed TWIP steels, contrary to the common belief that responses to the severe plastic deformation of metallic materials are not affected by initial grain size.

Effect of acoustic softening on the thermal-mechanical process of ultrasonic welding

Application of ultrasonic energy can reduce the static stress necessary for plastic deformation of metallic materials to reduce forming load and energy, namely acoustic softening effect (ASE). Ultrasonic welding (USW) is a rapid joining process utilizing ultrasonic energy to form a solid state joint between two or more pieces of metals. Quantitative characterization of ASE and its influence on specimen deformation and heat generation is essential to clarify the thermal-mechanical process of ultrasonic welding. In the present work, experiments were set up to found out mechanical behavior of copper and aluminum under combined effect of compression force and ultrasonic energy. Constitutive model was proposed and numerical implemented in finite element model of ultrasonic welding. Thermal-mechanical analysis was put forward to explore the effect of ultrasonic energy on the welding process quantitatively. Conclusions can be drawn that ASE increases structural deformation significantly, which is beneficial for joint formation. Meanwhile, heat generation from both frictional work and plastic deformation is slightly influenced by ASE. Based on the proposed model, relationship between ultrasonic energy and thermal-mechanical behavior of structure during ultrasonic welding was constructed.

Mechanical testing and fracture analyses of miniaturized ZnO-based multilayer components

Functional components are commonly fabricated combining a ceramic substrate with external and/or internal metallization (e.g. metal electrodes, vias, contact pads, etc.) using a tape casting process. Different layers are printed and/or fired (e.g. up to 800°C) onto the ceramic part to provide the component with a (certain) functionality. As a result of the combination of different materials (e.g. ceramic, glass, metal alloys) with different coefficients of thermal expansion, internal stresses may arise during the different fabrication steps. Although some of these tensile residual stresses may relax due to plastic deformation of metallic materials, stress concentrations generated in material junctions or terminations (imposed by geometrical constrains) may lead to cracks and/or reduce the component strength.In this work different architectures combining metal and glass layers on the surface of ZnO substrates were investigated experimentally and numerically in order to identify weak points in commercial components. Mechanical testing using three-point-bending was performed on samples taken after different steps. A FE model was developed to (i) calculate residual stresses generated during the manufacturing process, and (ii) simulate the propagation of initial crack/defect during manufacturing. Experimental results were compared with numerical predictions. These results in combination with fractographic analyses were used to validate the finite element model in order to assess location of failure.