Coble Creep Research Articles

Correlation between strengthening and damage of Cu refined by different SPD processing and friction in different lubricant regions

The aim of this work is to evaluate the strengthening and damage of surface layers of copper (Cu) after friction in different lubricant regions and to compare them with stress–strain properties of Cu refined by different methods of severe plastic deformation (SPD). Friction and wear tests were performed using a ring-on block rig. The structure and damage of Cu samples rubbed in Elastic Hydrodynamic Lubrication (EHL) and Boundary Lubrication (BL) were studied. It was shown that the formation of the laminar structure in surface layers under friction in the EHL region is accompanied by a generation of pores and their coalescence. The calculated value of the strain rate during friction in the EHL region was compared with the Coble creep and grain boundary sliding models. It was found out that the deformation hardening, nanocrystalline grain size and level of plastic deformation of thin surface layers under friction in the BL region are close to the parameters of refined structure of Cu obtained by different methods of SPD. Friction saturation in the EHL and BL regions is considered to be a result of the dynamic recovery based on thermally activated processes of accumulation and annihilations of dislocations. The relation between the dislocation accumulation and dynamic recovery determines the transition from one contact region to another. Constant values of the hardness, grain size and geometrical parameters of contact at the steady friction state in different lubricant regions indicate any connection between the structural and geometrical parameters of contact.

Importance of diffusional creep in fine grained Mg–3Al–1Zn alloys

Deformation mechanisms of fine-grained Mg–3Al–Zn (AZ31) alloys that were prepared by using severe plastic deformation were identified at elevated temperatures between 473K and 573K by examining their stress–strain rate relations at different grain sizes and temperatures. Unlike the previous reports where grain boundary sliding has been suggested to be the rate-controlling deformation mechanism in the fine-grained AZ31 alloy, the current analysis indicated that Coble creep competes with grain boundary sliding and the contribution of Coble creep to overall strain rate increases as grain size decreases and temperature increases. Making the efforts to minimize grain growth during sample heating and tensile deformation is, however, important for observing Coble creep. Texture variation took place during the tensile deformation and differed depending on the type of dominant deformation mechanism operating under the given testing condition.

Creep Behavior of 409L and 436 Ferritic Stainless Steels Applied for Automotive Exhaust System

In order to investigate the creep behavior and understand its deformation mechanism of automotive exhaust pipe materials, this study conducted creep experiments of 409L and 436 ferritic stainless steels at both 600°C and 750°C under different stress levels. After creep tests, crept specimens were analyzed on the microstructure and fracture surfaces by the use of scanning electron microscopy. From creep data, two important material coefficients, namely, M of Monkman-Grant relationship and K of Coble creep equation are calculated for predicting the material creep life. Results show the creep resistance of 436 stainless steel is better than that of 409L stainless steel, because the 436 stainless steel has more Ni, Cr, and Mo contents. As the creep mechanism, all tests show grain boundary diffusion or Coble creep is the dominate deformation mechanism, except at higher temperature 750 °C and higher stress levels.

Whisker and hillock growth via coupled localized Coble creep, grain boundary sliding, and shear induced grain boundary migration

A model incorporating the effects of grain geometry and size and grain boundary properties on the growth of whiskers and hillocks has been developed based on coupling between localized Coble creep and grain boundary sliding. For both whiskers and hillocks accretion of atoms by Coble creep on grain boundary planes normal to the growth direction is limited by grain boundary sliding on planes parallel to the direction of whisker growth. If the accretion-induced shear stresses are not coupled to grain boundary migration a whisker forms when sliding occurs. In the case of hillocks an additional coupling between grain boundary sliding and shear-induced grain boundary migration leads to the observed lateral growth. By incorporating grain size and geometry, a structure-dependent grain boundary sliding coefficient and measured film stresses the local conditions for whisker growth, including the growth rates, can be calculated. As described here, other commonly observed whisker and hillock morphologies and geometries are consistent with this model, as are the effects of a surface oxide film and thermal cycling.

Enhanced superplasticity and diffusional creep in ultrafine-grained Mg–6Al–1Zn alloy with high thermal stability

Ultrafine-grained Mg–6Al–1Zn alloys, which have β-Mg17Al12 70–140 nm particles preferentially located at the grain boundaries, exhibited excellent thermal stability and superplasticity at high temperatures. Diffusional creep (Coble creep), which has rarely been reported in other ultrafine-grained Mg alloys and composites, becomes dominant over the grain boundary sliding mechanism at strain rates near 10−4 s−1 at 523–573 K.

Transverse creep and stress relaxation induced by interface diffusion in unidirectional metal matrix composites

Abstract An analytical solution is developed to predict the transverse creep rate induced by interface diffusion in unidirectional fiber-reinforced composites. The driving force for the interface diffusion is the normal stress acting on the interface, which is obtained from rigorous Eshelby inclusion theory. The closed-form solution is an explicit function of the applied stress, volume fraction and radius of the fiber, as well as the modulus ratio between the fiber and the matrix. It is interesting that the solution is formally similar to that of Coble creep in polycrystalline materials. For the application of the present solution in the realistic composites, the scale effect is taken into account by finite element analysis based on a unit cell. Based on the solution, a closed-form solution is also given as a description of stress relaxation induced by interfacial diffusion under constant strain. In addition, the analytical solution for the interface stress presented in this study gives some insight into the relationship between the interface diffusion and interface slip.

High-speed creep process mediated by rapid dislocation absorption in nanocrystalline Cu

A high-speed creep process mediated by rapid dislocation absorption was found in the nanoindentation creep test on nanocrystalline Cu. The creep strain and creep strain rate depend strongly on the loading strain rate and are far higher than those predicted by the models of Coble creep and thermally activated grain boundary sliding. Our analysis revealed that grain boundary dislocation sources can be activated and emitted dislocations from grain boundaries can be stored effectively at a high loading strain rate, but cannot at a low loading strain rate. The observed high-speed creep process is mediated mainly by the rapid absorptions of the stored dislocations and the dislocations newly nucleated during the holding period. An implication of our experimental finding is that dislocation structure in nanocrystalline metals is highly unstable and dislocation activity can proceed after loading and lead to a significant post-loading plasticity.

Creep and stress relaxation in free-standing thin metal films controlled by coupled surface and grain boundary diffusion

An analytical solution is presented that interprets the effects of grain size, surface and grain boundary diffusivities, surface and grain boundary free energies, as well as grain boundary grooving on the creep rate in free-standing polycrystalline thin metal films. The Coble creep in the film plane is also taken into account; this has a significant effect on the creep rate of the film. The effects of diffusion ratio and free energy ratio between surface and grain boundary on film agglomeration are illustrated as well. A closed-form expression for stress relaxation in films under constrained strain conditions is derived from this solution. An exponential decay in stress is found as a function of the film microstructure. Results predicted by the solution are shown to be in agreement with the experiments.

Molybdenum Disilicide - Diffusion, Defects, Diffusion Correlation, and Creep

Molybdenum disilicide (MoSi2) is an interesting material for high-temperature applications. It has a high melting temperature, good thermal and electrical conductivity and an excellent oxidation resistance. For many years the primary use of MoSi2has been in heating elements, which can be used for temperatures up to 1800°C. Since the 1990s the potential of MoSi2as a high-temperature structural material has been recognized as well. Its brittleness at lower temperatures and a poor creep resistance above 1200°C have hindered its use as in load-bearing parts. These disadvantages may be offset at least partly by using it together with a second material in a composite or an alloy. Projected applications of MoSi2-based materials include, e.g. stationary hot section components in gas turbine engines and glow plugs in diesel engines. For future research and development directions of MoSi2-based composites diffusion is a crucial property because creep is closely connected with diffusion. This paper is devoted to the basic diffusion and defect properties of MoSi2. Data of Si and Mo as well as Ge diffusion from the Münster laboratory for both principal directions are briefly summarized. For all three kinds of atoms diffusion perpendicular to the tetragonal axis is faster than parallel to it. The diffusivities of Mo in both directions are many orders of magnitude slower than those of Si and Ge. The huge asymmetry between Mo and Si (or Ge) diffusion suggests that atomic motion of each constituent is restricted to its own sublattice. Positron annihilation studies on MoSi2from the Stuttgart laboratory are reviewed as well. They show that formation of thermal vacancies occurs primarily on the Si sublattice but cannot exclude vacancy formation on the Mo sublattice at higher temperatures. Correlation factors for Si and Mo diffusion via sublattice vacancies in the respective sublattices of MoSi2have been calculated recently mainly by Monte Carlo simulation techniques and are also briefly described. Diffusion, in particular self-diffusion, is discussed in connection with literature data on high-temperature creep, which is diffusion-controlled. Grain-size effects of creep have been reported and can be attributed to Nabarro-Herring and Coble creep. Power-law creep is attributed to diffusion-controlled dislocation creep. Some details are, however, not completely understood, presumably due to a lack of theoretical concepts for creep in uniaxial, stochiometric compounds and due to missing information on grain-boundary diffusion.

Fractal analysis of Palladium hillocks generated due to oxide formation

Understanding the controlling mechanism in growth of hillocks may lead to new processes enabling the growth of self-assembled hillocks. By processing the topographical images of the surface of Pd thin films, surface roughness was erased resulting to well-separated hillocks over the surface. As a result, strong correlation has been found between the total hillock surface area and residual stress generated in the films. Next, fractal analysis is introduced as effective mathematical approach to enhance accuracy of the aforementioned correlation when films are textured. This enables us to study hillocks geometrically and to monitor their distribution over the surface. Moreover, X-ray photoelectron spectroscopy revealed the generation of PdO selvedge layer in the interface of Pd thin films and SrTiO3 substrate as a result of lattice misfit. It could be concluded that oxygen diffusion into grain boundaries of Pd thin film leads to the thermal expansion coefficient mismatch between grains and grain boundaries resulting to hillock growth. Moreover, it has been shown using Coble creep that hillock formation is dominant stress relaxation process in thickness lower than 40nm whereas at higher thickness, selvedge layer generation becomes dominant.

High temperature deformation and fracture of tribo-layers on the surface of AA5083 sheet aluminum–magnesium alloy

Abstract Plastic deformation and fracture of tribo-layers covering surfaces of aluminum AA5083 alloy sheets subjected to tensile tests at elevated temperatures and different strain rates were studied. When tested at 420 °C and a strain rate of 4 × 10−2 s−1, tribo-layers failed predominantly by brittle fracture but about 10% of the fractured oxide patches were found to be held together by fibrous structures. These fibres maintained uniform cross-sections and were as long as 2.5 μm at higher temperatures consistent with superplastic behaviour. Typically each oxide fibre had a diameter of 200 nm and consisted of nanocrystalline MgO and MgAl2O4 grains of 4.5 ± 0.7 nm in diameter as revealed by high resolution TEM. It was suggested that a Coble type creep was responsible for the superplasticity of the oxide fibres.

Microstructure and high-temperature deformation behavior of Al2O3–TiO2 obtained from ultra-high-pressure densification of metastable powders

Metastable alumina composite powders with 13 wt.% titania were produced by plasma spraying using liquid nitrogen quenched substrates. The material consisted of a mixture of Al2O3 in a distorted spinel structure and a large amount of amorphous phases. High-pressure–high-temperature sintering below the crystallization temperature of the amorphous phase yielded a crystalline/amorphous composite that was used for creep experiments. During heating to the desired testing temperature, crystallization occurred and an in situ composite composed of corundum and rutile was obtained. This material exhibited superior deformation behavior compared to conventionally processed alumina materials. Comparably low temperatures of 1100°C could be utilized to achieve plastic deformation of the material under a pressure of 50MPa. Coble grain boundary creep with n=1 and a low activation energy for diffusion of 390kJmol−1 was assumed to be the deformation mechanism.

An Experimental and Theoretical Investigation of Creep in Ultrafine Crystalline Nickel RF-MEMS Devices

The creep behavior of ultrafine crystalline RF microelectromechanical systems (RF-MEMS) devices is investigated in this paper. The RF-MEMS devices are characterized through a highly accurate capacitance-sensing setup, under a special bi-state bias condition. In particular, the devices are kept continuously biased (on-state) for up to 1400 h, but the bias voltage is momentarily removed for 1 min every hour in order to record the off-state capacitance. Measurements at two bias voltages of 20 and 40 V are reported. The capacitance measurement uncertainty is less than 200 aF and the long-term stability is better than 4 fF throughout the measurement period. Furthermore, we report the creep behavior under the same bi-state bias condition through independent direct optical measurements conducted with a confocal microscope-based setup. The measurement uncertainty for the vertical displacement reported in these measurements is less than 50 nm. These measurements show for the first time the entire profile evolution of the deflected beams and membranes for up to 168 h. Both the capacitance and optical measurements reveal the same trends for the creep behavior of the devices. A physics-based creep model for the ultrafine crystalline nickel devices is also included in this paper. The required model parameters are obtained by fitting the simulation results to the measured creep curve under a bias voltage of 20 V. Using the same material parameters and geometry, this model correctly captures the rate of deformation during the steady-state creep stage at 40 V. The model reveals that the observed steady-state creep deformation at both 20 and 40 V is dominated by Coble creep.

Creep behavior inb a fine-grained Al -5356 alloy at low stress and intermediate temperature

m was investigated using the helicoid spring specimen technique. The testing temperatures ranged from 423 K to 523 K (0.47 to 0.58 Tm) and the applied stresses from 0.20 to . It was found that under the above conditions, viscous creep of Bingham type occurs, characterized by a threshold stress which decreases with increasing temperature. The activation energy is Qc = 44 7 kJ/mol. No microstructural changes were observed in recrystallized and the as-crept specimens. The measured creep rates were found to be four orders faster than that predicted by Coble creep model.

Spark Plasma Sintering Kinetics of Pure α‐Alumina

The microstructure development of sintered alumina at different stages in the spark plasma sintering process has been investigated. Based on classical kinetics laws and adapted Coble creep models of hot pressing, the densification and grain growth kinetics were analyzed as a function of various parameters such as heating rate, sintering temperature, and dwell duration. It is found that sintering kinetics are greatly influenced by heating rates. The local temperature gradients at interparticle contacts during high heating rate can be from three to 10 times higher than in low heating rate cycles. Plastic yield might lead to instantaneous densification at an early stage of sintering. However, grain‐boundary diffusion probably dominates at low heating rate whereas grain coarsening, in respect of thermal equilibrium establishment, is unavoidable with high heating rate during the initial‐stage sintering at low temperatures. During the final‐stage sintering, fast grain growth mechanisms such as surface diffusion and pore‐controlled grain‐boundary migration may dominate over densification controlled by grain‐boundary sliding or lattice diffusion. The grain size–density trajectory corroborates that low heating rate is much favorable to achieve near full density with fine grain size at low sintering temperatures.

Grainsize evolution in ductile shear zones: Implications for strain localization and the strength of the lithosphere

At high stresses and low temperatures, grainsize reduction by dynamic recrystallization profoundly modifies rock rheology. Strain energy driven grain-boundary migration (ρGBM) is involved both in the nucleation of new grains by the grain-boundary bulging mechanism (BLG), and in the subsequent evolution of the microstructure. Above the D min line, which is a line in stress/grainsize space that defines the minimum size of nucleus that can form by BLG, ρGBM dominates the microstructure, and grain growth by surface energy driven grain-boundary migration (γGBM) is inhibited. The recrystallized grainsize is therefore dominated by the nucleation process, possibly controlled by the size of subgrains or dislocation cells within the old grains. This provides a first-order explanation for the experimentally observed grainsize-stress relationship. ρGBM is an important agent of recovery in rocks deformed by dislocation creep, sweeping out dislocations and counteracting work-hardening. We have derived a new flow law (DRX-assisted dislocation creep) based on this process, which exhibits grainsize sensitivity as a result of the role of ρGBM. If grainsize obeys the empirically-determined grainsize-stress relationship, DRX creep has an effective stress exponent of a little over 4, consistent with experimental observations and inferences from naturally deformed rocks. DRX creep may be an important agent in weakening quartz at low temperatures, whereas current flow law data suggest it may not be important in olivine. Rocks deformed and dynamically recrystallized above the D min line may switch from climb-assisted dislocation creep to grainsize-sensitive creep (Coble creep, DRX creep, or creep dominated by grain-boundary sliding), resulting in weakening. Lithospheric-scale shear zones are likely to evolve at approximately constant stress; under these conditions weakening results in an increase in strain rate, not a stress drop. The rate of dislocation motion, the dislocation density, and the dynamically recrystallized grainsize all remain the same, and grain growth will be inhibited by the activity of ρGBM. Hence the switches in deformation mechanism and the weakening they cause will be permanent, so long as the tectonic boundary conditions remain unchanged. Grainsize reduction caused by dynamic recrystallization may therefore play a fundamental role in lithospheric weakening, and may be a key process in the maintenance of plate tectonics.

An Evaluation of Creep Mechanisms in Ultrafine-Grained Metals

Our earlier published creep data are analyzed for ultrafine-grained pure aluminium and copper processed by equal-channel angular pressing (ECAP). The analysis demonstrates conclusively that creep occurs in the investigated materials after ECAP by the same mechanism as in conventional coarse-grained materials with intergranular dislocation glide and climb as the dominant rate-controlling flow process. Under creep conditions examined in this work diffusion creep is not important in pure aluminium and copper because the ultrafine grains are unstable at elevated and/or high temperature creep and the grains grow sufficiently to preclude any significant contribution from Nabarro-Herring or Coble creep.

Deformation mechanism maps for micro-grained, ultrafine-grained, and nano-grained materials

There are several deformation mechanisms that depend on grain size and are controlled by grain boundary diffusion. These mechanisms include: Coble creep, superplastic flow (micrograin superplastic flow and high-strain rate superplastic flow), and nanograin deformation. By combining the rate-controlling equations of these mechanisms and by making assumptions regarding triple-junction creep, a deformation map based on grain size was constructed. It is demonstrated that this map can account for the locations of experimental data representing three types of deformation behavior: micrograin superplasticity, high-strain rate superplasticity, and nanograin deformation.

Size-dependent rate sensitivity and plasticity of nanocrystalline Ru films

The rate sensitivity of hexagonal close-packed nanocrystalline Ru films was investigated by nanoindentation, which is found to be a function not only of grain size but also of indentation depth. The enhanced magnitude of the sensitivity is comparable to that of face-centered cubic nanocrystalline metals. Experimental results suggest that the interaction of dislocations with grain boundaries (GBs) and the diffusion along the tip–sample interface are the dominant mechanisms, whereas both GB sliding and Coble creep are ruled out as the rate-controlling deformations.

The Influence of Titania on Creep in Superplastic Zirconia

Detailed high‐temperature compression creep experiments on a pure 3 mol% yttria‐stabilized tetragonal zirconia (3YTZ) and 3YTZ doped with 4.8 wt% TiO2 revealed that both materials exhibit a similar transition in stress exponents from n∼1 to n∼2 with a decrease in stress. The stress exponent of 1 and the inverse grain size dependence p of ∼3 are consistent with the Coble diffusion creep at high stresses; the increase in stress exponent at low stresses is attributed to an interface‐controlled diffusion creep process. Measurements revealed that grain‐boundary sliding contributes to >∼50% of the total strain in both regions with n∼1 and n∼2, indicating the operation of the same fundamental deformation process in both regions. The creep data indicate that doping with TiO2 leads to an increase in the grain‐boundary diffusion coefficients. The increase observed in the dihedral angle with doping is also consistent with the increase in grain boundary diffusion coefficient and the reported enhanced ductility in such materials.