Defect-free Channels Research Articles

Origin, parameters, and underlying deformation mechanisms of propagating deformation bands in irradiated 316L stainless steel

Lüders-type propagating deformation bands were observed in specimens of irradiated 316L stainless steel samples removed from Spallation Neutron Source target vessels after service. Mechanical testing with digital image correlation (DIC) and in-situ tensile testing with scanning electron microscopy electron backscatter diffraction showed that the observed Lüders-type behavior was not related to the known transformation-induced plasticity or twinning-induced plasticity behavior. Instead, the phenomenon occurs at small local strain values before a significant amount of martensite or deformation twins appear in the microstructure. Microstructural analysis and in-situ mechanical test results suggest Lüders-type band formation and propagation were related to the appearance and evolution of defect-free channels—analogous to slip bands. A modified Swift equation with a Ludwigson-like component was offered to rationalize the phenomenon and model the strain-softening processes at small strain values. The results indicate complex microstructural processes and deformation mechanisms were active at small strain values and underline the benefits of advanced mechanical test approaches such as DIC.

In-situ study of initiation and extension of nano-thick defect-free channels in irradiated nickel

Radiation defects-induced plastic flow localization is the origin of loss of ductility in irradiated metals. Defect-free channels (DFCs) are a typical form of strain localization that lead to crack initiation and premature failure. A comprehensive understanding of the DFC dynamics is key to managing radiation boosted property degradation. Despite great research efforts, a clear mechanism of DFC remains unknown. Here, our in-situ tests on irradiated Ni pillars provide a real-time observation of the dynamics of DFCs, including DFC initiation, extension and thickening. The merging and spreading of dislocation loops serve as an alternative mechanism of dislocation sources that emit massive dislocations and initiate nano-thick DFCs inside the grain. Nano-thick DFCs were formed through chopping up or sweeping away of loops by mobile dislocations. Annihilation of opposite loops and interactions between loops and vacancies accelerate DFC extension. Activation of multiple dislocation sources and dislocation cross-slips are the mechanisms for DFC thickening.

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.

Insights into the plastic behavior of irradiated Ni-based alloy through in-situ TEM experiments: Formation and evolution of defect-free channels

Foils of Ni-based alloys are deformed in situ in a TEM at 298 K after irradiation at the same temperature to 1014 ions/cm2 with 1 MeV Kr ions. The irradiation effects on the dislocation sources, dislocation motion, and on dislocation interactions with each other and the microstructure are assessed through in-situ dynamic observations. Dislocation motion through the irradiated material is observed to be jerky and discontinuous, as they get pinned by the irradiation-induced defects and the progression of the pile-ups proceeds in bursts of multiple dislocations. Active sources of dislocations originating from the walls of the channels are captured in-situ. They provide dislocations that glide in the channel and progressively clear it of the defects. Dislocation interactions with irradiation defects result in their increase in length by absorption of the defects; dislocations can become longer than the width of the channel, which can lead to congested channels and heavily jogged dislocations, resulting in a higher driving force for dislocation cross-slip out of the channels into the outside matrix, a phenomenon also captured in-situ. Similar observations were done in both Inconel 617 and Haynes 230, a similar Ni-based alloy with different composition but similar Stacking Fault Energy, indicating that the mechanisms at play are not strongly dependent on the chemical composition.

Role of irradiation induced defects in altering the micro-mechanical response of Zr domains during nano indentation: A molecular dynamics study

In the present study, the plastic flow behavior of Zr at 300 K in the presence of defects i.e. self-interstitials, vacancies and voids, which generally form during irradiation, has been investigated by molecular dynamics (MD) simulation of nano-indentation. For the purpose of comparison, a perfect defect-free domain was also probed under similar loading conditions. During nano-indentation various dislocation activities, i.e. nucleation, interactions, locking etc., involved during a typical loading cycle, were distinctly captured. This study has clearly demonstrated that the nature of defects present in the domain influences the dislocation activities during nano-indentation. Using Schmid factor calculation, it is shown that the nucleation of dislocation in all the domains, except the domain containing self-interstitials, occurs only along the directions of maximum resolved stress. In the case of domain containing self-interstitials, difference in the behavior was shown due to the presence of self-interstitials loops, which influenced the state of stress beneath the region of nano-indenter. The cyclic nano-indentation responses of all the domains were quantified in terms of the value of the nano-hardness in each loading stage. In all cases, except in the case of vacancies containing domain, the value of nano-hardness tends to saturate with the increasing number of cycles. In the case of domain containing vacancies the continuous drop in the nano-hardness was observed due to the formation of defect free channels resulting in continuous softening of domain. In the present study the role of vacancies over dislocation loops in the formation of these channels has been emphasized. The current MD simulation is the first such study which shows the feasibility of localized channel formation in the presence of large concentration of vacancies during cyclic nano-indentations.

Deformation twinning in irradiated ferritic/martensitic steels

Two different ferritic/martensitic steels were tensile tested to gain insight into the mechanisms of embrittlement induced by the combined effects of displacement damage and helium after proton/neutron irradiation in SINQ, the Swiss spallation neutron source. The irradiation conditions were in the range: 15.8–19.8 dpa (displacement per atom) with 1370–1750 appm He at 245–300 °C. All the samples fractured in brittle mode with intergranular or cleavage fracture surfaces when tested at room temperature (RT) or 300 °C. After tensile test, transmission electron microscopy (TEM) was employed to investigate the deformation microstructures. TEM-lamella samples were extracted directly below the intergranular fracture surfaces or cleavage surfaces by using the focused ion beam technique. Deformation twinning was observed in irradiated specimens at high irradiation dose. Only twins with {112} plane were observed in all of the samples. The average thickness of twins is about 40 nm. Twins initiated at the fracture surface, became gradually thinner with distance away from the fracture surface and finally stopped in the matrix. Novel features such as twin-precipitate interactions, twin-grain boundary and/or twin-lath boundary interactions were observed. Twinning bands were seen to be arrested by grain boundaries or large precipitates, but could penetrate martensitic lath boundaries. Unlike the case of defect free channels, small defect-clusters, dislocation loops and dense small helium bubbles were observed inside twins.

Comprehensive molecular dynamics simulations of the stacking fault tetrahedron interacting with a mixed dislocation at elevated temperature

The defect-free channels were frequently observed in irradiated materials, i.e. copper, as a result of the stacking fault tetrahedron (SFT) interactions with dislocations. However, the underlying mechanisms for this process are still unclear to date. To address them, a comprehensive study on the interactions between SFTs and mixed dislocations was performed using molecular dynamics simulations. In particular, eight interaction geometries were considered, in terms of the dislocation Burgers vector directions, dislocation gliding directions and intersection positions on SFT. Various interaction outcomes were revealed after dislocation detachment. (1) SFT is fully absorbed through the transformation into Lomer dislocations, and subsequently moves out of free surfaces along the dislocation. (2) SFT is partially absorbed with the absorbed SFT base moving out of free surfaces along the dislocation. (3) SFT is not absorbed but sheared with ledges left on the SFT faces. (4) SFT is unaffected by the mixed dislocation. The current simulations, especially the full SFT absorption, provide important insights into the forming mechanisms of defect-free channels in irradiated materials.

In situ measurements of a homogeneous to heterogeneous transition in the plastic response of ion-irradiated 〈1 1 1〉 Ni microspecimens

We report on the use of quantitative in situ microcompression experiments in a scanning electron microscope to systematically investigate the effect of self-ion irradiation damage on the full plastic response of 〈111〉 Ni. In addition to the well-known irradiation-induced increases in the yield and flow strengths with increasing dose, we measure substantial changes in plastic flow intermittency behavior, manifested as stress drops accompanying energy releases as the driven material transits critical states. At low irradiation doses, the magnitude of stress drops reduces relative to the unirradiated material and plastic slip proceeds on multiple slip systems, leading to quasi-homogeneous plastic flow. In contrast, highly irradiated specimens exhibit pronounced shear localization on parallel slip planes, which we ascribe to the onset of defect free channels normally seen in bulk irradiated materials. Our in situ testing system and approach allows for a quantitative study of the energy release and dynamics associated with defect free channel formation and subsequent localization. This study provides fundamental insight into the nature of interactions between mobile dislocations and irradiation-mediated and damage-dependent defect structures.

Towards further understanding of stacking fault tetrahedron absorption and defect-free channels – A molecular dynamics study

The mechanisms leading to stacking fault tetrahedron (SFT) absorption via interactions with dislocations, and subsequent formation of plastic flow localization in defect-free channels, which were frequently observed in irradiated materials in transmission electron microscopy experiments, are still unclear. To address this, screw dislocation interactions with SFTs in copper were investigated using molecular dynamics (MD) simulations. The interaction details reveal that a screw dislocation can fully absorb an SFT through the thermally activated transformation of Lomer–Cottrell lock into Lomer dislocations. After absorption, almost all the vacancies in the SFT are transferred into Lomer dislocations, which are able to move transversely under complex loading conditions. As a result, SFTs can be removed from the material (for SFTs near surface) or from defect-free channels (for SFTs in the bulk) with the aid of Lomer dislocations. In addition, it was shown that this absorption process is favorable only at high temperature, low applied shear stress and/or high SFT density. These results are in good agreement with in situ TEM observations. The current simulations and analyses provide useful insights into the formation mechanisms of defect-free channels in irradiated materials.

A complete absorption mechanism of stacking fault tetrahedron by screw dislocation in copper

It was frequently observed in experiments that stacking fault tetrahedron (SFT) can be completely absorbed by dislocation and generate defect-free channels in irradiated materials, but the mechanism is still open. In this paper, molecular dynamics (MD) was used to explore the dislocation mechanism of reaction between SFT and screw dislocation in copper. Our computational results reveal that, at high temperature, the SFT is completely absorbed by screw dislocation with the help of Lomer–Cottrell (LC) lock transforming into Lomer dislocation. This complete absorption mechanism is very helpful to understand the defect-free channels in irradiated materials.

A dislocation dynamics study of the transition from homogeneous to heterogeneous deformation in irradiated body-centered cubic iron

Low temperature irradiation of crystalline materials is known to result in hardening and loss of ductility, which limits the usefulness of candidate materials in harsh nuclear environments. In body-centered cubic (bcc) metals, this mechanical property degradation is caused by the interaction of in-grown dislocations with irradiation defects, particularly small dislocation loops resulting from the microstructural evolution of displacement cascades. In this paper, we perform dislocation dynamics simulations of bcc Fe containing various concentrations of dislocation loops produced by irradiation in an attempt to gain insight into the processes that lead to hardening and embrittlement. We find that a transition from homogenous to highly localized deformation occurs at a critical loop density. Above it, plastic flow proceeds heterogeneously, creating defect-free channels in its wake. We find that channel initiation and size are mediated by loop coalescence resulting from elastic interactions with moving dislocations.

In situ and tomographic observations of defect free channel formation in ion irradiated stainless steels

The effects of heavy-ion irradiation on dislocation processes in stainless steels were investigated using in situ irradiation and deformation in the transmission electron microscope as well as post mortem electron tomography to retrieve information on the three-dimensional dislocation state. Irradiation-induced defects were found to pose a strong collective barrier to dislocation motion, leading to dislocation pileups forming in grain interiors and at grain boundaries. The passage of multiple dislocations along the same slip plane removes the irradiation defects and leads to the eventual formation of a defect-free channel. These channels are composed of densely tangled dislocation networks which line the channel-matrix walls as well as residual dislocation debris in the channel interiors. The structures of the dislocation tangles were found to be similar to those encountered in later stages of deformation in unirradiated materials, with the exception that they developed earlier in the deformation process and were confined to the defect free channels. Also, defect free channels were found to widen through both source widening as well as complex cross-slip mechanisms.

Influence of Neutron Irradiation on Energy Accumulation and Dissipation during Plastic Flow and Hardening of Metallic Polycrystals

Abstract Deformation-calorimetric experiments have been performed on specimens of Armco iron, pure nickel, and two austenitic chromium-nickel alloys: 12Cr18Ni10Ti and 03Cr20Ni45Mo4Nb, each tested in both unirradiated and irradiated conditions. It has been shown that for neutron-irradiated 12Cr18Ni10Ti stainless steel and also for nickel and Armco iron, the latent energy (Es) versus σ curves shift towards lower values of Es, and these curves progressively lose their parabolic appearance that is typical for unirradiated metals and alloys. It has also been shown experimentally that “excess heat” is measured when stored energy associated with radiation-produced defect agglomerates is released during their annihilation in formation of defect-free channels. In effect more energy in released than the mechanical energy supplied externally to cause deformation of the specimen. When post-irradiation annealing is used to reduce the concentration of defect agglomerates, the excess energy phenomenon disappears.

Structural relaxation in metastable alloys

A mechanism of stress relaxation in metastable alloys is considered. It is demonstrated that structural transformations during a relaxation test lead to anomalously high stress relaxation. Two stages of structural relaxation are revealed: formation of defect-free channels and decomposition of a solid solution. A new method of program hardening of metastable alloys based on the structural relaxation processes is suggested.

Molecular dynamics study of the interactions between dislocation and imperfect stacking fault tetrahedron in Cu

The microstructure of irradiated face centered cubic alloys with low stacking fault energy is distinguished by the formation of a high number density of nanometer size stacking fault tetrahedra (SFT). A recent transmission electron microscopy investigation of high-energy proton irradiated copper [16] has shown that nearly 50% of the visible SFT population are not perfect SFTs, but rather consist of truncated SFT and/or groups of overlapping SFT. This paper presents the results of atomistic molecular dynamics simulations of the interaction between gliding dislocations, of either edge or screw character, and truncated SFT or overlapping SFT. The most common result of the edge dislocation interaction with a truncated SFT is defect shearing, ultimately leading to complete separation into two smaller defect clusters. Partial absorption of the truncated SFT is the most common result of the interaction with a screw dislocation, resulting in the formation of super-jog (or helical) segments as the defect is absorbed into the dislocation core. The resulting non-planar screw dislocation is self-pinned with reduced mobility and is re-emitted as a similar truncated SFT as the applied shear stress is increased. The re-emitted truncated SFT is often rotated and translated relative to the original position. These observations are consistent with the hypothesis that shearing (decreased defect cluster size) and dislocation dragging of the defect clusters by partial absorption into the dislocation core contributes to the formation of defect-free channels.

Welding-induced mechanical properties in austenitic stainless steels before and after neutron irradiation

The effects of neutron irradiation on the mechanical properties of welded joints made of austenitic stainless steels have been investigated. The materials are welded AISI 304 and AISI 347, so-called test weld materials, irradiated with neutrons at 573 K to doses of 0.3 and 1.0 dpa. In addition, an AISI 304 from a decommissioned pressurised water reactor, so-called in-service material, which had accumulated a maximum dose of 0.35 dpa at about 573 K, was investigated. The mechanical properties of heat-affected zones and base materials were analysed before and after irradiation. Tensile parameters were determined at room temperature and at 573 K, for all materials and irradiation conditions. In the test weld materials it is found that radiation hardening is lower and loss of ductility is higher in the heat-affected zone than in the base material. In the in-service material radiation hardening is about the same in heat-affected zone and base material. After irradiation, deformation takes place by stacking faults and twins, at both room temperature and high temperature, contrary to unirradiated materials, where deformation takes place by twinning at room temperature and by dislocation cells at high temperature. No defect free channels are observed.

Atomic-scale study of dislocation–stacking fault tetrahedron interactions. Part I: mechanisms

Stacking fault tetrahedra (SFTs) are formed under irradiation in fcc metals and alloys. The high number density of SFTs observed suggests that they should contribute to radiation-induced hardening and, therefore, be taken into account when estimating mechanical property changes of irradiated materials. The key issue in this is to describe the interaction between a moving dislocation and an individual SFT, which is distinguished by a small physical size of the order of ∼1–10 nm. We have performed atomistic simulations of edge and screw dislocations interacting with SFTs of different sizes at different temperatures and strain rates. Five possible interaction outcomes have been identified, involving either partial absorption, or shearing or restoration of SFTs. The mechanisms that give rise to these processes are described and their dependence on interaction parameters, such as SFT size, dislocation–SFT geometry, temperature and stress/strain rate are determined. Mechanisms that help to explain the formation of defect-free channels cleared by gliding dislocations, as observed experimentally, are also discussed. Hardening due to the various mechanisms and their dependence on loading conditions will be presented in a following paper (Part II).

Mechanism of recovery of the mechanical properties of neutron-irradiated metals in the course of thermocycling

The mechanism of recovery of the strength and strain characteristics of neutron-irradiated metals and alloys (up to their initial values observed prior to irradiation) during periodic quenching in the temperature range below the irradiation temperature is considered. It is assumed that the removal of radiation-induced defects (interstitial and vacancy loops) from radiation-hardened metals is associated with the formation of defect-free channels along the slip planes (the phenomenon of dislocation channeling) under the thermal stresses arising in each cycle of quenching. The relationships describing both the kinetics of the decrease in the yield strength σY and in the ultimate strength σU and the kinetics of the increase in the uniform strain eU of a preliminarily irradiated material with increasing number of quenching cycles are derived using the equations of dislocation kinetics. The theoretical results obtained are compared with experimental data on the kinetics of recovery of the mechanical properties of neutron-irradiated samples (the austenitic FeNiCr and ferritic FeCrMo structural steels and the titanium alloy TiAlZr) in the course of periodic quenching.

Dynamic observations and atomistic simulations of dislocation–defect interactions in rapidly quenched copper and gold

Dislocation interactions with quenched-in defects, which are either partial or complete stacking-fault tetrahedra, in copper and gold have been studied by combining static and dynamic transmission electron microscope studies with molecular dynamics computer simulations. The interaction of a dislocation with a stacking-fault tetrahedron can result in the tetrahedron being sheared into two defects, converted to another defect type, or annihilated. These observations provide insight into defect interaction and annihilation mechanisms that are responsible for creating the defect-free channels in deformed irradiated material.

Atomic-scale details of dislocation–stacking fault tetrahedra interaction

Stacking fault tetrahedra (SFTs) are formed during irradiation of f.c.c. metals and alloys with low stacking fault energy. The high number density of SFTs observed suggests that they should contribute to radiation-induced hardening and, therefore, be taken into account when estimating mechanical property changes of irradiated materials. The key issue is to describe the interaction between a moving dislocation and an individual SFT, which is characterized by a small physical scale of about 100 nm. In this paper we present results of an atomistic simulation of edge and screw dislocations interacting with small SFTs at different temperatures and strain rates and present mechanisms which can explain the formation of defect-free channels observed experimentally.