Dislocation-defect Interaction Research Articles

Interactions between irradiation-induced defects and dislocations in concentrated solid solution alloys

Dislocation is a common structural feature in metallic alloys. Under irradiation, interactions between dislocations and irradiation-induced defects play a crucial role in governing various microstructural changes and the irradiation response of materials. Though such interaction has been intensively studied in elemental metals, little is known about it in concentrated solid solution alloys (CSAs). In this work, we study defect-dislocation interactions in NiFe CSAs through both elastic theory and atomistic simulations. Unexpectedly, we find that dislocations interact more strongly with defects in CSAs than in pure metals due to their higher elastic moduli, which is adverse to their irradiation performance based on conventional understanding. By comparing random models (Rand) and average atom models (Ave) for NiFe, we show that the random occupancy of atoms in the Rand model compared to the Ave model can reduce the interaction between dislocations and defects, thereby diminishing the dislocation bias factor. We further perform defect kinetic simulations and find that the complex energy landscape for defect motion in NiFe tends to restrict the movement of defects and weaken the ability of dislocations to absorb interstitials. Therefore, the chemical disorder in CSAs can partly reduce the defect-dislocation interaction strength thermodynamically and, more importantly, limit the movement of defects kinetically, both of which suppress the absorption ability of dislocations toward defects. Our integrated efforts shed light on the mechanisms governing defect-dislocation interactions in CSAs, providing insight into the reasons for the superior irradiation resistance of CSAs and offering possible directions to tune their irradiation performance.

Calculation of dislocation binding to helium-vacancy defects in tungsten using hybrid ab initio-machine learning methods

Calculations of dislocation-defect interactions are essential to model metallic strength, but the required system sizes are at or beyond ab initio limits. Current estimates thus have extrapolation or finite size errors that are very challenging to quantify. Hybrid methods offer a solution, embedding small ab initio simulations in an empirical medium. However, current implementations can only match mild elastic deformations at the ab initio boundary. We describe a robust method to employ linear-in-descriptor machine learning potentials as a highly flexible embedding medium, precisely matching dislocation migration pathways whilst keeping at least the elastic properties constant. This advanced coupling allows dislocations to cross the ab initio boundary in fully three dimensional defect geometries. Investigating helium and vacancy segregation to edge and screw dislocations in tungsten, we find long-range relaxations qualitatively change impurity-induced core reconstructions compared to those in short periodic supercells, even when multiple helium atoms are present. We also show that helium-vacancy complexes, considered to be the dominant configuration at low temperatures, have only a very weak binding to screw dislocations. These results are discussed in the context of recent experimental and theoretical studies. More generally, our approach opens a vast range of mechanisms to ab initio investigation and provides new reference data to both validate and improve interatomic potentials.

Dislocations Stabilized by Point Defects Increase Brittleness in PbTe

AbstractDislocations and the residual strain they produce are instrumental for the high thermoelectric figure of merit, zT ≈ 2, in lead chalcogenides. However, these materials tend to be brittle, barring them from practical green energy and deep space applications. Nonetheless, the bulk of thermoelectrics research focuses on increasing zT without considering mechanical performance. Optimized thermoelectric materials always involve high point defect concentrations for doping and solid solution alloying. Brittle materials show limited plasticity (dislocation motion), yet clear links between crystallographic defects and embrittlement are hitherto unestablished in PbTe. This study identifies connections between dislocations, point defects, and the brittleness (correlated with Vickers hardness) in single crystal and polycrystalline PbTe with various n‐ and p‐type dopants. Speed of sound measurements show a lack of electronic bond stiffening in p‐type PbTe, contrary to the previous speculation. Instead, varied routes of point defect–dislocation interaction restrict dislocation motion and drive embrittlement: dopants with low doping efficiency cause high defect concentrations, interstitial n‐type dopants (Ag and Cu) create highly strained obstacles to dislocation motion, and highly mobile dopants can distribute inhomogeneously or segregate to dislocations. These results illustrate the consequences of excessive defect engineering and the necessity to consider both mechanical and thermoelectric performance when researching thermoelectric materials for practical applications.

Coupling crystal plasticity and stochastic cluster dynamics models of irradiation damage in tungsten

Irradiation damage is known to alter a material’s microstructure due to the accumulation of high densities of defect clusters. Such irradiated microstructures change the mechanical response of the material due to dislocation-defect interactions, which leads to a host of issues such as hardening, swelling, irradiation creep, embrittlement, etc. Traditionally, the effect of irradiation on the mechanical response of materials is evaluated via tensile tests of pre-irradiated specimens at different doses and temperatures. From a modeling perspective, methods exist that simulate irradiation and deformation as separate processes, with the former based on kinetic transport theory and the latter on crystal plasticity (CP). Generally, these are connected by a state variable, usually in the form of a characteristic length scale, that represents the defect concentration and strength and its effect on dislocation-mediated slip. However, cases where deformation takes place during irradiation are also important despite being less common. In this paper, we develop a coupled CP and stochastic cluster dynamics (SCD) approach capable of treating all instances of irradiation/deformation in irradiated materials. We apply the methodology to tungsten crystals due to its importance as a high-temperature candidate structural material and to its extensive defect and mechanical data base. SCD evolves the defect microstructure stochastically, providing a statistically-averaged defect cluster spacing parameter that informs CP calculations of the material’s mechanical deformation. The coupling is bi-directional in the sense that the SCD method updates the obstacle density and furnishes a resistance stress to the CP model, while CP feeds updated dislocation densities that act as defect sinks in the SCD calculation cycles. The coupling can be done sequentially, as in standard tensile tests of pre-irradiated materials, or concurrently, as in in situ straining tests during irradiation. We carry out simulations of realistic irradiation/deformation scenarios and highlight the differences between the present method and past works considering similar situations.

Analysis of damping spectra of silver-plated brass from a Weltklang saxophone manufactured in 1969

The damping of wall vibrations in material of musical instruments influences its sound, but the damping capacity of the materials is rarely investigated. One of the most used material for musical wind instruments is α-brass. Therefore, samples from a saxophone manufactured of silver plated Cu28%Zn with small Pb content were investigated by scanning electron microscopy, energy dispersive spectroscopy, X-ray fluorescence, thermogravimetric analyses coupled with Fourier transform infrared spectroscopy, and mechanical spectroscopy. Pb particles are located at grain boundaries. Damping spectra show three relaxation peaks which can be attributed to Zener and/or solvent grain boundary relaxation, a peak due to dislocation defect interactions and a solute grain boundary peak. The peak temperatures of these peaks are higher in silver plated brass than for brass without silver cover due to AgCO3 particles. The silver plating process led to the formation of Zn and Cu carbonates. Thermal decomposition of these carbonates to ZnO and CuO together with lead particles leads to blocking of the solute grain boundary peak.

Mechanical properties of neutron-irradiated single crystal tungsten W(100) studied by indentation and FEM modelling

The aim of this work is to investigate the effect of high-temperature neutron irradiation on the mechanical properties of single-crystal tungsten W(100) studied by mechanical testing and modelling. Neutron irradiation up to 0.12 displacements per atom was performed at 600, 800, 900 and 1200 °C in the Belgium material test reactor BR2 at SCK•CEN in Mol. The mechanical properties of the irradiated tungsten were assessed by impulse excitation and depth-sensing indentation measurements. The values of Young's modulus, shear modulus and Poisson ratio are not affected by the irradiation. The mean hardness of the irradiated tungsten increases by 48, 45, 46, and 34% after irradiation at 600, 800, 900 and 1200 °C, respectively. The results obtained by the instrumented hardness tests were used as input for the finite element method model, applied to deduce the contributions coming from the neutron irradiation defects to the plastic deformation upon the indentation process, which include the impediment of gliding dislocations by sessile defects and modification of surface morphology due to the inhibited strain hardening ability through the dislocation-defect interaction. The validation of the fitted constitutive laws was realized by the crystal plasticity finite element method (CP-FEM) model which was applied to simulate the indentation load-depth curves, with certain assumptions on the irradiation-induced microstructure, and to calculate the distribution of stress under the indenter to investigate the extension of the plastic zone in the process of the indentation.

High-strength and tunable plasticity in sputtered Al–Cr alloys with multistage phase transformations

To raise mechanical strength of metallic materials, methods such as grain refinement and amorphization are usually utilized but sacrifice plasticity. Strength and plasticity are largely dictated by dislocation-defect interactions in crystalline materials and by shear banding in metallic glasses, depending significantly on the length scales of microstructural features. Here we report on the evolution of microstructures and multistage phase transformations from micro/nanocrystalline to an entirely amorphous structure, realized by tailoring the composition of Cr in co-sputtered Al100-xCrx (x = 0–25 at%) alloys. The associated Cr segregation caused the formations of different dual-phase nanocomposites. In-situ micromechanical experiments revealed that the flow stress of Al–Cr alloys can reach 2.4 GPa and the deformation behaviors varied drastically with Cr composition. This study established the mechanistic connection between the Cr composition-dependent evolution of microstructure and ultrahigh strength as well as plasticity, and revealed the benefits of building nanocomposites through a multistage phase transformation to improve the plasticity of nanocrystalline and amorphous materials.

Screw dislocation core structure in the paramagnetic state of bcc iron from first-principles calculations

Iron-based alloys are widely used as structural components in engineering applications. This calls for a fundamental understanding of their mechanical properties, including those of pure iron. Under operational temperatures the mechanical and magnetic properties will differ from those of ferromagnetic body-centered-cubic iron at 0 K. In this theoretical work we study the effect of disordered magnetism on the screw dislocation core structure and compare with results for the ordered ferromagnetic case. Dislocation cores control some local properties such as the choice of glide plane and the associated dislocation mobility. Changes in the magnetic state can lead to modifications in the structure of the core and affect dislocation mobility. In particular, we focus on the core properties of the $\frac{1}{2}\ensuremath{\langle}111\ensuremath{\rangle}$ screw dislocation in the paramagnetic state. Using the noncollinear disordered local moment approximation to address paramagnetism, we perform structural relaxations within density functional theory. We obtain the dislocation core structure for the easy and hard cores in the paramagnetic state, and compare them with their ferromagnetic counterparts. By averaging the energy of several disordered magnetic configurations, we obtain an energy difference between the easy- and hard-core configurations, with a lower, but statistically close, value than the one reported for the ferromagnetic case. The magnetic moment and atomic volume at the dislocation core differ between paramagnetic and ferromagnetic states, with possible consequences on the temperature dependence of defect-dislocation interactions.

Discrete stochastic model of point defect-dislocation interaction for simulating dislocation climb

Dislocation climb is an important high temperature process in metals plasticity, responsible for the phenomena such as creep, swelling, or hardening. Climb is defined by the ability of dislocations to leave their original glide plane by interacting with point defects. As such, dislocation climb is controlled by point defect diffusion/absorption/emission, all of which involve thermal activation. The existing thermodynamically consistent models for climb are generally formulated in a continuum framework, through the definition of effective defect fluxes and climb propensities in response to thermodynamic driving forces. However, the point-wise discrete nature of vacancies (and/or self-interstitials) confers a highly discrete nature to the climb dynamics, which is also strongly affected by elastic forces. The combination of discreteness, thermal activation, and elasticity makes this process too challenging for direct atomistic methods such as molecular dynamics. Here we develop a kinetic Monte Carlo model that captures vacancy generation and transport kinetics acting in conjuction with the evolving elastic fields provided by discrete dislocation dynamics simulations. The two models are coupled via the applied stresses and stress gradients generated by dislocation structures at vacancy locations. Our simulations reveal two surprising results. First that climb is dominated by vacancy emission even when the background vacancy concentration is much higher than the equilibrium one. And, second, that climb velocities might be much faster than otherwise believed when one uses the classical theories of climb. These effects are due to the locality of vacancy-dislocation processes, which are not captured in classical treatments that assume smooth vacancy fluxes and homogeneous concentrations. We apply the method to study elementary climb processes in body-centered cubic iron and furnish climb mobility functions to be used in parametric dislocation dynamics and/or crystal plasticity simulations. We apply the technique to study non-conservative plastic bypass of spherical precipitates by edge dislocations and point out the differences between our discrete approach and existing continuum formulations.

Fundamental Mechanisms for Irradiation-Hardening and Embrittlement: A Review

It has long been recognized that exposure to irradiation environments could dramatically degrade the mechanical properties of nuclear structural materials, i.e., irradiation-hardening and embrittlement. With the development of numerical simulation capability and advanced experimental equipment, the mysterious veil covering the fundamental mechanisms of irradiation-hardening and embrittlement has been gradually unveiled in recent years. This review intends to offer an overview of the fundamental mechanisms in this field at moderate irradiation conditions. After a general introduction of the phenomena of irradiation-hardening and embrittlement, the formation of irradiation-induced defects is discussed, covering the influence of both irradiation conditions and material properties. Then, the dislocation-defect interaction is addressed, which summarizes the interaction process and strength for various defect types and testing conditions. Moreover, the evolution mechanisms of defects and dislocations are focused on, involving the annihilation of irradiation defects, formation of defect-free channels, and generation of microvoids and cracks. Finally, this review closes with the current comprehension of irradiation-hardening and embrittlement, and aims to help design next-generation irradiation-resistant materials.

Atomistic simulation of defect-dislocation interactions in concentrated solid-solution alloys

The interaction between point defects and dislocations plays a crucial role in governing material properties and microstructural evolutions under external stimuli, such as mechanical deformation and irradiation. Here, we present an atomistic study of the interactions between point defects and dislocations in concentrated solid-solution alloys (CSAs). Using molecular statics and kinetic Monte Carlo methods, we demonstrate that the strain energy and stress field distribution induced by a dislocation in CSAs are highly inhomogeneous along the dislocation line, which leads to heterogeneity of defect-dislocation interactions. Specifically, the interactions are spatially different and screened by the random arrangement of different elemental species. Such localization of defect-dislocation interaction indicates that the ``dislocation-bias'' mechanism that is a driving force for radiation-induced void swelling can be suppressed in concentrated alloys.

Atomistic simulation of the obstacle strengths of radiation-induced defects in an Fe–Ni–Cr austenitic stainless steel

A molecular dynamics method is employed to simulate the interaction of an edge dislocation with a combined solute cluster-dislocation loop defect, as well as independent faulted interstitial dislocation loops and solute clusters in an Fe–12Ni–20Cr at a temperature of 300 K. The examined defects are typical radiation-induced defects in austenitic stainless steels used as core structural components in nuclear reactors, while the selected composition matches with commercial grade 304L alloy. The dislocation-defect interaction is examined, and the peak shear stress required to overcome the obstacle array, and its dependence on obstacle size, solute concentration, and orientation is determined. The peak shear stress is then converted to an effective obstacle strength (α). Results show that dislocation loops are stronger obstacles than solute clusters, and their strength is heavily dependent on orientation, while the strength of solute clusters is largely dependent on the solute concentration. The total strength of the combined solute cluster-dislocation loop defect is largely contributed by the dislocation loop, though a fraction of the cluster strength is added as well.

The Formation of Microcrystal in Helium Ion Irradiated Aluminum Alloy

A microstructure variation in Al-1060 alloy after helium ion irradiation was revealed by a transmission electron microscope (TEM). The result shows that ion irradiation produced dislocations, dislocation loops, cavities and microcrystals in the irradiated layer. Dislocation-defect interactions were portrayed, especially the pinning effect of a dislocation loop and cavity on moving dislocation. Irradiation-induced stress was recognized as the main factor which impacted on the interaction of defect. Based on the dislocation inhibited with irradiation defects, the mechanism of microcrystal formation was proposed.

Integrated Experimental-Modelling Strategy to Understand Dislocation–Defect Interactions During Hot Working of Face-Centred Cubic Alloys

The interactions of moving dislocations with various point, line and planar defects during hot working of polycrystalline face-centred cubic alloys are studied. Experimental parameters from three austenitic steels and a Ni-base superalloy are integrated with a dislocation model within a multi-scale framework. The combination of these inputs is used to explain the microstructure evolution and flow behaviour through atomic-scale phenomena and their manifestation at higher length scales.

Thermal activation of dislocations in large scale obstacle bypass

Dislocation dynamics simulations have been used extensively to predict hardening caused by dislocation-obstacle interactions, including irradiation defect hardening in the athermal case. Incorporating the role of thermal energy on these interactions is possible with a framework provided by harmonic transition state theory (HTST) enabling direct access to thermally activated reaction rates using the Arrhenius equation, including rates of dislocation-obstacle bypass processes. Moving beyond unit dislocation-defect reactions to a representative environment containing a large number of defects requires coarse-graining the activation energy barriers of a population of obstacles into an effective energy barrier that accurately represents the large scale collective process. The work presented here investigates the relationship between unit dislocation-defect bypass processes and the distribution of activation energy barriers calculated for ensemble bypass processes. A significant difference between these cases is observed, which is attributed to the inherent cooperative nature of dislocation bypass processes. In addition to the dislocation-defect interaction, the morphology of the dislocation segments pinned to the defects play an important role on the activation energies for bypass. A phenomenological model for activation energy stress dependence is shown to describe well the effect of a distribution of activation energies, and a probabilistic activation energy model incorporating the stress distribution in a material is presented.

Scale transition using dislocation dynamics and the nudged elastic band method

Microstructural features such as precipitates or irradiation-induced defects impede dislocation motion and directly influence macroscopic mechanical properties such as yield point and ductility. Dislocation-defect interactions involve both atomic scale and long range elastic interactions. Thermally assisted dislocation bypass of obstacles occurs when thermal fluctuations and driving stresses contribute sufficient energy to overcome the energy barrier. The Nudged Elastic Band (NEB) method is typically used in the context of atomistic simulations to quantify the activation barriers for a given reaction. In this work, the NEB method is generalized to coarse-grain continuum representations of evolving microstructure states beyond the discrete particle descriptions of first principles and atomistics. This method enables the calculation of activation energies for a 1/2[111](11¯0) glide dislocation bypassing a [001] self-interstitial atom loop of size in the range of 4–10 nm with a spacing larger than 150 nm in α-iron for a range of applied stresses and interaction geometries. Further, the study is complemented by a comparison between atomistic and continuum based prediction of barriers.

Does irradiation enhance or inhibit strain bursts at the submicron scale?

Irradiation-induced defects have been experimentally observed either to inhibit or promote strain bursts and lead to plastic flow localization. Through three dimensional discrete dislocation dynamics simulations, this work unravels the mystery of how and why irradiation-induced defects enhance or inhibit strain bursts in submicron BCC iron and FCC Cu single crystals. It is shown that smaller strain burst amplitudes in irradiated nano- and micro-pillars are obtained under stress control conditions. However, under strain control conditions, bursts are found not to be sensitive to irradiation, despite the arresting effect of radiation defects. This feature is a result of rapid stress relaxation truncating the strain burst, compared with the influence of irradiation-induced defects. In heavily irradiated materials, dislocation avalanches and strain bursts are shown to be promoted. The influence of dislocation-defect interaction mechanisms, cross slip, irradiation dose, as well as loading mode on strain bursts is systematically investigated, and the results compared with experimental observations.

Modelling irradiation-induced softening in BCC iron by crystal plasticity approach

Crystal plasticity model (CPM) for BCC iron to account for radiation-induced strain softening is proposed. CPM is based on the plastically-driven and thermally-activated removal of dislocation loops. Atomistic simulations are applied to parameterize dislocation-defect interactions. Combining experimental microstructures, defect-hardening/absorption rules from atomistic simulations, and CPM fitted to properties of non-irradiated iron, the model achieves a good agreement with experimental data regarding radiation-induced strain softening and flow stress increase under neutron irradiation.

Mechanical properties of irradiated multi-phase polycrystalline BCC materials

Structure materials under severe irradiations in nuclear environments are known to degrade because of irradiation hardening and loss of ductility, resulting from irradiation-induced defects such as vacancies, interstitials and dislocation loops, etc. In this paper, we develop an elastic–viscoplastic model for irradiated multi-phase polycrystalline BCC materials in which the mechanical behaviors of individual grains and polycrystalline aggregates are both explored. At the microscopic grain scale, we use the internal variable model and propose a new tensorial damage descriptor to represent the geometry character of the defect loop, which facilitates the analysis of the defect loop evolutions and dislocation-defect interactions. At the macroscopic polycrystal scale, the self-consistent scheme is extended to consider the multiphase problem and used to bridge the individual grain behavior to polycrystal properties. Based on the proposed model, we found that the work-hardening coefficient decreases with the increase of irradiation-induced defect loops, and the orientation/loading dependence of mechanical properties is mainly attributed to the different Schmid factors. At the polycrystalline scale, numerical results for pure Fe match well with the irradiation experiment data. The model is further extended to predict the hardening effect of dispersoids in oxide-dispersed strengthened steels by the considering the Orowan bowing. The influences of grain size and irradiation are found to compete to dominate the strengthening behaviors of materials. Comparison of numerical modeling results (solid lines) with experimental results (square lines) for the polycrystalline BCC iron. We develop an elastic–viscoplastic model for irradiated multiphase polycrystalline BCC materials, based on a dislocation-based model at the grain scale, and the self-consistent scheme at the macroscopic scale. This model can be easily applied to predict the hardening behaviors of irradiated multi-phase polycrystalline materials.

Interaction between mobile dislocations and perfect dislocation loops in Fe-Ni-Cr austenitic alloy systems

The classical molecular dynamics method is employed to simulate the interaction of screw and edge dislocations with interstitial perfect dislocation loops (of 2 and 5 nm in diameter) in the austenitic model alloy Fe70Ni10Cr20 at temperatures of T = 300–900 K. Perfect loops can be created from Frank loops during the plastic deformation of irradiated austenitic steels applied in nuclear reactors. As a result, the dislocation-defect interaction mechanisms are established and classified. The loop absorption mechanisms, which are related to the formation of free channels capable of enhancing radiation-induced steel embrittlement, are revealed. The effectivenesses of loop absorption observed during their interaction with screw and edge dislocations, as well as unpinning stresses required for a dislocation to overcome the defect acting as an obstacle, are compared versus the material temperature, defect size, and interaction geometry.