Low-temperature Dislocation Research Articles

Plastic deformation of dry fine-grained olivine aggregates under high pressures

Abstract This study investigates the effect of pressure on diffusion creep of dry San Carlos and synthetic (prepared by sol-gel method) olivine. We prepared dry (water content < 9 ppm wt) fine-grained (< 1 μm grain-size) olivine and deformed the samples (both San Carlos and solgel olivine in the same assembly) in the same sample assembly under high-pressure (P = 2.9–8.8 GPa) and modest temperatures (T = 980–1250 K) at a fixed strain-rate. Evolution of strength was studied using the radial X-ray diffraction from various diffraction planes. We found that San Carlos and sol-gel olivine show similar rheological behaviour (when normalized to the same grain-size). Stress estimated by the radial X-ray diffraction increases with time and initially shows similar values for all diffraction planes. In many cases, stress values start to depend on the diffraction planes in the later stage and time dependence becomes minor. The micro-structural observations show that grain-size increases during an experiment. The results are interpreted using a theory of radial X-ray diffraction and the theoretical models of diffusion and dislocation creep. We conclude that the initial stage of deformation is by diffusion creep, but deformation in the later stage is by dislocation creep. For dislocation creep, our results are in reasonable agreement with previous low temperature dislocation creep results after a correction of temperature effect. For diffusion creep, we obtain an activation volume of 7.0 ± 2.4 cm3/mol that is substantially smaller than the values reported on dislocation creep but agrees well with the results on grain-growth. By comparing the present results on dry olivine with the previous results on wet (water-saturated) olivine, we found that water enhances diffusion creep but only modestly in comparison to dislocation creep. The difference in the pressure and water content dependence between diffusion and dislocation creep has an important influence on the dominant deformation mechanisms of olivine in the upper mantle.

Power‐Law Viscoelastic Flow of the Lower Accretionary Prism in the Makran Subduction Zone Following the 2013 Baluchistan Earthquake

AbstractSubduction zone accretionary prisms are commonly modeled as elastic structures where permanent deformation is accommodated by faulting and folding of otherwise elastic materials, yet accretionary prisms may exhibit other deformation styles over relatively short time scales. In this study, we use 6.5‐year (2014–2021) Sentinel‐1 interferometric synthetic aperture radar (InSAR) time‐series of post‐seismic deformation in the Makran accretionary prism of southeast Pakistan to characterize non‐linear viscoelastic deformation within an active accretionary prism on short timescales (months to years). We constructed a series of 3‐D finite‐element models of the Makran subduction zone, including an accretionary prism, and constrained the elastic thickness of the upper wedge and the flow‐law parameters (power‐law exponent, activation enthalpy, and pre‐exponential constant) of the lower wedge through forward model fits to the InSAR time‐series. Our results show that the prism is elastically thin (8–12 km) and the non‐linear viscoelastic relaxation of the deep portions of the prism alone can sufficiently explain the post‐seismic surface deformation. Our best fitting flow‐law parameters (n = 3.76 ± 0.39, Q = 82.2 ± 37.73 kJ mol−1, and A = 10−3.36±4.69) are consistent with triggering of low temperature dislocation creep within fluid‐saturated siliciclastic rocks. We believe that the fluids necessary for this weakening originate from sedimentary underplating and/or the presence the hydrocarbons. The presence of power‐law rheology within the lower wedge impacts the estimated plate coupling and the stress state in the subduction system, with respect to the conventional elastic wedge model, and hence should to be considered in future earthquake cycle models.

Dislocation mechanisms of low-temperature acoustic relaxation in iron

An integrated approach to the study of the mechanisms of low-temperature dislocation relaxation during low-temperature cyclic deformations of crystalline materials has been developed and tested. The approach is based on the combined use of experimental methods of mechanical spectroscopy in wide frequency-temperature ranges and theoretical methods of statistical and thermoactivation analysis of experimental results. The efficiency of this approach was demonstrated by studying low-temperature relaxation resonances in iron crystals whose dislocation structure was varied by preliminary plastic deformation. In this study, the previous mechanical spectroscopy results for iron in the temperature range of 4 K < T < 150 K at vibrational frequencies of about 1 Hz and 105 Hz were supplemented by a detailed study of the temperature spectra of internal friction and the Young’s modulus of a single-crystal iron plate at intermediate frequencies of about 103 Hz. A two-mode dislocation relaxator model was suggested for interpreting the entire set of experimental results. Its first component is a linear segment of a dislocation line in a first-kind Peierls relief whose relaxation properties are determined by thermal activation of kink pairs. The second component is a chain of geometric kinks capable of thermally activated diffusion movement in a second-kind Peierls relief. Empirical estimates of the energy, force, inertial, and geometric characteristics of both components of such a relaxator have been obtained. This study complements the earlier analysis of the processes of mechanical relaxation in crystals caused by nucleation and movements of kinks on dislocation lines [a review: A. Seeger and C. Wüthrich, Nuovo Cimento B 33, 38 (1976)].

Examination of rate-controlling mechanisms for plastic deformation of pearlitic steel at low homologous temperatures

The rate-controlling mechanisms of plastic deformation of pearlitic steel before and after cold drawing at low homologous temperatures (T/Tm) were explored because cold-drawn pearlitic steel is an important structural material that requires long-term sustainability at low T/Tm. Both the undrawn and the cold-drawn pearlitic steels were deformed plastically under constant loads at low T/Tm of 0.25–0.30. The strain rate-stress data were analyzed in terms of a power law model based on low temperature dislocation climb, a lattice friction controlled plasticity model, and an obstacle controlled plasticity model. The first two models were inadequate to explain low T/Tm plastic deformation of the present steels: (a) much higher activation energy than that for core diffusion resulted from the low temperature dislocation climb model, and (b) athermal stress comparable to that for pure α-Fe was estimated from the lattice friction controlled plasticity model, so the role of cementite on athermal stress in pearlite was unexplainable. By contrast, the obstacle controlled plasticity model estimated activation energy on the same order as that for overcoming forest dislocations which were regarded as the main obstacles in the present steels, as evidenced by microstructural observation. In addition, this model not only gave an estimate of the athermal stress for the present pearlitic steel that was higher than that of pure α-Fe (reflecting the effect of cementite on athermal stress), but also provided an estimate of the athermal stress of the cold-drawn steel that was higher than that of the undrawn steel (reflecting the effect of pre-existing dislocations caused by cold-drawing in the former). Therefore, it is probable that low T/Tm plastic deformation of the present pearlitic steels is rate-controlled by dislocation glide overcoming forest dislocations generated by low T/Tm deformation and/or by cold drawing.

Creep behavior of high-pressure die-cast Mg-4Al-4La-0.4Mn alloy under medium stresses and at intermediate temperatures

Tensile creep behavior of the high-pressure die-cast Mg-4Al-4La-0.4Mn alloy under stresses of 50–100MPa and at 150–225°C was thoroughly investigated. This alloy exhibits a stress exponent of approximately 6 and the activation energy Q falls within the range of 80–120kJ/mol at 150–200°C. From fitting results, the commonly adopted threshold stress approach is not suitable for ALa44 alloy, and the normalized stress exponent is approximately 7, suggesting that creep deformation of ALa44 alloy was controlled by low-temperature dislocation climb (core diffusion). Furthermore, the microstructural analysis results indicate that the rate-controlling creep mechanism of ALa44 alloy is dislocation climb, but the cell boundary disintegration and the dislocation cross-slip also assisted deformation during creep processes.

Crystal geometry of screw dislocation glide in tungsten nanocrystals

A zigzag pattern of low-temperature dislocation glide occurring in tungsten nanocrystals in the intersecting planes {110} and {211}, which belong to the 〈111〉 crystallographic zone, has been revealed using field ion microscopy. It has been shown that cores of 1/2[111] screw dislocations are undissociated within the limits of the resolution of the field ion microscope. It has been found experimentally that surface atoms are displaced into metastable positions in the region of the trace of screw dislocation motion.

The Micromechanics of Westerley Granite at Large Compressive Loads

The micromechanical damage mechanics formulated by Ashby and Sammis (Pure Appl Geophys 133(3) 489–521, 1990) has been shown to give an adequate description of the triaxial failure surface for a wide variety of rocks at low confining pressure. However, it does not produce the large negative curvature in the failure surface observed in Westerly granite at high confining pressure. We show that this discrepancy between theory and data is not caused by the two most basic simplifying assumptions in the damage model: (1) that all the initial flaws are the same size or (2) that they all have the same orientation relative to the largest compressive stress. We also show that the stress–strain curve calculated from the strain energy density significantly underestimates the nonlinear strain near failure in Westerly granite. Both the observed curvature in the failure surface and the nonlinear strain at failure observed in Westerly granite can be quantitatively fit using a simple bi-mineral model in which the feldspar grains have a lower flow stress than do the quartz grains. The conclusion is that nonlinearity in the failure surface and stress–strain curves observed in triaxial experiments on Westerly granite at low loading rates is probably due to low-temperature dislocation flow and not simplifying assumptions in the damage mechanics. The important implication is that discrepancies between experiment and theory should decrease with increased loading rates, and therefore, the micromechanical damage mechanics, as formulated, can be expected to give an adequate description of high strain-rate phenomena like earthquake rupture, underground explosions, and meteorite impact.

Depinning of a discrete elastic string from a two-dimensional random array of weak pinning points

The present work is essentially concerned with the development of statistical theory for the low temperature dislocation glide in concentrated solid solutions where atom-sized obstacles impede plastic flow. In connection with such a problem, we compute analytically the external force required to drag an elastic string along a discrete two-dimensional square lattice, where some obstacles have been randomly distributed. The corresponding numerical simulations allow us to demonstrate a remarkable agreement between simulations and theory for an obstacle density ranging from 1 to 50 % and for lattices with different aspect ratios. The theory proves efficient on the condition that the obstacle-chain interaction remains sufficiently weak compared to the string stiffness.

Deformation mechanisms in Cr20Ni80 alloy at elevated temperatures

The mechanisms of plastic deformation of Cr20Ni80 nichrome with an initial grain size of 80 μm were studied in the temperature range 600–950°C and the strain-rate range 1.5 × 10−6−5 × 10−2s−1. Nichrome is shown to exhibit anomalously high values of stress exponent n and a high deformation activation energy Q. These unusual properties were found to be caused by “threshold” stresses below which deformation does not occur. An analysis of the deformation behavior with allowance for threshold stresses reveals the regions of hot, warm, and cold deformation in nichrome. At normalized strain rates $$ \dot \varepsilon $$ kT/D 1 Gb < 10−8, the true values of n and Q are ∼4 and 285 ± 30 kJ/mol, respectively. In the normalized-strain range 10−8−10−4 n ∼ 6 and the deformation activation energy decreases to 175 ± 30 kJ/mol. This change in the deformation-behavior characteristics is explained by the transition from high-temperature dislocation climb, which is controlled by lattice self-diffusion, to low-temperature dislocation climb, which is controlled by pipe diffusion, as the temperature decreases. At $$ \dot \varepsilon $$ kT/D 1 Gb = 10−4, a power law break-down takes place and an exponential law (which describes the deformation behavior in the range of cold deformation) becomes operative.

Attenuation of seismic waves and the universal rheological model of the Earth’s mantle

Analysis of results of laboratory studies on creep of mantle rocks, data on seismic wave attenuation in the mantle, and rheological micromechanisms shows that the universal, i.e., relevant to all time scales, rheological model of the mantle can be represented as four rheological elements connected in series. These elements account for elasticity, diffusion rheology, high temperature dislocation rheology, and low temperature dislocation rheology. The diffusion rheology element is described in terms of a Newtonian viscous fluid. The high temperature dislocation rheology element is described by the rheological model previously proposed by the author. This model is a combination of a power-law non-Newtonian fluid model for stationary flows and the linear hereditary Andrade model for flows associated with small strains. The low temperature dislocation rheology element is described by the linear hereditary Lomnitz model.

Relationships between melt-induced rheological transitions and finite strain: Observations from host rock pendants of the Tuolumne Intrusive Suite, Sierra Nevada, California

Finite strain analysis in the northern (Piute Meadow pendant (PMP)) and eastern (Saddlebag Lake pendant (SLP)) host rock pendants of the Tuolumne Intrusive Suite (TIS) reveals plane strain geometries in the pendants and a steep strain gradient in the SLP. Average regional z-axis shortening in both pendants is ca. 43% and increases to ca. 70 and 85% in the SLP within ca. 100 m to the pluton contact. Microstructures in the SLP adjust to increasing temperatures and partial melting near the margin. Beyond the pluton margin, weakly elongate quartz grains with lobate grain boundaries, patchy to sweeping undulose extinction, deformation bands, and incipient subgrains are consistent with low-temperature dislocation creep. Towards the margin, quartz grain boundaries become increasingly irregular and display “chessboard” patterns indicating high-temperature dislocation creep. Grain size is markedly smaller near the margin and despite large finite strain grains do not show evidence for crystal-plastic deformation. Cuspate strain-free interstices are interpreted to mimic former anatectic melt. Outcrop-scale migmatization, coincidence with abruptly increasing finite strain, and microfracturing towards the margin implies that fracture-induced mechanical grain size reduction may have triggered the transition to melt-assisted granular flow, ultimately resulting in drastic rheological weakening.

Mechanical Spectroscopy of High Pressure Torsion Deformed Fe-Based Alloys and Ti

Low temperature dislocation- and point defect-related anelasticity in high pressure torsion (HPT) deformed metallic materials bcc Fe-26Al, fcc Fe-36Ni, hcp Ti is studied by mechanical spectroscopy. Internal friction (IF) peaks, which correspond to these phenomena, have different stability against heating. Up to five IF peaks are recorded, at least some of these peaks can be classify as Hasiguti peaks. Mechanical spectroscopy gives a useful tool to study early stages of severely plastic deformed alloys study.

Determination of the pre-exponential factor and the activation energy for the low-temperature dislocation relaxation in aluminum

The ultrasonic attenuation and the sound velocity at 10 MHz were measured in deformed Al single crystals in the temperature range between 1.5 and 50 K. In the high purity specimen (RRR=20 000) the change in sound velocity due to deformation shows a step in temperature dependence at 17 K, which corresponds to the 11 K relaxation peak at 50 kHz found by Kosugi and Kino [J. Phys. Soc. Jpn. 58 (1989) 4269], although the change in attenuation due to deformation was masked by the large background attenuation due to phonon–electron interaction. From the present experiment and that of Kosugi and Kino, the activation energy and the attack frequency of the relaxation process are estimated to be 14 meV and 1.0×10 12/s, respectively. If the kink pair formation process of dislocations on the Peierls potential is responsible for this relaxation, the Peierls stress is 5.4×10 −5 G, where G is the shear modulus, and this value does not conflict with the flow stress at low temperatures.

Low-temperature dislocation and magnetomechanical acoustic effects in high purity Fe single crystals

The effect of preliminary plastic deformation εpl&amp;lt;3% and an external magnetic field H⩽3.35 kOe on linear and nonlinear acoustic properties of high purity Fe single crystals is studied. Measurements were made at frequencies ∼90 kHz in the temperature interval 5.5–300 K using the two-component composite oscillator technique. The wave vector of the longitudinal standing wave and the magnetic field direction coincide with the crystallographic axis 〈731〉. In the linear range, the main attention is paid to the influence of the deformation level and subsequent low-temperature recovery on parameters of the relaxation α-peak near 50 K. The increase in εpl leads to a considerable rise in the height of the peak, while the height and temperature of the peak are considerably lowered in the recovery process. An increase in the amplitude ε0 of the ultrasonic deformation beyond a certain threshold ε0c is accompanied by the emergence of strong amplitude dependences of acoustic parameters of the crystals under investigation. As a result of plastic deformation, the amplitude dependences of the decrement and the Young’s modulus are displaced towards lower amplitudes. Magnetoacoustic effects in a saturating external field are practically independent of the amplitude and display a weak monotonic dependence on temperature in the region of the α-peak. It is concluded that the amplitude dependences as well as the α-peak are of the dislocation nature. Possible microscopic mechanisms of the observed effects are discussed.

Low-frequency internal friction studies of nanocrystalline copper

The low-frequency internal friction behavior of OFHC copper and mechanically milled copper was studied over a temperature range of 100–600 ° K at a frequency of approximately 3 Hz. The nanocrystalline copper was prepared by consolidating mechanically milled powders using hot isostatic pressing to form compacts from which bars were machined for testing. Samples studied included milled copper and commercial grade 101 copper bar which had been recrystallized. Powders were consolidated at 600 °C. The final particle size of the consolidated material was approximately 60 nm and the 101 material had a grain size of 32 μm. The internal friction studies indicated the presence of a large grain boundary peak in both materials at approximately 600 ° K which was superimposed on a high temperature background. The peak was significantly larger for the nanocrystalline material because of the increased grain boundary/interfacial area. The OFHC material exhibited at low temperature dislocation peak at approximately 125 °K which was absent in the nanocrystalline material, which is believed to be due to the lack of dislocation structures in heavily milled nanocrystalline materials.

Low-Temperature Dislocation Relaxation Processes in Niobium of Different Purity

Low-Temperature Dislocation Relaxation Processes in Niobium of Different Purity

Low-temperature dislocation internal friction in crystals

An analysis of the experimental and theoretical results related to studies of the low-temperature dislocation mobility by the internal friction method is given. It is shown that a number of unusual experimental results obtained at low and very low temperatures might be explained in the framework of the existing theories modified by taking into account the influence of quantum effects and viscous drag on the overcoming by dislocations of various potential barriers in crystals.

Recombination-induced motion of dislocations in III-V compounds

The methods of in situ cathodoluminescence and scanning electron microscopy were used in a study of stimulated dislocation glide. Dislocations generated by deliberate surface damage were found to be highly mobile when excited above a certain threshold. A study was made of the dependence of the glide velocity on the excitation rate and the first quantitative results on low-temperature dislocation motion are reported.

Mechanical stresses in low pressure chemically vapour deposited silicon films

Stresses vs. deposition temperature and annealing time have been studied in silicon films 0.15 and 0.30 μm thick. The films were deposited onto oxidized silicon wafers by low pressure chemical vapour deposition from a gas mixture containing 5% SiH 4 and 95% Ar in a horizontal-type reactor at temperatures of 793–853 K. The films were annealed during 0.25–1.0 h flowing argon at 1223 K. Stresses have been found to increase from about 300 to about 500 MPa as the deposition temperature increases from 833 to 853 K. This is due to oxidation of film crystallites during deposition and a change in the condensation mechanism with temperature. The validity of a model allowing for crystallization of the amorphous phase during deposition has been confirmed by X-ray diffraction, reflection high energy electron diffraction and scanning electron microscopy. It is shown that a decrease in compressive stresses in the films containing a considerable fraction of amorphous phase is caused by crystallization and dynamic recrystallization of the films owing to annealing. A general equation has been derived which describes changes in the film elastic energy during crystallization taking into account plastic deformation. By using the deformation mechanism map it is shown that the stress reduction during annealing of polycrystalline films is caused by low temperature dislocation (power law) creep.

Low-temperature dislocation relaxations in high-purity iron single crystals deformed at 295 and 78 K

Low-temperature dislocation relaxations in high-purity iron single crystals deformed at 295 and 78 K