Evolution Of Dislocation Structure Research Articles

On the stable/metastable nature of the γ-hydride phase in Zircaloy-2: Microstructural characterization by electron diffraction, electron energy-loss spectroscopy, and diffraction line profile analysis

This work investigates the potential metastable versus stable nature of the γ-hydride in Zircaloy-2. Specimens were hydrided, and hydrides were precipitated through water-quenching. Synchrotron X-ray diffraction of the water-quenched sample revealed a diffraction peak with the d-spacing value of ∼2.70 Å. While this peak is conventionally attributed to {111}-γ planes, it could also stem from the (0004)-ζ plane. To clarify this ambiguity, the crystal structure of nano-hydrides was characterized by nano-beam electron diffraction (NBED) and electron energy-loss spectroscopy (EELS). While EELS detected nano-hydrides with plasmon energy (PE) values associated with the ζ-,γ-, and δ-phases, suggesting all three types of phases might be present, complementary NBED analysis revealed that regardless of the measured PE values, the examined nano-hydrides were of only γ- or δ-nature.Repeating the heating/quenching cycles reduced the γ-phase volume fraction until its (almost) complete disappearance after three cycles. δ-phase, however, was observed after each heating/quenching cycle. This observation, in accordance with previous reports, indicates that the γ-phase is metastable in Zircaloy-2, such that even during water-quenching (which is conventionally believed to facilitate the formation of γ-phase) only δ-hydrides form in cases where microstructural conditions are suitable.Diffraction line profile analysis and transmission electron microscopy revealed an increase in dislocation density during the first heating/quenching cycle, with no noticeable variations during subsequent cycles. A mechanism is proposed that links microstructure (i.e., dislocation structure) evolution during heating/quenching cycles to the suppression of the (metastable) γ-phase.

Dislocation structure evolution during room temperature dwell loading of a Ti-6Al-4 V alloy

The current work examines the evolution of the dislocation structure, within both the α and β phases, of a Ti-6Al-4 V alloy subjected to room temperature dwell loading performed at about 96% of the macroscopic yield stress level for several durations, as part of a synchrotron in-situ deformation experiment. The obtained findings represent a vital contribution to creating a scientific basis for improving lifing predictions for aero-propulsion components. The dislocation configuration, type and density were examined post-mortem using transmission electron microscopy (TEM). Despite the relatively low applied stress level, well-developed dislocation structures were already formed within the α phase at a dwell duration of 10 min and changed only modestly up to 60 min. The plastic deformation of the above phase took place solely via dislocation slip. To facilitate a comparison between different dwell durations, the TEM investigation focused on the regions oriented for the prismatic glide that represented the main deformation mode due to the starting crystallographic texture. After deformation, the regions oriented for the prismatic slip contained planar slip bands largely consisting of dislocations of a screw type. There was rather limited interaction between different bands, as the dislocation arrays passed through the slip band intersections with only minor difficulty. The screw-type dislocations seemed to preferentially form during straining. Dislocation Burgers vectors experimentally determined within the β phase occasionally departed from the Schmid rule and such dislocations typically showed a prevalent screw component. Some deformation banding and {112} 〈111〉 − type mechanical twin formation were locally detected within the β phase.

Topology and evolution of dislocation structures mediated by glissile reactions in face-centered cubic metals

The strength and hardening of metallic materials are dictated by the motion and interactions of dislocations. Individual dislocations intersect, react, and frequently form junctions, defining a defect topology that is the basis of subsequent deformation. While immobilized dislocation locks are intuitively considered as potent strengthening structures, simulations suggest that glissile reactions are a predominant contributor to hardening among the four types of dislocation reactions in face-centered cubic crystals, even though the resulting dislocations are inherently mobile. To date, the prevailing understanding of glissile reactions has been primarily based on classical geometric models of perfect dislocations and simulations thereof. Understandings of the reaction pathways and detailed experimental characterization of glissile reactions are lacking, leaving the potential topological variations shrouded in mystery. This study details molecular dynamics simulations of glissile reaction involving dissociated partial dislocations, and the direct experimental characterization of the dislocation configurations resulting from glissile reactions in deformed pure aluminum using transmission electron microscopy. The experimentally-determined structure was reconstructed in 3D and parametrically studied in discrete dislocation dynamics simulations, revealing varying topological evolutions under different loading conditions. Further statistical analyses on an ensemble of simulated dislocations revealed the essential role of stress states and cross-slip in affecting the probability of glissile reaction and the fraction of mobile dislocation nodes. These findings point to avenues for the development of dislocation-based constitutive theories of plasticity.

Effect of prestrain on tensile behavior of HS800 steel sheet

With the development of advanced high strength steels like HS800 and HS1000, it is imperative to develop an understanding of their tensile properties. The effect of 0%, 5%, 7.5%, and 10% prestrain on the tensile behaviour of HS800 steel sheet was investigated in the current research. The optical micrograph of as received HS800 steels reveals the presence of ferrite grains with polygonal morphology without the presence of pearlite or cementite. Moreover, in higher pre-straining conditions, both polygonal and acicular ferrite phases are observed to coexist with the second phase (martensite/austenite). Tensile tests were performed on base HS800 steel sheets in three different directions (rolling, transverse, and diagonal), and it was found that the strength and ductility in the transverse direction were greater than the rolling and diagonal direction. Tensile strength had a linear relationship with the prestrain condition, whereas ductility had an inverse relationship. XRD analysis was used for phase identification and determination of dislocation density. A scanning electron microscope (SEM) was used to observe the ductile fracture behaviour in tensile specimens. The strain hardening exponent of HS800 steel was determined by comparing tensile data to Hollomon and Ludwik's mathematical equations. To understand the dislocation structure evolution of as received and various prestraining conditions samples, a transmission electron microscope (TEM) study was performed. The prestraining study of the HS800 steel gives a new horizon to high strength materials that can be utilized to design the current and future automobile industries requirements.

Micropillar compression using discrete dislocation dynamics and machine learning

Discrete dislocation dynamics (DDD) simulations reveal the evolution of dislocation structures and the interaction of dislocations. This study investigated the compression behavior of single-crystal copper micropillars using few-shot machine learning with data provided by DDD simulations. Two types of features are considered: external features comprising specimen size and loading orientation and internal features involving dislocation source length, Schmid factor, the orientation of the most easily activated dislocations and their distance from the free boundary. The yielding stress and stress-strain curves of single-crystal copper micropillar are predicted well by incorporating both external and internal features of the sample as separate or combined inputs. It is found that the Machine learning accuracy predictions for single-crystal micropillar compression can be improved by incorporating easily activated dislocation features with external features. However, the effect of easily activated dislocation on yielding is less important compared to the effects of specimen size and Schmid factor which includes information of orientation but becomes more evident in small-sized micropillars. Overall, incorporating internal features, especially the information of most easily activated dislocations, improves predictive capabilities across diverse sample sizes and orientations.

Atomic mixing mechanisms in nanocrystalline Cu/Ni composites under continuous shear deformation and thermal annealing

Molecular dynamics (MD) simulations are used to reveal the mechanisms of defect substructure evolution and atomic mixing in nanocrystalline Cu/Ni composites under severe shear deformation and subsequent thermal annealing. A continuous shear scheme of MD simulation utilizing an on-the-fly periodic boundary adjustment approach enables it to reach any large shear strain. The comprehensive evaluation on the evolution of dislocation structures at various strain states indicates partial dislocation-mediated plastic deformation and triple junction slide via partial dislocation nucleation and emission from triple junctions and grain boundaries. The analysis of atomic structures in unique mixing regions suggests that triple junction sliding can result in a long-range region of atomic mixing facilitated by net dislocation flux, which is the key mechanism of atomic mixing in nanocrystalline Cu/Ni composite under severe shear, while dislocation emissions at interfaces result in short-range mixing. It is also found that thermal annealing causes the evolution of non-equilibrium defect substructures which assists atomic mixing.

Retraction Note: Evolution of Dislocation Structure of Polycrystals of Homogeneous Copper-Manganese Solid Solutions Under Conditions of Plastic Deformation

Retraction Note: Evolution of Dislocation Structure of Polycrystals of Homogeneous Copper-Manganese Solid Solutions Under Conditions of Plastic Deformation

Fatigue damage assessment using nonlinear critically refracted longitudinal (LCR) wave in pure iron: Experiments and FEM

To achieve early-stage microstructure evolution assessment and damage monitoring of fatigue, a new thickness-independent and nondestructive method based on nonlinear critically refracted longitudinal (LCR) wave is proposed. The nonlinear ultrasonic (NLU) parameter is sensitive to the accumulated fatigue damage, showing two peaks around the 5th and 1000th cycles (0.03% and 6% of fatigue life), respectively. This phenomenon is comprehensively discussed in terms of dislocation structure evolution based on the dislocation string model. A correction considering the weakening of nonlinearity induced by the rough surface is applied to the experiment measurements. The corresponding results are closer to the finite element modeling (FEM) quantitation based on the dislocation-dependent nonlinear constitutive relation. Compared to linear LCR wave, nonlinear LCR wave has weaker sensitivity to roughness effect and higher sensitivity to damage such as dislocation, which offers a new method for monitoring fatigue in engineering components.

Modeling the evolution of representative dislocation structures under high thermo-mechanical conditions during Additive Manufacturing of Alloys

Mesoscale simulations of discrete defects in metals provide an ideal framework to investigate the micro-scale mechanisms governing the plastic deformation under high thermal and mechanical loading conditions. To bridge size and time-scale while limiting computational effort, typically the concept of representative volume elements (RVEs) is employed. This approach considers the microstructure evolution in a volume that is representative of the overall material behavior. However, in settings with complex thermal and mechanical loading histories careful consideration of the impact of modeling constraints in terms of time scale and simulation domain on predicted results is required. We address the representation of heterogeneous dislocation structure formation in simulation volumes using the example of residual stress formation during cool-down of laser powder-bed fusion (LPBF) of AISI 316L stainless steel. This is achieved by a series of large-scale three-dimensional discrete dislocation dynamics (DDD) simulations assisted by thermo-mechanical finite element modeling of the LPBF process. Our results show that insufficient size of periodic simulation domains can result in dislocation patterns that reflect the boundaries of the primary cell. More pronounced dislocation interaction observed for larger domains highlight the significance of simulation domain constraints for predicting mechanical properties. We formulate criteria that characterize representative volume elements by capturing the conformity of the dislocation structure to the bulk material. This work provides a basis for future investigations of heterogeneous microstructure formation in mesoscale simulations of bulk material behavior.

High-resolution reciprocal space mapping reveals dislocation structure evolution during 3D printing

High-resolution reciprocal space mapping reveals dislocation structure evolution during 3D printing

RETRACTED ARTICLE: Evolution of Dislocation Structure of Polycrystals of Homogeneous Copper-Manganese Solid Solutions Under Conditions of Plastic Deformation

RETRACTED ARTICLE: Evolution of Dislocation Structure of Polycrystals of Homogeneous Copper-Manganese Solid Solutions Under Conditions of Plastic Deformation

A Novel Method for Early Fatigue Damage Diagnosis in 316L Stainless Steel Formed by Selective Laser Melting Technology.

Early fatigue damage is an important factor affecting the service safety of 316L stainless steel parts formed by selective laser melting (SLM) technology. Nonlinear ultrasonic testing for early fatigue damage in SLM 316L stainless steel specimens was carried out. A new method for evaluation of early fatigue damage based on nonlinear ultrasonic testing was proposed. Empirical mode decomposition (EMD) was applied to the unsteady ultrasonic testing signal, and the signal was decomposed into multiple intrinsic mode functions (IMFs) that meet certain conditions; then, the specific IMF (ESI) containing the effective fatigue damage information was extracted. Lastly, fast Fourier transform (FFT) was applied to the specific IMF signal to obtain the required information to evaluate the damage in the measured part caused by fatigue. The results of nonlinear ultrasonic testing agreed well with transmission electron microscope experimental analysis and theoretical model of acoustic nonlinearity caused by dislocations. The change in nonlinear ultrasonic testing results reflected the generation and evolution of dislocation structure during the low-cycle fatigue regime of the SLM 316L stainless steel specimen and revealed the early fatigue damage mechanism of this metal part. Compared with the classical FFT method, the EMD-ESI-FFT method is more sensitive in identifying the early damage in SLM 316L stainless parts induced by fatigue loading, which is equivalent to improving the early fatigue damage identification and diagnosis ability and can better ensure the service safety of important metal parts.

Phase field modeling of precipitation in Zr-1%Nb-1%Sn alloy: a study of statistical properties and mechanical response

We develop the phase field model to simulate precipitation of secondary phase in ternary alloys with extra-small content of doping. This approach is applied to study β-phase precipitation in the model system of commercial alloy Zr-Nb-Sn at thermal treatment. An analysis of local rearrangement of doping and equilibrium vacancies during precipitation has shown that the dissolved Tin is mostly segregated around phase interface by trapping vacancies. Kinetics of precipitation, size and distribution of the precipitates, concentration of the species in precipitates and matrix are studied where it is revealed that Lifshits-Slyozov-Wagner distribution can be used to predicate statistical properties of precipitates. Mechanical response including the plastic deformations in precipitated solid is discussed. It is shown that yield strength change increases during precipitation. Yield and ultimate stresses are studied at different shear rates for the annealed alloy. A transition to plastic flow is described by means of dislocation structure evolution. Formation and growth of slip planes and dislocation loop-precipitate interaction governed by elastic moduli difference is analyzed. It is shown that emergence of dislocation loops around precipitates follows the Orowan mechanism.

Hydrogen-prompted heterogeneous development of dislocation structure in Ni

First documented in 1875, the deterioration of mechanical properties of hydrogen-containing metals is a longstanding yet unsolved problem in materials science. In this work, the evolution of dislocation structures in differently orientated grains (i.e., near [100], [110], and [111]) of the uncharged and hydrogen-charged (400 and 1200 ppm) polycrystalline Ni were systematically investigated by combining electron backscatter diffraction, focused ion beam and scanning transmission electron microscopy. By using site-specific characterization methods, for the first time, we discover that hydrogen-enhanced localized plasticity (HELP) is orientation-dependent, with the following sequence: [100] > [111] > [110]. Massive incompatibility between differently orientated grains, induced by the orientation dependence of HELP, contributes to the premature intergranular fracture of Ni, especially for the 400 ppm H-charged Ni. Our results suggest that optimizing orientation distribution is a potential approach for enhancing metals' resistance to hydrogen damage. The relative contribution of HELP and hydrogen-enhanced decohesion (HEDE) mechanisms in hydrogen embrittlement of Ni is also analyzed quantitatively for 400 and 1200 ppm H-charged samples. In the 400 ppm H-charged Ni, a strong synergistic interaction exists between HELP and HEDE mechanisms, and the HELP mechanism plays a critical role in premature fracture. By contrast, in the 1200 ppm H-charged Ni, the HELP effect on final failure is much less significant and HEDE is the dominant embrittlement mechanism.

[1 0 0] Dislocation core extension and decomposition in BCC bicrystal under biaxial loading

The evolution of grain boundary dislocation structure under biaxial tension–compression loading is simulated by the phase field crystal (PFC) method for small angle symmetric tilt grain boundary (STGB) of body center cubic (BCC) bi-crystal. Extension of the STGB dislocation core under the strain and the extended width of the core with the strain increasing are studied. In the elastic deformation process under the loading, the extension of the grain boundary dislocation core is accompanied by increasing the strain energy of the system. When the extension of the dislocation core reaches maximum critical width, the extended dislocation core occurs to decompose into three dislocation cores, in which two cores is in no-extended state, the Burgers vector direction of which is in the same direction of the original extended dislocation. The other dislocation core is of the extended type which Burgers vector direction is opposite to the Burgers vector direction of the original extended dislocation core. By establishing the macroscopic energy equation of the extended dislocation core system, it can be well revealed that the elastic strain energy is mainly stored in the extended dislocation defects and the complete lattice of the matrix. At the same time, it also reveals the function relationship between the energy of the extended dislocation core and the width of the extended dislocation during the biaxial loading process of the bi-crystal, as well as the critical condition of energy instability and the change of the energy barrier of the system for the dislocation extension core decomposition.

Size effect in nanoindentation: Taylor hardening or dislocation source-limited effect?

In the present work, a 3D discrete dislocation dynamics (DDD) method was used to explore the indentation size effect (ISE) in the nanoindentation of single crystals. The distribution of geometrically necessary dislocation (GND) underneath the indent obtained from the DDD simulations was compared with that in experiments. The results show that the GND density increases as the indentation depth increases, which is inconsistent with the Nix-Gao model that is usually used to interpret the ISE phenomenon. The spatial-temporal evolution of dislocation structures indicates that the available dislocation sources are limited at smaller indentation depths, thus requiring higher stress to accommodate the plastic deformation. The authors do not have permission to share data. • The indentation size effect was investigated by 3D discrete dislocation dynamics. • The patterns of GNDs are correlated with the dislocation motions on slip planes. • The GND density increases as the indentation depth increases. • The indentation size effect is caused by the limitation of dislocation sources.

Evolution of dislocation structure determined by neutron diffraction line profile analysis during tensile deformation in quenched and tempered martensitic steels

The role of the dislocation structure on the work-hardening behavior during the tensile deformation of quenched and tempered martensite was studied. The evolution of the dislocation structure during tensile deformation at room temperature in ultralow-carbon 18 mass%Ni martensitic steels under the conditions of as-quenched by sub-zero-treatment (SZ) and quenched-and-tempered at 573 and 773 K (T573 and T773, respectively) was monitored using in situ time-of-flight neutron diffraction combined with the convolutional multiple whole profile (CMWP) procedure. The changes in the dislocation parameters due to tempering and deformation obtained by the CMWP procedure were explained by the metallurgical phenomena of body-centered cubic iron. The elastic limit increased in the order SZ, T573, and T773, whereas the dislocation density decreased in the opposite order, indicating that the elastic limit is not always dependent on the total dislocation density of martensite. The dislocation density in SZ, which showed a high level of work hardening after yielding, hardly changed during tensile deformation, whereas that in T573 and T773 increased with tensile straining. The dislocation arrangement parameter that represents the interaction among the dislocations was high before deformation and decreased during deformation in materials SZ and T573, whereas the parameter was maintained at a low value during the entire deformation in material T773. Large and small values of the dislocation arrangement parameter indicate weak and strong interactions, respectively. The dislocation arrangement parameter is considered helpful for estimating the increment in strength of martensitic steels by dislocation strengthening, as the coefficient α in Taylor’s equation, for both the as-quenched and tempered conditions.

Macro-micro behaviors of Ti–22Al–26Nb alloy during warm tension

In this paper, the plastic flow rules and fracture evolution mechanisms of the nearly O + B2 Ti–22Al–26Nb alloy during warm tension have been studied within a wide temperature range of 20–800 °C. The macro-micro deformation behaviors of the material have been investigated by means of SEM, EBSD and TEM. It is found that the stress-strain curve at 400 °C exhibits serrated patterns, indicating the occurrence of dynamic strain aging at mild temperature. The overall yield strength decreases and the uniform elongation increases with the temperature in general. However, an unexpected peak of yield strength and also an unexpected valley of uniform elongation are exhibited at 600 °C. Additionally, the uniform elongation shows a positive correlation with strain rate at 600 °C. With the comprehensive analyses of fracture surface characteristics, the morphological variation of the constituent phases and the dislocation structure evolution, the causations of the above phenomena are further explained as follows. The B2 phase is mainly responsible for deformation coordination between O laths with the original {110}B2∥(001)O orientation barely changed, and act as a media for propagating localized slip deformation to O grains. The abnormal mechanical responses at 600 °C are mainly attributed to the change of fracture mode caused by the precipitation of fine acicular O grains, while the softening mechanism at 800 °C is mainly dominated by the dynamic globularization of the O phase.

Microstructure transition and mechanical properties of friction stir processed CoCrFeMnNi high entropy alloy fabricated by laser powder bed fusion

In this work, Friction stir processing (FSP) was conducted on the CoCrFeMnNi alloy fabricated via laser powder bed fusion (LPBF) to modify the surface microstructure, during which the evolution of grain structures, dislocation structures, and texture as well as the dynamic recrystallization (DRX) mechanism were investigated. The experimental results demonstrated that continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) occurred simultaneously to refine the average grain size from 15.4 μm to 4.8 μm, while the distribution of grain size and area became more uniform in the stir zone (SZ). Due to the inhomogeneous distribution of material flow and friction heat, the thermo-mechanically affected zone (TMAZ) exhibits microstructure gradients and the grain structures are nonuniform throughout the SZ. In addition, the relationship between the microstructure and mechanical properties of the FSP region was analyzed. The hardness and yield strength (YS) of the SZ (∼190 ± 3 HV and 501 ± 2 MPa, respectively) are lower than those of the LPBF material (∼229 ± 10 HV and 563 ± 3 MPa, respectively). On the other hand, the elongation increased from 23.8 ± 0.4% to 37.5 ± 0.3%, while the ultimate tensile strength (UTS) increased from 665 ± 5 MPa to 719 ± 3 MPa. A better combination of strength and ductility than the LPBF-built CoCrFeMnNi alloy was achieved in the SZ after FSP.

Dislocation Structure Evolution in Low-Stability States of Solid Solutions of Alloys Under Their Deformation

Dislocation Structure Evolution in Low-Stability States of Solid Solutions of Alloys Under Their Deformation