Dislocation Loop Generation Differences between Thin Films and Bulk in EFDA Pure Iron under Self-Ion Irradiation at 20 MeV

In the present investigation, high-energy self-ion irradiation experiments (20 MeV Fe+4) were performed on two types of pure Fe samples to evaluate the formation of dislocation loops as a function of material volume. The choice of model material, namely EFDA pure Fe, was made to emulate experiments simulated with computational models that study defect evolution. The experimental conditions were an ion fluence of 4.25 and 8.5 × 1015 ions/cm2 and an irradiation temperature of 350 and 450 °C, respectively. First, the ions pass through the samples, which are thin films of less than 100 nm. With this procedure, the formation of the accumulated damage zone, which is the peak where the ions stop, and the injection of interstitials are prevented. As a result, the effect of two free surfaces on defect formation can be studied. In the second type of experiments, the same irradiations were performed on bulk samples to compare the creation of defects in the first 100 nm depth with the microstructure found in the whole thickness of the thin films. Apparent differences were found between the thin foil irradiation and the first 100 nm in bulk specimens in terms of dislocation loops, even with a similar primary knock-on atom (PKA) spectrum. In thin films, the most loops identified in all four experimental conditions were b ±a0<100>{200} type with sizes of hundreds of nm depending on the experimental conditions, similarly to bulk samples where practically no defects were detected. These important results would help validate computational simulations about the evolution of defects in alpha iron thin films irradiated with energetic ions at large doses, which would predict the dislocation nucleation and growth.

Generalized stacking fault energy surface mismatch and dislocation transformation

Even though the fundamental rules governing dislocation activities have been well established in the past century, we report a phenomenon, dislocation transformation, governed by the generalized-stacking-fault energy surface mismatch (GSF mismatch for short) between two co-existing phases. By carrying out ab-initio-informed microscopic phase-field simulations, we demonstrate that the GSF mismatch between a high symmetry matrix phase and a low symmetry precipitate phase can transform an array of identical full dislocations in the matrix into an array of two different types of full dislocations when they shear through the precipitates. The precipitates serve as a passive Shockley partial source, creating new Shockley partial dislocations that are neither the ones from the dissociation of the full dislocation. This phenomenon enriches our fundamental understanding of partial dislocation nucleation and dislocation-precipitate interactions, offering additional opportunities to tailor work-hardening and twinning processes in alloys strengthened by low-symmetry precipitate phases.

Relative mobility of screw versus edge dislocations controls the ductile-to-brittle transition in metals

Body-centered cubic metals including steels and refractory metals suffer from an abrupt ductile-to-brittle transition (DBT) at a critical temperature, hampering their performance and applications. Temperature-dependent dislocation mobility and dislocation nucleation have been proposed as the potential factors responsible for the DBT. However, the origin of this sudden switch from toughness to brittleness still remains a mystery. Here, we discover that the ratio of screw dislocation velocity to edge dislocation velocity is a controlling factor responsible for the DBT. A physical model was conceived to correlate the efficiency of Frank–Read dislocation source with the relative mobility of screw versus edge dislocations. A sufficiently high relative mobility is a prerequisite for the coordinated movement of screw and edge segments to sustain dislocation multiplication. Nanoindentation experiments found that DBT in chromium requires a critical mobility ratio of 0.7, above which the dislocation sources transition from disposable to regeneratable ones. The proposed model is also supported by the experimental results of iron, tungsten, and aluminum.

Atomistic processes of surface-diffusion-induced abnormal softening in nanoscale metallic crystals

Ultrahigh surface-to-volume ratio in nanoscale materials, could dramatically facilitate mass transport, leading to surface-mediated diffusion similar to Coble-type creep in polycrystalline materials. Unfortunately, the Coble creep is just a conceptual model, and the associated physical mechanisms of mass transport have never been revealed at atomic scale. Akin to the ambiguities in Coble creep, atomic surface diffusion in nanoscale crystals remains largely unclear, especially when mediating yielding and plastic flow. Here, by using in situ nanomechanical testing under high-resolution transmission electron microscope, we find that the diffusion-assisted dislocation nucleation induces the transition from a normal to an inverse Hall-Petch-like relation of the strength-size dependence and the surface-creep leads to the abnormal softening in flow stress with the reduction in size of nanoscale silver, contrary to the classical “alternating dislocation starvation” behavior in nanoscale platinum. This work provides insights into the atomic-scale mechanisms of diffusion-mediated deformation in nanoscale materials, and impact on the design for ultrasmall-sized nanomechanical devices.

Twisting of a Pristine α-Fe Nanowire: From Wild Dislocation Avalanches to Mild Local Amorphization

The torsion of pristine α-Fe nanowires was studied by molecular dynamics simulations. Torsion-induced plastic deformation in pristine nanowires is divided into two regimes. Under weak torsion, plastic deformation leads to dislocation nucleation and propagation. Twisting-induced dislocations are mainly 12<111> screw dislocations in a <112>-oriented nanowire. The nucleation and propagation of these dislocations were found to form avalanches which generate the emission of energy jerks. Their probability distribution function (PDF) showed power laws with mixing between different energy exponents. The mixing stemmed from simultaneous axial and radial dislocation movements. The power-law distribution indicated strongly correlated ‘wild’ dislocation dynamics. At the end of this regime, the dislocation pattern was frozen, and further twisting of the nanowire did not change the dislocation pattern. Instead, it induced local amorphization at the grip points at the ends of the sample. This “melting” generated highly dampened, mild avalanches. We compared the deformation mechanisms of twinned and pristine α-Fe nanowires under torsion.