In situ TEM study of twin boundary migration in sub-micron Be fibers

Deformation twinning in hexagonal crystals is often considered as a way to palliate the lack of independent slip systems. This mechanism might be either exacerbated or shut down in small-scale crystals whose mechanical behavior can significantly deviate from bulk materials. Here, we show that sub-micron beryllium fibers initially free of dislocation and tensile tested in-situ in a transmission electron microscope (TEM) deform by a $\{ 10\bar{1}2 \}$ $\langle 10\bar{1}1 \rangle$ twin thickening. The propagation speed of the twin boundary seems to be entirely controlled by the nucleation of twinning dislocations directly from the surface. The shear produced is in agreement with the repeated lateral motion of twinning dislocations. We demonstrate that the activation volume ($V$) associated with the twin boundary propagation can be retrieved from the measure of the twin boundary speed as the stress decreases as in a classical relaxation mechanical test. The value of $V \approx 8.3 \pm 3.3 \times 10^{-29}m^3$ is comparable to the value expected from surface nucleation.Comment: 13 pages, 9 figure

Probing grain boundary mechanisms by in-situ TEM

In small grained metals, specific elementary deformation mechanisms such as GB sliding, dislocation emission from GB, shear coupled GB migration, grain rotation are expected to be active. However, their relative preponderance and activation during the overall plastic deformation is still difficult to assess, mainly because identifying these mechanisms in small crystallite at the appropriate time scale is a challenge. To that respect, in-situ TEM experiments have proven to be an adequate tool to probe the dynamics of these mechanisms. In this talk, I would like to report observations obtained during several in-situ straining TEM experiments in small grained Al (grain size between 100nm and 1µm) on MEMS-supported polycrystalline thin films, bicrystals, and bulk polycrystals. In polycrystalline thin films, several inter- (dislocation emission from internal or GB sources) and intra- granular mechanisms (i.e. GB sliding and grain growth) are activated during plastic deformation in a complex manner that can be first qualitatively retrieved [1]. Grain growth under stress can be interpreted by an unusual mechanism where the GB migration is coupled to a strain. The amount of deformation produced, also called the coupling factor, can be measured during an experiment by image correlation and then tentatively modeled [2]. We have shown, thanks to dedicated experiments in bicrystals, that this mechanism can be understood by the motion of step-dislocations along the GB, a conclusion supported also by recent MD simulations and high resolution observations [3-5]. Because these step dislocations can result from the dissociation of lattice dislocations in the GB [5], we highlight a possible interplay between intra- and inter-granular deformation mechanisms. Contrary to grain growth that can be easily evidenced, observations of grain rotation remains limited. Experimental evidence are hard to capture in small-grained materials and might lead to artefacts like sample rigid rotation. Recent efforts to unravel elementary GB mechanics mostly focused on individual straight low index coincident GB, and the analysis of the collective behavior of a realistic GB network is still in its infancy. We have developed an original methodology combining sequential in-situ straining experiments followed by automated crystal orientation mapping in a TEM and a custom-made data processing. This configuration allows both dynamical observations of elementary mechanisms, analysis of GB evolution in individual grain and statistical analysis at large scale. We have shown in polycrystalline thin films that grain rotation can be indeed observed and as for grain growth, appears to be a direct consequence of GB dislocation motion with a Burgers vector out of the film plane [6]

Plastic deformation of sub-micron Al and Be wires: A TEM and in situ TEM study

The origin of the improved strength of sub-micron single crystals and whiskers is still debated, but after studies concentrated solely on size effects, it appeared that an as important parameter was the dislocation content of these small crystals. In this presentation, the importance of the dislocation content and the role played by the external surface on the triggering of plasticity in both Al and Be sub-micron wires investigated by in-situ transmission electron microscopy (TEM) will be highlighted. The wires, obtained by selective etching of Al/Al2Cu and Al/Be eutectic alloys (Fig.1), all exhibit a thin Al oxide outer layer. Al wires present a large variability in dislocation density while Be wires parallel to their c-axis are usually dislocation free or contain very few dislocations. In Al, we show that multiplication of dislocations through intermittent spiral sources directly causes a power-law increase of the yield stress with decreasing cross-sectional size. The size effect and resulting mechanical response are directly linked to the initial defect density and the distance between the source and the surface. In the absence of dislocations, fibers elastically reach high stresses with limited to no plasticity, reminiscent of whisker behavior. Please click Additional Files below to see the full abstract

Cast aluminium single crystals cross the threshold from bulk to size-dependent stochastic plasticity

Metals are known to exhibit mechanical behaviour at the nanoscale different to bulk samples. This transition typically initiates at the micrometre scale, yet existing techniques to produce micrometre-sized samples often introduce artefacts that can influence deformation mechanisms. Here, we demonstrate the casting of micrometre-scale aluminium single-crystal wires by infiltration of a salt mould. Samples have millimetre lengths, smooth surfaces, a range of crystallographic orientations, and a diameter D as small as 6 μm. The wires deform in bursts, at a stress that increases with decreasing D. Bursts greater than 200 nm account for roughly 50% of wire deformation and have exponentially distributed intensities. Dislocation dynamics simulations show that single-arm sources that produce large displacement bursts halted by stochastic cross-slip and lock formation explain microcast wire behaviour. This microcasting technique may be extended to several other metals or alloys and offers the possibility of exploring mechanical behaviour spanning the micrometre scale

Multiscale modelling for fusion and fission materials: the M4F project

The M4F project brings together the fusion and fission materials communities working on the prediction of radiation damage production and evolution and its effects on the mechanical behaviour of irradiated ferritic/martensitic (F/M) steels. It is a multidisciplinary project in which several different experimental and computational materials science tools are integrated to understand and model the complex phenomena associated with the formation and evolution of irradiation induced defects and their effects on the macroscopic behaviour of the target materials. In particular the project focuses on two specific aspects: (1) To develop physical understanding and predictive models of the origin and consequences of localised deformation under irradiation in F/M steels; (2) To develop good practices and possibly advance towards the definition of protocols for the use of ion irradiation as a tool to evaluate radiation effects on materials. Nineteen modelling codes across different scales are being used and developed and an experimental validation programme based on the examination of materials irradiated with neutrons and ions is being carried out. The project enters now its 4th year and is close to delivering high-quality results. This paper overviews the work performed so far within the project, highlighting its impact for fission and fusion materials science.This work has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 755039 (M4F project)

Mécanismes de Plasticité aux joints de grains : Expériences et Modèles

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Les alliages métalliques à haute performance

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