Dislocation Loops In Iron Research Articles

An FFT based approach to account for elastic interactions in OkMC: Application to dislocation loops in iron

Object kinetic Monte Carlo (OkMC) is a fundamental tool for modeling defect evolution in volumes and times far beyond atomistic models. The elastic interaction between defects is classically considered using a dipolar approximation but this approach is limited to simple cases and can be inaccurate for large and close interacting defects. In this work a novel framework is proposed to include exact elastic interactions between defects in OkMC valid for any type of defect and anisotropic media. In this method, the elastic interaction energy of a defect is computed by volume integration of its elastic strain multiplied by the stress created by all the other defects, being both fields obtained numerically using a FFT solver. The resulting interaction energies reproduce analytical elastic solutions and show the limited accuracy of dipole approaches for close and large defects.The OkMC framework proposed is used to simulate the evolution in space and time of self-interstitial atoms and dislocation loops in iron. It is found that including the anisotropy has a quantitative effect in the evolution of all the type of defects studied. Regarding dislocation loops, it is observed that using the exact interaction energy results in higher interactions than using the dipole approximation for close loops.

Collision cascades overlapping with self-interstitial defect clusters in Fe and W

Overlap of collision cascades with previously formed defect clusters become increasingly likely at radiation doses typical for materials in nuclear reactors. Using molecular dynamics, we systematically investigate the effects of different pre-existing self-interstitial clusters on the damage produced by an overlapping cascade in bcc iron and tungsten. We find that the number of new Frenkel pairs created in direct overlap with an interstitial cluster is reduced to essentially zero, when the size of the defect cluster is comparable to that of the disordered cascade volume. We develop an analytical model for this reduced defect production as a function of the spatial overlap between a cascade and a defect cluster of a given size. Furthermore, we discuss cascade-induced changes in the morphology of self-interstitial clusters, including transformations between and dislocation loops in iron and tungsten, and between C15 clusters and dislocation loops in iron. Our results provide crucial new cascade-overlap effects to be taken into account in multi-scale modelling of radiation damage in bcc metals.

<110> Dislocation Junction Formation via the Coalescence between Nanoscale 1/2<111> Prismatic Dislocation Loops in Iron

<110> Dislocation Junction Formation via the Coalescence between Nanoscale 1/2<111> Prismatic Dislocation Loops in Iron

Evolution of dislocation loops in iron under irradiation: The impact of carbon

The impact of carbon content on the evolution of dislocation loops in iron–carbon was studied by object kinetic Monte Carlo, explicitly introducing carbon atoms and their atomic features for the first time. We demonstrate that the saturated loop density strongly depends on carbon content and temperature, in good agreement with in situ irradiation microscopy studies. The physical processes responsible for the accumulation and long-range migration of the loops are rationalized with implications for nanostructural evolution in commercial steels upon low-dose-rate neutron irradiation.

The magnetic origin of anomalous high-temperature stability of dislocation loops in iron and iron-based alloys

The dominant occurrence of the b = a ( 0 0 1 ) prismatic edge dislocation loops in iron and iron-based alloys irradiated at temperatures approaching 500 °C is a striking anomaly distinguishing iron and its alloys from other bcc metals. It is surprising that the a ( 0 0 1 ) dislocation loops form at all, since there is an alternative b = a / 2 ( 1 1 1 ) dislocation loop configuration that, according to the conventional isotropic treatment of elasticity, has a lower self-energy. In this paper we highlight the magnetic aspect of the problem and note the fundamental link between the α – γ phase transition, the elastic anisotropy of iron, which is particularly significant at elevated temperatures, and thermal magnetic fluctuations.

Effect of theα−γPhase Transition on the Stability of Dislocation Loops in bcc Iron

Body-centered-cubic iron develops an elastic instability, driven by spin fluctuations, near the alpha-gamma phase transition temperature T(c) = 912 degrees C that is associated with the dramatic reduction of the shear stiffness constant c' (c(11)-c(12))/2 near T(c). This reduction of c' has a profound effect on the temperature dependence of the anisotropic elastic self-energies of dislocations in iron. It also affects the relative stability of the a[100] and a/2[111] prismatic edge dislocation loops formed during irradiation. The difference between the anisotropic elastic free energies provides the fundamental explanation for the observed dominant occurrence of the a[100], as opposed to the a/2[111], Burgers vector configurations of prismatic dislocation loops in iron and iron-based alloys at high temperatures.

Process of the One-Dimensional Motion of Small Interstitial-Type Dislocation Loops in Iron

Extensive simulations based on classical molecular dynamics have shown that small interstitial-type perfect dislocation loops in various metals and alloys have the structure of bundles of crowdions and a loop can easily makes the one-dimensional glide motion due to almost independent motion of crowdions in the loop. However, the experimental knowledge on the motion of loops is not enough. The present study dynamically examined the motion process of loops in pure iron under 1000 keV electron irradiation and thermal annealing by using transmission electron microscopy under which loops could move. Two types of loops were formed by irradiation. Loops of one type possessed the Burgers vector of 1/2<111> and the habit plane of {011}, and loops of the other type were <001> {001}. Loops of the former type made back-andforth glide motion and expansion towards the direction along their Burgers vectors when they were smaller than about a few-ten nanometers in diameter. This strongly suggests that these small 1/2<111> loops have the structure of the bundle of crowdions. Loops of the latter type only rarely moved less frequently when they were smaller than about the same size. When loops of two types grew larger than about 50 nm, the characteristics of the motion of loops changed drastically. Dislocation segments of each large loop made long-distance glide independently of their opposite segments, and the habit plane deviated from the original ones. This kind of motion means that selfinterstitial atoms at the central region of such large loops are no longer the crowdions.

Detection of the Spontaneous Change in the Burgers Vector of Perfect Dislocation Loops in Iron

Detection of the Spontaneous Change in the Burgers Vector of Perfect Dislocation Loops in Iron

Effects of chromium on the one-dimensional motion of interstitial-type dislocation loops in iron

The one-dimensional motion of interstitial-type dislocation loops in Fe–9Cr ferritic alloy is studied by in situ transmission electron microscopy. Loop motion is examined under 1000 kV high-energy electron irradiation at temperatures ranging from 110 to 680 K and under thermal annealing from 295 to 960 K without introduction of additional displacement subsequent to the irradiation at 110 K. The results are compared with those in pure bcc Fe, and the effects of Cr on the motion of loops are extracted. Under high-energy electron irradiation, the motion frequency of loops is only slightly reduced by Cr. In contrast, under annealing, the motion frequency of loops in Fe–9Cr is significantly reduced by Cr. The origins of such two-way effects of Cr on the motion frequency of loops are discussed.

Formation process of dislocation loops in iron under irradiations with low-energy helium, hydrogen ions or high-energy electrons

Formation processes of interstitial-type dislocation loops (I loops) in high-purity Fe under irradiations with 5 keV H + ions or 1000 keV electrons are examined by in situ transmission electron microscopy at temperatures below room temperature, and the results are compared with that obtained under He + ion irradiation. For the electron irradiation, conventional model of I-loop nucleation based on the assumption that di-interstitial atoms are stable nuclei of I loops is questioned. The volume density of I loops by H + ion irradiation is one-order of magnitude higher than that by electron irradiation, and several times lower than that by He + ion irradiation. The temperature dependence of the volume density of I loops by H + ion irradiation supports the idea that such enhancement of I-loop formation is due to trapping of self-interstitial atoms by gas atom-vacancy complexes.

Formation of dislocation loops in iron by self-ion irradiations at 40K

This letter reports on the first observation of dislocation loops formed in iron by bombardment with self-ions. The irradiations were performed with 50 and 100 keV self-ions to maximum ion doses of 8 x 10/sup 13/ ions cm/sup -2/ in-situ in the High-Voltage Electron Microscope (HVEM) - Tandem accelerator facility at Argonne National Labs. During the irradiation and subsequent examination, the specimen temperature was maintained at 40 K (below stage I recovery in iron) to minimize the thermal rearrangement-of the defects. The microscope was operated at 200 kV to prevent the creation of additional Frenkel pairs; the threshold voltage for Frenkel pair production in iron is about 30 kV. These observations revealed that at low temperatures (40K) dislocation loops can be formed from displacement cascades generated by self-ion irradiation, but only at ion doses exceeding those of previous studies. It is thought that these dislocation loops are formed in iron, not from the direct collapse of individual displacement cascades as has been found in several other metals after both room temperature and low temperature, < 40 K, irradiations, but from the spatial overlap of cascades, as first suggested by. The relationship between the defect density and the ion dose ismore » not understood. A model of single overlap of cascades would require the loop density to vary as the square of the ion dose. However, the measured defect density was found to be proportional to ion dose to the 3/2 power. Further experiments are planned to investigate this relationship.« less