Localization of plastic strain in alloy 718 using digital image correlation

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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

Grain boundary-based plasticity mechanisms in nanostructured metals

The Hall-Petch relationship establishes the proportional dependency of a metal’s strength with the inverse of its grain size’s square root. This phenomena, is well understood using dislocation-based plasticity, until the grain size becomes too small. There plastic deformation mechanisms are usually related to grain boundaries (GBs). Previous studies have observed and simulated several plastic deformation mechanisms such as grain rotation, grain sliding and shear-migration coupling (Figure 1). Some models have been proposed to predict shear-migration coupling based on the initial GB misorientations[1], [4]. However, they have not yet been proven experimentally in the case of polycrystals. This study focuses on the shear-migration coupling, a mechanism which has fueled many recent studies in the field of plasticity [1], [2], [3]. To carry it out, we use polycrystals of Aluminum, Copper and Nickel, with ultrafine grains (\u3c1µm). We aim to find experimental evidences of such mechanism and characterize it in order to correlate it with initial grain misorientations, straining rate, chemical distribution, etc. Please click Additional Files below to see the full abstract

Micropillar compression study of Fe-irradiated 304L steel

Stainless steel used in nuclear reactors are experiencing heavy neutron irradiation that modify their microstructure, and therefore their mechanical properties. To assess the irradiation-induced hardening and the modification of deformation modes at the grain scale on 304L steels, indentations [1] and in situ microcompression tests were conducted on Fe-ions irradiated and non-irradiated FIB-made pillars [2]. 10 MeV and up to 8 dpa Fe irradiations were conducted at 450°C to surrogate neutron irradiation. Size effect was detected on unirradiated but not on irradiated pillars, revealing a strong impact of the microstructure on the mechanical behavior. Surprisingly, smoother plastic deformation took place in irradiated pillars while localized shear bands were observed in unirradiated ones. TEM investigations helped elaborating some hypothesis for this different behavior. Please click Additional Files below to see the full abstract

Nano mechanical and microstructural investigation of damage mechanisms in copper wire bonds

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Micro-mechanical approach of the intergranular stress corrosion cracking of austenitic stainless steels in PWR environment

Austenitic stainless steels are used in the nuclear industry to make the internals parts of Pressurized Water Reactors (PWR) such as baffle and former plates. Numerous Baffle-to-Former Bolts (BFB) intergranular failures have been reported as a result of Irradiation Assisted Stress Corrosion Cracking (IASCC) phenomenon. In order to predict the cracking of the grain boundary through a micro-mechanical approach, it is necessary to determine the intragranular mechanical behavior of the steel and the grain boundary strength. Please click Download on the upper right corner to see the full abstract

In-situ TEM straining experiments in Cantor’s alloy at room and LN2 temperatures

Cantor’s single-phase equiatomic FeNiCoCrMn alloy is a “high-entropy” alloy (or HEA) which crystallizes in the face-centered cubic (fcc) crystal structure. Its mechanical properties include high strength, particularly at low temperatures, good ductility and a large number of slip systems [1], on which its plasticity largely depends. To have a better understanding of these properties, in situ TEM straining experiments were carried out at room and liquid nitrogen temperatures, with a straining holder that applies mechanical stress on the specimen (locally measured using dislocations’ curvature) to analyze dislocation movements. According to previous studies, the planar slip of dislocations is responsible for the first stages of plasticity and twinning starts afterwards [1] [2] [3]. The strengthening mechanisms are a result of the classical dislocation/obstacle (grain boundary, twinning) interaction, but also of the local lattice distortions that may impede moving dislocations. These interactions seem to affect both perfect and partial dislocations. Please click Additional Files below to see the full abstract

In situ high temperature TEM tensile testing of pseudo single crystalline Si for PhotoVoltaic applications

Single crystalline Silicon has the highest efficiency to convert sunlight into electricity. Its production is however costly. On the other hand, cheap polycrystalline Si cells can be produced, with a 10% lower PV conversion efficiency. A promising technique, dubbed mono-like Si consists in growing pseudo-single crystalline Si ingots from a tile of single crystalline seeds aligned at the bottom of the crucible. At the present time, this technique is confronted to the high density of defects that multiply during solidification, fueled by the thermal gradients generated in the furnace. Some of these defects have small impact on the electrical properties but others are heavy recombination sites and should be avoided. Here, we focus on dislocations, micro twins and grain boundaries. Their mutual interaction may act as stress concentrators or sinks. We are working at various scales (Fig. 1a-b) to understand these interactions that occur both at long ranges and atomic-scale processes. To gain insight about the later, we have started to tensile strain dedicated micro-samples in situ in the TEM at temperatures between 900 and 1000°C (the melting temperature of Si is 1414°C). We have shown that multiplication processes are initiated at existing GBs and that mobile dislocations are poorly affected by Peierls stresses at this temperature. Interactions of dislocations with twins (Fig 1c) are currently investigated, along with twin terminations and initiation sites

An almost unifying theory for grain boundary‐based plasticity

Revealed in metallic nanocrystals, or thin films (Fig. 1) grain boundary (GB)-based plasticity has been studied for many years under various names: stress-assisted grain growth, grain rotation, grain boundary sliding or shear-coupled grain boundary migration. Based on MD simulations, TEM and in-situ TEM approaches, we will show that a key player in these mechanisms is the disconnection [1, 2]. This defect combines a step and a Burgers vector character, and belongs to GBs, especially real GBs. The motion of these defects can explain most of the above-mentioned mechanisms depending on the amplitude of both its step and dislocation components. But not all of them. Some observations suggest that local atomic shuffling also plays a role as clear non-conservative behaviours are detected, probably postponing the expected happy ending of a complete GB-based plasticity understanding. Please click Additional Files below to see the full abstract

Deep Learning of Crystalline Defects from TEM images: A Solution for the Problem of "Never Enough Training Data"

Crystalline defects, such as line-like dislocations, play an important role for the performance and reliability of many metallic devices. Their interaction and evolution still poses a multitude of open questions to materials science and materials physics. In-situ TEM experiments can provide important insights into how dislocations behave and move. During such experiments, the dislocation microstructure is captured in form of videos. The analysis of individual video frames can provide useful insights but is limited by the capabilities of automated identification, digitization, and quantitative extraction of the dislocations as curved objects. The vast amount of data also makes manual annotation very time consuming, thereby limiting the use of Deep Learning-based, automated image analysis and segmentation of the dislocation microstructure. In this work, a parametric model for generating synthetic training data for segmentation of dislocations is developed. Even though domain scientists might dismiss synthetic training images sometimes as too artificial, our findings show that they can result in superior performance, particularly regarding the generalizing of the Deep Learning models with respect to different microstructures and imaging conditions. Additionally, we propose an enhanced deep learning method optimized for segmenting overlapping or intersecting dislocation lines. Upon testing this framework on four distinct real datasets, we find that our synthetic training data are able to yield high-quality results also on real images-even more so if fine-tune on a few real images was done