Showing papers on "Stress relaxation published in 1995"
TL;DR: In this paper, the effect of hydrogen on dislocationdislocation and dislocationdashimpurity atom interactions is studied under conditions where hydrogen is in equilibrium with local stresses and in systems where hydrogen increases the shear modulus.
Abstract: The effect of hydrogen on dislocationdashdislocation and dislocationdashimpurity atom interactions is studied under conditions where hydrogen is in equilibrium with local stresses and in systems where hydrogen increases the shear modulus. In the case of two edge dislocations (plane strain) the effect of hydrogen is modeled by a continuous distribution of dilatation lines whose strength depends on the local hydrogen concentration. The hydrogen distribution in the atmospheres is adjusted to minimize the energy of the system as the dislocations approach each other. The iterative finite element analysis used to calculate the hydrogen distribution accounts for the stress relaxation associated with the hydrogen induced volume and the elastic moduli changes due to hydrogen. The interactions between the dislocations are calculated accounting for all the stress fields due to dislocations and hydrogen atmospheres. Modeling of the hydrogen effects on the edge dislocationdashinterstitial solute atom interaction and on the screw dislocationdashinterstitial solute atom interaction is discussed using a finite element analysis and the atom interaction energies are calculated in the presence of hydrogen. For the case where hydrogen increases the shear modulus, a significant hydrogendashrelated decrease of the edge dislocationdashinterstitial solute atom interaction energy was observed when the edge dislocationdashsolute distance is approximately less than two Burgers vectors. Depending on the orientation of the tetragonal axis of the interstitial solute distortion field, hydrogen may strengthen or weaken the interaction between the screw dislocationdashinterstitial solute.
309 citations
TL;DR: In this paper, the mechanical behavior of partially molten Westerly granite was investigated in the temperature range 800°-1100°C, 250 MPa confining pressure, by means of constant strain rate, creep, and stress relaxation tests.
Abstract: The mechanical behavior of partially molten Westerly granite was investigated in the temperature range 800°–1100°C, 250 MPa confining pressure, by means of constant strain rate, creep, and stress relaxation tests. The only water in the samples came from the breakdown of hydrous phases, biotite, minor chlorite and muscovite and alteration products of feldspars. Thus the amount of melt was controlled by the test temperature and ranged from 3% at 800°C to 50% at 1100°C. Over that temperature range, strength decreased from ≈500 MPa to less than 1 MPa, and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates. The comparative viscosity of the melt alone was estimated at 950° and 1000°C from the distance it could be made to penetrate into a porous sand under a known pressure gradient. Under all conditions, deformation of the matrix of solid grains was by brittle fracture only. Samples containing up to 10 vol % melt failed with the formation of a shear fault zone. At higher melt fractions, melt-filled “pores” collapsed by shear-enhanced compaction, squeezing the melt into axial cracks. Above 40 vol % melt, unfractured solid grains were carried about passively in the flowing liquid. There was no evidence of a “rheologically critical melt percentage” in this system. By analogy with the uniaxial compaction of water-saturated soils, a simple model is erected to describe a two-stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress. Shear-enhanced compaction is inferred to drive melt into a network of melt-filled veins, whereupon porous flow through the high-permeability vein network allows rapid drainage of melt to higher crustal levels.
298 citations
TL;DR: In this paper, the behavior of copper films is evaluated to determine effects of film texture, thickness, and the presence of a passivation layer, and an inverse relationship between film thickness and strength is quantified.
259 citations
TL;DR: In this article, the results of static strain aging, stress relaxation, and strain rate change tests on 310s stainless steel foils, with and without hydrogen, have been presented to complement the stress-strain curves.
Abstract: Microscopic observations and the results of static strain aging, stress relaxation, and strain rate change tests on 310s stainless steel foils, with and without hydrogen, have been presented to complement the stress-strain curves in a previous article. The hydrogen-free specimens showed minute yield points during static strain aging, while the hydrogen-containing specimens demonstrated “preyield microstrain. ” Thermal activation analysis of the strain rate change and stress relaxation plots led to the conclusion that the activation area for dislocation motion is decreased by hydrogen. Microstructural examination with the scanning electron microscope (SEM) revealed extensive strain localization, while transmission electron microscopy (TEM) studies showed microtwinning and austenite faulting in hydrogenated specimens tested at room temperature. The relation of hydrogen-induced changes in plastic deformation to hydrogen embrittlement is discussed.
182 citations
TL;DR: In this article, the influence of misfit dislocations on the surface morphology of partially strain relaxed Si1−xGex films is studied by atomic force microscopy and transmission electron microscopy.
Abstract: The influence of misfit dislocations on the surface morphology of partially strain relaxed Si1−xGex films is studied by atomic force microscopy and transmission electron microscopy. Surface steps arising from the formation of single and multiple 60° dislocations are identified. The role of such steps in the development of a cross‐hatch pattern in surface morphology is discussed.
155 citations
TL;DR: In this article, the authors compared the creep behavior of 30 vol pct SiC-6061 Al and that of its matrix alloy, 6061 Al, and found that the creep rates of the composite are more than one order of magnitude slower than those of the alloy, and the activation energy for creep in the composite is higher than that in the alloy.
Abstract: High-temperature strengthening mechanisms in discontinuous metal matrix composites were examined by performing a close comparison between the creep behavior of 30 vol pct SiC-6061 Al and that of its matrix alloy, 6061 Al. Both materials were prepared by powder metallurgy techniques. The experimental data show that the creep behavior of the composite is similar to that of the alloy in regard to the high apparent stress exponent and its variation with the applied stress and the strong temperature dependence of creep rate. By contrast, the data reveal that there are two main differences in creep behavior between the composite and the alloy: the creep rates of the composite are more than one order of magnitude slower than those of the alloy, and the activation energy for creep in the composite is higher than that in the alloy. Analysis of the experimental data indicates that these similarities and differences in creep behavior can be explained in terms of two independent strengthening processes that are related to (a) the existence of a temperature-dependent threshold stress for creep, τ0, in both materials and (b) the occurrence of temperature dependent load transfer from the creeping matrix (6061 Al) to the reinforcement (SiC). This finding is illustrated by two results. First, the high apparent activation energies for creep in the composite are corrected to a value near the true activation energy for creep in the unreinforced alloy (160 kJ/mole) by considering the temperature dependence of the shear modulus, the threshold stress, and the load transfer. Second, the normalized creep data of the composite fall very close to those of the alloy when the contribution of load transfer to composite strengthening is incorporated in a creep power law in which the applied stress is replaced by the effective stress, the stress exponent,n, equals 5, and the true activation energy for creep in the composite,Q
c
, is equal to that in the alloy.
131 citations
TL;DR: In this paper, the effects of stress and physical ageing on the creep compliance of high-density polyethylene (HDPE) were studied and a unified non-linear creep relation was obtained that incorporates the physical ageing effect and predicts the long-term creep behaviour with good agreement with experiments.
129 citations
TL;DR: A creep theory is developed in which the whole of tertiary creep and, in particular, the creep lifetime are predicted from measurements made at the onset of creep, when the tendon is undamaged.
Abstract: The tail tendons from wallabies (Macropus rufogriseus) suffer creep rupture at stresses of 10 MPa or above, whereas their yield stress in a dynamic test is about 144 MPa. At stresses between 20 and 80 MPa, the time-to-rupture decreases exponentially with stress, but at 10 MPa, the lifetime is well above this exponential. For comparison, the stress on a wallaby tail tendon, when its muscle contracts isometrically, is about 13.5 MPa. Creep lifetime depends sharply on temperature and on specimen length, in contrast to strength and stiffness as observed in dynamic tests. The creep curve (strain versus time) can be considered as a combination of primary creep (decelerating strain) and tertiary creep (accelerating strain). Primary creep is non-damaging, but tertiary creep is accompanied by accumulating damage, with loss of stiffness and strength. 'Damage' is quantitatively defined as the fractional loss of stiffness. A creep theory is developed in which the whole of tertiary creep and, in particular, the creep lifetime are predicted from measurements made at the onset of creep, when the tendon is undamaged. This theory is based on a 'damage hypothesis', which can be stated as: damaged material no longer contributes to stiffness and strength, whereas intact material makes its full contribution to both.
127 citations
TL;DR: In this article, the microscopic nature of a block-substrate interface during sliding of an elastic block on a substrate with a lubrication film of molecular thickness (boundary lubrication) was discussed.
Abstract: I discuss the microscopic nature of a block-substrate interface during sliding of an elastic block on a substrate with a lubrication film of molecular thickness (boundary lubrication). Arguments are given that the lubrication film at low sliding velocities has a granular structure, with pinned adsorbate domains accompanied by elastic stress domains in the block and substrate. At zero temperature, the stress domains form a ``critical'' state, with a continuous distribution P(\ensuremath{\sigma}) of local surface stresses \ensuremath{\sigma} extending to the critical stress ${\mathrm{\ensuremath{\sigma}}}_{\mathit{a}}$, necessary for fluidization of the pinned adsorbate structure. During sliding adsorbate domains will fluidize and refreeze. During the time period that an adsorbate domain remains in a fluidized state, the local elastic stresses built-up in the elastic bodies during ``sticking'' will be released, partly by emission of elastic wave pulses (sound waves) and partly by shearing the lubrication fluid. The role of temperature-activated processes (relaxation and creep) is studied and correlated with experimental observations. In particular, the model explains in a natural manner the logarithmic time dependence observed for various relaxation processes; this time dependence follows from the occurrence of a sharp steplike cutoff at \ensuremath{\sigma}=${\mathrm{\ensuremath{\sigma}}}_{\mathit{a}}$ in the distribution P(\ensuremath{\sigma}) of surface stresses. Finally, I suggest a simple experiment to test directly the theoretical predictions: by registering the elastic wave pulses emitted from the sliding junction, e.g., by a piezoelectric transducer attached to the elastic block, it should be possible to prove whether, during uniform sliding at low velocities, rapid fluidization and refreezing of adsorbate domains occur at the interface.
123 citations
TL;DR: In this paper, the crack-healing behavior of Al{sub 2}O{sub 3} and Alsub 2 O{Sub 3}-SiC nanocomposite was studied using Vickers indentations to generate precracks.
Abstract: The crack-healing behavior of Al{sub 2}O{sub 3} and Al{sub 2}O{sub 3}-SiC nanocomposite was studied using Vickers indentations to generate precracks. After annealing in argon for 2 h at 1,300 C, radial cracks in the nanocomposite healed: The cracks closed and there was a small degree of rebonding in the vicinity of the crack tip. In contrast, radial cracks in alumina grew when exposed to the same annealing treatment. The different responses are attributed to the fracture mode and toughening mechanism in each material: In the nanocomposite, the cracks close as the residual stresses surrounding the indentations relax. Radial cracks open and grow in Al{sub 2}O{sub 3} because microstructural toughening is diminished during heating to the annealing temperature. An implication is that strength-limiting machining flaws in these materials behave similarly, thereby accounting for the strengthening effect of annealing in this ``nanocomposite`` system.
120 citations
TL;DR: In this paper, in situ wafer curvature measurements were performed to study mechanical stress in amorphous SiO2 during Xe, Ne, and Er ion irradiation at energies in the 0.27-4.0 MeV range.
Abstract: In situ wafer curvature measurements were performed to study mechanical stress in amorphous SiO2 during Xe, Ne, and Er ion irradiation at energies in the 0.27–4.0 MeV range. Three phenomena are observed: network compaction, radiation‐induced viscous flow, and a nonsaturating anisotropic deformation phenomenon. The radiation‐induced viscosity is shown to be inversely proportional to the energy density deposited into atomic displacements. The relation between radiation‐induced flow and diffusion is discussed in the context of the Stokes–Einstein relation. Viscous flow serves to relax stress, yet a continuous nonsaturating anisotropic deformation effect causes the stress in the irradiated layer to saturate at nonzero values: Xe irradiation at an energy below 3.6 MeV results in a tensile saturation stress; for higher energies a compressive stress builds up. These effects are explained in terms of competing bulk and surface deformation processes resulting from local heating of the SiO2 around the ion tracks. T...
TL;DR: In this article, three elementary processes (elastic, plastic and dilatant deformations) in brittle rock under sustained load are considered. But their contributions to the creep strain of brittle rock are not equal and vary with the stages of creep.
TL;DR: Stress relaxation of bovine femur was investigated as a function of water content, phi and shear modulus values in the relaxation described by the KWW function (KWW relaxation) depend remarkably on phi while those in Debye relaxation are almost constant for increasing phi.
Patent•
15 Dec 1995TL;DR: In this article, a method of strengthening a glass article by developing compressive stress in a surface layer on the article through an exchange of alkali metal ions in the surface layer at an elevated temperature below the glass strain point is described.
Abstract: A method of strengthening a glass article by developing compressive stress in a surface layer on the article through an exchange of alkali metal ions in the surface layer at an elevated temperature below the glass strain point, the step of minimizing stress relaxation by carrying out the ion exchange in a glass essentially free from non-bridging oxygen atoms. Glasses having particular utility contain alumina in their compositions in such amount that the number of aluminum atoms in a glass are at least equal to the number of alkali metal ions, or contain both alumina and boric oxide in such amounts that the formula ##EQU1## is satisfied.
TL;DR: In this paper, the formation of crosshatch patterns is the result of a strain relaxation process that essentially consists of four subsequent stages: (i) elastic strain relaxation by surface ripple formation; (ii) nucleation of dislocations at the rim of the substrate followed by dislocation glide and deposition of a misfit dislocation at the interface; (iii) a locally enhanced growth rate at the strain relaxed surface above the misfit dislocation that results in ridge formation.
Abstract: We study the interplay of elastic and plastic strain relaxation of SiGe/Si(001). We show that the formation of crosshatch patterns is the result of a strain relaxation process that essentially consists of four subsequent stages: (i) elastic strain relaxation by surface ripple formation; (ii) nucleation of dislocations at the rim of the substrate followed by dislocation glide and deposition of a misfit dislocation at the interface; (iii) a locally enhanced growth rate at the strain relaxed surface above the misfit dislocations that results in ridge formation. These ridges then form a crosshatch pattern that relax strain elastically. (iv) Preferred nucleation and multiplication of dislocations in the troughs of the crosshatch pattern due to strain concentration. The preferred formation of dislocations again results in locally enhanced growth rates in the trough and thus leads to smoothing of the growth surface.
TL;DR: In this paper, the authors considered the antiplane elasticity problem for a functionally graded coating bonded to a homogeneous half space, and formulated the problem in terms of an integral equation with strongly singular kernels.
TL;DR: In this paper, the authors presented experimental and theoretical studies of the currentvoltage characteristics of symmetrically doped n-type GaN−AlN−GaN semiconductor-insulator-semiconductor (SIS) structures.
Abstract: Experimental and theoretical studies are presented of the current‐voltage characteristics of symmetrically doped n‐type GaN‐AlN‐GaN semiconductor‐insulator‐semiconductor (SIS) structures. The asymmetry caused by the strain‐induced electric field leads to the depletion layer barrier in addition to the barrier presented by a thin insulating layer of AlN. It is shown that the tunnel current depends on the degree of the elastic strain relaxation which, in turn, is related to the AlN film thickness. This dependence provides quantitative information about the film relaxation. This characterization technique is compared with the capacitance‐voltage characterization of the SIS structures. The data indicate that the low bound of the conduction‐band offset for the AlN/GaN heterointerface is close to 1 eV.
TL;DR: The mechanics for the development and relaxation of stresses in thin-film systems are discussed in this article, where the authors use constitutive models for different relaxation processes to predict the development of stresses during thermal cycling.
Abstract: The mechanics for the development and relaxation of stresses in films is briefly reviewed. The discussion initially focuses on how stresses are distributed within a film and substrate, and on the resultant deformations that these stresses induce. Although the particular case of an elastically homogeneous system with a uniform stress within the film is addressed in detail, the modifications that would result from a nonhomogeneous system, or a nonuniform stress, are also outlined. A brief discussion of stress relaxation in epitaxial systems and the glide of dislocations in thin films then follows. In polycrystalline films, diffusion of atoms as well as the motion of dislocations can contribute to the relaxation of stresses. It is possible to use constitutive models for the different relaxation processes to predict the development of stresses during thermal cycling. A comparison of these predictions with experimental observations allows general conclusions about the dominant relaxation mechanisms that operate in particular thin-film systems to be made. An additional mechanism of stress relief
TL;DR: In this article, the authors characterized the rheology of two concentrated, liquid crystalline solutions of poly(γ benzyl-L-glutamate) of molecular weight 238 000 in m-cresol.
Abstract: We have characterized the rheology of two concentrated, liquid crystalline solutions of poly(γ benzyl–L–glutamate) of molecular weight 238 000 in m–cresol. Comparing these results to previous work on less concentrated, liquid crystalline solutions separates the effects of concentration from the direct influence of defect texture on liquid crystal polymer rheology. This work also defines the limitations of current polydomain models and suggests improvements. The solution at C=37wt% PBLG behaves similarly to moderately concentrated nematic solutions (12wt%
TL;DR: In this paper, the authors used dynamic mechanical, stress relaxation, and viscosity measurements in a temperature range from just below the conventional glass transition to 100 °C above it to characterize the responses in both the glassy and terminal dispersions of the polymers.
Abstract: The rheological behavior of five polybutadienes having different microstructures has been characterized using dynamic mechanical, stress relaxation, and viscosity measurements in a temperature range from just below the conventional glass transition to 100 °C above it. The data covered a broad enough frequency (time) and temperature range that we were able to characterize the responses in both the glassy and terminal dispersions of the polymers and to address the question of the validity of thermorheological simplicity. Uncritical application of time-temperature superposition principles to these data resulted in reduced viscoelastic responses that cover 12-14 decades in frequency or time. Close examination of the data in the glassy and terminal dispersions shows that the temperature shift factors required to superpose the data in the two regions are, however, different. Such a deviation from thermorheological simplicity can be analyzed within the framework of the coupling model of Ngai 1-3 that relates the shapes of the dispersion to the temperature dependence of the viscoelastic spectrum. Comparison of the polybutadienes shows differences in glass transition temperature, shape of the segmental relaxation, and fragility that depend on microstructure. Increasing the content of vinyl side groups causes an increase of spectral broadening as well as an increase in fragility-two features which can be related within the coupling model.
01 Oct 1995-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, a single mechanism of interfacial viscous shearing between the two eutectic phases is proposed for both creep and stress relaxation on the RSA II instrument modified to use a 0.5 mm diameter cylindrical punch under 1.5-47 MPa punching stress and within a temperature range of 25 °C to 110 °C.
Abstract: Impression creep and stress relaxation experiments on the SnPb eutectic alloy were carried out in the RSA II instrument modified to use a 0.5 mm diameter cylindrical punch under 1.5–47 MPa punching stress and within a temperature range of 25 °C to 110 °C. Based on a power law between the impression velocity and stress or between the stress rate and stress, the exponent increased with stress from 1 to 3.5 within the temperature range 80–110 °C and 2.5 to 6 within the range 25–65 °C. These exponents were generally comparable to those reported in the literature. Because of the change of stress exponent, several mechanisms have been proposed. However, the stress dependence was found to obey a hyperbolic sine function of stress for all the stresses and temperatures studied. Similarly before using the hyperbolic sine function, the activation energy was found to increase with stress, an abnormal behaviour. Fortunately, after using the hyperbolic sine function, a single activation energy, 55 kJ mol−1 was obtained. Based on the present data, a single mechanism of interfacial viscous shearing between the two eutectic phases is proposed for both creep and stress relaxation. In addition to the effect of stress and temperature, the impression velocity based on this model should be directly proportional to the punch radius and inversely proportional to the nth (n = 1–3) power of the size of phase particles. These predictions are consistent with available information in the literature.
TL;DR: In this article, an effort is made to model the time-dependent behavior of high-density polyethylene (HDPE) with a one-dimensional integral representation, and an effective time concept is introduced for the plastic deformation, so that the response due to complex loading can be accounted for.
Abstract: In the present paper an effort is made to model the time-dependent behavior of high-density polyethylene (HDPE) with a one-dimensional integral representation. Owing to the plasto-viscoelastic behavior of the material, we assume that the total strain can be decomposed into a recoverable viscoelastic strain and an irrecoverable plastic strain. The viscoelastic deformation is represented by the Schapery thermodynamic theory. The plastic deformation is assumed to be accumulated during the loading history. An effective time concept is introduced for the plastic deformation, so that the response due to complex loading can be accounted for. The present representation gives a very good prediction of the responses of creep and recovery, two-step creep, and constant stress rate loading and unloading. It is also applied successfully to describe the process of preconditioning of semicrystalline polymers.
TL;DR: In this paper, the authors extended the work of McLean (1985) to include effects of fiber breaks and the consequential stress relaxation in the broken fibers, and calculated the creep strain and the creep rupture time when global load sharing occurs.
Abstract: Creep models for metal matrix composites reinforced by long brittle fibers with weak interfaces are presented. These models extend the work of McLean (1985) to include effects of fiber breaks and the consequential stress relaxation in the broken fibers. The creep strain and the creep rupture time are calculated when global load sharing occurs. Analyses are conducted for composites with a wide range of fiber volume fractions, Young's modulus of the fibers and the matrix, interfacial sliding stress and Weibull properties for the strength of the fibers. The results derived from this study are compared with those predicted by McLean's (1985) model. The creep life is found to be sensitive to the extent of fiber stress relaxation in the broken fibers. Models, which ignore this effect overestimate the creep rupture time especially when the composite is subjected to a low or moderate level of stress.
TL;DR: In this article, the authors studied the relaxation of compositionally graded InGaAs buffers, with and without uniform cap layers, and the effects of different cap layers on the relaxation behavior of the graded buffer has been studied by double crystal x-ray diffraction, TEM, and low temperature photoluminescence.
Abstract: The relaxation of compositionally graded InGaAs buffers, with and without uniform cap layers, has been studied. Simple InGaAs linear‐graded layers on GaAs substrates never reach complete relaxation. The residual strain in these structures produces a dislocation‐free strained top region while the rest of the buffer is nearly completely relaxed through misfit dislocations, as observed by transmission electron microscopy (TEM). This strained top region is analyzed and its thickness compared with theoretical calculations. The effects of different cap layers on the relaxation behavior of the graded buffer has been studied by double crystal x‐ray diffraction, TEM, and low temperature photoluminescence, and results compared with predictions of the models. The optical quality of the cap layer improves when its composition is close to the value that matches the lattice parameter of the strained surface of the grade. The design of linear graded buffers having a strain‐free cap layer with high crystalline quality is...
TL;DR: In this paper, the growth of grain boundary voids at elevated temperatures by coupled creep and grain boundary diffusion is studied numerically using a cylindrical unit cell model, where the influence of the remote stress triaxiality is taken to cover the full range of axisymmetric stress states, from purely effective to purely hydrostatic states of stress.
Abstract: The growth of grain boundary voids at elevated temperatures by coupled creep and grain boundary diffusion is studied numerically using a cylindrical unit cell model. Emphasis is on the influence of the remote stress triaxiality, which is taken to cover the full range of axisymmetric stress states, from purely effective to purely hydrostatic states of stress. The motivation for extending previous results stems from the need for an accurate cavity growth model to analyse damage due to hydrogen attack, where the grain boundary voids are internally pressurized. Because of the wide range of stress states considered, numerical stability requires the use of two normalizations of the variational principle for the coupled void growth problem; one when the effective stress is dominant and the other when the mean stress is dominant. In the regime where deformation is primarily by creep, two distinct modes of deformation appear for each level of porosity; one for low triaxialities and one that takes over for sufficiently high triaxialities. Approximate models found in the literature for a dilute concentration of voids, or for finite concentrations, are explored to check their ability to represent the stress state dependence of the volumetric void growth rate. A novel approximate formula is derived for creep dominated growth and is shown to give good agreement with numerically computed void growth rates in the high triaxiality regime and for finite concentrations. A fairly abrupt transition between creep dominated void growth and diffusion dominated void growth is found when the stress triaxiality is very high, so that the interaction between creep and diffusion is then relatively unimportant. Finally, formulae are presented which give an approximate, yet fairly accurate, expression for the void volume growth rate due to coupled diffusional and creep growth over the full range of axisymmetric stress states.
TL;DR: In this paper, the dynamic strain aging behavior of low carbon steel wire rod was examined at room temperature to 450°C using tensile testing at strain rates of 10-4 to 10-1 s-1.
Abstract: The dynamic strain aging behaviour of low carbon steel wire rod was examined at room temperature to 450°C using tensile testing at strain rates of 10-4 to 10-1 s-1 The effects of temperature and strain rate on the yield stress, flow stress, UTS, fracture stress, and fracture strain were investigated in detail. In agreement with previous studies, work hardening peaks, minima in ductility, and negative strain rate dependences of the flow stress were observed between 100 and 400°C, the positions of which depended on the strain rate. A model for dynamic strain aging is employed to predict whether or not it will occur at the strain rates and temperatures involved in commercial wire drawing. For a steel containing 32 ppm N, a temperature higher than about 315°C must be attained for dynamic strain aging to occur; this is higher than the temperatures usually encountered in drawing. However, the model also predicts that if the N content is increased to 115 ppm, the minimum temperature for dynamic strain aging decreases to about 250°C, which can be attained if the die and capstan cooling are not adequate. The negative rate dependence of the flow stress attributable to dynamic strain aging is considered to promote flow localization and, therefore, to be a possible cause of wire breaks during drawing.
TL;DR: In this paper, the authors measured biaxial stress and strain in (100) and (111) oriented grains and observed that grain growth was controlled by strain energy density minimization.
Abstract: Biaxial stress and strain in (100) and (111) oriented grains have been measured as a function of annealing temperature for a Cu film on an oxidized Si substrate which exhibits abnormal (100) grain growth. The observed behavior indicates isostrain averaging, which is consistent with grain growth that is controlled by strain energy density minimization. In contrast, two films which do not exhibit (100) abnormal grain growth appear to follow isostress averaging. Strain energy density minimization in this situation favors (111) grain growth.
TL;DR: In this paper, a visco-elastic model is described in which these phenomena are attributed to local shear stress relaxation in thermal spikes induced by the energy transfer from the projectile particle to the target, followed by freezing in of the associated strain increments during cooling down.
TL;DR: In this article, the authors performed deformation experiments on icosahedral Al-Pd-Mn single-quasicrystals to determine the thermodynamic activation parameters of the deformation process.
Abstract: Plastic deformation experiments were performed on icosahedral Al-Pd-Mn single-quasicrystals to determine the thermodynamic activation parameters of the deformation process. The stress exponent and the strain-rate sensitivity of the flow stress were obtained by means of stress relaxation experiments. The activation enthalpy of the deformation process was measured by temperature change experiments. In the range from 730 to 800°C a nearly constant value of about 7 eV was determined.
TL;DR: In this article, the initial plastic relaxation of highly mismatched layers grown along GaAs is governed by the growth mode, and it is shown that when InAs are forced to grow in a two-dimensional (2D) mode, only pure edge-type dislocations are generated and they are located exactly at the epilayer/substrate interfacial plane.
Abstract: Using cross‐section and plan‐view transmission electron microscopy we demonstrate that the initial plastic relaxation of highly mismatched layers grown along [100] is governed by the growth mode. During MBE growth of InAs on GaAs in the Stranski‐Krastanov (SK) mode, 60o ‐type dislocations are generated at the island edges and then glide to the interface to relieve the strain. The resulting interfacial microstructure consists of an inefficient arrangement of misfit dislocations. On the other hand, when InAs is forced to grow in a two‐dimensional (2D) mode, only pure edge‐type dislocations are generated and they are located exactly at the epilayer/substrate interfacial plane. These results are explained by a different dislocation nucleation mechanism imposed by the planar morphology of the highly strained film.