Showing papers on "Stress relaxation published in 1988"

Creep in metallic materials

Abstract: 1. Deformation and Creep. Deformation. Definition of creep. Time dependence of creep strain. Creep Curve. Mechanisms of plastic deformation: A general survey. Mechanical equation of state. Creep test compared with tensile test at constant strain rate and constant loading rate. Creep tests at constant load and constant stress. 2. Motion of Dislocations. Dynamic Recovery. Motion of dislocations. Free, mobile and moving dislocations. Dynamic recovery. 3. Temperature Dependence of Creep Rate. Activation energy of creep. Methods of determination of activation energy of creep. Correction of experimentally determined activation energy of creep for temperature dependence of elastic modulus. Activation energy or creep and activation enthalpy of diffusion. 4. Applied Stress Dependence of Creep Rate. Initial creep rate. Steady-state creep. Transient creep. 5. Influence of Grain Size and Stacking Fault Energy. Grain size. Stacking fault energy. 6. Orowan and Bailey-Orowan Equations. Orowan equation. Bailey-Orowan equation. Relation between Orowan equation and Bailey-Orowan equations. A consequence of the equivalence of Orowan and Bailey-Orowan equations. Experimental verification of Bailey-Orowan equation. Experimental determination of quantities r and h. Incubation period and ``Frictional'' stress. 7. Back Stress. Internal, threshold and frictional stress. Internal and effective stress. Concept of internal and effective stress and the mechanical equation of state. Definitions of experimental parameters. Interpretation of experimental parameters. 8. Dislocation Structure. Development of dislocation structure during creep. Basic quantitative characteristics of dislocation structure. Subgrain structure. Subgrain structure and long-range internal stress. Behaviour of sub-boundaries. Interaction of dislocations with sub-boundaries. Generation of dislocations. Structural steady state. Concept of hard and soft regions and measured internal stress. 9. Dislocation Creep in Pure Metals. Creep controlled by recovery. Creep controlled by dislocation glide. Models based on thermally activated glide and diffusion controlled recovery. Relation between constants A and n in the dorn creep equation and the natural third power law. Harper-Dorn creep. 10. Creep in Solid Solution Alloys. Introduction. Mechanisms of creep strengthening in solid solutions. Creep controlled by viscous dislocation glide. 11. Creep in Precipitation and Dispersion Strengthened Alloys. Models of Ansell and Weertman. Back stress concept. 12. Diffusional Creep. Nabarro-Herring and Coble creep. Subgrain boundaries as sources and sinks for vacancies. Diffusional creep and grain boundary sliding. Reactions on grain boundaries. Diffusional caritational creep. 13. Deformation Mechanism Maps. Equations used for construction of deformation mechanism maps. Examples of deformation mechanism maps. ``Generalized'' deformation mechanism map for pure metals. 14. Grain Boundary Sliding.

Structural instability and relaxation in liquid and glassy phases near the fragile liquid limit

Abstract: We consider two sorts of structural instability of glassy substances, each of which may be released by a relaxation process with its own characteristic relaxation time, and specific kinetic features. The first of these is the instability against relaxation out of the amorphous state into the crystalline state, while the second is the instability against relaxation within the amorphous state itself. The latter may often involve relaxation out of a homogeneous amorphous phase into a two-phase amorphous structure, but we will not specifically consider this liquid-liquid phase separation process here. In most glasses, the former (which is no more than the characteristic nucleation time) is much longer than the latter time. However, there are important classes of glasses, for instance the metallic glasses, in which the former is in fact the shorter time, a fact which is responsible for the inability to observe the glass transition phenomenon in such substances. In this paper we will be considering the relation between these two times and the specific kinetics of each. The nucleation time has been the subject of theoretical developments over a number of decades, and details will be omitted in order to concentrate on experimental studies of this phenomenon. We will described briefly the recently developed DSC techniques for determining the classical time-temperature-transformation curves for a variety of supercooled liquids, and the relation of these to the nucleation curves. The relaxation process within the amorphous state, which can be observed for cases where the nucleation time is relatively long, has a number of features which currently lack a complete explanation. In most cases the relaxation process is non-Arrhenius in its temperature dependence, nonexponential in its time dependence, and nonlinear in its structural state dependence. Some examples taken from glasses at the “fragile” edge of the deduced viscosity-temperature pattern for glassforming liquids are dealt with in detail, and the distinction between shear stress relaxation and thermodynamic stress relaxation is made. The possibility that near T g the latter relaxation time remains Vogel-Fulcher in form with T 0 ≡ T K (the Kauzmann temperature), in contrast with the common observations for (the decoupled) shear relaxation, is raised. Strong support for this notion is found in the current “specific heat spectroscopy” results of Nagel and co-workers. Microscopic relaxation processes, as observed using spectroscopic probes and neutron scattering techniques, are reviewed, and the difference in non-exponentiality from macroscopic relaxation are examined in the light of current theories. Finally, secondary relaxations in ionic and molecular glasses, and their relation to the fastest of all glassy state relaxation processes, the tunnelling modes (TLS), are briefly considered.

Damage‐Enhanced Creep in a Siliconized Silicon Carbide: Phenomenology

Abstract: The creep behavior of a commercial grade of reaction-bonded silicon carbide was characterized at a temperature of 1300°C. Creep occurred more easily in tension than in compression. At a given applied stress, the steady-state creep rate in tension was found to be at least 20 times that obtained in compression. In both tension and compression, the stress exponent for steadystate creep was found to increase with increasing applied stresses. At low applied stresses, the stress exponent was ∼4, suggesting some kind of dislocation mechanism operating in the two-phase composite. At high stresses, the stress exponent was ∼11 in tension. The increase in the stress exponent was attributed to damage accumulation in the form of cavities. An effective threshold stress for cavitation of less than 100 MPa was suggested. In compression, the cause of the increase of stress exponent with stress cannot be attributed to cavitation.

Experimental “sytectonic” dehydration of serpentinite under conditions of controlled pore water pressure

Abstract: We have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine + talc + water, at temperatures ranging between 300° and 600° and at total pressures from 100 to 300 MPa. Constant stress, stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed. Effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system. Stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction. At 500° and 600°C, however, the strength was linearly dependent on the inverse of the deformation rate, and independent of effective confining pressure over the range investigated. Microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine-grained (0.1 µm olivine produced in the breakdown reaction. The enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion-accommodated grain boundary sliding. These experimental results suggest that metamorphic transformations affect the deformability of rocks, and demonstrate enhancement of deformability through the transient existence of fine-grained reaction products. The results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain-size reduction by dynamic recrystallization in plastically deforming rocks.

Strain distribution in thin aluminum films using x-ray depth profiling

Abstract: Thin polycrystalline Al films deposited on oxidized Si(100) substrates have been studied using grazing incidence x‐ray scattering techniques. The films become strained after high‐temperature annealing due to the differential thermal expansion of the substrate and film. By varying the grazing angle of incidence on the surface, the penetration depth of the incident x rays can be varied from a minimum of roughly 30 A to a maximum which is deeper than the film thickness. By measuring the change in the Al in‐plane peak position with penetration depth, the strain distribution within the film can be determined. For thin films (less than 3000 A) the strain is uniform throughout much of the film, decreasing only very near the surface. Strain relaxation with time occurs uniformly throughout the film over the first 4000 min.

Modeling shock-wave deformation via molecular dynamics

Abstract: Molecular dynamics (MD), where the equations of motion of up to thousands of interacting atoms are solved on the computer, has proven to be a powerful tool for investigating a wide variety of nonequilibrium processes from the atomistic viewpoint. Simulations of shock waves in three-dimensional (3D) solids and fluids have shown conclusively that shear-stress relaxation is achieved through atomic rearrangement. In the case of fluids, the transverse motion is viscous, and the constitutive model of Navier-Stokes hydrodynamics has been shown to be accurate---even on the time and distance scales of MD experiments. For strong shocks in solids, the plastic flow that leads to shear-stress relaxation in MD is highly localized near the shock front, involving slippage along close-packed planes. For shocks of intermediate strength, MD calculations exhibit an elastic precursor running out in front of the steady plastic wave, where slippage similar in character to that in the very strong shocks leads to shear-stress relaxation. An interesting correlation between the maximum shear stress and the Hugoniot pressure jump is observed for both 3D solid and fluid shock-wave calculations, which may have some utility in modeling applications. At low shock strengths, the MD simulations show only elastic compression, with no permanent transverse atomic strains. This result for perfect 3D crystals is also seen in calculations for 1D chains. We speculate that, if it were practical, a very large MD system containing dislocations could be expected to exhibit more realistic plastic flow for weak shock waves too.

A stress relaxation method for following carbonitride precipitation in austenite at hot working temperatures

Abstract: Stress relaxation measurements were carried out on a plain carbon and four solution-treated Ti steels over the temperature range 850 to 1050 °C. The results show that the stress relaxation of plain carbon austenite after a 5 pct prestrain (i.e., in the absence of precipitation) can be described by the relation σ = σ0 -α ln(l + βt). By contrast, in the solution-treated Ti steels, relaxation is arrested at the start of precipitation and is resumed when precipitation is completed. As a result, this mechanical method is particularly suitable for following carbonitride precipitation in microalloyed austenite at hot working temperatures. A new model regarding the effect of precipitation on dislocation motion is proposed, on the basis of which, the phenomenology of the stress relaxation technique is clarified. Precipitation-time-temperature (PTT) diagrams were determined for the Ti bearing steels containing 0.05, 0.11, 0.18, and 0.25 pct Ti. The PTT curves obtained are C-shaped for all the steels. The upper parts of these curves are shifted to significantly longer times as the Ti and C concentrations are reduced. By contrast, the positions of the lower arms of the curves are relatively independent of the compositions of the steels tested.

SiO2 film stress distribution during thermal oxidation of Si

Abstract: A laser reflection technique is used to investigate the relaxation of SiO2 film stress which occurs during the dry thermal oxidation of Si between 700 and 1000 °C. Included is a determination of the stress distribution in the oxide by two independent methods: (1) measurement on oxides of various thicknesses from 100 to 800 A, and (2) repeated stress measurements on chemically thinned SiO2 films, viz. etch‐back analysis toward the interface. Agreement is found between these experiments. These thin film stress measurements are achieved by the use of ultrathin Si substrates (75 μm). Essentially, an increase in film stress with decreasing film thickness is observed. Rapid stress relaxation is observed for all temperatures studied and is attributed to a time‐dependent oxide viscosity. The influence of these measured properties on the kinetics of Si oxidation is discussed.

Rheological properties of rennet-induced skim milk gels

Abstract: The rheological properties of rennet-induced skim milk gels, which are viscoelastic materials, were studied under various conditions. Dynamic and stress relaxation experiments were performed at small deformations of the gel network, whereas constant stress (creep) experiments were performed at large deformations. Stress relaxation moduli calculated from the dynamic moduli agreed fairly well with stress relaxation moduli determined by means of stress relaxation experiments, implying that true material properties were determined. The effects of important variables, such as casein concentration, temperature, pH, calcium, phosphorus and ionic strength on the mechanical properties of the gels were studied. The results are discussed in relation to the types of bond present in the network. Special attention was paid to the time scale at which processes, especially relaxation of bonds, occurred.

Fatigue of solder joints in surface mount devices

Abstract: Lifetime studies of a 16 I/O surface-mounted solder joint array undergoing isothermal cyclic fatigue in torsion shear under fixed plastic strain range show a strong correlation with creep fatigue and a creep-cracking mechanism. Experimental lifetime data follow an inverse dependence on matrix creep. Experimental measurement of the steady-state shear creep rate versus shear stress defines the creep characteristic that is sensitive to changes in metallurgical structure. The amounts of grain boundary and matrix creep taking place during a fatigue cycle are derived from experimental creep data combined with stress-strain hysteresis data obtained in steady-state cycling. Initially, thicker solder joints have a larger grain size than thinner solder joints, giving more matrix creep during fatigue and a faster failure rate. Fatigue increases the mean grain size of the solder joint as determined by the creep-rate-versus-stress characteristic and microstructure. Effects of grain size and joint thickness on lifetime are discussed. A maximum in the creep fatigue rate occurs at 333 K (60°C).

Strain and temperature accelerated relaxation in polycarbonate

Abstract: The nonlinear viscoelastic behavior of glassy polymers and its relationship to ductile yielding is studied by single- and double-step stress relaxation experiments. In the latter case a small stress relaxation step is superimposed on a specimen at an elevated state of temperature or strain. The results show that the changes in the relaxation behaviors in the two cases closely parallel each other. The relaxation behavior at strains near yield closely approximates that at low strain but near Tg. The small strain relaxation response can be described well by a Kohlrausch-Williams-Watts (KWW) type function. The interpretation of these data in terms of a coupling model which includes the KWW form is discussed.

Crack-enhanced creep in polycrystalline material: strain-rate sensitive strength and deformation of ice

Abstract: A non-linear viscoelastic creep equation for polycrystalline material is presented. It incorporates the effect of cracking and is capable of describing primary, secondary and tertiary behaviour. The model predicts the formation of microcracks and thus the damage state due to the high-temperature grain-boundary embrittlement process. This paper describes its application in formulating crack-enhanced creep and material response under constant strain-rate loading conditions (theoretically the simplest case but actually the most difficult to maintain). The formulation makes it possible to define the rate effect on stress-strain response and the rate sensitivity of strength, failure time, failure strain, damage and damage rate, strain recovery, etc. Numerical correspondence between theory and experiment was observed when predictions were compared with available closed-loop, controlled, constant strain-rate strength and deformation data on pure ice. Calculations made use of material constants determined from independent constant-load creep tests.

Ti(CN) precipitation in microalloyed austenite during stress relaxation.

Abstract: Stress relaxation tests were carried out on a 0.05 pct C-0.007 pct N-0.25 pct Ti steel preheated to 1260 °C and then held in the temperature range 900 to 1050 °C. The precipitation of Ti(CN) that took place was followed by extraction replication. Dense precipitates were observed when stress plateaus appeared on the relaxation curves. The cube-shaped Ti(CN) precipitates were heterogeneously distributed in either a chain-like or a cell-like manner, indicating that the precipitates were nucleated on dislocations or on dislocation substructures. The changes in the size distribution of the precipitates during relaxation were also followed and analyzed in terms of the Zener diffusion controlled particle growth theory. The results suggest that Ti(CN) precipitation under the present experimental conditions proceeds in three sequential stages: (i) nucleation and growth according to a parabolic law; (ii) nucleation site saturation accompanied by growth alone; and (iii) Ostwald ripening. The time dependence of the mean Ti(CN) particle size is nonlinear during the first, and becomes linear during the second stage.

Longitudinal moisture-shrinkage coefficients of softwood at the mechano-sorptive creep limit *

Abstract: An adaptor for the conversion of a high-accuracy tensile creep machine to compression loading is described. It was found that a stable mechano-sorptive creep limit could be obtained by a suitable load reduction after moisture cycling; after which further creep and creep recovery just balanced each other. In this stable state the value of the longitudinal moisture-swelling coefficient depended on the strain; being less with tensile strain and more with compressive strain then in the unloaded condition. These differences in the swelling coefficient could explain the apparent recovery during subsequent sorptions in mechanosorptive creep in bending. Such a hypothesis was strongly supported by numerical comparisons of strains in bending, tension and compression.

An In-Vivo Measurement and Analysis of Viscoelastic Properties of the Spinal Cord of Cats

Abstract: An in-vivo experimental technique was employed to determine the linear and nonlinear characteristics of viscoelastic properties of the spinal cord of anesthetized cats. The stress relaxation and recovery curves were reproducible in a group of cat experiments. The data of linear viscoelastic properties were used to develop a power law model with Boltzmann's convolution integral. The model was capable of predicting a prolonged stress relaxation and recovery curve. For larger deformation, the results were quantified using a nonlinear analysis of viscoelastic response of the spinal cord under the uniaxial experiment.

Damage‐Enhanced Creep in a Siliconized Silicon Carbide: Mechanics of Deformation

Abstract: Continuum mechanics methods were employed to analyze creep deformation of a grade of siliconized silicon carbide at elevated temperatures. Three loading modes (tension, compression, and bending) are considered in this analysis. In tension, deformation is accompanied by cavitation at stresses in excess of a temperature-dependent threshold level, resulting in bilinear power-law creep. In compression, greater applied stresses are required to achieve the same rate of strain, and although bilinear creep behavior is also observed, a single power-law creep equation was assumed to simplify the mathematical analysis of the flexure problem. Asymmetrical creep in siliconized silicon carbide leads to a number of unique features in flexural creep. At steady state, a threshold bending moment exists below which no damage occurs. The neutral axis shifts from the geometric center toward the compressive side of the specimen by an amount that depends on the level of applied stress. Cavitation zone shapes, which are predicted to develop in a four-point bend specimen as a function of load, are found to be in qualitative agreement with those obtained experimentally. For transient creep under bending, the time-dependent neutral axes for stress and strain do not coincide, although they do converge toward a single axis at steady state. Quantitative predictions are given for relaxation of tensile stresses at the outer fiber, reverse loading in the midplane region, and the growth of the damage zone toward the compressive side of the flexural specimen. This load redistribution leads to a prolonged transient stage as compared to its counterpart in uniaxial creep.

Yield strength of the B1 and B2 phases of NaCl

Abstract: We have measured the quasi-static yield strength of NaCl to 41 GPa at room temperature. Between 0 and 28 GPa, the strength of the B1 phase increases eightfold, from 0.08 to 0.65 GPa. Across the B1-B2 transition, the strength of NaCl decreases about 50%. We show that this result is consistent with the measurements of Bridgman [1937] on the shear strength of the B1 and B2 potassium halides to 5 GPa. We propose that the simultaneous increases in coordination and nearest-neighbor distance have opposite effects on the change in shear strength across B1-B2 transitions. Increasing the atomic coordination makes the B2 structure stronger than the B1 while increasing the nearest-neighbor distance decreases the strength of the high pressure phase. In NaCl the large increase in nearest-neighbor distance more than offsets the effect of increased coordination, and thus the B2 phase is weaker than the B1 phase.

Relaxation of Stress and Birefringence in Polymers of High Molecular Weight

Abstract: Stress relaxation behavior of concentrated solutions of high molecular weight polystyrene following a step shear strain is studied using both mechanical and optical birefringence techniques. Using the stress‐optic law, which we find to be valid for our solutions, we obtain time‐dependent shear stress and first normal stress difference values from birefringence measurement that are free of transducer compliance effects. Similar to the previously reported experimental observations of Fukuda, Osaki and Kurata, we obtain unusually low values for nonlinear shear moduli, much lower than the predictions of Doi‐Edwards model, for the sample with more than about 60 entanglements per molecule. Moreover, the shear stress and first normal stress difference data measured on this sample do not conform to the Lodge‐Meissner relationship, especially at long times, suggesting the formation of regions in which the imposed strain is not homogeneously distributed.

Stress relaxation during thermal cycling in metal/polyimide layered films

Abstract: The stress relaxation behavior during thermal cycling of metal/quartz and metal/polyimide/quartz layered structures has been investigated using a cantilever bending beam technique. The metals chosen for this study include Cu and Cr, two materials with contrasting mechanical and interfacial bonding properties. A finite element analysis, as well as an analytical calculation, has been carried out to deduce the stress distribution in the layered structures. Results indicate that the extent of stress relaxation strongly depends on the intrinsic mechanical property of the metal film with significantly higher stresses residing in the Cr film than in the Cu film. The interfacial polyimide layer has been found to serve as an effective buffering layer to reduce the residual stress in metal films. Observations from transmission electron microscopy suggest that the stress is partially released through the deformation of the polyimide near the metal/polyimide interface.

Residual Stress Effects on Fatigue of Surface Processed Steels

Abstract: Procedures are presented for analyzing the cyclic stability and influence on fatigue resistance of residual stress patterns arising from mechanical and thermal surface processing treatments such as shot peening and induction hardening. Cyclic properties and behavioral trends developed using smooth, axial specimens of steels simulating the various microstructures found in surface processed componentry are used to develop criteria to predict cyclic stability. the rate of relaxation for prescribed straining levels, the failure initiation point (surface or subsurface), and expected fatigue lifetime. The validity of the approach is verified using experimental data from the literature. Finally, the incorporation of these procedures in modern computer-based fatigue analysis routines, and opportunities for further enhancements, are discussed.

Creep deformation of ceramics in four-point bending

Abstract: Flexural testing was investigated as a method of studying the creep of ceramic materials at elevated temperatures. Three techniques were used to evaluate the steady-state stress exponent: two of them were based on the measurement of the surface curvature of a flexure specimen after testing; the third was based on the more conventional technique of measuring displacement rate as a function of applied load and exposure time. Applied to a grade of commercial vitreous-bonded alumina, the techniques yielded disparate results for the steady-state stress exponent. This discrepancy in results is believed to be a consequence of the fact that ceramics tend to creep more readily in tension than in compression, leading to a shift in the neutral plane for stress and strain in flexural specimens, which results in extended primary creep. A local enhancement of creep under the loading points of the test specimens was observed in the materials tested; this creep enhancement was attributed to contact stresses at the loading points.

Kinetics of structural relaxation and hydrogen evolution from plasma deposited silicon nitride

Abstract: Infrared absorption measurements and the temperature dependence of stress have been used to establish the kinetics of structural relaxation and hydrogen evolution from plasma deposited a‐SixNy :H films. The Arrhenius rate law describes the dissociation of N–H and Si–H bonds which occurs on annealing the films above 600 °C. The activation energies deduced from the infrared data are lower than the respective bond dissociation energies. The films undergo a rapid stress relaxation in the temperature range 400–650 °C. The discussion of the experimental results highlights possible mechanisms for the evolution of hydrogen from a‐SixNy: H networks.

Stress relaxation studies of model silicone RTV networks

Abstract: The stress relaxation behavior of model silicone room temperature vulcanizing (RTV) elastomers has been used to examine the chemistry of the cured silicone network. It has been shown that the tin crosslinking catalyst, together with water, produces siloxane bond rearrangement which results in chemical stress relaxation. The measurement of the rate of stress relaxation of unfilled model elastomers at various temperatures gave an apparent activation energy of 10.3 kcal/mole−1 for the relaxation process. In addition, the effects of some of the constituents of the RTV on relaxation behavior have been examined.

Work hardening and strain relaxation in strained‐layer buffers

Abstract: Fully relaxed buffer layers are of considerable importance for growth of high quality crystals on substrates having different lattice parameters. Recent experiments by Biefeld et al. [R. M. Biefeld, C. R. Hills, and S. R. Lee, J. Cryst. Growth (in press)] in the InAsSb system show that the degree of relaxation is considerably less than expected using conventional equilibrium models, but is more complete in continuously graded than in step‐graded buffer layers. In the present letter, this observation is explained in terms of Taylor‐type (dislocation interaction) work hardening.

Effect of process parameters on stress development in two‐dimensional oxidation

Abstract: A simplified viscoelastic analysis has been made of the stress evolution during two‐dimensional (2D) oxidation of silicon substrates, with the objective of learning the effect of process parameters such as temperature and steam pressure. A cylindrical siliconsurface was chosen for simplicity of analysis, and yet it still has most of the essential elements pertinent to practical problems such as, e.g., the oxidation of trench corners in siliconintegrated circuits. With correlations between the viscosity and the hydroxyl content of SiO2, and between the hydroxyl content and the steam pressure, the analysis shows that stress reduction can be achieved by carrying out oxidation at high steam pressures. However, stresses remain rather high if the oxidation temperature is as low as 800 °C. For a linear‐parabolic oxidation kinetics, both the oxide and the substrate stresses do not increase indefinitely with the increase of oxide thickness, but reach their respective peaks at oxide thicknesses that are dependent on process parameters. The present results should be useful in serving as guidelines in the selection of 2D oxidation conditions. The accuracy of a previous 2D oxidation model based on the viscous flow of an incompressible fluid has also been assessed with reference to the viscoelastic model. The incompressible‐fluid model is found to be quite accurate at high temperatures ≳900 °C.

Dependence of critical thickness on growth temperature in GexSi1−x/Si superlattices

Abstract: We present direct evidence for the dependence of critical thickness on growth temperature in a lattice‐mismatched epitaxial system. Ge0.5Si0.5/Si strained‐layer superlattices have been grown by molecular beam epitaxy on (100) Si substrates at temperatures between 330 and 530 °C. The extent to which lattice mismatch is accommodated by elastic strain has been determined through x‐ray diffraction, channeled Rutherford backscattering spectroscopy, and transmission electron microscopy. Lattice mismatch is found to be accommodated purely elastically in a structure grown at 365 °C. Samples grown at higher temperatures are seen to display increasingly high densities of misfit‐accommodating dislocations. This growth‐temperature dependence may account for apparent inconsistencies in critical thickness data reported in the literature. Our results clearly demonstrate the need to account adequately for the kinetics of defect formation in the prediction of critical thicknesses.