Dislocation Mechanics of Crystal/Polycrystal Mechanical Strength Properties

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Inorganic Crystalline Materials".

Deadline for manuscript submissions: closed (31 January 2023) | Viewed by 27550

Special Issue Editor

Department of Mechanical Engineering, A. James Clark School of Engineering, University of Maryland, College Park, MD 20742, USA
Interests: dislocation mechanics; constitutive equations; Hall–Petch relations; Zerilli–Armstrong equations; microstructural stereology; high rate metal deformations; ductile-brittle transition behaviors; X-ray diffraction imaging
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Special Issue Information

Dear Colleagues,

The purpose of the present Crystals Special Issue is to assemble dislocation mechanics descriptions commensurate, in part or overall, with corresponding crystal/polycrystal material strength properties. The Special Issue is intended to include the major sub-topics of: (1) dislocation deformation dynamics and corresponding mechanistic descriptions over a range of loading rates and temperatures; and (2) dislocation mechanisms operative at internal structural levels from the macro- to nano-dimensional scales. Crystal stress–strain, strain hardening, creep, impact, shock, hardness, fatigue, and ductile and brittle fracturing properties are to be included, along with correspondingly measured and/or computationally simulated crystal/polycrystal strength property determinations.

Prof. Dr. Ronald W. Armstrong
Guest Editor

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Keywords

  • dislocation mechanics
  • plasticity
  • strength
  • strain hardening
  • fracturing

Published Papers (16 papers)

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Editorial

Jump to: Research, Review

6 pages, 3024 KiB  
Editorial
The Dislocation Mechanics of Crystal/Polycrystal Plasticity
by Ronald W. Armstrong
Crystals 2022, 12(9), 1199; https://doi.org/10.3390/cryst12091199 - 25 Aug 2022
Cited by 2 | Viewed by 1162
Abstract
A brief history and update are given in four examples demonstrating that polycrystals are generally stronger than their individual component crystal grains because of obstructed dislocation pile-ups at grain boundaries. The example cases constitute diverse applications of a Hall–Petch dependence involving one or [...] Read more.
A brief history and update are given in four examples demonstrating that polycrystals are generally stronger than their individual component crystal grains because of obstructed dislocation pile-ups at grain boundaries. The example cases constitute diverse applications of a Hall–Petch dependence involving one or another aspects of the full polycrystal stress–strain behavior: (1) a Hall–Petch based description for a compilation of delayed yielding measurements compiled for steel; (2) computations for an H-P grain size dependent, tensile, plastic instability behavior of copper; (3) an H-P relationship for the true maximum stress for the limit of uniform straining of aluminum; and (4) the onset of a ductile-to-brittle transition in steel cleavage fracturing measurements that are connected to the material fracture toughness properties. Full article
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Research

Jump to: Editorial, Review

15 pages, 7739 KiB  
Article
Effect of a Long-Range Dislocation Pileup on the Atomic-Scale Hydrogen Diffusion near a Grain Boundary in Plastically Deformed bcc Iron
by Yipeng Peng, Rigelesaiyin Ji, Thanh Phan, Xiang Chen, Ning Zhang, Shuozhi Xu, Ashraf Bastawros and Liming Xiong
Crystals 2023, 13(8), 1270; https://doi.org/10.3390/cryst13081270 - 17 Aug 2023
Cited by 1 | Viewed by 901
Abstract
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 [...] Read more.
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, Dpileuptip, 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. Full article
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14 pages, 7368 KiB  
Article
Mesomechanical Aspects of the Strain-Rate Sensitivity of Armco-Iron Pulled in Tension
by Mikhail Eremin, Artyom Chirkov and Vladimir Danilov
Crystals 2023, 13(6), 866; https://doi.org/10.3390/cryst13060866 - 25 May 2023
Viewed by 780
Abstract
The phenomenon of the strain-rate sensitivity of metallic materials has been a topic of interest since the first mechanical tests at different strain rates were performed. The problem of its theoretical description appeared simultaneously. Despite the significant number of studies covering this issue, [...] Read more.
The phenomenon of the strain-rate sensitivity of metallic materials has been a topic of interest since the first mechanical tests at different strain rates were performed. The problem of its theoretical description appeared simultaneously. Despite the significant number of studies covering this issue, it is necessary to rule out a few drawbacks of previously reported models, which is the goal of this work. Herein, an extension of the elastic–viscoplastic model to a generalized state of stress is proposed while aiming to describe the strain rate sensitivity of Armco-iron samples that were pulled in tension within the framework of the finite-difference method. A mathematical model was formulated using equivalent stress and strain, which alleviated the complexity of the relaxation-type constitutive equations. The critical shear stress (CSS) function describes S-type instability with a single equation. The plastic strain rate was calculated based on the well-known Orowan equation, which is related to dislocation dynamics. In addition, the model took the material’s microstructure into account based on the design of a representative volume element (RVE) using the step-by-step packing (SSP) method. The results of the modeling were compared with the available experimental data and were found to satisfactorily correlate with them. The results suggest that the misfit error between the model and experimental data did not exceed 10% in the range of strain rates under study, which is a reliable outcome. Full article
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19 pages, 7937 KiB  
Article
Interface-Dominated Plasticity and Kink Bands in Metallic Nanolaminates
by Abhishek Arora, Rajat Arora and Amit Acharya
Crystals 2023, 13(5), 828; https://doi.org/10.3390/cryst13050828 - 17 May 2023
Cited by 2 | Viewed by 1786
Abstract
The theoretical and computational framework of finite deformation mesoscale field dislocation mechanics (MFDM) is used to understand the salient aspects of kink-band formation in Cu-Nb nano-metallic laminates (NMLs). A conceptually minimal, plane-strain idealization of the three-dimensional geometry, including crystalline orientation, of additively manufactured [...] Read more.
The theoretical and computational framework of finite deformation mesoscale field dislocation mechanics (MFDM) is used to understand the salient aspects of kink-band formation in Cu-Nb nano-metallic laminates (NMLs). A conceptually minimal, plane-strain idealization of the three-dimensional geometry, including crystalline orientation, of additively manufactured NML is used to model NMLs. Importantly, the natural jump/interface condition of MFDM imposing continuity of (certain components) of plastic strain rates across interfaces allows theory-driven ‘communication’ of plastic flow across the laminate boundaries in our finite element implementation. Kink bands under layer parallel compression of NMLs in accord with experimental observations arise in our numerical simulations. The possible mechanisms for the formation and orientation of kink bands are discussed, within the scope of our idealized framework. We also report results corresponding to various parametric studies that provide preliminary insights and clear questions for future work on understanding the intricate underlying mechanisms for the formation of kink bands. Full article
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14 pages, 2055 KiB  
Article
Tensor Representation Method Applied to Magnesium Alloys
by Aleksander Zubelewicz
Crystals 2023, 13(5), 719; https://doi.org/10.3390/cryst13050719 - 24 Apr 2023
Viewed by 740
Abstract
The tensor representation method (TRM) offers tensorial tools suitable for streamlining the development of constitutive models. The TRM reduces the empiricism of phenomenological descriptions and provides physics-based justifications for the tensorial construction of material models. The method is presented in a stepwise manner, [...] Read more.
The tensor representation method (TRM) offers tensorial tools suitable for streamlining the development of constitutive models. The TRM reduces the empiricism of phenomenological descriptions and provides physics-based justifications for the tensorial construction of material models. The method is presented in a stepwise manner, thus giving the reader an opportunity to appreciate the details of the concept. The selected material is magnesium alloy AZ31B (wt% composition: Mg 95.8, Al 3.0, Zn 1.0, and Mn 0.2), and the choice is not coincidental. The hexagonal close-packed (hcp) structure of rolled sheets exhibits highly directional plastic flow, while the crystallographic reorientations add to the complexity of the material’s behavior. A generic structure of the deformation mechanisms is determined first. In the next step, the TRM tools enable the coupling of the mechanisms with proper stimuli. Lastly, the thermo-mechanical flow rules for plasticity and twinning complete the constitutive description. The model predictions for Mg AZ31B have been compared with experimental data, demonstrating a desirable level of predictability. Full article
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11 pages, 2549 KiB  
Article
Dislocation Mechanisms and Local Strength with a View towards Sleeper Screw Failures
by Kang Lin, Lichu Zhou, Dorte Juul Jensen and Xiaodan Zhang
Crystals 2023, 13(4), 656; https://doi.org/10.3390/cryst13040656 - 11 Apr 2023
Cited by 1 | Viewed by 915
Abstract
Dislocation structures can be directly related to the fatigue properties of metals, such as fatigue strength, including the fatigue limit and saturation stress. We present an indirect dislocation-structure-based method to evaluate the local stresses for an in-depth analysis of sleeper screw failures, as [...] Read more.
Dislocation structures can be directly related to the fatigue properties of metals, such as fatigue strength, including the fatigue limit and saturation stress. We present an indirect dislocation-structure-based method to evaluate the local stresses for an in-depth analysis of sleeper screw failures, as there is little knowledge about the load and local stresses related to these failures. The sleeper screw, fastening baseplates of rails to sleepers, is a small but critical component in the railway. High loads from passing trains are transferred to the screws, leading to cyclic straining. In the present study, three stress-level tension fatigue experiments are designed in the constant stress mode at a stress ratio R = 0 and a testing frequency of 10 Hz. The microstructures in the failed specimens are characterized and compared with those close to the fracture surface of screws that failed in the field. The dislocation structure similarities and differences are analyzed, and the potential of the proposed methodology is discussed. Full article
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12 pages, 3602 KiB  
Article
Geometrically Necessary Dislocation Analysis of Deformation Mechanism for Magnesium under Fatigue Loading at 0 °C
by Qizhen Li
Crystals 2023, 13(3), 490; https://doi.org/10.3390/cryst13030490 - 12 Mar 2023
Viewed by 1024
Abstract
This study focused on the analysis of geometrically necessary dislocation (GND) densities for five selected fine-grained magnesium samples. Among the samples, three were tested under different fatigue-loading conditions at 0 °C, one experienced quasi-static tensile loading at 0 °C, and one represented the [...] Read more.
This study focused on the analysis of geometrically necessary dislocation (GND) densities for five selected fine-grained magnesium samples. Among the samples, three were tested under different fatigue-loading conditions at 0 °C, one experienced quasi-static tensile loading at 0 °C, and one represented the as-rolled state. The fatigue-tested samples were chosen according to the relationship between the maximum loading stress of a test and the material’s yield strength. This study provides new insights on the deformation mechanism of fine-grained magnesium at 0 °C. It is observed that the average GND densities were increased by 95~111% for the tested samples when compared with the as-rolled sample. It is especially interesting that there is a significant increase in the average GND density for the sample that experienced the fatigue loading with a low-maximum applied stress, and the maximum applied stress was lower than the material’s yield strength. This observation implies that the grain boundary mediated the dislocation-emission mechanism. Full article
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13 pages, 4429 KiB  
Article
Non-Schmid Effect on the Fracture Behavior of Tungsten
by Zhijie Li and Yinan Cui
Crystals 2023, 13(3), 417; https://doi.org/10.3390/cryst13030417 - 28 Feb 2023
Viewed by 1397
Abstract
The fracture process of tungsten is dominated by the competition mechanism between the plastic deformation and the crack propagation near the crack tip. The non-Schmid (NS) effect, which considers the contribution of non-planar shear stress on the screw dislocation motion, is known to [...] Read more.
The fracture process of tungsten is dominated by the competition mechanism between the plastic deformation and the crack propagation near the crack tip. The non-Schmid (NS) effect, which considers the contribution of non-planar shear stress on the screw dislocation motion, is known to significantly influence the plastic deformation of tungsten at low and medium temperatures. However, how the NS effect influences the crack-tip plasticity and the fracture behavior of tungsten remains to be answered. In this work, the coupled crystal-plasticity and phase-field model (CP-PFM) was adopted to study the influence of the NS effect on the plastic deformation of un-notched tungsten and the fracture process of pre-notched tungsten at different temperatures. It was found that the lower the temperature, the more significant the NS effect on tungsten plasticity, which manifests in the lower yield stress and more unsymmetrical plastic deformation when the NS effect is considered. In contrast, the NS effect displayed the most obvious effect on the fracture behavior of pre-notched tungsten in the medium temperature regime, which manifested as higher fracture stress, a more significant crack-tip shielding effect, different fracture morphology, and lower crack propagation speed. The brittle fracture response at low temperature was not affected too much by the existence of the NS effect. Full article
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18 pages, 5788 KiB  
Article
Breaks in the Hall–Petch Relationship after Severe Plastic Deformation of Magnesium, Aluminum, Copper, and Iron
by Shivam Dangwal, Kaveh Edalati, Ruslan Z. Valiev and Terence G. Langdon
Crystals 2023, 13(3), 413; https://doi.org/10.3390/cryst13030413 - 27 Feb 2023
Cited by 12 | Viewed by 2500
Abstract
Strengthening by grain refinement via the Hall–Petch mechanism and softening by nanograin formation via the inverse Hall–Petch mechanism have been the subject of argument for decades, particularly for ultrafine-grained materials. In this study, the Hall–Petch relationship is examined for ultrafine-grained magnesium, aluminum, copper, [...] Read more.
Strengthening by grain refinement via the Hall–Petch mechanism and softening by nanograin formation via the inverse Hall–Petch mechanism have been the subject of argument for decades, particularly for ultrafine-grained materials. In this study, the Hall–Petch relationship is examined for ultrafine-grained magnesium, aluminum, copper, and iron produced by severe plastic deformation in the literature. Magnesium, aluminum, copper, and their alloys follow the Hall–Petch relationship with a low slope, but an up-break appears when the grain sizes are reduced below 500–1000 nm. This extra strengthening, which is mainly due to the enhanced contribution of dislocations, is followed by a down-break for grain sizes smaller than 70–150 nm due to the diminution of the dislocation contribution and an enhancement of thermally-activated phenomena. For pure iron with a lower dislocation mobility, the Hall–Petch breaks are not evident, but the strength at the nanometer grain size range is lower than the expected Hall–Petch trend in the submicrometer range. The strength of nanograined iron can be increased to the expected trend by stabilizing grain boundaries via impurity atoms. Detailed analyses of the data confirm that grain refinement to the nanometer level is not necessarily a solution to achieve extra strengthening, but other strategies such as microstructural stabilization by segregation or precipitation are required. Full article
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20 pages, 4717 KiB  
Article
Influence of Grain Size on Mechanical Properties of a Refractory High Entropy Alloy under Uniaxial Tension
by Orlando Deluigi, Felipe Valencia, Diego R. Tramontina, Nicolás Amigo, Javier Rojas-Nunez and Eduardo M. Bringa
Crystals 2023, 13(2), 357; https://doi.org/10.3390/cryst13020357 - 19 Feb 2023
Cited by 2 | Viewed by 2654
Abstract
There is a growing interest in High Entropy Alloys (HEAs) due to their outstanding mechanical properties. Most simulation studies have focused on face-centered cubic (fcc) HEAs; however, bcc HEAs can offer a larger elastic modulus and plastic yielding, thus, becoming possible candidates for [...] Read more.
There is a growing interest in High Entropy Alloys (HEAs) due to their outstanding mechanical properties. Most simulation studies have focused on face-centered cubic (fcc) HEAs; however, bcc HEAs can offer a larger elastic modulus and plastic yielding, thus, becoming possible candidates for the next generation of refractory materials. In this work, we focus on molecular dynamics (MD) simulations of bcc HfNbTaZr nanocrystalline samples, with a grain size (d) between 5 and 17 nm, deformed under tension at 300 K. The elastic modulus increases with the grain size and reaches a plateau near 10 nm. We find the typical inverse Hall–Petch (HP) behavior with yield strength, ultimate tensile stress (UTS), and flow stress increasing with d. Up to 12 nm, there are contributions from dislocations and twins; however, grain boundary (GB) activity dominates deformation. For the 5 nm grains, the GB disorder extends and leads to extensive amorphization and grain size reduction. For d>10 nm, there is a HP-type behavior with dislocations and twinning controlling deformation. For this regime, there is hardening at large strains. Compared to bcc single metal samples, the HP maximum of this HEA appears at a lower grain size, and this could be related to the chemical complexity facilitating dislocation nucleation. We use machine learning to help understand deformation regimes. We also compare our results to a single crystal (SC) HfNbTaZr HEA deformed along [001] and find that the single crystal is weaker than the nanocrystalline samples. The single crystal deforms initially by twinning and then rapidly by dislocation multiplication, leading to strong hardening. It has been proposed that edge dislocations play a major role in bcc HEA plasticity, and we also analyze the relative contributions of edge versus screw dislocations during deformation for both single crystal and nanocrystalline samples. Full article
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17 pages, 18455 KiB  
Article
The Effect of Deformation Temperature on the Deformation Mechanism of a Medium-Mn Advanced High-Strength Steel (AHSS)
by Po-Chung Chen, Tzu-Ting Peng, Yu-Cheng Chan, Jun-Ming Chen and Chih-Pu Chang
Crystals 2023, 13(2), 328; https://doi.org/10.3390/cryst13020328 - 15 Feb 2023
Viewed by 1227
Abstract
The deformation mechanism of a medium-Mn advanced high strength steel (AHSS) over a temperature range from 25 °C to 400 °C has been studied. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the microstructures of specimens after the [...] Read more.
The deformation mechanism of a medium-Mn advanced high strength steel (AHSS) over a temperature range from 25 °C to 400 °C has been studied. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the microstructures of specimens after the tensile test at different temperatures. Four deformation mechanisms were found, namely deformation-induced martensitic (DIM) transformation, deformation-induced bainitic (DIB) transformation, deformation twinning and dislocation glide. Among these deformation mechanisms, DIM and DIB were very effective mechanisms to contribute work hardening. The product of ultimate tensile strength (UTS) and total elongation (TEL) of the AHSS reached a value higher than 65 GPa%, when these two mechanisms occurred. The highest UTS × TEL value of 84 GPa% was obtained at 150 °C. From the results of the present research, it is suggested that warm working is a good processing route for obtaining a combination of high strength and high ductility in medium-Mn AHSS. Full article
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11 pages, 2291 KiB  
Article
Assessing Strain Rate Sensitivity of Nanotwinned Al–Zr Alloys through Nanoindentation
by Nicholas Allen Richter, Xuanyu Sheng, Bo Yang, Benjamin Thomas Stegman, Haiyan Wang and Xinghang Zhang
Crystals 2023, 13(2), 276; https://doi.org/10.3390/cryst13020276 - 05 Feb 2023
Viewed by 1336
Abstract
Nanotwinned metals have exhibited many enhanced physical and mechanical properties. Twin boundaries have recently been introduced into sputtered Al alloys in spite of their high stacking fault energy. These twinned Al alloys possess unique microstructures composed of vertically aligned Σ3(112) incoherent twin boundaries [...] Read more.
Nanotwinned metals have exhibited many enhanced physical and mechanical properties. Twin boundaries have recently been introduced into sputtered Al alloys in spite of their high stacking fault energy. These twinned Al alloys possess unique microstructures composed of vertically aligned Σ3(112) incoherent twin boundaries (ITBs) and have demonstrated remarkable mechanical strengths and thermal stability. However, their strain rate sensitivity has not been fully assessed. A modified nanoindentation method has been employed here to accurately determine the strain rate sensitivity of nanotwinned Al–Zr alloys. The hardness of these alloys reaches 4.2 GPa while simultaneously exhibiting an improved strain rate sensitivity. The nanotwinned Al–Zr alloys have shown grain size-dependent strain rate sensitivity, consistent with previous findings in the literature. This work provides insight into a previously unstudied aspect of nanotwinned Al alloys. Full article
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24 pages, 10952 KiB  
Article
Role of Persistent Slip Bands and Persistent Slip Markings in Fatigue Crack Initiation in Polycrystals
by Jaroslav Polák
Crystals 2023, 13(2), 220; https://doi.org/10.3390/cryst13020220 - 25 Jan 2023
Cited by 5 | Viewed by 4563
Abstract
The cyclic plastic deformation of polycrystals leads to the inhomogeneous distribution of the cyclic plastic strain. The cyclic plastic strain is concentrated in thin bands, called persistent slip bands (PSBs). The dislocation structure of these bands generally differs from the matrix structure and [...] Read more.
The cyclic plastic deformation of polycrystals leads to the inhomogeneous distribution of the cyclic plastic strain. The cyclic plastic strain is concentrated in thin bands, called persistent slip bands (PSBs). The dislocation structure of these bands generally differs from the matrix structure and is characterized by alternating dislocation-rich and dislocation-poor regions. The mechanisms of the dislocation motion in the PSBs and the formation of the point defects and their migration are quantitatively described. It is shown that, due to localized cyclic plastic straining in the PSBs, persistent slip markings (PSMs) are produced where the PSBs emerge on the surface. They typically consist of a central extrusion accompanied by one or two parallel intrusions. The deep intrusion is equivalent to the crack-like surface defect. The concentration of the cyclic strain in the tip of an intrusion leads to intragranular fatigue crack initiation. The mechanism of the early crack growth in the primary slip plane is proposed and discussed. Numerous PSMs are produced on the surface of the cyclically loaded materials. PSMs contribute to the formation of the surface relief, as well as the relief on the grain boundary. PSMs from one grain impinging the grain boundary are sufficient to create sharp relief on the grain boundary. Void-like defects weaken the grain boundary cohesion and extra material push both grains locally apart. The conditions necessary for the weakening of the grain boundary are enumerated and examples of grain boundary crack initiations are shown. The relevant parameters affecting grain boundary initiation are identified and discussed. The collected experimental evidence and analysis is mostly based on the papers published by the author and his colleagues in the Institute of Physics of Materials in Brno. Full article
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18 pages, 23590 KiB  
Article
Effect of Grain Sizes on Electrically Assisted Micro—Filling of SUS304 Stainless Steel: Experiment and Simulation
by Mingliang Men, Rui Zhao, Yizhe Liu, Min Wan and Bao Meng
Crystals 2023, 13(1), 134; https://doi.org/10.3390/cryst13010134 - 12 Jan 2023
Viewed by 1369
Abstract
The filling quality of micro-feature structures has a significant impact on the forming quality of micro-channels. The electrical-assisted forming technology can effectively improve the formability of difficult-to-deform materials. In this research, the electrically driven micro-compression constitutive model of SUS304 stainless steels was established [...] Read more.
The filling quality of micro-feature structures has a significant impact on the forming quality of micro-channels. The electrical-assisted forming technology can effectively improve the formability of difficult-to-deform materials. In this research, the electrically driven micro-compression constitutive model of SUS304 stainless steels was established to assign grain boundary and grain interior with different material properties. An electrical–thermal–mechanical coupling model was constructed to simulate the filling process considering the effect of grain boundary and grain size. Compared to the experimental results, the simulation indicated a good agreement in microstructure characteristics and higher filling height for the fine-grained material. The increase in grain boundary density makes the resistivity of the fine grain material larger, causing the current destiny and temperature of the specimen to increase with the decrease in grain size. An ellipsoidal gradient temperature distribution is observed due to the uneven current density. Because of the high geometric dislocation density near the grain boundary, a significant dislocation pile-up causes stress to concentrate. It is observed that the deformation coordination is enhanced between the grain boundary and grain core with the decrease in grain size, thus improving the material formability and forming quality. Full article
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36 pages, 7443 KiB  
Article
Rate-Controlling Microplastic Processes during Plastic Flow in FCC Metals: Origin of the Variation of Strain Rate Sensitivity in Aluminum from 78 to 300 K
by Shigeo Saimoto, Bradley J. Diak and Marek Niewczas
Crystals 2022, 12(12), 1811; https://doi.org/10.3390/cryst12121811 - 12 Dec 2022
Cited by 3 | Viewed by 1337
Abstract
The thermodynamic response of dislocation intersections with forest dislocations and other deformation products is recorded using the Eyring rate relation wherein the application of shear stress increases the probability of activation at a given strain rate and temperature. The inverse activation volume, 1/ν, [...] Read more.
The thermodynamic response of dislocation intersections with forest dislocations and other deformation products is recorded using the Eyring rate relation wherein the application of shear stress increases the probability of activation at a given strain rate and temperature. The inverse activation volume, 1/ν, can be directly determined by instantaneous strain-rate change and its dependence on flow stress, τ, defines the strain-rate sensitivity, S, through the Haasen plot slope. A linear slope over a large strain interval is observed even for a heterogeneous distribution of obstacles that could be of more than one type of obstacles encountered by the gliding dislocation. It was deduced that ν and τ at each activation site are coordinated by the internal stress resulting in constant activation work (k/S). The stress changes from down-rate changes become larger than that from up-rate changes due to the formation of weaker obstacles, resulting in a composite S, whereas only forest dislocations are detected by the up-change. The additivity of 1/ν was used to separate obstacle species in specially prepared AA1100 and super-pure aluminum from 78 to 300 K. The deduction that repulsive intersection is the rate-controlling process and creates vacancies at each intersection site depending on temperature was validated by observing the pinning and depinning of dislocations via pipe diffusion above 125 K. A new method to separate S for dislocation-dislocation intersections from the intersections with other obstacles and their temperature dependence is presented and validated. Full article
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Review

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18 pages, 4726 KiB  
Review
A Comprehensive Review of Large-Strain-Extrusion Machining Process for Production of Fine-Grained Materials
by Muralimohan Gurusamy and Balkrishna C. Rao
Crystals 2023, 13(1), 131; https://doi.org/10.3390/cryst13010131 - 11 Jan 2023
Viewed by 1472
Abstract
Bulk nanostructured metals and alloys are finding increasing structural applications due to their superior mechanical properties. The methods that rely on the severe plastic deformation technique for effecting microstructural refinement through imposing large strains are utilized mostly to produce nanostructured materials. The machining [...] Read more.
Bulk nanostructured metals and alloys are finding increasing structural applications due to their superior mechanical properties. The methods that rely on the severe plastic deformation technique for effecting microstructural refinement through imposing large strains are utilized mostly to produce nanostructured materials. The machining process has been demonstrated as a simple process for severe plastic deformation by imposing large strains through a single pass of the cutting tool where strains in a range of 1–15 can be imposed for a variety of materials by varying the cutting conditions and tool geometry. However, the geometry of the resulting chip subjected to severe plastic deformation during the machining process is not under control and, hence, a variant of the machining process, called the large-strain-extrusion machining process, has been proposed and utilized extensively for producing bulk nanostructured materials. Large-strain-extrusion machining possesses simultaneous control over microstructure refinement, through managing the strain during large-strain machining, and the shape and dimension of the resulting chip by the extrusion process. This study provides a comprehensive review of the large-strain-extrusion machining process by presenting the findings related to the utilization of this process for the production of fine-grained foils for various metals and alloys. Further research efforts related to finite-element modelling of large-strain-extrusion machining and their usefulness in designing the experimental setup and process conditions are also discussed. Full article
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