Macroplastic Deformation Research Articles

Термоактиваційний аналіз температурної залежності твердості квазікристалів системи Al—Cu—Fe

The results of thermoactivation analysis of the temperature dependence of hardness for icosahedral quasicrystals (QC) of the Al—Cu—Fe system obtained as a coating, massive compact and ingot are presented. QC as well as covalent crystals at room temperature are brittle without signs of macroplastic deformation at standard methods of mechanical testing and only the method of local indenter loading makes it possible to deform QC to significant degrees of deformation without fracture. In the studied temperature range 77—1073 K, the HV(T) hardness dependences have the same character, despite the state in which the QC was obtained. The HV(T) dependence consists from two sections: an athermal low-temperature section (77—600 K) and a section (>600 K) where the hardness decreases sharply with increasing temperature. The presence of a low-temperature athermal section on the HV(T) dependence is explained by the phase transition of the QC to a more plastic approximant phase. Phase transition of this type can be associated with a high density of phason defects, which are formed during the deformation of the QC that leads to violations of the atomic structure. Based on the experimental data of the temperature dependences of the Vickers hardness (HV) obtained by the authors and the literature data, the values of the activation energy of the dislocation motion U and the activation volume V of a number of icosahedral quasicrystals were calculated. It is shown that the value of U  0,97—1,83 eV, and V is (65—132)∙10-24cm3. Previously, the method of thermoactivation analysis of tempera¬ture dependence of a flow stress was applied to materials with different crystal structures (BCC, FCC metals, covalent crystals, refractory compounds, intermetallics, high entropy alloys). In comparison with crystalline materials, the values of thermal activation parameters of the deformation process for QC are close to refractory compounds (carbides, borides) which have a covalent component in the interatomic bond. Keywords: quasicrystals, activation energy of dislocation motion, activation volume, hardness, temperature.

Direct Characterization of the Relation between the Mechanical Response and Microstructure Evolution in Aluminum by Transmission Electron Microscopy In Situ Straining.

Transmission electron microscopy in situ straining experiments of Al single crystals with different initial lattice defect densities have been performed. The as-focused ion beam (FIB)-processed pillar sample contained a high density of prismatic dislocation loops with the <111> Burgers vector, while the post-annealed specimen had an almost defect-free microstructure. In both specimens, plastic deformation occurred with repetitive stress drops (∆σ). The stress drops were accompanied by certain dislocation motions, suggesting the dislocation avalanche phenomenon. ∆σ for the as-FIB Al pillar sample was smaller than that for the post-annealed Al sample. This can be considered to be because of the interaction of gliding dislocations with immobile prismatic dislocation loops introduced by the FIB. The reloading process after stress reduction was dominated by elastic behavior because the slope of the load–displacement curve for reloading was close to the Young’s modulus of Al. Microplasticity was observed during the load-recovery process, suggesting that microyielding and a dislocation avalanche repeatedly occurred, leading to intermittent plasticity as an elementary step of macroplastic deformation.

Analysis of the Fatigue Damage Behavior of AW2099-T83 Al-Li Alloy under Strain-Controlled Fatigue

Microstructural characteristics, monotonic and strain-controlled cyclic axial behaviors of AW2099-T83 Aluminum-Lithium alloy were investigated. Grain sizes and structures are not uniform in the different orientations studied. High strength and low ductility characterize the tensile behavior of the alloy under static loading. Strain-controlled fatigue testing was conducted at strain amplitudes ranging from 0.3% to 0.7%. Over this range, macro plastic deformation was only observed at 0.7%. Cyclic stress evolution was found to be dependent on both the applied strain amplitude and the number of cycles. Limited strain hardening was observed at low number of cycles, followed by softening, due probably to damage initiation. With low plastic strain, analytical approach was adopted to profile the damaging mechanism for the different applied strain amplitude. Because of the absence of fatigue ductility parameters due to low plasticity, a three-parameter equation was used to correlate fatigue life. Fractured specimens were studied under SEM to characterize the fracture surface and determine the controlling fracture mechanisms. The fractography analysis revealed that fracture at low strain amplitudes was shear controlled while multiple secondary cracks were observed at high strain amplitude. Intergranular failure was found to be the dominant crack propagation mode.

Evaluation of the structural strength and failure for floating collar of a single-point mooring fish cage based on finite element method

Deep-water fish cage floating collar also referred to as float, bears a load in the form of waves, fishing nets, and moorings, in which the longitudinal wave force is the main type of loading. The long-term continuous effects caused by wave force could cause reduction in strength, or vibration failure in the floating collar. In this study, the peak responses of a single-point mooring (SPM) cage floating collar, due to either static or vibration loadings, were calculated by the finite element method (FEM) based on an elastic model. In this model, the float exhibited macro-plastic deformation when the stress of 26.1 MPa, generated under 100 kN load, was greater than the yield limit (25 MPa). With the increase in order modes from 1 to 18 (1.2–7.9 Hz), the stress increased. Moreover, the harmonic stress corresponding to 8 and 0.1 Hz was clearly higher than that for 0 Hz under 100 kN loads, due to structural resonance. The range of stress values due to the random vibration was 16.0–63.7 MPa, when the angular velocities (k) were in the range of 0.01–16 rad s―1. Moreover, the transient response (34.4 MPa) was maximum when the angular velocity was 10 rad s―1 (range 1–100 rad s―1). The reliability rate of the entire body was 53.1% and 97.0% after 108 loading cycles for 100 kN and 10 kN loads, respectively. Furthermore, the fatigue modes of the key components were determined from the outer triangular tops and inner hexagonal intersections. In short, static analysis, vibration analysis, and fatigue analysis of FEM could be used as reliable ways to evaluate structural strength and failure for floating system. Finally, the floating parts could be strengthened with partially double pipes or optimized by increasing the stiffness, and appropriate damping could be widely used.

The Influence of Corrosion Attack on Grey Cast Iron Brittle-Fracture Behaviour and its Impact on the Material Life Cycle

The paper is concerned with brittle‑fracture behaviour of grey cast iron attacked by corrosion and its impact on the life cycle of a spare part made of grey cast iron. In a corrosion chamber, outdoor climatic conditions (temperature and relative air humidity) were simulated in which degradation processes, induced by material corrosion, degrading mechanical properties of a material and possibly leading to irreversible damage of a machine component, occur in the material of maintenance vehicles that are out of operation for the period of one year. The corrosion degradation of grey cast iron, which the spare parts constituting functional parts of an engine are made of grey cast iron, is described with regard to brittle‑fracture behaviour of the material. For the description of corrosion impact on grey cast iron, an instrumented impact test was employed. A corrosion degradation effect on grey cast iron was identified based on measured values of total energy, macro plastic deformation limit, initiation force of unstable crack propagation and force exerted on unstable crack arrest. In the first part of the experiment, a corrosion test of the material concerned was simulated in a condensation chamber; in the second part of the experiment, research results are provided for the measured quantities describing the material brittle‑fracture behaviour; this part is supplemented with a table of results and figures showing the changes in the values of the measured quantities in relation to test temperatures. In the discussion part, the influence of corrosion on the values of unstable crack initiation and arrest forces is interpreted. In the conclusion, an overview of the most significant research findings concerning the impact of corrosion on the life cycle of grey cast iron material is provided.

Structural transformations at the initial stages of fragmentation of plastically deformed polycrystals: A computer experiment

Results have been presented for a computer experiment on concurrent micro-, meso-, and macroscopic studies of the evolution of dislocation structure in a large (adjacent to one of the junctions) domain of a grain after its constant-rate macroplastic deformation to an extent that corresponds to the onset of the stage of developed plastic deformation. The type of dislocation-density and dislocation-charge distributions, as well as amounts and degrees of inhomogeneity in local plastic deformation, have been analyzed. The type of dislocation rearrangements at the junctions and fractures of high-angle grain boundaries has been established, which is responsible for the formation of the first dangling dislocation boundaries, which are mesodefects that trigger fragmentation.

Modeling plasticity by non-continuous deformation

Plasticity and failure theories are still subjects of intense research. Engineering constitutive models on the macroscale which are based on micro characteristics are very much in need. This study is motivated by the observation that continuum assumptions in plasticity in which neighbour material elements are inseparable at all-time are physically impossible, since local detachments, slips and neighbour switching must operate, i.e. non-continuous deformation. Material microstructure is modelled herein by a set of point elements (particles) interacting with their neighbours. Each particle can detach from and/or attach with its neighbours during deformation. Simulations on two- dimensional configurations subjected to uniaxial compression cycle are conducted. Stochastic heterogeneity is controlled by a single “disorder” parameter. It was found that (a) macro response resembles typical elasto–plastic behaviour; (b) plastic energy is proportional to the number of detachments; (c) residual plastic strain is proportional to the number of attachments, and (d) volume is preserved, which is consistent with macro plastic deformation. Rigid body displacements of local groups of elements are also observed. Higher disorder decreases the macro elastic moduli and increases plastic energy. Evolution of anisotropic effects is obtained with no additional parameters.

Following the deformation behavior of nanocrystalline Pd films on polyimide substrates using in situ synchrotron XRD

The deformation behavior of nanocrystalline Pd films on compliant polyimide substrates is investigated using a synchrotron-based in situ tensile testing technique. The combination of analytical and physical modeling of the evolution of peak shape during deformation allows to subdivide the deformation behavior into elastic, microplastic and macroplastic deformation. The results confirm that the deformation behavior of nanocrystalline Pd in the grain size regime of 50nm is still governed by dislocation plasticity, however, in addition, thermally activated grain boundary mediated deformation mechanisms are active. The deformation behavior of the nanocrystalline Pd films is thus given by a specific sequence of a highly heterogeneous elastic response of the nanocrystalline aggregate, its accommodation by grain boundary mediated deformation and upcoming dislocation activity within individual grains.

Combination of in situ straining and ACOM TEM: A novel method for analysis of plastic deformation of nanocrystalline metals

Nanocrystalline metals are expected to exhibit different deformation mechanisms when compared to their coarse grained counterparts because the dislocation storage capacity decreases and the grain boundary mediated processes become more pronounced with decreasing grain size. As a new approach to directly image and quantify the plastic deformation processes in nanocrystalline thin films, a combination of automated crystal orientation mapping in microprobe STEM mode with in situ straining inside a TEM was developed. ACOM-TEM closes the gap between EBSD and BF/DFTEM by providing full orientation maps with nanometer resolution. The novel combination with in situ straining provided for the first time the possibility to directly image and quantify the structural changes of all crystallites in the ensemble of a thin film at the nanometer scale during mechanical deformation. It was used to characterize the metallographic changes during tensile deformation of a nanocrystalline Au thin film prepared by magnetron sputtering. The investigation of the grain size, grain orientation and twinning on a global (grain average over a micron sized area) and local (assembly of selected grains) scale allowed for the development of an in depth picture of the deformation processes. Grain boundary motion and local grain rotation were two of the processes acting to dissipate the applied stress. Additionally, twinning/detwinning occurred simultaneously during straining. These processes, which occurred locally already in the micro-plastic regime, led to global grain growth starting at the transition to the macro-plastic deformation regime.

A synthetic computer model for plastic strain localization

A model for the dislocation kinetics of work and thermal hardening of an element of a deformed medium exemplified by L12 superstructure alloys exhibiting a sophisticated deformation behavior under different thermomechanical loading conditions is used in fusion with a model for the mechanics of macroplastic deformation described in terms of an elastoplastic medium to describe plastic strain localization.

Deformation Research of T10 Steel at Low Stress Repeated Impact

The research use an impact test enginery, choose T10 steel samples, and do a low stress repeated impact collisions experiment. By comparing two kinds of different heat treatment of T10 steel, analyze and research its macro plastic deformation phenomena and plastic accumulation law in low stress repeated collision impact load, and observe its hardness change by hardness microscopicand and its metallographic structure by metallographic microscope to discuss its deformation causes.

BEHAVIOR OF PLASTIC DEFORMATION AND CRACK PROPAGATION ON FULLY LAMELLAR GAMMA-TiAl ALLOY

Practical residual strain field around the microcrack tip of fully lamellar γ- TiAl alloy was estimate the by digital image correlation (DIC) technology with in-situ SEM observation. And the macro plastic deformation before fracture of such low ductility alloys was observed. The results showed that the size of plastic zone around microcrack tip was 2 ~ 10µ m which was much more smaller than the calculated value by fracture mechanics. During the magnified observation on the plastic deformation region, a series of microcracks were observed in the specimen without macro cracking. Furthermore, the size of distribution region of the microcracks were in the same order of magnitude with the calculated plastic zone near primary crack tip which just reflected the comprehensive effect of microcracks and the plastic deformation on the tips of them.

English

This paper deals with rotating bending fatigue tests at high speed (150 Hz) carried out on AISI-SAE 1018 steel with a high content of impurities (non metallic inclusions), for which the high experimental stress inside the specimen is close to the elastic limit of the material. Simulations of rotating loading are obtained by Visual NASTRAN software in order to determine the numerical stresse and strain distributions inside a hypothetical homogeneous specimen; later, this information is used for the experimental set up. A general description of experimental test machine and experimental conditions are developed and then, the experimental results are presented and discussed according the observed failure origin related to the non metallic inclusions and the associated high stress zones. Finally, a simple model is proposed to predict the fatigue life for this non homogeneous steel under high speed rotating bending fatigue tests close to the elastic limit, based on the rate between the visual macro-plastic deformation zone at fracture surface and the total fracture surface, together with the crack initiation inclusion (or inclusions) located at this zone.

Rotating Bending Fatigue Tests on Aluminum Alloy AISI-SAE 6061-T6 at High Speed (150 Hz), and Close to Elastic Limit

This work deals with rotating bending fatigue tests at high speed (150 Hz), carried out on aluminum alloy AISI-SAE 6061-T6 for which the highest experimental stress inside the specimen is close to the elastic limit of material. Simulation results are obtained by Visual Nastran software in order to determine the numerical stress and strain distributions inside the specimen; then, this information is used for the experimental set up. A general description of experimental test machine and experimental conditions are developed in first sections, following section present the experimental results and discussion about the observed failure origin related to discontinuities and the associated high stress zones. A principal contribution of this work is related to the analysis of fracture surfaces and its correlation with experimental fatigue endurance: a simple model is proposed for the prediction of fatigue life of this aluminum alloy under high speed rotating bending fatigue tests, based on the surface density of associated fatigue macro-plastic deformation zones close the crack initiation.

Mechanical properties of pulse discharge sintered Cr 2AlC at 25–1000 °C

Ternary compound Cr 2AlC was synthesized by a reactive sintering process and its mechanical properties in the 25–1000 °C temperature range were studied by 4-point bending tests. The flexural strength of Cr 2AlC decreases continuously from 555 ± 11 MPa at room temperature down to 100 ± 4 MPa at 1000 °C and this strength decreasing tendency is more obvious as the testing temperature is higher than 900 °C. The ductile-to-brittle transition temperature of Cr 2AlC locates in the range of 800–900 °C. The macro-plastic deformation of Cr 2AlC is mainly attributed to the initiation and propagation of large number of microcracks.

Dilatational and distortional behavior of cracks in magnetoelectroelastic materials

Dilatation and distortion are the two basic modes of deformation in continuum mechanics. They occur at different time and position depending on the size scale under consideration. At the macroscopic scale, dilatational waves are said to arrive prior to distortional waves at a given location. Macro-plastic deformation caused by distortion off to the side of a macro-crack precedes that of macro-fracture caused by dilatation ahead of a macro-crack. Micro-plasticity and micro-cracking can introduce non-linearity to the macroscopic material behavior. Such effects can be modeled directly as geometric defects (dislocations and/or micro-cracks) or indirectly by introducing effective material coefficients in the constitutive relations. Material anisotropy and/or inhomogeneity, however, do not separate hydrostatic tension (or compression) state of stress from that of shear on the octahedral planes. The latter is the basis of the von-Mises yield criterion. Stated in terms of the strain energy density function, linear separation into one part for dilatation and another for distortion can be made only if the material is linearly isotropic and homogeneous. Recent trends of miniaturization of electronic components requires a better knowledge of how the micro-structure would affect the material response due to mechanical stress as well as other sources of disturbances. Continuum mechanics results need to be interpreted over a wider range of time and size scales, and a better understanding of their limitations need to be achieved. In what follows, the stationary values of the strain energy density function d W/d V will be shown to yield an interpretation of the local dilatational and distortional behavior of anisotropic (or non-homogeneous) materials with or without cracks. This quantity is not only attractive from the view point of mathematics and mechanics because it is positive definite, it also makes physical sense for non-linear materials where the quotient d W/d V could still be used to separate dilatation from distortion. Application of the minimum local energy density function should be distinguished from the global energy minimization method used in molecular dynamics which is applied to determine the equilibrium configuration of atoms. The response of a line crack in a magnetoelectroelastic (MEE) material will be used for discussion in order to illustrate the character of multi-scaling. Expressed differently is that the time and size scales for the transfer of magnetic, electric and elastic energies may not be the same. An arbitrary limiting process that assumes vanishing crack segments created by all forms of energy may not be justified at the local scale levels. What has been assumed at the global scale for isotropic and homogeneous situations may not be valid at the local scale levels where anisotropy and non-homogeneity are the rule rather then the exception. Numerical results are obtained for the composite BaTiO 3–CoFe 2O 4. They show how magnetic and electric poling normal to a line crack could greatly affect the interplay between volume change and shape change of the local continuum elements at both the macroscopic and microscopic scale level. The micro-energy-density function is found to be three orders of magnitude higher than that at the macroscopic scale. The volume fraction of the inclusions made of BaTiO 3 also comes into play for both Mode I and II mechanical loading. Potential micro-crack bifurcation is predicted in a region very close to the crack tip while macro-crack initiation occurs in a straight line for the MEE material. The information is relevant for developing macro- and micro-crack models where quantities and situations of secondary importance can be weeded out.

The effects of macrolocalization of deformation in Al‐based composites with Al2O3 inclusions

ABSTRACTUsing a television optical TOMSC system, regular features of localization of macroplastic deformation are studied in composite aluminum‐based materials with hard inclusions of Al2O3 under tension. Before investigation, the specimens are subjected to cold rolling, tempering for 30 min at 500 °C with subsequent cooling in air, and annealing at 550 °C during 2 h with subsequent cooling in furnace. It is shown that the tempered and annealed alloys exhibit mobile centers of active macrolocalization long before the formation of a stationary neck preceding material fracture. Physical mesomechanics, relying on the concepts of the leading role of different‐scale stress concentrators that relax and develop in a loaded heterogeneous material, is capable of providing a qualitative description of macrolocalization development.

Evaluation of Local Deformation of AMg6 Alloy from Cracked Inclusion Displacements

By means of electron transmission microscopy, we inquire into the statistic parameters of cracking of brittle inclusions of the second phase in the AMg6 alloy matrix in tension. A linear dependence is established between macroplastic deformation of tested specimens and relative displacement of cracked inclusions. It is suggested that the cracked inclusions of the second phase be used as indicators of macroplastic deformation of the matrix. A procedure is proposed for expert evaluation of plastic tensile deformation of materials from the analysis of displacements of cracked inclusions of the second phase.

Motion of dislocations and interfaces during deformation of martensitic Cu–Al–Ni crystals

Investigations of the ultrasonic strain amplitude-dependent internal friction (ADIF) and of the influence of ultrasonic vibrations on the macroplastic strain (the acoustoplastic effect) have been performed in situ during deformation of quenched Cu–Al–Ni single crystals in the β′ 1 martensitic phase. The effect of the macroscopic plastic strain rate on the ADIF and acoustoplastic effect as well as the kinetics of the acoustoplastic effect has been studied. The results of in situ ADIF measurements are compared with data on the ADIF temperature dependence. Observed regularities are attributed to differences in the mechanisms of the macroplastic deformation of the martensitic phase, related to the motion of intervariant interfaces, and of the reversible anelastic strain, which, at ultrasonic frequencies and moderate strain amplitudes, is largely due to the oscillatory motion of the partial dislocations. The conclusion has been drawn that the dynamics of partial dislocations at temperatures of 210–300 K is to a great extent determined by their interaction with atmospheres of mobile pinners with saturated density. Simultaneous measurements of the ADIF and acoustoplastic effect allows the conclusion that, for high oscillatory strain amplitudes, the breakthrough of partial dislocations beyond the mobile atmospheres of pinners initiates the step-like accumulation of the macroplastic strain due to the motion of intervariant boundaries.

Radio-frequency paramagnetic resonance spectra, detected from dislocation displacement in NaCl single crystals

Results are reported here of a study of the resonance effect of a constant magnetic field and a variable magnetic field crossed with it on the rate of macroplastic deformation and motion of edge dislocations in NaCl crystals. The frequencies of the variable magnetic field at which the maximum variations in the plasticity of the crystals are observed correspond to the resonant frequencies for transitions between the Zeeman sublevels in paramagnetic complexes of point defects and complexes consisting of a point defect and a dislocation. Analysis of the radio-frequency spectra obtained enabled us to establish the role of intercrystal reactions in the formation of the mechanical properties of the crystals.