Initial Stage Of Plastic Deformation Research Articles

Influence of Dual-Phase Microstructures on the Properties of High Strength Grade Line Pipes

The influence of dual-phase microstructures on mechanical properties of X70, X80, and X90 line pipes is investigated. It is found that the line pipes with dual-phase microstructures possess both larger uniform elongation and higher hardening exponent, especially for high grade steel X90. The tensile deformation of dual-phase line pipe does not follow the trend predicted by the Hollomon formula, and a stable strain-hardening exponent is not found. This stress-strain behavior is different from the normal line pipe. In the initial stage of plastic deformation, the strain-hardening capacity of dual-phase line pipe increases rapidly. However, it reaches a stable stage after 2.0% total strain. The dual-phase pipeline steel is composed of soft phase (polygonal ferrite) and hard phase (bainite), and thus the relatively soft ferrite is good for its deformability. Besides, the fraction of large angle grain boundaries in the dual-phase microstructures is greater than that of the normal line pipe, which is proven to be critical for improving the resistance to plastic deformation and crack propagation.

Void-assisted plasticity in Ag nanowires with a single twin structure.

By employing the in situ transmission electron microscopy (TEM) technique, tensile deformation behaviors of a silver nanowire (NW) with a single twin structure were studied. Our observations revealed that the initial stage of plastic deformation was dominated by surface-mediated partial dislocation activities. Strikingly, the void formation and growth were shown to govern the later stage of plasticity, leading to the ductile type of fracture in NWs. Possible void nucleation and growth mechanisms were discussed. Additionally, TEM images show the transformation from bi-crystal to polycrystal in the fracture area, likely due to the void activity. Our results have implications in the assembly of functional structures applying nano-building blocks.

Multiscale simulation of onset plasticity during nanoshearing process of copper film

This paper presents a multiscale simulation of nanoshearing process of a single crystal copper film using the quasi-continuum (QC) method. The purpose of the simulation is to study the initial stages of plastic deformation. Three different punch-die clearances and two different crystallographic orientations of the film are studied. A series of dislocation activities, such as nucleation, slip, and deformation twinning are captured. We compared the stress-displacement or the load-displacement curves for different punch-die clearances and for different punch orientations. It is found that both the elastic deformation and dislocation nucleation largely depend on the punch-die clearance. Each dislocation nucleation is accompanied by a load drop in the load-displacement curves. In addition, the punch orientation has a great influence on the mechanism of plastic deformation.

XRD LINE PROFILE ANALYSIS OF LY12 ALUMINUM ALLOY UNDER DYNAMIC COMPRESSIVE EXPERIMENT

The plastic deformation of metals under dynamic loading is related to the evolution of dislocation structure and density which is thermal activation-dependent.The dynamic compressive experiment on LY12 aluminum alloy has been performed by Hopkinson bar at different temperatures. X-ray diffraction line profile analysis is adopted for the tested specimens to investigate the micro-and/or meso-scale structure evolution.The edge character of dislocations in the specimens was determined by analyzing the integral breadths of X-ray diffraction lines.The Fourier analysis of diffraction lines indicates that under dynamic loading,the dislocation density approaches to saturation rapidly at the initial stage of plastic deformation,dislocations are homogeneously distributed in the specimens.It also demonstrates that the dislocation density decreases with increasing testing temperature. Meanwhile the size of substructures has a tendency of broadening with temperature,especially in the range from 280℃to 300℃which corresponds to the temperature of dissolution of precipitated phase in the aluminum matrix.

Microstructure and Properties of Mo Microalloyed Cold Rolled DP1000 Steels

Two kinds of ultra-high strength cold rolled dual phase steels have been developed by designing C-Si-Mn-Cr and C-Si-Mn-Cr-Mo alloy systems. Tensile strength and elongation of both steels exceed 1100 MPa and 10%, respectively. The microstructures of both steels consist of massive martensite and ferrite. And the massive martensite of Mo-free steel disperses in the ferrite with volume fraction of 64%. However, the massive martensite of Mo-containing steel is connected or closed by small martensite islands each other, and martensite volume fraction is 69%. As to Mo-free steel, the yield strength, yield ratio, and work hardening exponent n are 548 MPa, 0.49, and 0.26, respectively. As for Mo-containing one, the yield strength, yield ratio, and n value are 746 MPa, 0.66, and 0.33, respectively. Besides, the ferrite of Mo-free steel is deformed at the initial stage of plastic deformation. However, for Mo-containing one, Mo solution strengthened ferrite and small overaged martensite islands are deformed preferentially at small strain, which causes the yield strength to reach approximately 200 MPa higher than that of Mo-free steel.

Sequential multiplication of dislocation sources along a crack front revealed by high-voltage electron microscopy and tomography

Abstract

Identification of energy storage rate components. Theoretical and experimental approach

The subject of the present paper is decomposition of energy storage rate into terms related to different mode of deformation. The stored energy is the change in internal energy due to plastic deformation after specimen unloading. Hence, this energy describes the state of the cold-worked material. Whereas, the ratio of the stored energy increment to the appropriate increment of plastic work is the measure of energy conversion process. This ratio is called the energy storage rate. Experimental results show that the energy storage rate is dependent on plastic strain. This dependence is influenced by different microscopic deformation mechanisms.It has been shown that the energy storage rate can be presented as a sum of particular components. Each of them is related to the separate internal microscopic mechanism. Two of the components are identified. One of them is the storage rate of statistically stored dislocation energy related to uniform deformation. Another one is connected with non-uniform deformation at the grain level. It is the storage rate of the long range stresses energy and geometrically necessary dislocation energy. The maximum of energy storage rate, that appeared at initial stage of plastic deformation is discussed in terms of internal micro-stresses.

Pre-strain direction effect on microstructure evolution and energy storage process during uniaxial tension of austenitic steel

In the present paper the influence of pre-strain direction on energy balance during deformation of austenitic steel was investigated and the analysis of microscopic phenomena responsible for this influence was performed. The specimens with different pre-strain directions were prepared and the ratio of the stored energy increment to plastic work increment, called energy storage rate, as a function of plastic strain was experimentally determined. At the initial stage of plastic deformation of annealed materials this quantity vs. plastic strain has a maximum. It has been shown that for specimens strained in the same direction as pre-strain the energy storage rate decreases monotonically with deformation while for specimens where strain path was changed, the maximum of the energy storage rate is observed (as in case of annealed material). The study of slip and microstructure evolution at meso- and micro-scales have shown that the change in pre-strain direction leads to the redistribution of internal stresses generated by incompatible slip in neighbouring grains of different orientation. Just after change in strain direction the accommodation of these stresses takes place not only by generation of geometrically necessary dislocations but also by micro-shear banding.

Simulation of plastic deformation initiation in crystal materials under dynamic loading

In the last decade the scope of experimental and theoretical investigations into the initial stage of plastic deformation in solids has been expanded significantly. Nevertheless its atomistic mechanisms are as yet imperfectly understood. One of the major open questions concerns the existence of proto-defects, i.e. local structural distortions which provide the basis for the formation of conventional structural defects — carriers of plastic deformation. The investigations were applied to crystallines with different boundary types (free surfaces and grain boundaries) and different type of crystal lattice. The influence of loading intensity and deformation scheme was studied. Simulation was performed for various temperatures from 50 K to 500 K. It has been demonstrated that there is a certain threshold value of strain at which zones with local structural changes grow almost abruptly. A detailed analysis of atomic displacements at the stage of nucleation of local structural distortions (protodefects) has revealed that their formation is related with specific rearrangement in the first and second coordination spheres of one of the atoms. It is realized in the conditions of local expansion of the atomic volume.

Effect of Plastic deformation and its localization zones on magnetic characteristics of steel 45

Attachable transducers were used to measure the demagnetizing current, which is proportional to the coercive force, and Barkhausen noise parameters for steel 45 specimens depending on the degree of their plastic deformation in measuring the magnitudes along and perpendicular to the tension direction. The demagnetizing current measured along the preliminary tension direction was shown to increase monotonically; the rate of its changes is maximal at the initial stage of plastic deformation. The demagnetizing current measured perpendicular to the applied tension direction also increases monotonically but within a narrower range. Near local inhomogeneities formed during the deformation, substantial changes in the coercive force and Barkhausen noise take place. The results obtained are explained on the basis of the current concept of the anisotropy of internal stresses in preliminarily plastically deformed materials.

On the localization of plastic flow under compression of NaCl and KCl crystals

The plastic flow localization patterns for alkali halide crystals under compression are investigated. The main spatiotemporal regularities of the strain localization at the stages of deformation hardening in these single crystals are established. The relation is traced between the orientation of localized strain zones and the crystallography of slip systems of the test specimens at the initial stages of plastic deformation. The velocity of motion of localized strain zones under compression is determined.

Atomistic simulation of yield mechanism of single crystal copper nanowires

Molecular statics simulation was used to study the yielding mechanism of 〈100〉 oriented single crystal copper nanowires under tension. The results show that, the yield of nanowires is initially realised via {111}〈112〉 twinning from random activation of partials, and subsequently via slip of {111}〈112〉 partial dislocation; in addition, the partial dislocations on different planes successively slip and meet to form stair-rod dislocation at the initial stage of plastic deformation, while some partial dislocations are piled up near the rigid boundary at the later stage of plastic deformation, both of which hinder dislocation slip, resulting in strengthening of the single crystal copper nanowires.

Failure Mechanisms during Periodic Cellular Metal Fabrication by Perforation Stretching

Periodic cellular metals (PCMs) are structurally efficient lightweight materials. Perforation stretching is a recently developed PCM fabrication method that plastically deforms perforated sheet precursors into a three-dimensional (3-D) array of load-supporting struts. The present study examines the onset and limit of useful plastic deformation during perforation stretching of a wrought 3003 aluminum alloy over a range of precursor microstructures and geometries. A compression platform was used to apply the forming forces such that force-displacement profiles could be obtained for each condition. The initial stage of plastic deformation was bending dominated, and it was found that a simple plastic bending cantilever model was able to account for the nearly order of magnitude range in plasticity onset force. On the other hand, unlike a previously reported pin punch through mechanism, it was found that for the present forming conditions, perforation-stretching failure could be attributed largely to tensile instability, suggesting that the tensile failure strain is a key parameter in determining the range of accessible perforation-stretching formed architectures.

Quantitative description of plastic deformation in nanocrystalline Cu: Dislocation glide versus grain boundary sliding

Uniaxial plastic deformation of polycrystalline Cu with grain sizes in the range of 5--20 nm was studied by using molecular dynamics computer simulations. We developed a quantitative analysis of plasticity by using localized slip vectors to separate the contributions of dislocation activity from grain boundary sliding. We conclude that the competition between these two mechanisms depends on strain rate and grain size, with the dislocation activity increasing with grain size but decreasing with increasing strain rate. For samples with a 5 nm grain size, dislocations contribute $\ensuremath{\approx}50%$ of the total plastic strain during steady state deformation at a rate of $1\ifmmode\times\else\texttimes\fi{}{10}^{8}\text{ }{\text{s}}^{\ensuremath{-}1}$, but this fraction decreases to 35% at a rate of $1\ifmmode\times\else\texttimes\fi{}{10}^{10}\text{ }{\text{s}}^{\ensuremath{-}1}$. When the grain size is increased to 20 nm, dislocations account for 90% of the strain, even at $1\ifmmode\times\else\texttimes\fi{}{10}^{10}\text{ }{\text{s}}^{\ensuremath{-}1}$. During the initial stages of plastic deformation, grain boundary sliding initially decreases with strain owing to strain-induced relaxation processes within the grain boundaries. The grains also rotate a few degrees during straining to 20%; the rate of rotation (per unit strain) slightly decreases with strain rate. Lastly, we computed the amount of forced atomic mixing during plastic deformation. The mean square separation distance between atom pairs within grain interiors increases with strain at a rate proportional to their distance apart (i.e., the mixing is superdiffusive), but for pair separations greater than the grain size, this rate becomes independent of the separation distance.

Energy accumulation and dissipation in metals as a result of structural-scaling transitions in a mesodefect ensemble

The localization of plastic deformation and its wave-like propagation are well-known and experimentally studied phenomena. Modern experiments carried out with highly sensitive infrared cameras have allowed us to investigate in detail energy accumulation and dissipation in the material which accompany the above process. It is found that plastic deformation occurs together with great energy accumulation in the material. The energy accumulation rate peaks on the initial stage of plastic deformation and vanishes at developed plastic flow. At the moment of elastic-plastic transition dissipative waves induced by plastic deformation localization can be generated and propagate in some metals. The given paper puts forward a theoretical model of energy accumulation and dissipation in metals under plastic deformation. The model is based on a statistical description of evolution of an ensemble of typical mesodefects (microshears). The derived constitutive equations adequately describe certain experimentally observed peculiarities of energy dissipation in metals.

Plastic properties and defect structure of LiF-LiF:Mg layered single crystals at T = 4.2 K

The compressive stress-strain diagrams are obtained for layered single crystals of the LiF-LiF:Mg type with different orientations of the reinforcing layers at T = 4.2 and 300 K. The strength characteristics and specific features of the defect and dislocation structures of the crystals strained in liquid helium are studied and compared with those for the crystals strained at 300 K. It is established that the layered single crystals remain plastic at T = 4.2 K. Under a strain e > 1%, there arise microcracks and macrocracks responsible for complete fracture of the crystals. It is revealed that long-term (for ten years) storage of the layered single crystals at 300 K substantially affects the magnesium impurity structure in the reinforcing layers and leads to a considerable enhancement of their strength characteristics. This effect is taken into account when analyzing the strength properties of the layered single crystals in the temperature range T = 300–4.2 K. The inference is made that the results obtained in studying the defect microstructure of the model layered single crystals at the initial stage of plastic deformation can be used to predict the strength properties of the composites at T = 4.2 K.

The effect of solution treatment on the spatial behavior of the Portevin-Le Chatelier effect in Al-Cu alloys

The effect of solution treatment on the spatial behavior of the Portevin-Le Chatelier effect in Al-Cu alloys is investigated under constant applied driving velocity. During the initial stage of plastic deformation, the PLC deformation band nucleates in the neighborhood of the middle of the transverse direction of specimen. And multiple bands will exist simultaneously in the specimen when deformation develops, whose width is narrower and incline at a larger angle than those in annealing treated material. At high applied velocity, the PLC deformation band propagates continuously first, and then nucleates randomly in the specimen.

Determination of initial compression stresses when extruding bimetallic tubes with porous internal layer

This paper presents an analytical approach of the extrusion process applied to biomaterial tubes made of two prebonded metallic materials. The outer component of the bimetallic is a pore-free layer, whilst the internal one is a porous sintered tube. A generalized “slab method” of analysis, properly modified to describe the plastic behaviour of a porous-sintered material, developed in combination with the theory of thin shells, in order to investigate the initial stage of plastic deformation in the cylindrical chamber of the extrusion die. Numerical solution of the formulation suggested is conducted for components made of sintered and pore-free steel, respectively, and the effect of porosity on the stress–strain state of the bimetal is examined, allowing, in this manner, application of the method in relevant practical problems in the field of bimetallics.

On the initial stage of plastic deformation of SrTiO3

On the initial stage of plastic deformation of SrTiO₃

Dislocation-based modeling of the mechanical behavior of epitaxial metallic multilayer thin films

A 3D dislocation cellular automaton model is employed to simulate yield and hardening in nanostructured metallic multilayer thin films during in-plane, biaxial tensile loading. The films consist of two types of alternating, single-crystalline FCC layers with (0 0 1) epitaxy, a mismatch in stress-free lattice parameter, but no elastic modulus mismatch. The simulations monitor the operation of interfacial and threading sources with lengths greater than the individual layer thickness. At larger layer thickness, strength increases with decreasing layer thickness, due to slip confinement to individual layers. At sufficiently small layer thickness, slip confinement is not possible, even during initial stages of plastic deformation. Consequently, strength is not controlled by layer thickness but rather by source length, as well as coherency stress and interfacial barrier strength. Here, strength may increase, decrease, or reach a plateau depending on how source length and barrier strength vary with layer thickness.