Initial Stage Of Plastic Deformation Research Articles

Initial stages of plastic deformation in miniature samples

The procedure developed here and the automation of the system of measuring the parameters of microplastic deformation can be used for investigating the initial stages of plastic deformation as well as for evaluating the mechanical properties of large objects from the test results on miniature samples.

引張変形した多結晶アルミニウムにおけるすべり帯観察

Slip bands formed at lower strain region ranged from 0.01 to 2% in polycrystalline aluminum were investigated by optical microscopy and crystallographic orientation analysis using a SEM-ECP method. Many of grains exhibited slip bands caused by the primary slip system at 0.01% strain. Half of them produced new additional slip bands caused by the secondary slip system with increased plastic deformation. While, some grains showing slip bands formed by the secondary slip system alone at the initial stage of plastic deformation (0.01% strain) produced slip bands due to the primary slip system with further plastic deformation. Short slip bands were frequently observed at 2% strain near the grain boundaries. The slip system of this slip bands had very small Schmid factor. The phenomenon of this slip system becoming active could not be explained by the several theories proposed for the bicrystal deformation.

On the initial stage of plastic deformation of sulphur‐doped gallium phosphide

AbstractThe overall deformation behaviour between room temperature and 1000 K has been investigated by bending and uniaxial compression at small strains ≦ 0.1%. Ductile‐to‐brittle transition temperature was found at 44% of the melting point. Dislocations are generated mainly at surface irregularities. Whereas screw dislocations predominate at the beginning 30° and 60° segments become more important after cross slip and interaction of different slip systems.

Behavior of block boundaries during ultrasonic vibration in alkali halide crystals at temperatures 20?300�C

The behavior of block boundaries was investigated in alkali halide crystals of various orientations deformed by ultrasound at frequencies 40–73 kHz in the temperature interval 20–300°C. The specimen orientation was defined by the angle between the fourfold axis and the propagation direction of the ultrasonic wave. The initial stage of plastic deformation at room temperature is due to the generation of sources at the block boundaries in KCl and KBr, and by the heterogeneous nucleation of dislocation in NaCl and LiF. In the temperature interval 200–300°C, in NaCl and LiF dislocation multiplication begins already with the generation of sources at the block boundaries. In 30° orientation specimens the beginning of multiplication is preceded by the motion initiated by the ultrasound of sections of the block boundaries in the {1 0 0} secondary glide system.

Energy storage during the tensile deformation of Armco iron and austenitic steel

A modification of the single-step method based on continuous detection of IR radiation emitted directly from a strained sample was employed to study energy storage during the initial stage of plastic deformation of Armco iron and austenitic stainless steel. The existence of a maximum in the dependence of the ratio of stored energy to expended energy on strain was confirmed for both materials in the range of homogeneous deformation and was interpreted in terms of variations in the contributions of endo-energetic and exo-energetic processes; the contribution of endo-energetic processes is related to the increase in the dislocation density due to the generation of dislocations and the contribution of exo-energetic processes is related to energy dissipation due to movement, rearrangement and annihilation of dislocations.

Influence of temperature on the initial stage of plastic deformation of LiF crystals

The temperature dependence of the critical resolved shear stress, the stress relaxation parameters, and effects of the strain rate on the flow stress at the initial stage of plastic deformation in the temperature range 1.7 to 330 K are studied on LiF crystals containing below 10−3 wt% of divalent metal impurities. The velocity of individual screw dislocations versus stress in the range of 10−8 to 1 cm/s at temperatures 4.2, 77, and 300 K is measured. The experimental results allow to conclude that the plasticity of LiF crystals can be described by at least two basic thermally activated mechanisms in the temperature range studied. One, the overcoming of the Peierls barriers characterized by a Peierls stress τ*p = 35 MPa and an ultimate activation energy of ΔG0 = 0.08 eV prevails at lower temperatures. The other is dominant in the temperature range from about 100 K to room temperature (the overcoming by mobile dislocations of randomly arrayed impurities, as well as strain induced defects). [Russian Text Ignored].

Initial stage of plastic deformation of substrates in heterostructures

The model of the generation of misfit dislocations by dislocations gliding in a substrate covered with a crystalline or amorphous film is discussed. The critical film thickness (hd*) is shown to be the same for the beginning of plastic deformation both, in the film and in the substrate. If only one side of the substrate is covered with the film, there is one more critical film thickness (hd**) at which dislocation configurations with straight segments in the neutral surface of the substrate appear. Theoretical calculations are confirmed by experimental results. Both types of dislocation configurations in the substrate theoretically predicted for the initial stage of plastic deformation are found by X-ray topography. [Russian Text Ignored].

Modeling of the evolution of a dislocation structure and development of microplastic deformation in single crystals of fcc substitutional solid solutions

An analysis is made of evolution of a dislocation structure during the initial stage of plastic deformation and the stress-strain diagrams are derived theoretically for single crystals of fcc substitutional solid solutions. The example of Cu-Al alloys is used to confirm the theoretically predicted relationship between the stress and strain, and also between the flow stress and the resistance to the motion of a single dislocation.

Plastic constitutive relations for unidirectionally reinforced composites: Application to large deflection of plates

The large deflection behavior of the laminated composite plates beyond the elastic limit are investigated. A modified version of Hill's theory for anisotropic metals is used in the analysis of a baron epoxy cross ply symmetric plate under a uniform load and having simply supported immovable edges. An alternate set of plastic stress-strain relations is also employed wherein the effects of hydrostatic pressure on yielding and compressibility in the plastic range of composite materials in the initial stage of plastic deformation are considered. The solutions of both approaches are thus compared.

Dislocation mechanism of the initial stage of plastic deformation of polyethylene

A unified treatment of the initial stages of the plastic deformation of the polyethylene single crystal — phase transformation from the orthorhombic to the monoclinic lattice, twinning, and crack formation — is proposed. This treatment, which makes use of dislocation theory, is based on an analysis of experiments on the deformation of the polyethylene single crystal.

Mécanismes de déformation de composites de solidification interprétation du comportement macroscopique à l'aide d'essais de traction in situ effectués dans un microscope électronique à haute tension

Abstract DEFORMATION MODES OF UNIDIRECTIONALLY SOLIDIFIED CARBIDE-REINFORCED COMPOSITES: IN SITU EXPERIMENTS IN THE HVEM RELATED TO MACROSCOPIC BEHAVIOUR In situ investigation of the plastic deformation in tension was performed in the high voltage electron microscope on a unidirectionally solidified metal matrix carbide-reinforced fibrous composite. This technique was particularly suitable for observing dislocation pile-ups against the fibres during the initial stage of plastic deformation. The dislocation configurations were found to be in perfect accordance with previous observations on bulk material. A theoretical analysis of the observed dislocation pile-ups led to an unambiguous determination of the deformation modes of such composites. La déformation plastique de composites de solidification, à matrice métallique renforcée par des fibres monocristallines parfaites de monocarbures de métaux de transition, a pu être étudiée grâce à. des essais de traction in Situ effectués dans un microscope électronique à haute tension. Cette technique a permis d’observer, dans lee tout premiers stades de la déformation plaatique, la formation d’empilements de dislocations contre les fibres. Ces configurations de dislocations correspondent aux prévisions déduites des essais de traction sur échantillons massifs. L’étude théorique de ces empilements de dislocations a permis de déterminer sans ambiguité le mode de déformation de ces composites.

The Hardening of Fe–Mo Single Crystals by Non-Deforming Precipitated Particles

Single crystals of iron base molybdenum alloys containing a high volume fraction of precipitated particles Fe2Mo noncoherent with the matrix were extended at various temperatures between −110° and 200°C. Critical resolved shear stresses in these single crystals depended on the particle size, mean interparticle spacing, and deformation temperature, but not on the crystal orientation of the single crystals used. The increment of the macroscopic yield stresses by the precipitated particles was obtained by subtracting the solid solution strengthening of the matrix from the observed yield stresses in the aged specimens. It was found that the stress increment by the noncoherently precipitated particles was explained fairly well by the Orowan’s mechanism in this alloy system. At the initial stage of plastic deformation, neither primary nor cross slip lines could be resolved by replica-electron microscopy. On the other hand, at a large strain of about 8%, cross slips were clearly observed to take place over a distance of the order of particle size. The changes in crystal orientation with extension were barely perceptible in the over-aged Fe-10%Mo alloys, probably less than 1 degree even after 10% elongation. The increases in flow stress due to work hardening seemed to be rather independent of the crystal orientation of aged specimens and deformation temperature. This can better be described in terms of a secondary slip theory by Ashby than a model proposed by Fisher et al..

Equilibrium State of Slip Bands in Plastically Bent NaCl Crystals

AbstractThe slip bands known from ionic crystals and being typical for the initial stage of plastic deformation are found to have a limited, narrow width and a defined long‐range stress field in the present case of NaCl crystals tested in bending. This deformation structure has been investigated by combining special methods of photoelasticity, interferometry, and electron microscopy. The stress field of the slip band is described by a pile‐up of edge dislocations which are necessarily in excess within a bent crystal. The superposition of the slip‐band stress field to the applied stress leads to a stress equilibrium which can be reached “statically” or “dynamically” and which explains the limitation of the width and of the integral step height of the slip band. As a consequence of this limitation, the number of slip bands increases considerably during further bending. Thereby the atomic slip steps will cover the surface more and more homogeneously so that after a surface strain of about 1% individual slip bands can no longer be distinguished.

Stress field caused by a slid crystal at a free surface

The initial stage of plastic deformation of a fine grained aggregate with slip occurring only in the most favourably-oriented crystals is considered. The elastic modulus of the individual crystals is taken to be isotropic. The sliding crystals are assumed to be cubical in shape and located at a free surface of the aggregate. Strain hardening is considered. The analogy of plastic strain gradient and body force in causing stress field in an elastic medium, and Mindlin's solution of stress field caused by a point force in a semi-infinite solid, are applied. The calculated slip distribution and the stress field caused by the slip are shown. The slip near the free surface and other boundaries of the slid crystals is less than that at the interior.

Stress field in metals at initial stage of plastic deformation

The initial stage of plastic deformation of a fine grained aggregate with slip occurring only in the most favourably oriented crystals is studied. The elastic modulus of the individual crystals is taken to be isotropic. The sliding crystals are assumed to be cubical in shape and located at the interior of the aggregate. Work-hardening in the sliding crystal is considered. The analogy of plastic strain gradient and body force in producing stress field in an elastic medium is developed. This analogy is applied to calculate the slip and stress fields caused by (1) one sliding crystal and (2) two sliding crystals spaced at a clear distance of half the crystal width. This shows the interaction effect of slips in two nearby crystals. From these stress fields, the residual stress may be easily obtained and the Bauschinger effect is clearly shown.

Initial stages of plastic deformation in copper and aluminum

The early stages of plastic flow and microcreep have been studied in single crystals and in polycrystalline copper and aluminum using a method with a strain sensitivity of 2 × 10 −6 in the temperature range −196 to 25°C. The temperature dependence of the 10 −6 yield stress is interpreted as arising from a thermal activation of stacking faults under stress. This model results in a value of 25 ergs/cm 2 for the stacking fault energy of copper and 100 ergs/cm 2 for aluminum.

Mechanical deformation of aluminium bicrystals

Aluminium specimens composed of two symmetrically oriented crystals having a common 〈111〉 axis were subjected to a tensile test. It was observed that the yield stress and the rate of work hardening increased with the orientation difference between the crystals. In the initial stages of plastic deformation a linear stress-strain region was observed; the length of this region became shorter as the rate of work hardening (i.e., the orientation difference) increased.

The Initial Stage of Plastic Deformation in Lead Under Compression

In the steady-creep stage in metals, stress (or resistance to deformation) is related to the velocity of deformation, in accordance with the hyperbolic-sine law, or the exponential law. These laws, however, fail completely to describe the initial stage of plastic deformation in lead. The present series of tests is an investigation of this initial stage in compression. An optical lever system and high speed kymograph have been employed. The photographic records obtained show the progress of deformation with time. The duration of the initial stage increases with the compressing load. For a load of 120.0 kg the duration is about 0.16 sec. It is found that a unique, initial deformation, independent of time, is associated with each load. From measurements on the deformation curves, resistances as functions of time have been calculated. These data indicate a linear relationship between the resistance R and time t: R=l+mt,where l=a parameter of small value, evidently a correction term, and m=rate of increase of resistance. Using this equation and the dynamic constants of the apparatus, deformation curves in good agreement with experiment have been derived. Resistance in the initial stage appears to be only approximately a function of deformation alone