Search

The present investigation deals with the microstructural modification following the Bi + Sr additions to the squeeze-cast AZ91 alloy and its effect on impression creep response. The Bi + Sr additio...

FeCrAl alloys have been proposed as nuclear fuel cladding materials in hope of offering great potential in accident tolerance for light water reactors. However, only a limited amount of data has been published on their creep properties. This work aims to provide an atomic-scale insight into the creep behavior and deformation mechanism for nanocrystalline FeCrAl alloys, using molecular dynamics method. Both primary and steady state creeps are observed in all creep curves, and tertiary creep occurs at stresses above 2.5 GPa in the time period of the present simulation. As stress increases, the creep mechanism transits from Coble creep to grain boundary sliding, and then to the synergy of grain boundary sliding and dislocation creep. The turning points are about 0.8 GPa and 1.8 GPa, respectively. The dislocation motions are controlled by viscous gliding at low temperature, and then by climbing when temperature exceeds 1000 K. Moreover, the grain size and alloy composition have little impact on the transition of creep mechanism within the present research scope. The proposed creep mechanisms are further analyzed not only from the comparison between activation energies for diffusion and creep, but also from the representative atomic snapshots.

This is the first report on creep properties of biodegradable zinc alloys at human body temperature. The extruded Zn-0.1Li (wt.%) alloy exhibits appreciable creep deformation at 37°C under stresses ranging 80–230 MPa within 500 h. The creep deformation at low stresses (80–155 MPa, about 0.4–0.8 yield strength) is characterized by a stress exponent of about 4 and an activation energy close to that for self-diffusion of zinc, which are typical of dislocation creep. The implications of creep at human body temperature for durability of biodegradable zinc alloys as stents are discussed.

The microstructure and texture evolutions of 2A97 Al Li alloy with initial unrecrystallized structure during superplastic deformation were investigated. A total elongation of 300% was obtained at 390 °C at an initial strain rate of 3 × 10−3 s−1. The initial banded structure gradually transformed into a recrystallized structure, characterized by equiaxed grains, random boundary misorientation distribution and a weak texture at high strains. The true strain-stress curve exhibited three stages: work hardening (stage I), steady-state (stage II), and deformation instability regions (stage III). The corresponding deformation mechanisms varied at different stages. The strain rate sensitivity index (m) remained constant of 0.35 and the texture density increased with strain at stage I, where the dislocation creep and subgrain rotation were the dominant mechanisms. During stage II, the m-value increased to 0.44, and the texture density decreased drastically with strain. The combination of grain boundary sliding (GBS) and dislocation creeep could explain the behavior of the alloy. Grain growth and cavitation resulted in the decrease in m-value at stage III, where GBS was the main deformation mechanism.

In the present article, the effect of Cu on the microstructure and creep properties of Al-15Si-0.5Mg cast alloy was studied by using optical microscopy, scanning electron microscopy, energy dispersion spectrometry and the impression creep test. The microstructure is changed considerably in the presence of copper. The number of the Cu-rich phases increased noticeably with rising Cu content. Formation of a dendritic structure of α(Al) and modification of the eutectic Si were the other effects of increasing the Cu amount. The results showed that creep properties of the alloy increase considerably with the Cu addition. Calculating the stress exponent (n) and creep activation energy (Q) revealed that pipe diffusion climb-controlled creep is the dominant creep mechanism of the main alloy. Although Cu had no effect on the dominant creep mechanism at the lower stress, it was changed to lattice diffusion climb-controlled dislocation creep with increasing stress.

It is shown that the dislocation creep strength of a solid-solution-strengthened Ni-base superalloy is critically dependent upon the cooling rate from the annealing temperature. This behavior is correlated with the effect of cooling rate on the morphology of M23C6-type carbide which precipitates at grain boundaries during cooling. Experiment shows that the creep strength is enhanced by rapid cooling which results in discrete carbide particles with irregular shape following a zigzag path along the grain boundaries. However, the creep strength is degraded as the morphology is changed into lamellar type by a discontinuous grain boundary reaction which occurs during slower cooling rates. Due to the transition from the discrete particle into the lamellar morphology, plastic deformation becomes more localized alongside the grain boundaries which accelerates the creep rate leading to premature intergranular failure. On the other hand, it is also shown that the stacking fault energy of the alloy increases with temperature which can allow dislocations to rearrange themselves into subgrain structure with relative ease during steady-state creep.

The mechanical characteristics of electrochemically deposited copper coatings have been examined by application of two hardness composite models: the Chicot-Lesage (C-L) and the Cheng-Gao (C-G) models. The 10, 20, 40 and 60 &micro, m thick fine-grained Cu coatings were electrodeposited on the brass by the regime of pulsating current (PC) at an average current density of 50 mA cm&minus, 2, and were characterized by scanning electron (SEM), atomic force (AFM) and optical (OM) microscopes. By application of the C-L model we determined a limiting relative indentation depth (RID) value that separates the area of the coating hardness from that with a strong effect of the substrate on the measured composite hardness. The coating hardness values in the 0.9418&ndash, 1.1399 GPa range, obtained by the C-G model, confirmed the assumption that the Cu coatings on the brass belongs to the &ldquo, soft film on hard substrate&rdquo, composite hardness system. The obtained stress exponents in the 4.35&ndash, 7.69 range at an applied load of 0.49 N indicated that the dominant creep mechanism is the dislocation creep and the dislocation climb. The obtained mechanical characteristics were compared with those recently obtained on the Si(111) substrate, and the effects of substrate characteristics such as hardness and roughness on the mechanical characteristics of the electrodeposited Cu coatings were discussed and explained.

A constitutive model for crushed salt is presented in this paper. A creep constitutive model is developed first and compared with test results. The constitutive model presented here concentrates on creep deformation because saline media behave basically in a ductile and time-dependent way. An idealized geometry is used as a common framework to obtain stress–strain macroscopic laws based on two deformation mechanisms: fluid-assisted diffusional transfer creep and dislocation creep. The model is able to predict strain rates that compare well with results from laboratory tests under isotropic and oedometric conditions. Macroscopic laws are written using a non-linear viscous approach, which incorporates also a viscoplastic component, based on critical state theory. The viscoplastic term is intended for non-creep deformation mechanisms such as grain reorganization and crushing. Copyright © 2002 John Wiley & Sons, Ltd.

Depth-sensing (instrumented) indentation testing technique is a robust, reliable, convenient and non-destructive characterization method to study small-scale mechanical properties and rate-dependent plastic deformation in metals and alloys at ambient and elevated temperatures. In the present paper, depth-sensing indentation creep behavior of an additively manufactured, via laser powder bed fusion (L-PBF) method, Ti-6Al-4V alloy is studied at ambient temperature. Indentation creep tests were performed through a dual-stage scheme (loading followed by a constant load-holding and unloading) at different peak loads of 250 mN, 350 mN, and 450 mN with holding time of 400 s. Creep parameters including creep rate, creep stress exponent, and indentation size effect were analyzed, according to the Oliver and Pharr method, at different additive manufacturing scan directions and scan sizes. To assess processing parameter/ microstructure/ creep property correlations in the additively manufacture Ti-6Al-4V alloy, microstructural quantitative analyses (i.e. optical microscopy and scanning electron microscopy) were performed as well. The findings of this study, according to stress exponent values, showed that the controlling mechanism of the creep at ambient temperature for the examined L-PBF Ti-6Al-4V is mainly glide-controlled dislocation creep. These findings were compared against traditionally processed Ti-6Al-4V as well.

In this work, using the framework of (three-dimensional) Eshelbian dislocation mechanics, we derive the $J$-, $M$-, and $L$-integrals of a single (edge and screw) dislocation in isotropic elasticity as a limit of the $J$-, $M$-, and $L$-integrals between two straight dislocations as they have recently been derived by Agiasofitou and Lazar [Int. J. Eng. Sci. 114 (2017) 16-40]. Special attention is focused on the $M$-integral. The $M$-integral of a single dislocation in anisotropic elasticity is also derived. The obtained results reveal the physical interpretation of the $M$-integral (per unit length) of a single dislocation as the total energy of the dislocation which is the sum of the self-energy (per unit length) of the dislocation and the dislocation core energy (per unit length). The latter can be identified with the work produced by the Peach-Koehler force. It is shown that the dislocation core energy (per unit length) is twice the corresponding pre-logarithmic energy factor. This result is valid in isotropic as well as in anisotropic elasticity. The only difference lies on the pre-logarithmic energy factor which is more complex in anisotropic elasticity due to the anisotropic energy coefficient tensor which captures the anisotropy of the material., 8 pages. arXiv admin note: text overlap with arXiv:1702.00363

A dilute Al-0.06Sc-0.02Zr-0.005Er (at%) alloy, to which 0.09 at% Si was added, was peak-aged to create a high number density of (Al,Si)3(Sc,Er,Zr) precipitates, 3.6 nm in radius. The alloy shows a high resistance to dislocation creep, with a threshold stress of 18 MPa at 400 °C. After further aging under applied stress for ~ 1000 h at 400 °C, the threshold stress increases to 22 MPa, with the precipitates growing to a radius of 4–8 nm. This represents a very substantial improvement in creep resistance as compared to a similar alloy with one-third the Si content, 0.03 at%, whose threshold stress at 400 °C is 9–14 MPa. Atom probe tomography reveals that, for the new higher-Si alloy, the precipitates have an average Si concentration of 3.3 at% and show a broad core with uniform Sc-, Si- and Er concentrations and a thin Zr-enriched shell. By contrast, the low-Si alloy exhibits precipitates with half the average Si content, showing an Er-enriched core, a Sc-enriched inner-shell and a Zr-enriched outer-shell. A possible explanation for the higher creep resistance of the high-Si alloy is that the enhanced chemical homogeneity of Sc and Er in the core, as compared to the highly segregated core/shell/shell structure of the low-Si alloy, modifies the elastic strain field around precipitates so as to increase the repulsive force from the precipitate on the matrix dislocations climbing over them, thus enhancing the threshold creep stresses.

In this study, aging behavior, tensile property and creep performance of the ring rolled AZ80-Ag magnesium alloy at different aging temperatures were systematically investigated. Results show that the microstructure is mainly occupied by discontinuous precipitation (DP) and continuous precipitation (CP) of β-Mg17Al12 after T6 treatment. The peak aging at low temperature (175 °C) results in a larger volume fraction of β precipitation and a lower CP/DP ratio than peak aging at high temperature (250 °C). With raising aging temperature, the tensile strength at ambient temperature reveals a descending tendency, whereas the creep resistance at 120–175 °C under 70–90 MPa exhibits an enhancement. Based on the analysis of creep stress exponent and activation energy values, the dominant creep mechanism of both aged specimens is dislocation creep controlled by competing lattice diffusion and pipe diffusion. Basal and non-basal dislocations can be activated in the creep deformation.

Grain refinement is an effective technique to improve the mechanical properties of steels. In the present work, single-pass hot compression experiments have been conducted on two different compositions of high phosphorous steels to study the microstructural evolution and ferrite grain refinement at various strain rates and deformation temperatures, i.e., 0.01-10 s−1 and 750-1050 °C, respectively. Optical metallography has been employed to understand the physical processes that take place during hot deformation process. The results indicate that when these compositions of high phosphorous steels are worked at relatively low temperatures in the intercritical regions, a ferrite grain size of 5-7 µm is obtained. It is observed that the grain size decreases with an increase in strain rate and with the decrease in deformation temperature. Based on the values of stress exponent (n) obtained in the present work, dislocation creep is identified as the deformation mechanism. The activation energies for deformation of these two types of high phosphorous steels have been calculated. The effect of the alloying elements on the stress–strain curve, microstructure, and grain refinement has been discussed.

There is considerable interest currently in developing and understanding microstructural evolution, stability and deformation in the new class of highly concentrated, multi-principal element solid solution high entropy alloys. This overview provides a brief description of potential high temperature deformation mechanisms in such fine grained alloys, discusses the significance of diffusion, and compares the available experimental results with appropriate theoretical models. It is shown that diffusion is not substantially slower in such alloys, and that superplastic deformation follows the standard model established for conventional polycrystals. The data also suggest that deformation at higher stresses occurs by dislocation glide creep, followed by a power-law breakdown regime. It is necessary to exercise caution in interpreting spherical nanoindentation creep data, although some data on a nanocrystalline HEA suggest that deformation at room temperature occurs by Coble diffusion creep. Based on the available data and understanding, a deformation mechanism map is developed highlighting the dominance over different regimes of grain size and stress of Coble diffusion creep, superplastic flow, dislocation creep and power-law breakdown. (C) 2017 Elsevier B.V. All rights reserved.

The elevated temperature compressive deformation behavior of typical β solidifying γ-TiAl alloys with two microstructures was investigated. Combining with the microstructure characterization, the variation of the stress exponent n and the apparent activation energy Q with isothermal compression parameters indicates that a transition of deformation mechanism from dislocation creep to grain boundary sliding (GBS) occurred in both two alloys, but for the (β + γ) alloy, the domination of the compressive deformation by GBS began at a lower temperature and a higher strain rate than (α 2 +γ) alloy. The compressive deformation of (α 2 +γ) alloy might be mainly controlled by the γ phase lattice diffusion. But for the (β + γ) alloy, in the dislocation creep region it is because of the introduction of β/β o phase lattice diffusion that the Q value of the (β + γ) alloy is below that of the (α 2 +γ) alloy. However, in the GBS region the compressive deformation may also be only controlled by the γ lattice diffusion similar with (α 2 +γ) alloy, which leads to the rise of the Q value. The constitutive equations considering the compensation of the strain predicts well the flow curves. The power dissipation maps were developed on the basis of the above equations and the dynamic material model (DMM).

This study presented length-scale-dependent room temperature creep behaviour of Ti/Al multilayers by nanoindentation. Ti/Al multilayers with individual layer thickness ranging from 10 nm to 250 nm were prepared by a direct current magnetron sputtering method. Microstructural analysis showed that the prepared Ti/Al multilayers were polycrystalline, with strong Ti (0002) and Al (111) texture. Nanoindentation creep tests were performed to examine the time-dependent creep deformation, stress exponent, creep strain rate sensitivity and contact creep compliance of the multilayers at room temperature. It was found that both the maximum creep deformation and the creep strain rate sensitivity increase with an increase in the individual layer thickness, indicating a length-scale-dependent creep behaviour of the Ti/Al multilayers. The values of stress exponent were found to lay in the range of 15.74–61.46, implying that the creep behaviour of the multilayers was controlled by the dislocation creep mechanism. Finally, the contact creep compliances of the Ti/Al multilayers were well modelled by a two-element Kelvin-Voigt model. From this, it was determined that the creep numbers and retardation times increased with increased individual layer thickness.

The compressive creep behavior of hot-pressed Mg1.96Al0.04Si0.97Bi0.03, a promising thermoelectric material, is investigated at 500 °C. At stress levels between 81 and 212 MPa, dislocation creep with stress exponent n = 7.6 ± 0.3 is observed. No diffusional creep is observed, likely attributable to a dispersion of ~1 μm Bi-, Al-, and O- rich particles which pin grain boundaries. Mg1.96Al0.04Si0.97Bi0.03 exhibits similar creep behavior to previously studied silicides, but is significantly more creep resistant than other thermoelectric materials, PbTe and Bi2Te3. This makes Mg1.96Al0.04Si0.97Bi0.03 an excellent material for thermoelectric power generation systems subjected to high stresses and temperatures.

Evolution of the deformation behaviour, surface and bulk structures at superplastic flow of the АА7475 aluminium-based alloy were studied by scanning and transmission electron microscopes and a focused ion beam technique. Differences between deformation behaviour at a primary stage with strains below 0.69 and a steady stage with strains above 0.69 were discussed. The research showed the grain growth and grain elongation to the stress direction at a primary stage of deformation. Stabilisation of both mean grain size and grain aspect ratio was found at a steady stage of deformation. Grain neighbour switching, grain rotation, dispersoid free zones and some insignificant intragranular strain were observed at both stages. Appearing and disappearing of the grains on the sample surface, with increased dislocation activity occurred at the steady stage of deformation. The current results highlighted the importance of diffusion creep as a dominant mechanism at the beginning of superplastic deformation and as an accommodation mechanism of the grain boundary sliding at the steady stage of deformation. The dislocation creep as an additional accommodation mechanism of the grain boundary sliding at the steady stage of superplastic deformation is suggested.

On the basis of theoretical consideration and analysis of molecular dynamic (MD) simulation data, we show that the dislocation motion is determined by the stress field in its local environment. These stresses differ from the values averaged over even such tiny microscopic regions, which are usually used in MD study of dislocation motion. As a result, the slip velocity of dislocations can remain virtually constant with a gradual decrease in the average stresses in the calculation area. When a dislocation enters the trace of the previous dislocation, that is, into a region plastically relaxed by the slip of the previous dislocation, its velocity, on the contrary, decreases sharply, even if the average stresses in the region vary slightly. The revealed complex behavior leads to variable final average stress after the completion of the movement of dislocations; the average stress sometimes even changes its sign in the course of plastic relaxation. All these features can influence the response of the dislocation system to mechanical loading. Therefore, the action of the local stresses should be taken into account when analyzing the MD results, in the development of continuum models of plasticity, as well as in discrete dislocation dynamics. A dislocation motion equation is proposed with accounting of local stresses, and the constants for the edge dislocation in Al are determined by comparison with the results of MD simulations. An asymptotic solution of this equation is proposed, which can be used in the numerical solution of the equations of continuum dislocation plasticity. Alternative dependencies of the drag force on the dislocation velocity are analyzed; it is shown that they describe the results of MD simulations worse than the equation proposed by us.

The influence of Al (0.5wt.%, 1wt.%, 2wt.%) on the microstructural evolution and creep resistance of Mg-2.85Nd-0.92Gd-0.41Zr-0.29Zn (El21) alloy was systematically investigated. The creep results revealed that the additions of 0.5wt.% and 1wt.% Al significantly decreased the creep rate of El21 by more than an order of magnitude, whereas 2wt.% Al in El21 led to the reduction of creep properties. Microstructural analyses indicated that the additions of 0.5wt.% and 1wt.% Al led to significant grain coarsening due to the consumption of Zr via the formation of Al2Zr3 and Al2Zr phases. In contrast, the addition of 2wt.% Al caused distinct grain refinement, resulting from the additional formation of lumpy Al2RE in the centre of α-Mg grains. Additionally, the increase of Al content in the El21 gradually led to the disappearance of the Mg3RE phase and left Al2RE as the only dominant phase. The main Al–Zr phase was also changed from Al2Zr3+Al2Zr to Al2Zr phase. Creep data analysis showed that the dominant creep mechanism was dislocation creep for all alloys, which was in agreement with the EBSD and TEM characterizations. The enhanced creep resistance via the addition of 0.5wt.% and 1wt.% Al was ascribed to the high area fraction of intermetallic phases and the additional formation of the thermally stable Al2RE phase. El21 + 0.5Al has better creep resistance than El21+1Al, which was attributed to its stronger dynamic precipitation strengthening from γ precipitates. The deteriorated creep properties caused by adding 2wt.% Al in El21 alloy arose from the bimodal inhomogeneous distribution of grains and the laminar Al2RE phase. Such microstructure might cause significant stress concentrations and could not effectively impede dislocation motion or reinforce the grain/dendritic boundaries during creep, thus deteriorating the creep properties of El21+2Al.

To investigate the brittle creep failure process of rock material, the time-dependent properties of brittle rocks under the impact of homogeneity are analyzed by the numerical simulation method, RFPA-Creep (2D). Deformation is more palpable for more homogeneous rock material under the uniaxial creep loading condition. At a low stress level, diffusion creep may occur and transition to dislocation creep with increasing applied stress. The law for increasing creep strain with the homogeneity index under a constant confined condition is similar to the uniaxial case, and dislocation creep tends to happen with increasing confining pressure for the same homogeneity index. The dilatancy index reaches its maximum at a high stress level when rock approaches failure, and the evolution of the dilatancy index with the homogeneity index under the same confining pressure is similar to the uniaxial case and is more marked than that under the unconfined condition. Both uniaxial and triaxial creep failure originate from the ductile damage accumulation inside rock. The dominant shear-type failure is exhibited by uniaxial creep and the conventional compression case presents the splitting-based failure mode. Under confining pressure, the creep failure pattern is prone to shear, which is more notable for the rock with higher homogeneity.

U3Si2 is an advanced fuel candidate due to its relatively high fissile density and attractive thermal properties. Compared to standard UO2 fuel, there are significant data gaps for the thermophysical and thermomechanical properties of U3Si2. Point defect concentrations and mobilities under irradiation govern a number of important fuel performance properties, such as creep and fission gas release. In this work, we utilized density functional theory (DFT) data to inform a cluster dynamics framework to predict point defect concentrations in U3Si2 under irradiation. Molecular dynamics (MD) simulations were used to examine the contribution of atomic mixing during ballistic cascades to diffusion, as well as the diffusivity of U and Si at grain boundaries. These atomic scale models for diffusivity were then used to inform a creep model based on bulk (Nabarro-Herring) and grain boundary (Coble) diffusional creep, and climb-limited dislocation creep. The model compares well against available experimental data and has been implemented in the BISON fuel performance code. A demonstration case using simple power profiles has been carried out, showing that negligible creep occurs due to the low temperatures experienced by U3Si2 in-reactor, a consequence of its high thermal conductivity.

The influences of applied stress (120–760 MPa) and temperature (700–1000 °C) on the creep deformation mechanisms of the newly developed nickel-base superalloy M951G have been studied. The dominant deformation mechanisms of M951G alloy after different creep tests were systematically investigated and the reasons for their transition were well discussed. A creep mechanism map at various creep conditions was summarized and the theoretical critical resolved shear stresses (CRSSs) of various creep mechanisms under different temperatures were also calculated. Results show that the CRSSs of different creep mechanisms display different dependencies of temperature and the favorable deformation mechanisms at different creep conditions are different. τ APB , τ OB and τ CL are decreased to varying degrees with the temperature increasing; on the contrary, there is a positive correlation between τ SF and temperature. At low temperature region, the favorable deformation mechanism is shearing of γ′ precipitates by stacking faults. However, it changes to antiphase boundaries (APBs) coupled dislocation pairs shearing in the γ′ precipitates and dislocation climbing in the γ matrix channel at high temperatures. At intermediate temperatures, both stacking faults and APBs are observed owing to the alternate leading of the CRSSs for APBs and stacking faults shearing in γ′ precipitates.

This study investigated the features of grain boundary regions in a Ni-based superalloy which underwent creep deformation. Large grain boundary zones were denuded in fine γʹ precipitates but contained coarsened microscale γʹ rods. The coarsening direction of these rods was along the of the γ matrix and γʹ precipitates. The orientation of the grain boundary zone was the same as that of the adjacent grain behind the migrating boundary. This result indicated that the formation of these zones was accompanied by the migration of the grain boundary. Then, numerous slip features were observed through detailed local disorientation analysis. It is proposed that the driving force for the grain boundary migration was the local stress induced by the dislocation creep. The dissolution–reprecipitation of the precipitates at the boundary migration front resulted in the directional coarsening of the γʹ rods.

The isothermal compression of a Ti-43.5Al-8 Nb-0.2 W-0.2 B (at.%) alloy with (α2 + γ) microstructure has been examined at temperature range of 950–1050 °C under strain rate range of 10−2–10−5 s−1. The results showed that the value of m (strain rate sensitivity factor) increased from 0.18 through 0.33 to 0.48 with decreasing diffusion compensated strain rates. This indicates a transition of deformation mechanism from dislocation creep to grain boundary sliding (GBS), which was observed in the microstructure characterization. The transition took place over about one and a half orders of magnitude in diffusion-compensated strain rate may also be predicted by employing unified rate constitutive equations for dislocation creep and GBS in an additive manner.

A microstructure-sensitive model that predicts the strain-rate sensitive flow behavior as well as creep-strain life of a refractory Ni-base superalloy, Low Solvus High Refractory (LSHR), is presented. The model is based on discrete dislocation simulations that are computationally expensive, but the design tool derived from the simulations is fast acting. The model employs experimental data on microstructure and mechanical behavior, as well as the thermodynamic model PANDAT™, to calibrate, validate and verify the use of the model as a design tool. The mechanical properties predicted include flow stress as a function of temperature and strain-rate, as well as time for 0.1-0.2% creep strain as a function of stress and temperature. The model extends prior work of a strain rate sensitive flow stress model developed for IN100 alloy, by calibrating the model to data on LSHR, and by including dislocation creep as well as grain size dependent diffusional creep behavior and predicting time to reach a design creep life in terms of creep strain. The resultant model was found to capture reported data on LSHR, in both subsolvus and supersolvus heat treated conditions. The calibrated model was validated using additional data on yield and creep of LSHR with two other microstructures. The validation makes the model a promising design tool in the engineering of complex heat treat disks, where location-specific properties are desired.

To investigate whether the formation of rafts in Co-base superalloys has a hardening or softening effect, the single crystal deformation behavior was studied in the high temperature/low stress creep regime. The large positive lattice misfit of the investigated Co-base superalloy led to a pronounced directional coarsening of the microstructure normal to the stress axis during compressive creep at 950 °C/150 MPa. Two creep minima were observed similar to Ni-base superalloys under tensile creep. Rafting was accompanied by a decreasing creep rate indicating a hardening effect, although cutting of γ′ rafts by partial dislocations under the formation of stacking faults occurred.

Atomistic simulation of hydride formation under dislocation strain field piled up at inclusions in α-Fe was performed using a new Finnis–Sinclair-type embedded atom method potential. Two 1/2 [1 1 1] ( 1 0 1 ¯ ) edge dislocations were introduced in BCC α-Fe to study the effects of dislocation interaction on the formation of hydride. Our simulation demonstrated that the interaction of dislocation-inclusion could produce ultrahigh stress that resulted in the formation of iron hydride. In addition, the dissociation of one of the two 1/2 [1 1 1] edge dislocations into [0 0 1] + 1/2 [ 1 1 1 ¯ ] two perfect dislocations can occur under a large applied shear (5%) or smaller shear (0.5%) when a hydride plate forms, despite the dissociation not satisfying the energy condition of dislocation reaction. The [0 0 1] perfect dislocation is usually considered the origin of cleavage on {1 0 0} planes, which is not stable without large applied load or hydrogen. In our model, the densities of both dislocation and inclusion on the glide plane could be changed to correspond to the dislocation density and inclusion (carbide) density in real martensitic steels. The results indicated that low dislocation density and small size of inclusions could prevent the formation of hydride. From these findings, tempering was suggested as a measure of preventing hydrogen embrittlement because proper tempering can effectively reduce the residual stress caused by quenching, precipitate dispersive and fine carbides (inclusion) and, in addition, decrease dislocation density in quenching and tempering martensitic steels.

Dislocation structures forming during cyclic loading of fcc metals are fatigue damage precursors. Their specific structures are caused by the motion and interactions of dislocations. Depending on the load conditions, the grain orientation, the stacking fault energy, a variety of different dislocation structures appear in the material such as labyrinths, cells, veins and persistent slip bands. We present a continuum dislocation-based model for cyclic fatigue and incorporate it into a crystal plasticity finite element solver. A method for the simulation of dislocation junction formation is introduced, which reproduces the behaviour of discrete objects, such as dislocations, in a continuum framework. The formation of dislocation walls after 50 and 100 deformation cycles at 0.95% and 0.65% strain amplitude starting from an initial random dislocation distribution is predicted for 〈 001 〉 and 〈 1 1 ¯ 0 〉 oriented crystals. Simulations and cyclic tension-compression experiments of polycrystalline 316L stainless steel are performed to compare our model with another model based on edge and screw dislocation densities. The simulated dislocation structures and experimental results, obtained with the electron channeling contrast imaging technique, are compared using a 2D orientation distribution function of the dislocation structures. The dominant orientation of dislocation walls is predicted by the new model; it turns out to be perpendicular to the intersection line between the two slip planes involved in their formation and at an angle of around 45 o from the loading axis. This agrees well with the experimental observations and represents a step forward for understanding the formation mechanism of these dislocation structures.

Creep deformation behavior and microstructure evolution of G115 steel were systematically investigated for temperatures of 625–675 °C under uniaxial tensile stress of 120–220 MPa. The relationship between minimum creep rate and applied stress followed the Bird–Mukherjee–Dorn (BMD) equation. The modified BMD equation was proposed using threshold stress to elucidate the actual creep deformation mechanism. The values of the threshold stress were determined to be 177.8, 91.4 and 87.6 MPa at 625, 650, and 675 °C, respectively. The true creep activation energy and the true stress exponent were 275.76 kJ/mol and 6, respectively. Thus, the dominant creep deformation mechanism was identified as dislocation climb. Three types of precipitates can be revealed after creep deformation: W-rich Laves, Nb-rich MX, and Cu-rich phases. The creep damage of G115 steel after creep deformation was characterized by martensite cracks and martensite fractures owing to the hardness and brittleness of the lath martensite structure. Further, a dense array of deep and equiaxed dimples appeared in the central region of fracture surfaces under the tested creep conditions. Ductile fracturing was the main fracture mechanism during creep deformation.

The effect of temperature and cyclic loading on stress relaxation (SR) behavior of Ti–6Al–4V titanium alloy was studied by uniaxial tensile test. SR limit and rate vary with increasing the temperature and cyclic loading times. The microstructural variations were observed by scanning and transmission electron microscopes. Unstable SR behavior happens at 450–550 °C, which becomes obvious with the increase of cyclic loading times or the decrease of relaxation temperature. The stress instability maybe attributes to the activation energy improvement. Based on dislocation distribution and morphology as well as stress exponent values varying from 1.8 to 2.4, SR mechanism of the alloy is dislocation creep in the tested temperature range. The mechanical properties of the alloy at room temperature almost maintains invariable after SR process. However, the discontinuous yielding in as-received alloy changes to continuous yielding in relaxed ones.

In this work, characteristics of premature creep failure in the over-tempered base metal (OT-BM) of a Grade 91 steel weldment are investigated with specially designed creep experiments and advanced microstructure characterization. In situ digital image correlation (DIC) strain measurements reveal that local creep strain as high as 100% accumulated in the OT-BM, compared with only 10% nominal strain over the gauge length. The creep strain rate in the OT-BM is identical to that in the intercritical heat-affected zone and fine-grained heat-affected zone at the secondary creep stage and early tertiary stage, but faster at the late tertiary stage. Microstructural analysis shows that highly recovered microstructure in the OT-BM leads to the lowest hardness, the largest grain size, the lowest fraction of coincidence site lattice boundaries, and the lowest localized strain energy. Dislocation creep and transgranular creep fracture are the dominant deformation and fracture mechanisms in the OT-BM under the current creep testing condition of a low temperature (550 °C) and a high stress (215 MPa).

Recent experimental studies have shown that metals at submicron scale may harden rather than soften after high temperature annealing, which is contrast to the typical behavior at macro level. In the present work, a coupled dislocation glide-climb model is developed to study the intrinsic mechanism of high temperature anneal hardening in the submicron pillar. Both thermally activated dislocation glide and climb are dealt with in the framework of three dimensional (3D) discrete dislocation dynamics (DDD). A modified discrete-continuous method (DCM) is proposed for solving dislocation climb. The climb rate is determined by the vacancy volumetric flux across the dislocation core which is obtained by solving vacancy diffusion equations using the finite element method (FEM). The vacancy concentrations are transferred between the solving domain of DDD and continuum FEM by a new localization method. Through carrying out coupled dislocation glide-climb simulation, remarkable softening effect after pre-straining and hardening effect after annealing is observed in the submicron pillars. Microstructure analysis demonstrates that the anneal hardening can be ascribed to two major aspects: (1) The dislocation climb during high temperature annealing promotes the dislocation annihilation, leading to a decrease of dislocation density; (2) The jogs, nucleated during annealing, which have very weak mobility, act as obstacles to dislocation glide motion and result in a decrease of mobile dislocation density. Compared with the pre-strained pillars, the combination effect of these two aspects significantly decreases the dislocation mobility and results in higher flow strength, which agrees well with the experiment data.

In this paper, a combination of quantum mechanical and interatomic potential-based atomistic calculations are used to predict the core structures of screw and edge dislocations in tungsten in the presence of a particular concentration of hydrogen atoms. These configurations of the core structures are the results of two competing energies: the interaction between the partial dislocations and the corresponding generalized stacking fault energy in between the two partial dislocations, which are presented in this work. With this, we can precisely predict the configurations of the hydrogen-doped dislocation core structures.

The creep behaviors of M951G alloy were carried out under stress ranging from 240 MPa to 400 MPa at 900 °C, and the corresponding deformation mechanisms and fracture behaviors after rupture had been investigated by various techniques. Results showed that both the deformation mechanisms and fracture behaviors were dependent on the applied stress. According to the transmission electron microscope (TEM) observations, the dominant deformation mechanism changed from a combined process of slip and climb of dislocations in matrix channel to shearing of dislocations in γ′ precipitates and cross-slip of dislocations in matrix channel with the applied stress increasing. Fracture behaviors of M951G alloy were characterized using optical microscope (OM) and scanning electron microscope (SEM), which changed from intergranular to transgranular with the increase of applied stress. At low applied stress, M951G alloy was failure in the form of intergranular owing to coalescence of micropores along the grain boundaries. However, at higher applied stress the microcracks initiated at broken carbides in the grain interior, and finally resulted in transgranular fracture. Additionally, creep strain rate also played a key role in determining the transition of creep fracture modes by effect the corresponding temperature of equal strength for grain boundary and grain interior. The values of apparent stress exponent at low and high stress regions were calculated to be 5.14 and 11.13 respectively, which was due to the change of deformation mechanisms and fracture modes with the increase of applied stress.

In this work, the microstructure evolution and fracture mechanism of a novel 9% chromium tempered martensite ferritic steel G115 were investigated over the temperature range of 625–675 °C using uniaxial creep tests. The creep curves consist of a primary transient stage followed by an apparent secondary stage, and an accelerated tertiary creep regime. The relationship between the minimum creep rate and the applied stress followed Norton's power law. Based on the EBSD analysis, there were no obvious textural features formed after creep deformation, and with the increase in creep time, the number of subgrains slightly increased, and then sharply increased, indicating dynamic recrystallization (DRX) occurs after creep deformation. In addition, three types of precipitates can be observed after creep deformation: W-rich Laves phase, Nb-rich MX, and Cu-rich precipitates. The Nb-rich MX with a square shape and Cu-rich precipitates with an ellipsoidal shape remain very stable. However, the W-rich Laves phases distributed mainly on the grain boundaries have rod-like, chain-like, and bulky shape, which are coarsened significantly. Representative fractographs of the G115 steel after creep deformation exhibit significant necking with an elliptical shape. A dense array of deep and equiaxed dimples appear in the central region under the tested creep conditions. Ductile fracturing is the dominant fracture mechanism during short-term creep deformation.

Symmetric tilt Lomer -type low-angle grain boundaries (LLAGBs) are distinct grain boundaries formed by Lomer dislocation locks in face-centered cubic (fcc) metals. To reveal their mechanical behaviors, interactions between lattice dislocation and LLAGB are studied with molecular dynamics simulations. Dislocation reaction and slip transmission depend on the tilt angle of LLAGB, the character of incident dislocation and the particular glide planes inhabiting the incoming slip. For LLAGBs with relatively small tilt-angles, a free slip-transmission zone can be identified where dislocations can be forced to penetrate through the LLAGB without inducing dislocation reaction. Otherwise, the incident slip tends to be trapped, triggering a number of dislocation reactions and leading to indirect slip transmission across the boundary in the reaction zone. Critical strains for dislocation reactions are examined. Raising up the temperature will widen the dislocation reaction zone and the free transmission zone tends to disappear.

It is well recognized that hydrostatic pressure loading can significantly affect the mechanical properties of solids. One of the strongest effects of pressure is the increase in elastic properties of solids. In this article, we address the effect of high pressure on plastic properties through typical examples of the effect of hydrostatic pressure on dislocation core properties of solids.

Ultrasound has long been used as a non-destructive tool to test for the brittle fracture of materials. Could it be used as a similar tool to test for ductile failure? As a first step towards answering this question, we report results of local measurements of the speed of transverse waves in aluminum under standard testing conditions at two different probe locations and continuously as a function of applied load. The result, as expected, is independent of stress in the elastic regime, but there is a clear change, consistent with a proliferation of dislocations, as soon as the yield strength is reached. We use a model that blames the change in wave speed on the interaction of elastic waves with oscillating dislocation segments, which quantitatively relates the change in wave velocity with dislocation density Λ and segment length L, thus obtaining a continuous relation between dislocation density and externally applied stress. We took off samples from the probe before, at intermediate, and high loading, and we measured their dislocation density using standard X-ray diffraction and transmission electron microscopy techniques. The results agree well with the acoustic measurements, and the relation between stress and dislocation density is consistent with the Taylor rule. This indicates that monitoring the speed of transverse waves could become a useful diagnostic of dislocation density for metallic pieces in service as well as a tool to test models of plastic behavior.

3D discrete dislocation dynamics is employed to investigate motion of general mixed dislocation segments subjected to high temperature loadings in microstructures with impenetrable particles. The implementations of the model first address several benchmark processes including shrinkage of a glissile dislocation loop driven by self-stresses and an annihilation of mutually interacting co-axial prismatic dislocation loops. In particular, we show that models of the microstructure with planar and/or translational symmetry improve efficiency, speed and stability of the calculations. Our simulations then focus on migration of low angle dislocation boundaries in an array of particles while taking into account all mutual dislocation-dislocation interactions and the action of an externally applied stress. The results show for the first time that the migration of tilt dislocation boundaries in crystals with particles can be associated with threshold stresses. The calculated thresholds are in a good agreement with experimental threshold stresses that characterize creep behaviour of precipitation hardened alloys.

The present paper studies the hardening mechanism in fcc metallic structures during high rate deformations by incorporating the dislocation network properties. The large scale atomistic simulation is used to study the variations of dislocation length distribution and its characteristic lengths as the strain rate changes. First, the dislocation length distributions at different strain rates are studied to qualitatively capture the relation between the material strength and applied strain rate. It is observed that increasing the strain rate decreases the dislocation network lengths. Accordingly, the required stress to activate the dislocation sources increases. Since dislocation movements sustain the imposed plastic flow, higher activation stress leads to material hardening. Furthermore, the results show that the properties of dislocation length distribution at high deformation rates are different from those of lower strain rates due to the activation of cross-slip mechanism at high strain rates. In order to quantitatively describe the relation between the material strength and dislocation network properties as the strain rate varies, the variations of average and maximum dislocation length are investigated in the cases of different applied strain rates. The relation between the material strength and average length of mobile dislocations is captured using an inverse linear equation which shows a good agreement with the atomistic simulation results.

It is generally accepted that the addition of Fe deteriorates the creep behavior of titanium alloys at elevated temperature. However, the effect of Fe at ambient temperature is not well documented. In this paper, the creep performance of an α/β titanium alloy with different Fe contents (0.05 and 0.20 wt%) was systematically studied at temperatures from the ambient to 500 °C. Creep resistance of samples with 0.20 wt% Fe was found to decrease at temperatures above 300 °C, but to increase at ambient temperature and 200 °C. The resulting stress exponent was in the range of 4.7–14.4, corresponding with dislocation creep. The fitted creep activation energy fell into two categories, i.e., 1) comparable with the self-diffusion activation energy of α-Ti at elevated temperature, and 2) remarkably higher than the former at 200–400 °C. The underlying mechanisms controlling creep processes at different temperatures were discussed.

The creep behavior and microstructural evolution of 8030 alloy at 90–150 °C and 50–90 MPa of applied tensile stress were investigated by creep testing and transmission electron microscopy. The 8030 alloy possesses excellent creep resistance at low temperatures. The sizes of a small number of subgrains increase during the creep process due to subgrain merging. An Al 3 Fe phase non-uniformly pinned on the subgrain boundary made the pinned subgrains difficult to merge with the surrounding subgrains. At 90–120 °C/50–90 MPa, the stress exponent n was 5.1–5.7 and the activation energy Q c was 49.7–66.5 kJ/mol, suggesting that dislocation climb controlled by the grain boundary diffusion is the primary creep mechanism. At 150 °C, the ability of Al 3 Fe secondary phase to hinder the dislocations significantly decrease. When the tensile stress is 50–70 MPa, n = 6.8 and Q c = 49.7–66.5 kJ/mol, but n = 10.0 at a stress of 90 MPa. A creep activation energy of 123.2 kJ/mol is close to that of the lattice self-diffusion in aluminum, implying that a lattice self-diffusion mechanism is dominant at 150 °C/90 MPa.

In this paper, bending creep deformation mechanism for nickel nanobeam has been investigated using molecular dynamics simulation. Low temperature creep deformation (T m ) is found to be guided by jog formation and glide motion of grain boundary whereas lattice diffusion, grain boundary migration and sliding are the controlling mechanism for high temperature deformation (T > 0.5 T m ). The occurrence of tertiary creep regime is observed only at high temperature deformation due to creep instability caused by cavity formation. It is revealed through dislocation analysis that intrinsic Frank partial dislocations are the driving factor for cavity generation leading to intergranular fracture.

This work presents a coarse-grained atomistic study of the effect of phonons on the mobility of edge dislocations. A variety of phenomena, including phonon focusing and phonon-induced dislocation drag, are reproduced in the simulations. Results show that interaction with phonons slows down the dislocations and phonon focusing results in the arrest of dislocations. A wavelet analysis, together with visualization, reveals that the phonon-dislocation interaction leads to a reduction of the energy associated with the dislocation core, with some energy lagging behind the decelerated dislocation or dispersed around the arrested dislocation through emission of secondary phonon waves, thus clarifying the underlying physics.

In the present work, creep deformation and fracture behaviours of Sanicro 25 alloy were obtained based on long-term creep-strain tests. The multiscale precipitation behaviours were calculated thermodynamically and inspected by examination of the microstructure of the as-crept alloy. Creep cavitation nucleation near the M 23 C 6 precipitates and an increase in amount of Laves phase with increased creep time were observed. The nanoscale MX-type and Cu-rich phases were found distributed evenly within the as-crept alloy, which leads to the high creep resistance of Sanicro 25 at elevated temperatures. The dislocation substructure evolution was surveyed, and dislocation cells were discovered in abundance of the alloy during creep. The grain orientation transformation of the alloy before and after creep were analysed, and 〈111〉//RD and 〈001〉//RD were detected as the preferred orientations during creep. The number of low-misorientation angles grain boundaries increased in as-crept alloy due to the presence of grain rotation and dislocation cells.

To understand the strengthening mechanism of a metallic material with high dislocation density, the plastic deformation behavior of lath martensite was studied by means of in situ neutron diffraction measurements during tensile deformations using a 22SiMn2TiB steel and a Fe-18Ni alloy. The characteristics of dislocation were analyzed and were discussed with the relation of stress-strain curves. The dislocation densities (ρ) induced by martensitic transformation during heat-treatment in both materials were found to be originally as high as 1015 m-2 order, and subsequently to increase slightly by the tensile deformation. The parameter M value which displays the dislocation arrangement dropped drastically at the beginning of plastic deformation in both materials, indicating that the random arrangement became more like a dipole arrangement.

In single crystal Nickel-based superalloys subject to creep loading, the γ / γ ′ phase microstructure co-evolves with the system of dislocations under load. Computational modeling thus requires multiphysics approaches capable of describing and simulating both phase and defect microstructures within a common conceptual framework. To do so, we formulate a coupled continuum model of the evolution of phase and dislocation microstructures. The simulated γ / γ ′ phase microstructure accounts for concentration as well as crystallographic order parameters. Dislocation microstructure evolution is described in terms of dislocation densities and associated stress-driven dislocation fluxes. The creep strain curve is obtained as a natural by-product of the microstructure evolution equations. We perform simulations of γ / γ ′ evolution for different dislocation densities and establish the driving forces for microstructure evolution by analyzing in detail the changes in different contributions to the elastic energy and chemical free energy density, as well as the evolution of stress concentrations that may trigger the transition from dislocation flow in the γ channels towards shearing of the γ ′ precipitates. Our investigation reveals the mechanisms controlling the process of directional coarsening (rafting) and demonstrates that the kinetics of rafting significantly depends on characteristics of the dislocation microstructure. In addition to rafting under constant load, we investigate the effect of changes in loading conditions and explore the possibility of improving creep properties by pre-rafting along a different loading path.

Understanding the mechanism of martensitic transformation is of great importance in developing advanced high strength steels, especially TRansformation-Induced Plasticity (TRIP) steels. The TRIP effect leads to enhanced work-hardening rate, postponed onset of necking and excellent formability. In-situ transmission electron microscopy has been performed to systematically investigate the dynamic interactions between dislocations and α′ martensite at microscale. Local stress concentrations, e.g. from notches or dislocation pile-ups, render free edges and grain boundaries favorable nucleation sites for α′ martensite. Its growth leads to partial dislocation emission on two independent slip planes from the hetero-interface when the austenite matrix is initially free of dislocations. The kinematic analysis reveals that activating slip systems on two independent {111} planes of austenite are necessary in accommodating the interfacial mismatch strain. Full dislocation emission is generally observed inside of austenite regions that contain high density of dislocations. In both situations, phase boundary propagation generates large amounts of dislocations entering into the matrix, which renders the total deformation compatible and provide substantial strain hardening of the host phase. These moving dislocation sources enable plastic relaxation and prevent local damage accumulation by intense slipping on the softer side of the interfacial region. Thus, finely dispersed martensite distribution renders plastic deformation more uniform throughout the austenitic matrix, which explains the exceptional combination of strength and ductility of TRIP steels.

Low-angle grain boundaries (LAGBs) may show up frequently as distinct dislocation products such as in the processes of work hardening, recovery and recrystallisation of metals and alloys. To reveal...