Experimental observations during in situ tensile tests experiments have shown that a tiny amount of sulfur as low as 5 weight ppm induces a strong reduction of dislocation velocity when they interact with diffusing oxygen, amplifying the dynamic strain aging associated with oxygen in zirconium alloys above 300 °C. Using ab initio calculations, we study the interaction of sulfur and oxygen solute atoms with 〈a〉 dislocations in hexagonal close-packed zirconium. We show that the impact of sulfur on dislocation mobility can be rationalized by a strong short-range attraction of sulfur to the dislocation core, with attractive insertion sites existing in the stacking faults created through dissociation of 〈a〉 dislocations in two partials lying either in prismatic or pyramidal planes. Ab initio calculations further show that the energy barriers, which need to be overcome by sulfur atoms to reach these attractive sites, is lower than the activation energies for sulfur bulk diffusion. With sulfur diffusing faster than oxygen in zirconium, kinetics allow for sulfur segregation at temperatures where oxygen form enriched atmospheres around dislocations. This sulfur segregation in the dislocation core, which comes on top of the elastic attraction of sulfur atoms in the dislocation strain field, explains the enhancement by sulfur of the dynamic strain aging caused by oxygen in zirconium.

Refractory concentrated complex alloys (RCCAs) offer interesting mechanical properties with aprolonged high yield stress at high temperature. In NbTiZr alloys a previous study revealed a long athermal stress plateau from 400◦ C to 600 ◦ C [1]. Here, we complement this study by investigating the dislocation dynamics by in-situ tensile straining tests in a TEM from -170◦ C up to 700 ◦ C. Up to 300◦ C, dislocation motion is jerky and dislocations appear highly pinned due to the formation of superjog, dipoles and loops by cross-slip. Edge segments appear also heavily pinned but are in general more mobile than the screw segments. In the athermal plateau, the alloy presents dynamic strain aging presumably due to interstitial impurities, a result attested by macroscopic experiments showing negative strain rate sensitivity in that temperature range.

The softening of materials subjected to irradiation in reactors is known for a long time but still difficult to interpret. One reason for such a situation is that post-irradiation microstructural observations mainly show different types of irradiation defects which are potential sources of hardening, not softening. Irradiation softening must accordingly be studied in situ, as a function of temperature, irradiation energy and flux. Simple sources of irradiating particles can be used, such as ions or electrons, but in situ experiments remain difficult to perform, and to combine with clear microstructural observations. For those reasons, and in order to measure irradiation effects at the scale of individual dislocations, a simple procedure has been used: straining magnesium micro-samples in a transmission electron microscope working between 80 kV (close to the irradiation threshold of magnesium) and 180 kV. The in situ observations reveal a huge effect of the electron beam corresponding to a 10,000 times enhancement of the velocity of screw dislocations in the prismatic planes under a 160 kV electron beam, and the dependence of this enhancement factor with the accelerating voltage. These results are interpreted in terms of something equivalent to a temperature increase of 100 K or more, but clearly different from a true heating process.

Sulfur addition to zirconium alloys is known to have a tremendous strengthening effect, with only a few ppm leading to a strong hardening and a noticeable decrease of the creep rate above 300°C. Although previous works have shown an interdependent effect of oxygen and sulfur, with sulfur amplifying the strengthening of oxygen in the domain of dynamic strain aging, the physical mechanism behind this impact of sulfur addition on zirconium mechanical properties is still unknown. Using in situ straining experiments in a transmission electron microscope, we study how sulfur and oxygen modify the glide motion of dislocations for temperatures between 200°C and 550°C in zirconium samples with an oxygen concentration between 80 and 1300 weight ppm and a sulfur concentration up to 25 wppm. These experiments show that the velocity of gliding dislocations is reduced by the dragging of their enriched oxygen atmosphere, when the temperature is high enough for oxygen to diffuse and segregate on dislocations. Once the motion of dislocations is slowed down, segregation of sulfur on dislocations becomes also possible despite the low sulfur content, leading to a further reduction of dislocation velocity. This synergistic effect of sulfur and oxygen segregation on dislocation explains the enhancement by sulfur of oxygen strengthening in zirconium alloys.

Marc Legros, Frédéric Mompiou and Daniel Caillard discuss the different aspects that influence the reproducibility and reliability of characterizations performed using in situ mechanical tests in transmission electron microscopes.

The kinetics of elastically interacting screw dislocations has been studied in pure iron strained in situ at low temperature. Annihilating and expanding screw dipoles yield macroscopic activation areas which are substantially smaller than those deduced from conventional mechanical stress, but consistent with theoretical estimates. The kinetics of attractive intersecting screw dislocations indicates that their velocity is determined by the velocity of their most stressed parts. Repulsive screw dislocations with different Burgers vectors can move cooperatively at a surprisingly high velocity, probably on account of elastic torque interactions and twinning-anti-twinning effects. All these interactions are shown to play an important role in the description of macroscopic mechanical properties in terms of individual dislocation mechanisms.

We investigate the mobility of screw dislocations decorated by carbon solutes in body-centred cubic iron based on density functional theory (DFT) calculations and transmission electron microscopy (TEM) observations of in-situ straining experiments. We focus on the high-temperature domain, between 500 and 800 K, where plasticity is controlled by the slow glide of decorated screw dislocations with a mobility similar to the Peierls mechanism existing below 300 K, at variance with the athermal regime observed at intermediate temperatures. We propose that due to the strong pinning of reconstructed dislocation lines by carbon solutes, dislocation glide occurs by the formation and migration of kinks, both controlled by the jumps of carbon atoms. We calculate the associated kink-pair nucleation and formation energies as well as the migration energies of isolated kinks and use these DFT values to predict the dislocation velocity as a function of temperature, applied stress and dislocation length. The validity of the mobility law is assessed by the comparison with dislocation velocities measured in TEM observations at different temperatures and local shear stresses. The quantitative agreement between the experimental measurements and the analytical model supports the proposed glide mechanism and the DFT values obtained for kink energies.

The interplay of screw dislocations with carbon atoms is investigated in tungsten at high temperature using in situ straining experiments in a transmission electron microscope (TEM) and through ab initio calculations. When the temperature is high enough to activate carbon diffusion, above 1373 K, carbon segregates in the core of screw dislocations and modifies their mobility, even for a carbon concentration as low as 1 appm. TEM observations reveal the reappearance of a Peierls mechanism at these high temperatures, with screw dislocations gliding viscously through nucleation and propagation of kink-pairs. The mobility of screw dislocations saturated with carbon atoms is then investigated with ab initio calculations to determine kink-pair formation, nucleation and migration energies. These energies are used in kinetic Monte-Carlo simulations and in an analytical model to obtain the velocity of screw dislocations as a function of the temperature, the applied stress and the dislocation length. The obtained mobility law parametrised on ab initio calculations compares well with experiments.

Crystal strength and plastic flow are controlled by the motion and interaction of dislocations, the line defects carrying atomic shear increments. Whereas, in most crystals, deformation develops in the crystallographic planes in which the glide force acting on dislocations is maximum, plasticity in body-centred cubic metals is more complex. Slip systems in which the resolved shear stress is not the highest can dominate at low temperature, leading to anomalous slip1,2. Using in situ tensile tests in a transmission electron microscope we show that anomalous slip arises from the high mobility of multi-junctions3, that is, junctions between more than two dislocations, which glide at a velocity several orders of magnitude larger than single dislocations. These multi-junctions result from the interaction of a simple binary junction with a gliding dislocation. Although elasticity theory predicts that these binary junctions should be unstable in crystals with a weak elastic anisotropy such as tungsten, both experiments and atomistic simulations reveal that such junctions can be created under dynamic conditions, in agreement with the existence of anomalous slip in almost all body-centred cubic metals, including tungsten4,5.

Work hardening in bcc single crystals at low homologous temperature shows a strong orientation-dependent hardening for high symmetry loading, which is not captured by classical dislocation density based models. We demonstrate here that the high activation barrier for screw dislocation glide motion in tungsten results in repulsive interactions between screw dislocations, and triggers dislocation motion at applied loading conditions where it is not expected. In situ transmission electron microscopy and atomistically informed discrete dislocation dynamics simulations confirm coupled dislocation motion and vanishing obstacle strength for repulsive screw dislocations, compatible with the kink pair mechanism of dislocation motion in the thermally activated (low temperature) regime. We implement this additional contribution to plastic strain in a modified crystal plasticity framework and show that it can explain the extended work hardening regime observed for [100] oriented tungsten single crystal. This may contribute to better understanding the increase in ductility of highly deformed bcc metals.

Plasticity in hexagonal close-packed zirconium is mainly controlled by the glide of dislocations with 1/3〈12¯10〉 Burgers vectors. As these dislocations cannot accommodate deformation in the [0001] direction, twinning or glide of 〈c+a〉 dislocations, i.e. dislocations with 1/3〈12¯13〉 Burgers vector, have to be activated. We have performed in situ straining experiments in a transmission electron microscope to study the glide of 〈c+a〉 dislocations in two different zirconium samples, pure zirconium and Zircaloy-4, at room temperature. These experiments show that 〈c+a〉 dislocations exclusively glide in first-order pyramidal planes with cross-slip being activated. A much stronger lattice friction is opposing the glide of 〈c+a〉 dislocations when their orientation corresponds to the ⟨a⟩ direction defined by the intersection of their glide plane with the basal plane. This results in long dislocations straightened along ⟨a⟩ which glide either viscously or jerkily. This ⟨a⟩ direction governs the motion of segments with other orientations, whose shape is merely driven by the minimization of the line tension. The friction due to solute atoms is also discussed.

Transmission electron microscopy in situ straining experiments have been carried out in a W-8% Re alloy, between 100 K and 300 K, in order to determine the origin of softening by alloying with Re. The geometry and kinetics of glide show that Re solutes do not induce any change of main slip plane. The only clearly visible effect of Re is the pinning of screw dislocations at super-jogs, which should a priori induce some hardening, but which in fact has only a weak effect. Local measurements of the applied shear stress at the scale of individual dislocations nevertheless confirm that softening is well reproduced in the micro-samples. It is thus concluded that softening results from a decrease of the strength of Peierls valleys with respect to pure W. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

This paper presents a study of the creep at 850 degrees C under 150 MPa of the IRIS alloy (Ti-Al48-W2-B0.1) densified by spark plasma sintering. The dislocation microstructure in a sample strained up to 1.5% was studied by post-mortem transmission electron microscopy. The deformation is mainly due to ordinary dislocations. Several populations of dislocations are evidenced. Their Burgers vectors, the plane in which they are moving and the corresponding deformation mechanisms are determined. In the discussion section, the deformation mechanisms, the factors controlling their activation and the role of tungsten as hardening element are examined. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Physical deformation and fracture mechanisms of a duplex high strength, low density steel have been examined, both in individual phases using TEM in-situ tensile tests and in bulk specimens using tensile and impact tests. Deformation in ferrite concentrated into wavy bands. In both ferrite and austenite, strong pinning induced non-continuous motion of dislocations even at low temperature. The stability of austenite against transformation into martensite was varied using either the test temperature or prior low temperature treatment, while keeping the chemistry, processing parameters and microstructural size and morphology constant. It was shown that the brittle-to-ductile transition in this alloy is driven by mechanical interactions between phases, and that cleavage fracture of coarse delta-ferrite is governed by the presence of hard martensite in the fine-grained regions, before or during the mechanical test.

The mechanical behaviour and fracture were studied in a Fe–5Mn–2.5Al–0.2C (in wt-%) Medium Mn steel. Metallographic and magnetic measurements confirm the significant influence of the transformation kinetics of the strain-induced martensite on the mechanical properties and strain heterogeneities (Lüders and Portevin-Le Chatelier-like phenomena). An accurate study of isothermal evolutions of the microstructure, associated with atomistic calculations, complements current thermodynamic databases to quantify the nature and volume fraction of phases at different temperatures. A kinetic approach then predicts the influence of annealing conditions on the composition of retained austenite, key parameter for the martensitic transformation kinetics. This supports quantitative modelling of the influence of the intercritical annealing temperature on the ultimate tensile strength for industrial developments of these new grades.

In situ straining experiments were carried out in pure tungsten in order to study the geometry and kinetics of glide of dislocations as a function of stress and temperature. For tensile axes different from <110>, screw dislocations move by a combination of steady and jerky motion in planes which cannot be identified unambiguously. For a tensile axis close to <110>, however, screw dislocations have a much jerkier motion with jumps over large distances, in {112} planes exactly but only in the twinning direction, and in {123} planes. This involves a strong violation of the Schmid law, in agreement with slip line observations reported in the 60's by different authors. Jerky {112} slip is combined with a more classical steady motion in planes close to {110}, but the proportion of steady motion decreases rapidly to zero at decreasing temperature, which leads to a gradual transition between the two mechanisms. These results account for the specific mechanical properties measured in case of a <110> straining axis. They also bring new elements to understand the discrepancy between theoretical and experimental stress values at low temperatures. Indeed, like in Fe, local stress values as a function of temperature and dislocation velocity do not obey to the classical rules of thermodynamics, in agreement with possible quantum effects.

Slip systems involving dislocations with Burgers vectors have been studied in hexagonal close packed Zr and Ti, by means of in situ straining experiments in a transmission electron microscope, at various temperatures and as a function of resolved shear stress. The results show that Zr and Ti are very similar in many respects. Prismatic slip is activated at rather low resolved shear stresses, and is controlled by the interaction between mobile dislocations and solute atoms (presumably oxygen). Pyramidal slip requires substantially higher resolved shear stresses and is characterized by straight screw dislocations moving by a kink-pair mechanism. Basal slip is activated at and above room temperature, for resolved shear stresses equal or higher than those in the prismatic planes. The slip traces are always wavy, presumably due to intensive cross slip from basal to prismatic planes. It also involves straight screw dislocations moving by a kink-pair mechanism. These microscopic observations are discussed in the light of some aspects of the mechanical behavior, in particular the increase of yield-stress at decreasing temperature and the discontinuity of activation area close to room temperature.

Dynamic strain ageing in iron due to various interstitial (carbon) and substitutional (nickel, silicon, chromium, aluminum) solute atoms has been studied by in situ straining in a transmission electron microscope. The effect of carbon in solid solution is characterized by two different carbon-dislocation interactions in the jerky flow and serrated flow temperature domains, and by a low mobility of screw dislocations controlled by a “high-temperature Peierls mechanism”. Substitutional atoms in solid solution can either move the domain of dynamic strain ageing to higher temperatures or not, depending on their chemical affinity for carbon. The results are interpreted by a shielding effect of mobile carbon atoms, inhibiting the interaction between dislocations and substitutional solute atoms.

The ease of a metal to deform plastically in selected crystallographic planes depends on the core structure of its dislocations. As the latter is controlled by electronic interactions, metals with the same valence electron configuration usually exhibit a similar plastic behaviour. For this reason, titanium and zirconium, two transition metals of technological importance from the same column of the periodic table, have so far been assumed to deform in a similar fashion. However, we show here, using in situ transmission electron microscopy straining experiments, that plasticity proceeds very differently in these two metals, being intermittent in Ti and continuous in Zr. This observation is rationalized using first-principles calculations, which reveal that, in both metals, dislocations may adopt the same set of different cores that are either glissile or sessile. An inversion of stability of these cores between Zr and Ti is shown to be at the origin of the profoundly different plastic behaviours.