Special Issue "Dislocation Mechanics of Metal Plasticity and Fracturing"

A special issue of Metals (ISSN 2075-4701).

Deadline for manuscript submissions: closed (10 February 2020).

Special Issue Editor

Prof. Dr. Ronald W. Armstrong
Website
Guest 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 modern understanding of metal plasticity and fracturing began in the 1920s with the pioneering work, first, on crack-induced fracturing by Griffith and, secondly, on dislocation-enhanced crystal plasticity by Taylor, Orowan, and Polanyi. Modern counterparts of this work are fracture mechanics as invented by Irwin and dislocation mechanics initiated in large part by Cottrell. No less important was the breakthrough development of optical characterization of sectioned polycrystalline metal microstructures begun by Sorby in the late 19th century, leading eventually to modern x-ray and electron microscopy methods of assessing crystal fracture surfaces, via fractography, and internal dislocation behaviors. A major current effort is to match computational simulations of metal deformation/fracturing behaviors with experimental measurements made over extended ranges of metal microstructures and over varying external conditions of stress-state, temperature, and loading rate.  The relationship between such simulations and the development of constitutive equations for a hoped-for predictive description of material deformation/fracturing behaviors is an active topic of research. The purpose of the present Special Issue is to offer a publication venue for current reports on the two subjects of understanding metal failures and understanding corresponding deformation strengths relating to metal processing.

Prof. Dr. Ronald W. Armstrong
Guest Editor

Manuscript Submission Information

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Keywords

  • crystal plasticity
  • metal fracturing
  • dislocation mechanics
  • fracture mechanics
  • crystal/polycrystal microstructures
  • crystal deformation/fracturing simulations
  • constitutive equations

Published Papers (11 papers)

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Editorial

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Open AccessEditorial
Dislocation Mechanics Pile-Up and Thermal Activation Roles in Metal Plasticity and Fracturing
Metals 2019, 9(2), 154; https://doi.org/10.3390/met9020154 - 31 Jan 2019
Cited by 1
Abstract
Dislocation pile-up and thermal activation influences on the deformation and fracturing behaviors of polycrystalline metals are briefly reviewed, as examples of dislocation mechanics applications to understanding mechanical properties. To start, a reciprocal square root of grain size dependence was demonstrated for historical hardness [...] Read more.
Dislocation pile-up and thermal activation influences on the deformation and fracturing behaviors of polycrystalline metals are briefly reviewed, as examples of dislocation mechanics applications to understanding mechanical properties. To start, a reciprocal square root of grain size dependence was demonstrated for historical hardness measurements reported for cartridge brass, in line with a similar Hall-Petch grain size characterization of stress-strain measurements made on conventional grain size and nano-polycrystalline copper, nickel, and aluminum materials. Additional influences of loading rate (and temperature) were shown to be included in a dislocation model thermal activation basis, for calculated deformation shapes of impacted solid cylinders of copper and Armco iron materials. Connection was established for such grain size, temperature, and strain rate influences on the brittle fracturing transition exhibited by steel and other related metals. Lastly, for AISI 1040 steel material, a fracture mechanics based failure stress dependence on the inverse square root of crack size was shown to approach the yield stress at a very small crack size, also in line with a Hall-Petch dependence of the stress intensity on polycrystal grain size. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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Research

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Open AccessArticle
Effects of Testing Temperature on the Serration Behavior of an Al–Zn–Mg–Cu Alloy with Natural and Artificial Aging in Sharp Indentation
Metals 2020, 10(5), 597; https://doi.org/10.3390/met10050597 - 03 May 2020
Abstract
Serration phenomena, in which stress fluctuates in a saw-tooth shape, occur when a uniaxial test is performed on an aluminum alloy containing a solid solution of solute atoms. The appearance of the serrations is affected by the strain rate and temperature. Indentation tests [...] Read more.
Serration phenomena, in which stress fluctuates in a saw-tooth shape, occur when a uniaxial test is performed on an aluminum alloy containing a solid solution of solute atoms. The appearance of the serrations is affected by the strain rate and temperature. Indentation tests enable the evaluation of a wide range of strain rates in a single test and are a convenient test method for evaluating serration phenomena. Previously, the serrations caused by indentation at room temperature were clarified using strain rate as an index. In this study, we considered ambient temperature as another possible influential factor. We clarify, through experimentation, the effect of temperature on the serration phenomenon caused by indentation. An Al–Zn–Mg–Cu alloy (7075 aluminum alloy) was used as the specimen. The aging phenomenon was controlled by varying the testing temperature of the solution-treated specimen. Furthermore, the material properties obtained by indentation were evaluated. By varying the testing temperature, the presence and amount of precipitation were controlled and the number of solute atoms was varied. Additionally, the diffusion of solute atoms was controlled by maintaining the displacement during indentations, and a favorable environment for the occurrence of serrations was induced. The obtained results reveal that the variations in the serrations formed in the loading curvature obtained via indentation are attributed to the extent of interaction between the solute atoms and the dislocations. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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Open AccessArticle
Quantifying the Contribution of Crystallographic Texture and Grain Morphology on the Elastic and Plastic Anisotropy of bcc Steel
Metals 2019, 9(12), 1252; https://doi.org/10.3390/met9121252 - 22 Nov 2019
Cited by 1
Abstract
The influence of grain shape and crystallographic orientation on the global and local elastic and plastic behaviour of strongly textured materials is investigated with the help of full-field simulations based on texture data from electron backscatter diffraction (EBSD) measurements. To this end, eight [...] Read more.
The influence of grain shape and crystallographic orientation on the global and local elastic and plastic behaviour of strongly textured materials is investigated with the help of full-field simulations based on texture data from electron backscatter diffraction (EBSD) measurements. To this end, eight different microstructures are generated from experimental data of a high-strength low-alloy (HSLA) steel processed by linear flow splitting. It is shown that the most significant factor on the global elastic stress–strain response (i.e., Young’s modulus) is the crystallographic texture. Therefore, simple texture-based models and an analytic expression based on the geometric mean to determine the orientation dependent Young’s modulus are able to give accurate predictions. In contrast, with regards to the plastic anisotropy (i.e., yield stress), simple analytic approaches based on the calculation of the Taylor factor, yield different results than full-field microstructure simulations. Moreover, in the case of full-field models, the selected microstructure representation influences the outcome of the simulations. In addition, the full-field simulations, allow to investigate the micro-mechanical fields, which are not readily available from the analytic expressions. As the stress–strain partitioning visible from these fields is the underlying reason for the observed macroscopic behaviour, studying them makes it possible to evaluate the microstructure representations with respect to their capabilities of reproducing experimental results. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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Open AccessArticle
Analysis of Ductile Fracture Obtained by Charpy Impact Test of a Steel Structure Created by Robot-Assisted GMAW-Based Additive Manufacturing
Metals 2019, 9(11), 1208; https://doi.org/10.3390/met9111208 - 10 Nov 2019
Cited by 1
Abstract
In this study, gas metal arc welding (GMAW) was used to construct a thin wall structure in a layer-by-layer fashion using an AWS ER70S-6 electrode wire with the help of a robot. The Charpy impact test was performed after extracting samples in directions [...] Read more.
In this study, gas metal arc welding (GMAW) was used to construct a thin wall structure in a layer-by-layer fashion using an AWS ER70S-6 electrode wire with the help of a robot. The Charpy impact test was performed after extracting samples in directions both parallel and perpendicular to the deposition direction. In this study, multiple factors related to the resulting absorbed energy have been discussed. Despite being a layered structure, homogeneous behavior with acceptable deviation was observed in the microstructure, hardness, and fracture toughness of the structure in both directions. The fracture is extremely ductile with a dimpled fibrous surface and secondary cracks. An estimate for fracture toughness based on Charpy impact absorbed energy is also given. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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Open AccessArticle
Strain Rate Contribution due to Dynamic Recovery of Ultrafine-Grained Cu–Zr as Evidenced by Load Reductions during Quasi-Stationary Deformation at 0.5 Tm
Metals 2019, 9(11), 1150; https://doi.org/10.3390/met9111150 - 26 Oct 2019
Cited by 1
Abstract
During quasi-stationary tensile deformation of ultrafine-grained Cu-0.2 mass%Zr at 673 K and a deformation rate of about 10−4 s−1 load changes were performed. Reductions of relative load by more than about 25% initiate anelastic back flow. Subsequently, the creep rate turns [...] Read more.
During quasi-stationary tensile deformation of ultrafine-grained Cu-0.2 mass%Zr at 673 K and a deformation rate of about 10−4 s−1 load changes were performed. Reductions of relative load by more than about 25% initiate anelastic back flow. Subsequently, the creep rate turns positive again and goes through a relative maximum. This is interpreted by a strain rate component ϵ ˙ associated with dynamic recovery of dislocations. Back extrapolation indicates that ϵ ˙ contributes the same fraction of ( 20 ± 10 ) % to the quasi-stationary strain rate that has been reported for coarse-grained materials with high fraction of low-angle boundaries; this suggests that dynamic recovery of dislocations is generally mediated by boundaries. The influence of anelastic back flow on ϵ ˙ is discussed. Comparison of ϵ ˙ to the quasi-stationary rate points to enhancement of dynamic recovery by internal stresses. Subtraction of ϵ ˙ from the total rate yields the rate component ϵ ˙ + related with generation and storage of dislocations; its activation volume is in the order expected from the classical theory of thermal glide. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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Open AccessFeature PaperArticle
Quasi-Stationary Strength of ECAP-Processed Cu-Zr at 0.5 Tm
Metals 2019, 9(11), 1149; https://doi.org/10.3390/met9111149 - 26 Oct 2019
Abstract
The influence of the grain structure on the tensile deformation strength is studied for precipitation-strengthened Cu-0.2%Zr at 673K. Subgrains and grains are formed by equal channel angular pressing (ECAP) and annealing. The fraction of high-angle boundaries increases with prestrain. Subgrains and grains coarsen [...] Read more.
The influence of the grain structure on the tensile deformation strength is studied for precipitation-strengthened Cu-0.2%Zr at 673K. Subgrains and grains are formed by equal channel angular pressing (ECAP) and annealing. The fraction of high-angle boundaries increases with prestrain. Subgrains and grains coarsen during deformation. This leads to softening in the quasi-stationary state. The initial quasi-stationary state of severely predeformed, ultrafine-grained material exhibits relatively high rate-sensitivity at relatively high stresses. This is interpreted as a result of the stress dependences of the quasi-stationary subgrain size and the volume fraction of subgrain-free grains. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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Open AccessArticle
Nanoscale Hierarchical Structure of Twins in Nanograins Embedded with Twins and the Strengthening Effect
Metals 2019, 9(9), 987; https://doi.org/10.3390/met9090987 - 06 Sep 2019
Cited by 2
Abstract
Hierarchical structures of 20 nm grains embedded with twins are realized in electrodeposited Au–Cu alloys. The electrodeposition method allows refinement of the average grain size to 20 nm order, and the alloying stabilizes the nanoscale grain structure. Au–Cu alloys are face-centered cubic (FCC) [...] Read more.
Hierarchical structures of 20 nm grains embedded with twins are realized in electrodeposited Au–Cu alloys. The electrodeposition method allows refinement of the average grain size to 20 nm order, and the alloying stabilizes the nanoscale grain structure. Au–Cu alloys are face-centered cubic (FCC) metals with low stacking fault energy that favors formation of growth twins. Due to the hierarchical structure, the Hall–Petch relationship is still observed when the crystalline size (average twin space) is refined to sub 10 nm region. The yield strength reaches 1.50 GPa in an electrodeposited Au–Cu alloy composed of 16.6 ± 1.1 nm grains and the average twin spacing at 4.7 nm. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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Open AccessArticle
From Statistical Correlations to Stochasticity and Size Effects in Sub-Micron Crystal Plasticity
Metals 2019, 9(8), 835; https://doi.org/10.3390/met9080835 - 27 Jul 2019
Cited by 1
Abstract
Metals in small volumes display a strong dependence on initial conditions, which translates into size effects and stochastic mechanical responses. In the context of crystal plasticity, this amounts to the role of pre-existing dislocation configurations that may emerge due to prior processing. Here, [...] Read more.
Metals in small volumes display a strong dependence on initial conditions, which translates into size effects and stochastic mechanical responses. In the context of crystal plasticity, this amounts to the role of pre-existing dislocation configurations that may emerge due to prior processing. Here, we study a minimal but realistic model of uniaxial compression of sub-micron finite volumes. We show how the statistical correlations of pre-existing dislocation configurations may influence the mechanical response in multi-slip crystal plasticity, in connection to the finite volume size and the initial dislocation density. In addition, spatial dislocation correlations display evidence that plasticity is strongly influenced by the formation of walls composed of bound dislocation dipoles. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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Open AccessArticle
Influence of Size on the Fractal Dimension of Dislocation Microstructure
Metals 2019, 9(4), 478; https://doi.org/10.3390/met9040478 - 25 Apr 2019
Abstract
Three-dimensional (3D) discrete dislocation dynamics simulations are used to analyze the size effect on the fractal dimension of two-dimensional (2D) and 3D dislocation microstructure. 2D dislocation structures are analyzed first, and the calculated fractal dimension ( n 2 ) is found to be [...] Read more.
Three-dimensional (3D) discrete dislocation dynamics simulations are used to analyze the size effect on the fractal dimension of two-dimensional (2D) and 3D dislocation microstructure. 2D dislocation structures are analyzed first, and the calculated fractal dimension ( n 2 ) is found to be consistent with experimental results gleaned from transmission electron microscopy images. The value of n 2 is found to be close to unity for sizes smaller than 300 nm, and increases to a saturation value of ≈1.8 for sizes above approximately 10 microns. It is discovered that reducing the sample size leads to a decrease in the fractal dimension because of the decrease in the likelihood of forming strong tangles at small scales. Dislocation ensembles are found to exist in a more isolated way at the nano- and micro-scales. Fractal analysis is carried out on 3D dislocation structures and the 3D fractal dimension ( n 3 ) is determined. The analysis here shows that ( n 3 ) is significantly smaller than ( n 2 + 1 ) of 2D projected dislocations in all considered sizes. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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Open AccessFeature PaperArticle
Size Effects of High Strength Steel Wires
Metals 2019, 9(2), 240; https://doi.org/10.3390/met9020240 - 17 Feb 2019
Cited by 6
Abstract
This study examines the effects of size on the strength of materials, especially on high strength pearlitic steel wires. These wires play a central role in many long span suspension bridges and their design, construction, and maintenance are important for global public safety. [...] Read more.
This study examines the effects of size on the strength of materials, especially on high strength pearlitic steel wires. These wires play a central role in many long span suspension bridges and their design, construction, and maintenance are important for global public safety. In particular, two relationships have been considered to represent strength variation with respect to length parameters: (i) the strength versus inverse square-root and (ii) inverse length equations. In this study, existing data for the strength of high strength pearlitic steel wires is evaluated for the coefficient of determination (R2 values). It is concluded that the data fits into two equations equally well. Thus, the choice between two groups of theories that predict respective relationships must rely on the merit of theoretical developments and assumptions made. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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Review

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Open AccessFeature PaperReview
Dislocation Emission and Crack Dislocation Interactions
Metals 2020, 10(4), 473; https://doi.org/10.3390/met10040473 - 03 Apr 2020
Abstract
An understanding of the crack initiation and crack growth in metals spanning the entire spectrum of conventional and advanced has long been a major scientific challenge. It is known that dislocations are involved both in the initiation and propagation of cracks in metals [...] Read more.
An understanding of the crack initiation and crack growth in metals spanning the entire spectrum of conventional and advanced has long been a major scientific challenge. It is known that dislocations are involved both in the initiation and propagation of cracks in metals and alloys. In this review, we first describe the experimental observations of dislocation emission from cracks under stress. Then the role played by these dislocations in fatigue and fracture is considered at a fundamental level by considering the interactions of crack and dislocations emitted from the crack. We obtain precise expression for the equilibrium positions of dislocations in an array ahead of crack tip. We estimate important parameters, such as plastic zone size, dislocation free zone and dislocation stress intensity factor for the analysis of crack propagation. Finally, we describe very recent novel and significant results, such as residual stresses and relatively large lattice rotations across a number of grains in front of the crack that accompanies fatigue process. Full article
(This article belongs to the Special Issue Dislocation Mechanics of Metal Plasticity and Fracturing)
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