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.

Quantitative insights into the dislocation source behavior of twin boundaries suggest a new dislocation source mechanism

Pop-in statistics from nanoindentation with spherical indenters are used to determine the stress required to activate dislocation sources in twin boundaries (TBs) in copper and its alloys. The TB source activation stress is smaller than that needed for bulk single crystals, irrespective of the indenter size, dislocation density and stacking fault energy. Because an array of pre-existing Frank partial dislocations is present at a TB, we propose that dislocation emission from the TB occurs by the Frank partials splitting into Shockley partials moving along the TB plane and perfect lattice dislocations, both of which are mobile. The proposed mechanism is supported by recent high resolution transmission electron microscopy images in deformed nanotwinned (NT) metals and may help to explain some of the superior properties of nanotwinned metals (e.g. high strength and good ductility), as well as the process of detwinning by the collective formation and motion of Shockley partial dislocations along TBs. Graphic abstract

Growth Kinetics of the (110) Faces of Complex Potassium Cobalt–Nickel Sulphate K2CoxNi1−x(SO4)2·6H2O Crystals

The normal growth rate, the steepness of polygonized growth hillocks and the velocity of step movement on the (110) faces of potassium cobalt–nickel sulphate crystals in aqueous solutions with cobalt to nickel ratios of 1:1 and 1:2 were investigated as a function of supersaturation by the geometry of growth hillocks using laser interferometry. It was found that the morphologies of growth hillocks on the (110) faces of the crystals grown from 1:1 and 1:2 solutions are similar and that the growth hillocks are formed by multiple screw dislocation sources. The experimental data on the growth kinetics of the (110) faces of the crystals were analyzed by using the Burton–Cabrera–Frank theory. It was found that (1) there is a critical supersaturation for the growth of the (110) faces, and the value of this supersaturation in the 1:2 solution is higher than that in the 1:1 solution, and (2) the kinetic coefficient of the step movement in the sectors of growth hillocks is highly anisotropic, and the values of this coefficient are larger in 1:2 solution than in 1:1 solution. These results are discussed in the presented work.

Interpretation of volcanic surface deformation using a 3D multi-source approach

<p>Volcano geodetic observation is a valuable tool to infer location, strength and geometry of magmatic plumbing systems. The availability of high precision and spatial resolution, spanning decades, deformation data from satellite radar observation and Global Navigation Satellite Systems (GNSS) can give us important information for detecting and characterizing their temporal variations as well as other possible geodynamic sources acting in the volcanic area. For this objective inversion techniques are necessary which help us to obtain the maximum of information from these new datasets. We present a new, original methodology to carry out a multi-source inversion of ground deformation data to better understand the subsurface causative processes (Camacho et al., 2020). The methodology uses a nonlinear approach which permits the determination of location, size and three-dimensional configuration, without any a priori assumption as to the number, nature or shape of the potential sources. The proposed method identifies a combination of pressure bodies and different types of dislocation sources (dip-slip, strike-slip and tensile) representing magmatic sources and other processes such as earthquakes, landslides or groundwater-induced subsidence through the aggregation of elemental cells. This approach carries out a simultaneous inversion of the deformation components and/or line-of-sight (LOS) data; and a simultaneous determination of diverse structures such as pressure bodies or dislocation sources, representing local and regional effects. Both things are done in a fully 3D context and without any initial hypothesis about the number, geometry or types of the causative sources is necessary. We show results from the application of this new methodology to synthetic and real test cases (e.g., Mt. Etna).</p><p>This research has been primarily supported by the Spanish Ministerio de Ciencia, Innovación and Universidades research project DEEP-MAPS (RTI2018-093874-B-I00) and is part of the CSIC-PTIs TELEDETEC and POLARCSIC activities.</p><p>References<br>Camacho, A.G., Fernández, J., Samsonov, S.V., Tiampo K.F., Palano, M., 2020. Multisource 3D modelling of elastic volcanic ground deformations. Earth and Planetary Science Letters, 547C, 116445. https://doi.org/10.1016/j.epsl.2020.116445.</p>

Deciphering the interactions between single arm dislocation sources and coherent twin boundary in nickel bi-crystal

The introduction of a well-controlled population of coherent twin boundaries (CTBs) is an attractive route to improve the strength ductility product in face centered cubic (FCC) metals. However, the elementary mechanisms controlling the interaction between single arm dislocation sources (SASs), often present in nanotwinned FCC metals, and CTB are still not well understood. Here, quantitative in-situ transmission electron microscopy (TEM) observations of these mechanisms under tensile loading are performed on submicron Ni bi-crystal. We report that the absorption of curved screw dislocations at the CTB leads to the formation of constriction nodes connecting pairs of twinning dislocations at the CTB plane in agreement with large scale 3D atomistic simulations. The coordinated motion of the twinning dislocation pairs due to the presence of the nodes leads to a unique CTB sliding mechanism, which plays an important role in initiating the fracture process at a CTB ledge. TEM observations of the interactions between non-screw dislocations and the CTB highlight the importance of the synergy between the repulsive force of the CTB and the back stress from SASs when the interactions occur in small volumes.

Detection of volcanic unrest onset in La Palma, Canary Islands, evolution and implications

La Palma island is one of the highest potential risks in the volcanic archipelago of the Canaries and therefore it is important to carry out an in-depth study to define its state of unrest. This has been accomplished through the use of satellite radar observations and an original state-of-the-art interpretation technique. Here we show the detection of the onset of volcanic unrest on La Palma island, most likely decades before a potential eruption. We study its current evolution seeing the spatial and temporal changing nature of activity at this potentially dangerous volcano at unprecedented spatial resolutions and long time scales, providing insights into the dynamic nature of the associated volcanic hazard. The geodetic techniques employed here allow tracking of the fluid migration induced by magma injection at depth and identifying the existence of dislocation sources below Cumbre Vieja volcano which could be associated with a future flank failure. Therefore they should continue being monitored using these and other techniques. The results have implications for the monitoring of steep-sided volcanoes at oceanic islands.

Nanoscale precipitates as sustainable dislocation sources for enhanced ductility and high strength

Traditionally, precipitates in a material are thought to serve as obstacles to dislocation glide and cause hardening of the material. This conventional wisdom, however, fails to explain recent discoveries of ultrahigh-strength and large-ductility materials with a high density of nanoscale precipitates, as obstacles to dislocation glide often lead to high stress concentration and even microcracks, a cause of progressive strain localization and the origin of the strength–ductility conflict. Here we reveal that nanoprecipitates provide a unique type of sustainable dislocation sources at sufficiently high stress, and that a dense dispersion of nanoprecipitates simultaneously serve as dislocation sources and obstacles, leading to a sustainable and self-hardening deformation mechanism for enhanced ductility and high strength. The condition to achieve sustainable dislocation nucleation from a nanoprecipitate is governed by the lattice mismatch between the precipitate and matrix, with stress comparable to the recently reported high strength in metals with large amount of nanoscale precipitates. It is also shown that the combination of Orowan’s precipitate hardening model and our critical condition for dislocation nucleation at a nanoprecipitate immediately provides a criterion to select precipitate size and spacing in material design. The findings reported here thus may help establish a foundation for strength–ductility optimization through densely dispersed nanoprecipitates in multiple-element alloy systems.