Grain Boundary Slip Research Articles

Effect of a Long-Range Dislocation Pileup on the Atomic-Scale Hydrogen Diffusion near a Grain Boundary in Plastically Deformed bcc Iron

In this paper, we present concurrent atomistic-continuum (CAC) simulations of the hydrogen (H) diffusion along a grain boundary (GB), nearby which a large population of dislocations are piled up, in a plastically deformed bi-crystalline bcc iron sample. With the microscale dislocation slip and the atomic structure evolution at the GB being simultaneously retained, our main findings are: (i) the accumulation of tens of dislocations near the H-charged GB can induce a local internal stress as high as 3 GPa; (ii) the more dislocations piled up at the GB, the slower the H diffusion ahead of the slip–GB intersection; and (iii) H atoms diffuse fast behind the pileup tip, get trapped within the GB, and diffuse slowly ahead of the pileup tip. The CAC simulation-predicted local H diffusivity, Dpileup−tip, and local stresses, σ, are correlated with each other. We then consolidate such correlations into a mechanics model by considering the dislocation pileup as an Eshelby inclusion. These findings will provide researchers with opportunities to: (a) characterize the interplay between plasticity, H diffusion, and crack initiation underlying H-induced cracking (HIC); (b) develop mechanism-based constitutive rules to be used in diffusion–plasticity coupling models for understanding the interplay between mechanical and mass transport in materials at the continuum level; and (c) connect the atomistic deformation physics of polycrystalline materials with their performance in aqueous environments, which is currently difficult to achieve in experiments.

The microstructure evolution and the effect of grain size, temperature, and TiBw on the deformation mechanism of TiBw/TA15 composites during high temperature deformation

In this paper, the deformation mechanism and microstructure evolution of TiB whisker (TiBw)/TA15 composites during high temperature (HT) rolling and tensile were investigated. The effects of grain size and temperature on the HT mechanical properties, deformation mechanism, and strengthening/softening effects were studied. The influence of temperature on the TiBw fracture mode was analyzed within the range of 600–750 °C. The results show that grain boundary slip (GBS) and dislocation motion were more prone to promote the occurrence of discontinuous dynamic recrystallization (DRX) and continuous DRX, respectively. TiBw could withstand higher forces and fractured when the orientation was consistent with the loading direction and at a lower temperature, while it was easier to debond with temperature increasing. TiBw/TA15 composites had Hall-Petch (H–P) and inverse H–P (I–H–P) regions in the range of 4.5–7.0 μm with a critical grain size of 5.8 μm under HT tensile at 600 °C and 0.001 s−1. The grain boundary strength decreased with the temperature increased, which caused the transformation of the deformation mechanism and the H–P/I–H–P regions at different temperatures. The deformation was dominated by GBS when the strength of grain boundary was lower than that of intracrystalline. The deformation mechanism transformation temperature of TiBw/TA15 composites was 700 °C under HT deformation at 0.001 s−1.

Special hot working plastic deformation behavior and microstructure evolution mechanism of single-phase BCC structure AlFeCoNiMo0.2 high-entropy alloy

The single-phase body-centered cubic (BCC) structured high-entropy alloys are considered to be typically difficult-to-deform processing alloy because they can still crack during hot deformation at high temperatures and low strain rates. However, we found that the hot working formability of this type of alloy is very sensitive to temperature, and there is a unique microscopic transformation. In this paper, the as-cast single-phase BCC-structured AlFeCoNiMo0.2 high-entropy alloy was investigated with the single-pass hot-compression simulation experiment (deformation of 0.6), at temperatures and strain-rate ranges of 900 – 1150 ℃ and 0.001 – 0.1 s−1, respectively. The hot-deformation behavior and microstructure-evolution mechanisms were studied. The Arrhenius constitutive relation model was revised and established. The processing maps of Prasad, Gegel, Malas, and Murty with different instability criteria were constructed. The optimal thermal-processing parameters (temperatures of 1070 – 1150 ℃ and strain rates of 0.001 – 0.1 s−1) were provided. It was great to find that the alloy has a narrow temperature window effect of hot working with the Ruano-Wadsworth-Sherby (R-W-S) deformation-mechanism map established by incorporating the dislocation quantity. The deformation mechanisms of the alloy at 900 °C, 1000 °C, 1050 °C, and 1100 °C were predicted. The single-phase (BCC) structure of the alloy has strong stability during hot deformation. In a narrow range of hot-working temperatures, the microstructure has a necklace-like structure, and its substructure has a strong textured effect, impeding grain-boundary slip. In the optimized processing interval, discontinuous dynamic recrystallization (DDRX) occurs, and the necklace-like structure disappears. The recrystallization mechanism is related to grain-boundary sliding, caused by dislocation sliding.

Structural Evolution and Transitions of Mechanisms in Creep Deformation of Nanocrystalline FeCrAl Alloys.

FeCrAl alloys have been suggested as one of the most promising fuel cladding materials for the development of accident tolerance fuel. Creep is one of the important mechanical properties of the FeCrAl alloys used as fuel claddings under high temperature conditions. This work aims to elucidate the deformation feature and underlying mechanism during the creep process of nanocrystalline FeCrAl alloys using atomistic simulations. The creep curves at different conditions are simulated for FeCrAl alloys with grain sizes (GS) of 5.6-40 nm, and the dependence of creep on temperature, stress and GS are analyzed. The transitions of the mechanisms are analyzed by stress and GS exponents firstly, and further checked not only from microstructural evidence, but also from a vital comparison of activation energies for creep and diffusion. Under low stress conditions, grain boundary (GB) diffusion contributes more to the overall creep deformation than lattice diffusion does for the alloy with small GSs. However, for the alloy with larger GSs, lattice diffusion controls creep. Additionally, a high temperature helps the transition of diffusional creep from the GB to the dominant lattice. Under medium- and high-stress conditions, GB slip and dislocation motion begin to control the creep mechanism. The amount of GB slip increases with the temperature, or decreases with GS. GS and temperature also have an impact on the dislocation behavior. The higher the temperature or the smaller the GS is, the smaller the stress at which the dislocation motion begins to affect creep.

Grain-boundary migration induced abnormally increased flow stress under large strain in a high-temperature compressed Ti-22Al-25Nb alloy

• Impeded grain boundary (GB) slip results in visibly increased flow stress. • The GB slip mechanism was impeded by zigzag GBs. • The zigzag GB result from the GB migration. • Energy difference between <001> and <111> fiber textures leads GB migration. • DDRX is a concomitant phenomenon with increased flow stress. The flow stress of Ti-22Al-25Nb alloy abnormally increased during uniaxial compression at large strains in the B2 phase field. This phenomenon is dependent on the influence of oriented grain boundary (GB) migration on GB sliding. In the early stages of deformation, straight and smooth GBs gradually evolved facilitating GB sliding, and the <001>/<111> double fiber texture has formed. There is a stored energy difference between <001> and <111> grains due to their different Taylor factors. Under low strain rates, the energy difference between the two fibers will drive GB to migrate, making the GBs zigzag and impeding GB sliding. Thus, greater stress is required to achieve further deformation, namely, the flow stress increased. The dislocation density in the shear band near the GB rapidly increased. Although the current alloy has a high stacking fault energy (SFE), the high dislocation density is sufficient to cause discontinuous dynamic recrystallization (DDRX).

Hot tensile deformation mechanism and microstructure evolution of Mg 2Nd alloy with heterostructure

The deformation mechanism and microstructure evolution of the extruded Mg2Nd alloy were investigated in the hot tensile deformation process at strain rate of 2.2 × 10−4 s−1 and deformation temperatures of 150 °C–300 °C. the results showed that the peak stress declines and the elongation to fracture increases with the increasing of deformation temperature. By observation of the microstructure, the fibrous extrusion zones (FEZs) and non-fibrous extrusion zones (non-FEZs) coexist in the alloy, and the microstructure deformation of the FEZs is more obvious than that of the non-FEZs. In the initial deformation stage, the grain deformation first occurs in the FEZs. In the later deformation stage, recrystallization occurs in both FEZs and non-FEZs. Moreover, the deformation mechanism of the FEZs is different from that of the non-FEZs. The deformation mechanism of the FEZs is dominated by intragranular slip, supplemented by grain boundary slip (GBS). The recrystallization mechanism in FEZs is continuous dynamic recrystallization (CDRX). On the contrary, the deformation of the non-FEZs is dominated by GBS, while the intragranular slip plays a coordinated role. The recrystallization mechanism in non-FEZs is discontinuous dynamic recrystallization (DDRX). Therefore, the dominated hot tensile deformation mechanisms of the alloys are alternated among intragranular slip, GBS and dynamic recrystallization (DRX)

Modelling of cavity growth during the superplastic flow of a fine-grained Ti–6Al–4V titanium alloy processed by direct rolling

ABSTRACT Ti–6Al–4V fine-grained plates were manufactured using a rolling method and then subjected to superplastic tensile tests at varying temperatures and strain rates on an AG 250KNE electronic tensile testing machine. The superplastic behaviours of the plates were also tested. A cavity-growth model was established and the changing laws of energy during cavity growth and microstructure evolution of superplastic deformation were predicted. The Ti–6Al–4V alloy possessed the maximum elongation rate of 886% at 840°C and a strain rate of 5 × 10−4 s−1. The strain-rate sensitivity index m for this alloy was 0.54. The mechanism of superplastic deformation was established to be strain-induced grain-boundary slip, and the mechanism of cavity growth to be plasticity-controlled cavity coalescence and growth.

Grain-size effects on the deformation in nanocrystalline multi-principal element alloy

Multi-principal element alloys (MPEAs) continue to garner great interest due to their potentially remarkable mechanical properties, especially at elevated temperatures for key structural and energy applications. Despite extensive literature examining material properties of MPEAs at various compositions, much less is reported about the role of grain size on the mechanical properties. Here, we examine a representative nanocrystalline BCC refractory MPEA and identify a crossover from a Hall-Petch to inverse-Hall-Petch relation. While the considered MPEA predominantly assumes a single-phase BCC lattice, the presence of grain boundaries imparts amorphous distributions that increase with decreasing grain size (i.e., increasing grain boundary volume fraction). Using molecular dynamics simulations, we find that the average flow stress of the MPEA increases with decreasing average grain size, but below a critical grain size of 23.2 nm the average flow stress decreases. This change in the deformation behavior is driven by the transition from dislocation slip to grain-boundary slip as the predominant mechanism. The crossover to inverse-Hall-Petch regime is correlated to dislocation stacking at the grain boundary when dislocation density reaches a maximum.

Hot tensile deformation behavior and microstructure evolution of Mg-1Al-6Y alloy

In this study, the hot tensile test was carried out using the extruded and annealed Mg-1Al-6Y alloy. The effect of temperature and strain rate on the hot tensile deformation behavior of the alloy was systematically studied at different temperatures (200 °C ∼ 350 °C) and different strain rates (8 × 10−5 s−1 ∼ 1.6 × 10−3 s−1). In addition, the effect of temperature on the evolution of microstructure when the strain rate is 1.6 × 10−3 s−1 was investigated. The results showed that as the temperature increased or the strain rate decreased, the peak stress decreased and the elongation increased. Hot tensile at different temperatures all increased the texture intensity, and the microstructure after deformation showed obvious characteristics of basal fiber texture ([0001]⊥ED). Correspondingly, the weaker [−15–40]//ED texture before deformation transformed into a stronger [01–10]//ED fiber texture. After deformation, the average Schmid factor (SF) of each non-basal slip was significantly increased compared with the average SF before deformation, indicating that abundant non-basal slip was activated during the deformation. When the deformation temperature was 300 °C, dynamic recrystallization (DRX) occurred significantly, and the DRXed grains accounted for 15.9%. DRX was a combination of continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). Furthermore, the calculated activation energy of the alloy was about 98.8 kJ mol−1. Comprehensive research showed that the hot tensile deformation mechanism mainly included intragranular slip, grain boundary slip (GBS) and DRX.

Hot Tensile Deformation Behavior of Mg-4Li-1Al-0.5Y Alloy

The microstructure evolution and deformation mechanism of the as-extruded-annealed Mg-4Li-1Al-0.5Y alloy (denoted as LAY410) were investigated during the hot tensile deformation at the temperatures between 150°C and 300°C with strains from 8 × 10−5 s−1 to 1.6 × 10−3 s−1. The results show that when the strain rate decreases and/or the deformation temperature increases, the peak stress of the alloy gradually decreases, and the elongations to fracture gradually increases. The true stress–strain curves show typical dynamic recrystallization (DRX) softening characteristics. It is observed that the microstructure in the magnesium (Mg) alloy deformed at 150°C is mainly composed of the deformed grains and a few recrystallized grains. The microstructures in the Mg alloy deformed at 200°C consisted of substructures and a slightly increasing number of dynamic recrystallized grains. When the deformation temperature reaches 250°C, the number of recrystallized grains increases significantly, and the microstructures are dominated by recrystallized grains. Moreover, through theoretical calculation and result analysis, the activation energy was about 99.3 kJ/mol, and the hot tensile deformation mechanism was the alternate coordinated deformation mechanism among grain boundary slip (GBS), intragranular slip, and DRX.

Grain-Size Effects on the Deformation in Nanocrystalline Multi-Principal Element Alloy

Multi-principal element alloys (MPEAs) continue to garner interest due to their remarkable mechanical properties, especially at elevated temperatures. Here, we examine a representative nanocrystalline refractory MPEA and identify a crossover from a Hall-Petch to inverse-Hall-Petch relation. While the considered MPEA predominantly assumes a single-phase BCC lattice, the presence of grain boundaries imparts amorphous phase distributions that increase with decreasing grain size (i.e., increasing grain boundary volume fraction). Using molecular dynamics simulations, we find that the yield strength of the MPEA increases with decreasing average grain size, but below a critical grain size < 23.2 nm the yield strength decreases. This change in the deformation behavior is driven by the transition from dislocation slip to grain-boundary slip as the predominant mechanism. Our results reveal that the change from Hall-Petch to inverse-Hall-Petch regime is correlated to dislocation stacking at the grain boundary when dislocation density reaches a maximum.

Hot tensile deformation behavior of extruded LAZ532 alloy with heterostructure

Hot tensile test was performed at deformation conditions of 150–°C300 °C and 8.33 × 10−5 s−1~1.67 × 10−3 s−1, respectively, to systematically study the microstructure development and deformation mechanism of extruded Mg–5Li–3Al–2Zn (LAZ532) alloy with a heterostructure of both the fibrous extrusion zones (FEZs) and non-fibrous extrusion zones (non-FEZs) coexisted. The results showed that the peak stresses decreased gradually, while the fracture strains increased gradually with the decrease of strain rates or the increase of deformation temperatures, and the alloy exhibited superplastic characteristics at deformation conditions of 300 °C and 8.33 × 10−5 s−1~1.67 × 10−3 s−1. By microstructure observation, the alloy showed that in the initial deformation stage, the deformation of the grains in the FEZs was prior to that in the non-FEZs. In the middle deformation stage, the deformation of the grains in the FEZs was dominated by intragranular slip and accompanied with grain boundary slip (GBS), while the deformation of the grains in the non-FEZs was dominated by GBS and accompanied with intragranular slip. In the later deformation stage, the continuous dynamic recrystallization (CDRX) generated in the coarse lamellar grains in the FEZs due to dislocation pile-up, in contrast, the discontinuous dynamic recrystallization (DDRX) generated at the grain boundaries in the non-FEZs. Moreover, based on theoretical calculation and result analysis, the activation energy was about 110.0 kJ/mol, and the hot tensile deformation mechanism was each other alternating and coordinated deformation mechanism among GBS, intragranular slip and dynamic recrystallization (DRX).

Effect of Heating on the Structure and Properties of a Solid-Phase Diffusion Bond Based on a Forced Fit in OT4-1 Alloy

The production of a solid-phase diffusion bond in OT4-1 alloy on the basis of a cold forced fit of components in a shaft–hole configuration is considered. The influence of the maximum stress–strain state in the cold forced fit and subsequent heat treatment (in autonomous vacuum) on the structural evolution and properties of the contact region is investigated. In cold plastic deformation of the alloy with the formation of a solid-phase diffusion bond, deformational relief (traces of grain-boundary slip) is observed in the microstructure of the contact region. In addition, decrease is noted in the area of the contact surfaces, and fewer bulk interactions are noted in the contact plane (grain distortion) and in the contact volume (regions of dislocation departure). The basic characteristics of the structural interface—the unit parameter of structural organization, the grain density, the mean grain-boundary density, and the development of the grain boundaries—exceed by factors of 10, 4, 1.8, and 1.5, respectively, those of the basic metal in the initial state. Heating in autonomous vacuum in the range of α → β phase transitions leads to stepwise structural changes in the basic metal and in the contact region of the solid-phase diffusion bond. Initially, a globular component appears in the microstructure. With further heating, this component reverts to the initial acicular structure (with some increase in microhardness). The formation of globular structure on heating plastically deformed metal is observed not only at temperatures corresponding to phase transition but also at higher temperatures; this has not previously been noted. With increase in temperature, the duration of this stage decreases. In addition, with less developed plastic deformation, the formation of globular structure is observed close to the polymorphic-transformation temperature Tpt, with shorter holding. For the basic metal (with little deformation), the globular structure disappears practically completely after heating for 10 min at 950°C. In the cold-deformed contact region of the solid-phase diffusion bond, the globular structure disappears on heating for 1 h at 950°C, for 40 min at 975°C, and for 20 min at 1000°C. At those temperatures the sealing of discontinuities is practically complete. In other words, the weld line disappears: the metal with continuous microstructure in the contact region does not differ from the basic metal, except for a slight enlargement of the microstructure. Quantitative assessment of the structural changes in terms of the basic characteristics of the structural interface permits determination of their mechanism and kinetics and also the dependence of the structure on the plastic strain and the heat treatment. On that basis, it is possible to identify conditions such that the discontinuities are eliminated, the boundaries disappear, and the properties of the solid-phase diffusion bond are no worse than those of the basic metal.

Development of a novel strength ductile Mg–7Al–5Zn alloy with high superplasticity processed by hard-plate rolling (HPR)

In the present study, the application of hard–plate rolling (HPR) on the hard-to-deform Mg–7Al–5Zn (AZ75) alloy resulted in a homogeneous fine grained (∼6 μm) microstructure, where numerous micron/nano Mg17(Al, Zn)12 precipitates with spherical morphology uniformly dispersed both at grain boundaries and within grain interiors. More importantly, the HPRed AZ75 alloy exhibited superior mechanical properties with a simultaneous high strength and ductility at room temperature, i.e. yield strength (YS) of ∼218 MPa, ultimate tensile strength (UTS) of ∼345 MPa and elongation of ∼19%, which was comparable to the commercial wrought magnesium alloys. According to the contribution from the several strengthening mechanisms estimated by simplified models, the grain boundary strengthening is the predominant mechanism for the high YS of the alloy. The high ductility is benefited from the strong work-hardening capacity resulted from Zn solid solutes and the presence of numerous well-dispersed nanosized precipitates as well as weakened texture. Moreover, the fine grained AZ75 alloy exhibits an optimum superplasticity of ∼615% at 300 °C at 1 × 10−3 s−1. It is attributed to the enhanced grain boundary slip (GBS) promoted by a well maintained fine grain structure resulted from the pining effect by numerous Mg17(Al, Zn)12 particles segregating along grain boundaries during tension. The results will be helpful for the development and processing of high alloying element content Mg alloys with high strength and ductility as well as enhanced formability.

Determining characteristic plastic-relaxation times using micro- and nanocrystalline nickel as an example

Based on the concept of the incubation time of plastic deformation, an integral yield criterion is introduced and time effects of irreversible deformation are considered. The efficiency of the approach is demonstrated using micro and nanocrystalline nickel as an example. The parameters of the phenomenological model are treated physically from the viewpoint of the behavior of the defect structure of the material, which is controlled by the dislocation sliding and grain-boundary slip mechanisms in a wide range of the rate of deformation.

Microstructure based fatigue life prediction framework for polycrystalline nickel-base superalloys with emphasis on the role played by twin boundaries in crack initiation

Fatigue crack initiation in polycrystalline materials is dependent on the local microstructure and the deformation mechanism, and can be attributed to various mechanistic and microstructural features acting in concert like the elastic stress anisotropy, plastic strain accumulation, slip-system length, and grain boundary character. In nickel-base superalloys, fatigue cracks tend to initiate near twin boundaries. The factors causing fatigue crack initiation depend on the material's microstructure, the variability of which results in the scatter observed in the fatigue life. In this work, a robust microstructure based fatigue framework is developed, which takes into account i) the statistical variability of the material's microstructure, ii) the continuum scale complex heterogeneous 3D stress and strain states within the microstructure, and iii) the atomistic mechanisms such as slip-grain boundary (GB) interactions, extrusion formations, and shearing of the matrix and precipitates due to slip. The quantitative information from crystal plasticity simulations and molecular dynamics is applied to define the energy of persistent slip bands (PSB). The energy of a critical PSB and its associated stability with respect to the dislocation motion is used as the failure criterion for crack initiation. This unified framework provides us with insights on why twin boundaries act as preferred sites for crack initiation. In addition to that, the computational framework links scatter observed in fatigue life to variability in material's microstructure.

Microscopic incompatibility controlling plastic deformation of bicrystals

Grain boundary (GB) resistance to slip transmission was investigated computationally and experimentally as a function of GB type. A geometry-based computational method quantified the microscopic incompatibility of two GBs: one high angle GB (HAGB) with elastic and plastic incompatibility and one low angle GB (LAGB) with elastic and plastic compatibility. Compression tests of bicrystalline micropillar (BCMPs) with 1–2 μm diameters containing the same GBs correlated the calculated GB slip transmission resistance to the total bicrystals (BCs) mechanical properties. While large resistance values calculated for HAGB were in good agreement with experimentally obtained strengthening, the small GB resistance values confirmed the weakening effect of the LAGB. It is shown that the deformation mechanisms proposed for single crystalline micropillars (SCMPs) are applicable to BCMPs, however, GB type should also be considered. Besides the misorientation angle, the spatial geometry of the GB relative to the direction of loading axis is a determining factor. This parameter can change the transferability of the GB, while the misorientation angle between the grains is kept constant.

Grain boundary engineering and superstrength of nanocrystals

A new paradigm of hardening of nanocrystals is proposed based on the competing influence of various mechanisms of plastic deformation, i.e., dislocation sliding and grain-boundary slip. It has been confirmed using the results of computer modeling and the experimental data that the use of grain boundary engineering on the basis of the proposed ideas makes it possible to enhance substantially the strength of titaniumbased materials up to ultimate (theoretical) values.

Microstructures and Mechanical Properties of AZ31+Sr+Y Magnesium Alloy Sheets Produced by Twin-Roll Casting and Sequential Hot Rolling

In this paper, the deformation properties of AZ31+Sr+Y magnesium alloy sheets produced by twin-roll casting (TRC) and sequential hot rolling were studied by the tensile testing at a strain rate of 7×10-4s-1and various temperatures: room temperature (RT), 200°C, 300°C and 400°C, respectively. The result shows that the microstructure of AZ31+Sr+Y alloy was refined obviously by adding elements Sr and Y, the elongation of the alloy increased with increasing temperature, and the fracture behavior of the alloy changed from brittle fracture to ductile fracture with increasing temperature. During the process of plastic deformation of AZ31+Sr+Y alloy, the twin plays a leading role at room temperature; the dislocation movement is regarded as the main deformation mechanism at 200° C; at the higher temperature (above 300°C) the grain boundary slip (GBS) plays a dominant role .

Fatigue damage of ultrafine-grain copper in very-high cycle fatigue region

Ultrafine-grained copper produced by ECAP technique was tested in ultrasonic fatigue in the region of lives from 108 to 2 × 1010 cycles. The fatigue strength of ultrafine-grained copper is by a factor of 2 higher than that of conventional-grain copper. The occurrence of surface fatigue slip markings is rare. No grain coarsening was observed either in bulk or in areas underneath the surface slip markings. Localized collective grain-boundary slip of near-by oriented grains is considered to be the main damage mechanism leading to fatigue microcrack initiation either directly in the roots of surface intrusions or underneath the surface slip markings.