Mechanisms of high-temperature creep of nickel-based superalloys under low applied stresses

[001] single-crystal specimens of the superalloys CMSX-4 and CMSX-10 were tested for creep at 1100°C under tensile stresses between 105 and 135?MPa, where they show pronounced steady creep. The deformed superalloys were analysed by density measurements, scanning electron microscopy and transmission electron microscopy which supplied information about porosity growth, evolution of the γ–γ′ microstructure, dislocation mobility and reactions during creep deformation. It is shown that, under the testing conditions used, steady creep strain mostly results from transverse glide–climb of (a/2) ?011? interfacial dislocations. A by-product of the interfacial glide–climb are vacancies which diffuse along the interfaces to growing pores or to a ?100? edge dislocations climbing in the γ′ phase. Climb of a ?100? dislocations in the γ′ phase is a recovery mechanism which reduces the constraining of the γ phase by the γ′ phase, thus enabling further glide of (a/2) ?011? dislocations in the matrix. Moreover the γ′ dislocations act as vacancy sinks facilitating interfacial glide–climb. The creep rate increases when the γ–γ′ microstructure becomes topologically inverted; connection of the γ′ rafts results in extensive transverse climb and an increase of the number of a?100? dislocation segments in the γ′ phase.     

Spannungsverläufe Nach Wechseln der Verformungsgeschwindigkeit an Metalleinkristallen I. Ursachen von Übergangsvorgängen Nach Wechseln der Abgleitgeschwindigkeit

The exact shape of the stress variations after a change in strain rate is influenced not only by the rate dependence of effective glide obstacles, but also by the changes in dislocation multiplication mechanism. Especially the density of active sources and the total length of dislocations emitted by one source per unit of time are rate sensitive. Various types of sources are discussed and also the strain dependence of source density is considered.

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Ke Karlovu 5, Praha 2, Czechoslovakia.

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Teilweise vorgetragen auf dem Reinststoff-Symposium Dresden 1970.     

Numerical calculations of the radiation-induced strain rate of interstitial solid solutions. II Allowance for the main channels for the influence of impurities on radiationinduced creep

A previously developed numerical method for calculating the radiation-induced creep rate [Yu. S. Pyatiletov and A. D. Lopuga, Tech. Phys. 45 (1999)] is used to study the influence of impurity atmospheres around dislocations and pores, impurity traps, and mobile impurity-vacancy and impurity-interstitial complexes on the radiation-induced strain rate of interstitial alloys. Quantitative data are obtained on the creep rate as a function of impurity concentration, and a physical interpretation allowing for the recombination of interstitial atoms and vacancies directly with one another, on impurity traps, and on mobile complexes is put forward. Zh. Tekh. Fiz. 69, 19–23 (March 1999)     

Observations of glide and decomposition of a⟨101⟩ dislocations at high temperatures in Ni-Al single crystals deformed along the hard orientation

Ni-44 at.% Al and Ni-50 at.% Al single crystals were tested in compression in the hard d001 ¢orientation. The dislocation processes and deformation behaviour were studied as a function of temperature, strain and strain rate. A slip transition in NiAl occurs from a?111? slip to non-a?111? slip at intermediate temperatures. In Ni-50 at.% Al single crystals, only a?010? dislocations are observed above the slip transition temperature. In contrast, a a?101?{101} glide has been observed to control deformation beyond the slip transition temperature in Ni-44 at.% Al. a?101? dislocations are observed primarily along both ?111? directions in the glide plane. High-resolution transmission electron microscopy observations show that the core of the a?101? dislocations along these directions is decomposed into two a?010? dislocations, separated by a distance of approximately 2 nm. The temperature window of stability for these a?101? dislocations depends upon the strain rate. At a strain rate of 1.4 210^?4 s^?1, a?101? dislocations are observed between 800 and 1000 K. Complete decomposition of a?101? dislocations into a?010? dislocations occurs beyond 1000 K, leading to a?010? climb as the deformation mode at higher temperatures. At lower strain rates, decomposition of a?101? dislocations has been observed to occur along the edge orientation at temperatures below 1000 K. Embedded-atom method calculations and experimental results indicate that a?101? dislocations have a large Peierls stress at low temperatures. Based on the present microstructural observations and a survey of the literature with respect to vacancy content and diffusion in NiAl, a model is proposed for a?101?{101} glide in Ni-44 at.% Al, and for the observed yield strength versus temperature behaviour of Ni-Al alloys at intermediate and high temperatures.     

Grain boundary sliding and lattice dislocation emission in nanocrystalline materials under plastic deformation

A theoretical model is proposed to describe the physical mechanisms of hardening and softening of nanocrystalline materials during superplastic deformation. According to this model, triple interface junctions are obstacles to glide motion of grain boundary dislocations, which are carriers of grain boundary glide deformation. Transformations of an ensemble of grain boundary dislocations that occur at triple interface junctions bring about the formation of partial dislocations and the local migration of triple junctions. The energy characteristics of these transformations are considered. Pileups of partial dislocations at triple junctions cause hardening and initiate intragrain lattice sliding. When the Burgers vectors of partial dislocations reach a critical value, lattice dislocations are emitted and glide into adjacent grains, thereby smoothing the hardening effect. The local migration of triple interface junctions (caused by grain boundary sliding) and the emission of lattice dislocations bring about softening of a nanocrystalline material. The flow stress is found as a function of the total plastic strain, and the result agrees well with experimental data.     

A parametric simulation method for discrete dislocation dynamics

A new computer simulation method employed in discrete dislocation dynamics is presented. The article summarizes results of an application of the method to elementary interactions among glide dislocations and dipolar dislocation loops. The glide dislocations are represented by parametrically described curves moving in glide planes whereas the dipolar loops are treated as rigid objects. All mutual force interactions are considered in the models. As a consequence, the computational complexity rapidly increases with the number of objects considered. This difficulty is treated by advanced computational techniques such as suitable accurate numerical methods and parallel implementation of the algorithms. Therefore the method is able to simulate particular phenomena of dislocation dynamics which occur in crystalline solids deformed by single slip: generation of glide dislocations from the Frank-Read source, interaction of glide dislocations with obstacles, their encounters in channels of the bands, sweeping of dipolar loops by glide dislocations and a loop clustering.     

Kinematics of edge dislocations. I. Involutive distributions of local slip planes

The geometry of continuous distributions of dislocations and secondary point defects created by these distributions is considered. Particularly, the dependence of a distribution of dislocations on the existence of secondary point defects is modeled by treating dislocations as those located in a time-dependent Riemannian material space describing, in a continuous limit, the influence of these point defects on metric properties of a crystal structure. The notions of local glide systems and involutive distributions of local slip planes are introduced in order to describe, in terms of differential geometry, some aspects of the kinematics of the motion of edge dislocations. The analysis leads, among others, to the definition of a class of distributions of dislocations with a distinguished involutive distribution of local slip planes and such that a formula of mesoscale character describing the influence of edge dislocations on the mean curvature of glide surfaces is valid.     

In situ TEM observations of dislocation/anti-phase boundary interactions

L2₁-ordered Fe₅₉Mn₁₇Al₂₄ (in at%) single crystals were in situ strained at either 300?K or 700?K in a transmission electron microscope. At 300?K, the strain was accommodated by the glide of four-fold dissociated super-dislocations, whereas, at 700?K, the strain was accommodated by the glide of a/2?111? partials. Dislocation pile-ups occurred at a/4?111? thermal anti-phase boundaries (APBs). Screw super-dislocations frequently cross-slipped when they encountered the thermal APBs, while mixed dislocations tended to be pinned at them. This impediment is attributed to the creation of new APB segments when dislocations pass through the curvy thermal APBs.     

Experimental and computational creep characterization of Al–Mg solid-solution alloy through instrumented indentation

Carefully designed indentation creep experiments and detailed finite-element computations were carried out in order to establish a robust and systematic method to extract creep properties accurately during indentation creep tests. Samples made from an Al–5.3?mol%?Mg solid-solution alloy were tested at temperatures ranging from 573 to 773?K. Finite-element simulations confirmed that, for a power-law creep material, the indentation creep strain field is indeed self-similar in a constant-load indentation creep test, except during short transient periods at the initial loading stage and when there is a deformation mechanism change. Self-similar indentation creep leads to a constitutive equation from which the power-law creep exponent n, the activation energy Q _c for creep, the back or internal stress and so on can be evaluated robustly. The creep stress exponent n was found to change distinctively from 4.8 to 3.2 below a critical stress level, while this critical stress decreases rapidly with increasing temperature. The activation energy for creep in the stress range of n = 3.2 was evaluated to be 123?kJ?mol^?1, close to the activation energy for mutual diffusion of this alloy, 130?kJ?mol^?1. Experimental results suggest that, within the n = 3.2 regime, the creep is rate controlled by viscous glide of dislocations which drag solute atmosphere and the back or internal stress is proportional to the average applied stress. These results are in good agreement with those obtained from conventional uniaxial creep tests in the dislocation creep regime. It is thus confirmed that indentation creep tests of Al–5.3?mol%?Mg solid-solution alloy at temperatures ranging from 573 to 773?K can be effectively used to extract material parameters equivalent to those obtained from conventional uniaxial creep tests in the dislocation creep regime.     

Instability of plastic flow in the microstructure of alkali halide crystals

Spatial localization of deformation bands in LiF and KCl single crystals caused by instability of plastic flow in the strain rate range from 5 × 10^?6 to 2 × 10^?4 s^?1 was studied experimentally. The geometrical parameters of localized shift bands (LSB) were studied as a function of strain rate and temperature. To study the LSB relief, a surface profilometry technique was used for the first time, which made it possible to determine the LSB parameters at the early stages of plastic flow (for strains in the range from 0.5 to 2%). The formation and branching of LSB steps on the surface of a deformed crystal due to the generation and motion of dislocations were found to be scaled. It was shown experimentally that the LSB formation is a thermally activated process that occurs through dislocation glide and is limited by dislocation creep.     

Dislocation-mediated creep process in nanocrystalline Cu

Nanocrystalline Cu with average grain sizes ranging from ～ 24.4 to 131.3 nm were prepared by the electric brushplating technique.Nanoindentation tests were performed within a wide strain rate range,and the creep process of nanocrystalline Cu during the holding period and its relationship to dislocation and twin structures were examined.It was demonstrated that creep strain and creep strain rate are considerably significant for smaller grain sizes and higher loading strain rates,and are far higher than those predicted by the models of Cobble creep and grain boundary sliding.The analysis based on the calculations and experiments reveals that the significant creep deformation arises from the rapid absorption of high density dislocations stored in the loading regime.Our experiments imply that stored dislocations during loading are highly unstable and dislocation activity can proceed and lead to significant post-loading plasticity.     

Local heating and quasiathermal plastic deformation of crystals at low temperatures

A theoretical discussion is given of the relationship between local heating of a crystal by glide lines and bands and deviations observed in the temperature dependences of the flow stresses and their rate coefficients at low temperatures (<10 K) from the dependences characteristic of thermally activated plastic deformation. The appearance of plateaus in these dependences is currently explained by the onset of quantum-mechanical, athermal mechanisms for overcoming local barriers. In this paper it is shown that softening and the apparent athermicity of low-temperature deformation are caused by heating of sites where the strain is localized. Fiz. Tverd. Tela (St. Petersburg) 39, 2019–2022 (November 1997)     

Intermediate temperature creep mechanisms in Ni 3 Al

The intermediate-temperature creep response of single-crystal Ni 3 Al(Ta) has been investigated along both [ ] and [001] axial orientations. The effect of the existing deformation structure (i.e. pre-straining) on the [ ] creep response was reported. The creep responses of virgin specimens and specimens prestrained at room temperature (RT) and 520°C are compared. In order to compare the dislocation structures prior to creep, the microstructure of specimens which had been deformed at a constant strain rate at RT and 520°C, but not subjected to creep, was also examined. Creep curves show that the temperature of pre-strain influences the subsequent creep properties. The primary creep response, like the yielding response, appears to be controlled by the kink size distribution, while the secondary creep response is thought to be controlled by the kink separation (or the length of the Kear-Wilsdorf locks). Specimens crept along [ ] display steady state creep properties and rectangularly oriented [ ](010) dislocations, while a virgin specimen crept along [001] displays an increasing secondary creep rate (inverse creep) and d110 ¢{100}-type dislocations. Inverse creep along [001] is thought to be the result of an increasing density of edge kink octahedral sources where there is little resolved shear stress on the cube planes.     

Interfaces between nanostructures and stepwise creep in highly oriented polymers

A comparative laser-interferometric study of steady creep in oriented ultrahigh-molecular-weight polyethylene films differing in the structure of interfaces between nanosized structural units has been carried out to gain a better understanding of the creep mechanism in oriented polymer materials. In contrast to conventional methods, laser interferometry permits measurement of creep rates from very small strain increments (0.3 μm) to within 1%. This technique made it possible to detect the stepwise nature of plastic deformation in creep. The data obtained suggest that the creep rate and its periodic changes are controlled by the structure of the interfaces, and that the plastic deformation itself occurs to a considerable extent through shear of nanosized structural units relative to one another by an “acceleration-deceleration” type. It is proposed that the “deceleration” phase is due to a glide resistance created by some “stoppers” having either physical or chemical nature, which become destroyed and reappear again in the course of creep. Fiz. Tverd. Tela (St. Petersburg) 41, 1788–1791 (October 1999)     

Slip-accommodated superplastic flow in Zn-22 wt%Al

Experiments were conducted on the Zn-22 wt% Al eutectoid that contained nanometre-scale dispersion particles. These particles were introduced in the matrix of the alloy via powder metallurgy followed by cryomilling. Transmission electron microscopy observations made on specimens crept at a strain rate near the centre of the superplastic region (the intermediate-stress region or region II in the sigmoidal relationship between stress and strain rate) reveal clear evidence for lattice dislocation activities during superplastic flow. Such evidence is demonstrated in part by the presence of attractive particle-dislocation interactions that are only noted in some of the grains. It is suggested that each one of these grains serves as an obstacle for a group of grains sliding together as a unit. In addition, the configurations of the lattice dislocations in the interiors of the blocking grains are suggestive of viscous glide and single slip in the blocking grain. Combining the present findings with earlier observations reported for superplastic deformation leads to the conclusion that the generation and movement of lattice dislocations provide an accommodation process for grain-boundary sliding.     

Interaction of Lomer–Cottrell locks with screw dislocations

In fcc crystals, dislocations are dissociated into partial dislocations and, therefore, restricted to move on {111} glide planes. By junction reactions with dislocations on two intersecting {111} planes, Lomer–Cottrell dislocations along ?110? directions can be formed which are barriers for approaching screw dislocations. Treating the interaction between a dissociated screw dislocation and a LC lock conventionally, using classical continuum theory and assuming the partials to be Volterra dislocations, leads to erroneous conclusions. A realistic result can only be obtained in the framework of the Peierls model, treating the partials as Peierls dislocations and explicitly taking account of the change in atomic misfit energy in the glide plane. At even moderate stresses (at less than 3 × 10^?3 µ in Cu), the screw will combine with the LC lock to form a Hirth lock. As a result, the nature of the repulsive force will change drastically.     

Dislocation glide through non-randomly distributed point obstacles

Abstract

Classical meso-scale models for dislocation–obstacle interactions have, by and large, assumed a random distribution of obstacles on the glide plane. While a good approximation in many situations, this does not represent materials where obstacles are clustered on the glide plane. In this work, we have investigated the statistical problem of a dislocation sampling a set of clustered point obstacles in the glide plane using a modified areal-glide model. The results of these simulations show two clear regimes. For weak obstacles, the spatial distribution does not matter and the critically resolved shear stress is found to be independent of the degree of clustering. In contrast, above a critical obstacle strength determined by the degree of clustering, the critical resolved shear strength becomes constant. It is shown that this behaviour can be explained semi-analytically by considering the probability of interaction between the dislocation line and obstacles at a given level of stress. The consequences for alloys exhibiting solute clustering are discussed.     

Formation mechanism for nanodefects on surfaces of loaded metals

Tunneling profilometry is used to investigate the shape and orientation of defects that form at the surfaces of Cu, Au, Mo and Pd under loading. The defects have the shape of an indented prism. The value of the angles at the tip of the defects coincide with the angles between glide planes, while the orientation of the walls coincide with the orientation of these planes. At the edges of the defects there exist “swellings” caused by expulsion of material at the surface. Based on these results, the creation of these defects is explained by the exit of dislocations as they burst through barriers formed at intersecting glide planes. Fiz. Tverd. Tela (St. Petersburg) 40, 668–671 (April 1998)     

Low-temperature motion of dislocations as a possible mechanism of formation of one-dimensional electronic structures in semiconductor crystals

A new mechanism is proposed for the formation of one-dimensional electronic structures in semiconductor crystals. The mechanism is based on controllable low-temperature glide of dislocations. Moving dislocations generate associations of intrinsic point defects in the form of one-dimensional chains, and the decay of the associations is impeded by low temperature. Experimental results and numerical estimates are presented for cadmium sulfide crystals. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 639–644 (25 November 1997)     

Climb via vacancy diffusion of edge dislocations in 2D dislocation microstructures

The prevention of strength degradation of components is one of the great challenges in solid mechanics. In particular, at high temperatures material may deform even at low stresses, a deformation mode known as deformation creep. One of the microstructural mechanisms that governs deformation creep is dislocation motion due to the absorption or emission of vacancies, which results in motion perpendicular to the glide plane, called dislocation climb. However, the importance of the dislocation network for the deformation creep remains far from being understood. In this study, a climb model that accounts for the dislocation network is developed, by solving the diffusion equation for vacancies in a region with a general dislocation distribution. The definition of the sink strength is extended, to account for the contributions of neighbouring dislocations to the climb rate. The model is then applied to dislocation dipoles and dislocation pile-ups, which are dense dislocation structures and it is found that the sink strength of dislocations in a pile-up is reduced since the vacancy field is distributed between the dislocations. Finally, the importance of the results for modelling deformation creep is discussed.