Onset Of Plastic Deformation Research Articles

Influence of Surface Segregation on the Mechanical Property of Metallic Alloy Nanowires

ABSTRACTThe influence of surface segregation on the elastic properties of Pt-M (M = Ni, Co, or Fe) nanowires (NWs) are examined by comparing the predicted Young’s moduli of the segregated and non-segregated nanowires using density functional theory (DFT) calculations and the computed stress-strain curves under tensile loading using molecular dynamics (MD) simulation method. The moduli of the segregated NWs were found to be higher than that of the non-segregated ones. It is believed that the surface segregation increases the number of Pt-M bonds across the outermost and second surface layers, and thus enhances the Young’s modulus of the segregated Pt-M nanowires. MD results confirm our DFT results and it is found that onset of plastic deformation could be altered by the surface segregation process, as well.

In situ study of plastic deformation in a Mg–Ce alloy using polychromatic micro X-ray diffraction

Abstract Synchrotron micro X-ray diffraction (micro-XRD) has been applied to map, at an intragranular length scale, the evolution of deviatoric strain and dislocation distribution across an embedded grain in a polycrystalline Mg(0.2 wt.%)Ce alloy sample during uni-axial tensile loading. Employing a normal distribution, elastic strain accumulation was quantified by calculation of the mean axial ( ɛ ¯ xx ) and normal ( ɛ ¯ zz ) components, parallel and perpendicular to the loading direction, respectively. In the axial direction, tensile strain increased from 2.3× 10 −3 at an applied macroscopic load of 15 MPa to 6.6× 10 −3 at140 MPa. Whereas, in the normal direction an initial compressive strain of −1.1× 10 −3 at 15 MPa increased to −4.2× 10 −3 at 140 MPa. A noticeable increase in the strain distribution (variance) with load was also accompanied by dislocation slip, identified by Laue spot streaking. Simulation of observed lattice rotation streaking of individual Laue spot patterns was used to determine the active dislocation slip mechanism. At the onset of plastic deformation (90 MPa) relatively uniform streaking was identified. After comparison of observed and simulated patterns, streaking was attributed to ensembles of edge geometrically necessary dislocations (GNDs) undergoing a >-type basal slip 1 1 2 ¯ 0 > (0 0 0 2). At 140 MPa non-uniform streaking and splitting of individual spots was prevalent. It is anticipated the inhomogeneous nature of the streaking arises from distinctly different loading conditions imposed by neighbouring grain boundaries. Onset of Laue spot splitting was attributed to ordering of dislocations into geometrically necessary boundaries (GNBs). Up to 4 individual sub-grains were identified at some locations while in the grain center only 2 distinct sub-grains were identified. Newly formed small-angle tilt boundaries produced a maximum misorientation of up to 1.4 ° within the original parent grain.

Effect of strain rate and temperature on the plastic deformation behaviour of a bulk metallic glass composite

► A BMG composite is compressed at different strain rates and temperatures. ► Initially, dendrites deform and the participation of the matrix increases progressively. ► At temperatures above 600 K deformation is fully controlled by the matrix. ► Higher strain rates allow more participation of dendrites in the deformation process. ► Transition from homogeneous to heterogeneous flow occurs at higher strain rates. The composites consisting of amorphous matrix reinforced with crystalline dendrites offer extraordinary combinations of strength, stiffness, and toughness and can be processed in bulk. Hence, they have been receiving intense research interest, with a primary focus to study their mechanical properties. In this paper, the temperature and strain rate effects on the uniaxial compression response of a tailored bulk metallic glass (BMG) composite has been investigated. Experimental results show that at temperatures ranging between ambient to 500 K and at all strain rates; the onset of plastic deformation in the composite is controlled by that in the dendrites. As the temperature is increased to the glass transition temperature of the matrix and beyond, flow in the amorphous matrix occurs readily and hence it dictates the composite's response. The role of the constituent phases in controlling the deformation mechanism of the composite has been verified by assessing the strain rate sensitivity and the activation volume for deformation. The composite is rate sensitive at room temperature with values of strain rate sensitivity and activation volume being similar to that of the dendrites. At test temperatures near to the glass transition temperature, the composite however becomes rate-insensitive corresponding to that of the matrix phase. At low strain rates, serrated flow akin to that of dynamic strain ageing in crystalline alloys was observed and the serration magnitude decreases with increasing temperature. Initiation of the shear bands at the dendrite/matrix interface and propagation of them through the matrix ligaments until their arrest at another interface is the responsible mechanism for this.

Peculiar behavior of structure rearrangement in nanofiber of intermetallic Ni3Al, containing long-period paired thermal anti-phase boundaries, under high-rate tensile uniaxial loading along

Using molecular dynamics simulations, peculiarities of structure rearrangement in nanofiber of intermetallic Ni3Al containing long-period, paired, thermal (nonconservative) anti-phase boundarties (APBs is investigated in the course of high-rate, tensile uniaxial loading along . Four main deformation stages are determined (quasi-elastic, plastic, material flow and rupture), with each stage revealing particular features of structure transformations and energy transfer. The presence of periodic thermal planar defects in the long-period nanostructure (combined thermal anti-phase boundaries) significantly affects the onset of plastic deformation. A change in the type of thermal APBs in the long-period structure in turn affects the time to total rupture of nanofiber under plastic deformation condition. For the thermal AA 1/2 {001}APBs, the time to total nanofiber rupture is slightly decreased, while that for the thermal AB 1/2 {001} APBs is considerably increased.

Selective Self-Assembly of Polymer Structures Using Templated Assembly by Selective Removal

This paper presents the first models and experimental demonstrations of the selective assembly of deformable polymer structures onto rigid assembly templates using templated assembly by selective removal (TASR). Polystyrene microspheres with 2 μm diameter were successfully assembled with nearly 100% assembly yield into assembly sites that are uniformly well-matched to the microspheres' size and shape. The experiments also demonstrated selective assembly of microspheres into well-matched assembly sites when both well-matched and poorly matched assembly sites were available. In order to identify the limits of the TASR process for assembly in deformable systems, an analytical model was created. This model assesses the applicability of TASR to different materials systems using Hertzian contact theory to identify the onset of plastic deformation in a loaded, deformable sphere on a surface of given geometry. Using this model, TASR's effectiveness was predicted to include many polymers on rigid substrates. Quantitative comparison of the deformable system assembly data obtained here with existing TASR models for nondeformable systems shows significant agreement.

Effects of Vacancies on the Onset of Plasticity in Metals—An Atomistic Simulation Study

As strength of crystalline materials strongly depends on their ability to nucleate new dislocations or move the existing ones, a proper knowledge of factors that play a major role at the first stages of plastic deformation is highly desirable. However, fundamental understanding of the role of point defects, such as vacancies or inclusions, on the onset of plastic deformation in otherwise defect-free solids is still lacking. In this work, atomistic simulations are applied to study the role of vacancies in the inception of plasticity in metals. In particular, the effect of single vacancy as well as different vacancy distributions on the onset of plastic deformation in Ni single crystal during the nanoindentation test is explored. The combined effect of vacancy concentration and temperature on the onset of plasticity is also studied. The increase in vacancy concentration typically results in a decrease of load at the onset of plasticity. However, it is found that this is not always the case, and a particular location of one vacancy can be more important for the onset of plasticity than the higher total number of vacancies distributed in the crystal. It is also found that at higher temperatures, the effect of vacancy concentration on the load at the onset of plasticity is less pronounced than at low temperature.

Defect and surface asperity dependent yield during contact loading of an organic molecular single crystal

The onset of plastic deformation was investigated using nanoindentation in single crystals of the explosive cyclotrimethylene trinitramine (RDX). Cleavage and habit planes were tested revealing a range of yielding behaviors. Smooth habit planes of unprocessed single crystals exhibited distinct yield points near the theoretical shear strength; planes produced by cleavage yielded at lower applied stresses. Cumulative probability distributions of maximum shear stresses at yield were used to illustrate the representative yielding behavior for samples prepared by the different methods. A statistically significant difference was observed for cleavage and habit planes. This suggested that structural defects, such as dislocations from growth and sample preparation, were being probed and nanoindentation can be used to correlate the mechanical response of organic molecular crystals with defect density. This capability may help explain the observed range of measurement differences in fundamental properties in this class of materials, such as sensitivity to the initiation of detonation in explosives, and disparate tablet integrity and stability responses in polymorphs of some pharmaceutical materials.

Neutron Diffraction Analysis of Load Transfer in DP 600 Steel During <i>In Situ</i> Tensile Tests

The load transfer among ferrite orientations and between ferrite and martensite was analysed in DP 600 steel by means of neutron diffraction duringin situtensile tests on the multiple pulse overlap time-of-flight strain scanner POLDI. The material had 0.07 wt% C and a martensite volume fraction of 15%.In situtests were done in “Young” as well as in “Poisson setup”. The martensite phase could not be probed due to its low tetragonality. The curves of the lattice plane strains as a function of the externally applied macroscopic stress reveal (1) plastic relaxations of transformation and intergranular stresses in the compliant <100> oriented grains, and (2) a second inflection point in the fully plastic part indicating the onset of plastic deformation of the hard phase.

In situ micro-Raman compression: characterization of plasticity and fracture in GaAs

In situ Raman micro-pillar compression has been shown to be capable of determining the onset of plastic deformation as well as characterizing plastic deformation and fracture. The compression experiments were carried out on 10 µm diameter single crystalline GaAs micro-pillars at room temperature. The deformation and fracture events were revealed by a shift in position and broadening of the characteristic Raman peak in agreement with the load drops measured by the load cell and post-mortem observations from scanning electron microscopy.

Inception of plasticity in the presence of vacancies in FCC single crystals: indenter size effect

ABSTRACTAtomistic simulations of nanoindentation tests were used to study the indenter radius size effect in the presence of vacancies in a (111) single crystal of nickel. For radii from 2 nm to 8 nm, the maximum shear stresses under the indenter at the onset of plastic deformation in crystals with vacancies were compared to those which cause yield in perfect crystals by placing a single vacancy in a position near the maximum shear stress underneath the indenter tip. The effect of the presence of vacancies is lowered by decreasing the indenter radius. Results obtained for several random distributions of vacancies, in the range 3.3e-4 to 0.0033, show that placing a single vacancy near a specific location produces similar results as using larger numbers of vacancies while simplifying the complexity of the simulation. Finally, visualizations of atomic configurations of a single crystal with vacancy concentration of 3.3e-4 for radii of 4 nm and 6nm show that the heterogeneous nucleation is a size dependent phenomenon.

Acoustic emission from deformed magnesium alloy based composites

Magnesium alloys based composites were deformed at ambient temperature. The acoustic emission (AE) was detected in situ during deformation. The time dependence of the AE count rate was measured at two threshold levels. Two AE peaks were found. The peak observed at the onset of plastic deformation was ascribed to massive twinning and dislocation slip, and the second, broader maximum was caused mainly by fibre breakage and debonding at the fibre-matrix interfaces. The AE activity was significantly influenced by the load transfer from the matrix to the fibres.

Characterization of porous, net-shaped NiTi alloy regarding its damping and energy-absorbing capacity

Porous NiTi alloys are highly attractive for energy absorbers, damping devices and biomedical implants. In the present work, metal injection moulding (MIM) in combination with the application of a suitable space holder material was used for the production of NiTi parts with well defined pore sizes and porosities in the range of 30–70vol.%. For comparing the properties, porous titanium and Ti–6Al–4V samples were prepared in the same manner.Focus of the present work was a detailed investigation of the mechanical properties of porous NiTi to estimate its potential regarding the abovementioned applications. For a Ni-rich NiTi alloy with a porosity of 50vol.%, fully pronounced pseudoelasticity after 6% compression was demonstrated. An energy dissipation of 1.5MJ/m3 was measured, which could be directly related to the reversible austenite–martensite phase transformation. At higher deformations, pseudoelasticity becomes more and more superposed by the onset of plastic deformation. Nevertheless, even at deformations of up to 50%, a clearly pronounced amount of pseudoelastic shape recovery still remained. Fatigue of pseudoelasticity was investigated by conducting of up to 230,000 load cycles to 4% compression at a frequency of 1Hz.

Variable thermal loading analysis on (111) plane of single crystal gold

The temperature response on the properties of single crystal gold (111) plane at elevated temperatures was considered in this work. The ability to perform nanoindentation experiments at elevated temperatures opens up significant new possibilities in nanotechnology. The experiments are performed at various temperatures of 373 K, 473 K and 573 K with tailor made Berkovitch tip of radius 100 nm to study the behavior of single crystal gold. The new phenomenon of material bouncing back under the indenter at the end of unloading was clearly noted, due to the accumulation of high energy. The results for different temperatures were compared. Our experiments clearly show the onset of the first burst of dislocation glide, which is indicated by a sudden increase of displacement with no increase of loading, the onset of plastic deformation in connection with the periodic bursts, and the strain hardening, softening and release effects. Pile up was also observed. The complete elastic range was found. There was significant drop in the hardness, elastic modulus and the increase in depth with increasing temperature. The elastic recovery was reduced at higher temperatures.

An investigation of the onset of elastoplastic deformation during nanoindentation in MgO single crystal (0 0 1) and (1 1 0) planes

Nanoindentation was performed in the (0 0 1) and (1 1 0) planes of magnesium oxide single crystal. The results showed that pop-in events were observed in both cases, which were accompanied by acoustic emissions. The energy released in a pop-in event was related to the acoustic emission energy. The pop-in indicated the onset of plastic deformation. Using the Hertzian contact theory and the measured pop-in load, the critical shear stresses for slipping on the (0 0 1) and (1 1 0) planes were estimated. The stress value for the (0 0 1) plane was about 17% higher than that of the (1 1 0) plane.

Quantification of release from microcapsules upon mechanical deformation with AFM

We present a novel method combining fluorescence microscopy with AFM force spectroscopy to investigate release of encapsulated materials from polyelectrolyte multilayer capsules upon mechanical deformations. The experimental approach allows independent measurement of release and plastic deformation thresholds. Our results reveal a well defined critical load upon which release takes place and let us correlate release with the onset of plastic deformations.

The application of acoustic emission technique to plastic deformation of low carbon steel

In present paper, using Acoustic Emission (AE) as an ultrasonic technique, the onset of plastic deformation has been investigated on smooth and notched specimens of AISI 1010 steel. AE activities (number of counts and signal energy) and stress–strain curve were measured during tests. The results show that activity of AE increases near the yield point and reaches to maximum at beginning of plastic deformation. AE activity decreases significantly with further straining. The main source of AE at all stages of deformation is associated with massive dislocation motion.

Dislocation generation in heat-treated nitrogen-doped silicon wafers when applying external loads

The features of generation and motion of dislocations in nitrogen-doped dislocation-free silicon wafers after their multistage thermal treatments are investigated. It is established that doping with nitrogen leads to a substantial increase in stresses of the onset of plastic deformation from the external and internal heterogeneous sources in heat-treated wafers. The motion velocity of dislocations in nitrogen-doped silicon wafers is lower than in undoped wafers. The strengthening effect of nitrogen is apparently caused by its activating effect on the decomposition process of a supersaturated solid solution of oxygen during heat treatment.

Load capacity of components with initial point contact

Formulas have been derived for the critical loads corresponding to the onset of plastic deformation in different loading stages. Likewise, a formula has been obtained for the limiting load corresponding to sharp deviation of the size of the surface impression from the values predicted by the Hertz equations, which describes elastic behavior. A means of increasing the load capacity with initial localized contact is considered.

In situ strain profiling of elastoplastic bending in Ti–6Al–4V alloy by synchrotron energy dispersive x-ray diffraction

Elastic and plastic strain evolution under four-point bending has been studied by synchrotron energy dispersive x-ray diffraction. Measured strain profiles across the specimen thickness showed an increasing linear elastic strain gradient under increasing four-point bending load up to ∼2 kN. The bulk elastic modulus of Ti–6Al–4V was determined as 118 GPa. The onset of plastic deformation was found to set in at a total in-plane strain of ∼0.008, both under tension and compression. Plastic deformation under bending is initiated in the vicinity of the surface and at a stress of 1100 MPa, and propagates inward, while a finite core region remains elastically deformed up to 3.67 kN loading. The onset of the plastic regime and the plastic regime itself has been verified by monitoring the line broadening of the (100) peak of α-Ti. The effective compression/tension stress-strain curve has been obtained from the scaling collapse of strain profile data taken at seven external load levels. A similar multiple load scaling collapse of the plastic strain variation has also been obtained. The level of precision in strain measurement reported herein was evaluated and found to be 1.5×10−5 or better.

Evolution of deformation structures under varying loading conditions followed in situ by high angular resolution 3DXRD

With high angular resolution three-dimensional X-ray diffraction, individual subgrains are traced in the bulk of a polycrystalline specimen and their dynamics is followed in situ during varying loading conditions. The intensity distribution of single Bragg reflections from an individual grain is analyzed in reciprocal space. It consists of sharp high-intensity peaks arising from subgrains superimposed on a cloud of lower intensity arising from dislocation walls. Individual subgrains can be distinguished by their unique combination of orientation and elastic strain. The responses of polycrystalline copper to different loading conditions are presented: during uninterrupted tensile deformation, formation of subgrains can be observed concurrently with broadening of the Bragg reflection shortly after onset of plastic deformation. With continued tensile deformation, the subgrain structure develops intermittently. When the traction is terminated, stress relaxation occurs and number, size and orientation of subgrains are found to be constant. The subgrain structure freezes and only a minor clean-up of the dislocation structure is observed. When changing the tensile direction after pre-deformation in tension, a systematic correlation between the degree of strain path change and the changes in the dislocation structure quantified by the volume fraction of the subgrains is established. For obtaining the subgrain volume fraction, a new fitting method has been developed for partitioning the contributions of subgrains and dislocation walls.