Search

International audience

International audience

International audience

International audience

The heterogeneous precipitation of the eta (MgZn2) phase on the E (Al18Mg3Cr2) dispersoids of the 7075 aluminum alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectrometry (EDX). It is found that coarse B particles are heterogeneously precipitated at the E particle interface after water quenching and isothermal aging at 120 degrees C. The incoherent E/Al interface is responsible for the high tendency of heterogeneous precipitation of the B phase. Two different orientation relationships (ORs) between the eta, E and Al matrix are identified: OR1 [2 (11) over bar0](eta)[011](E)//[(1) over bar 12](Al), (01 (1) over bar0)(eta)//(13 (3) over bar)(E)//(201)(Al), OR2 [(1) over bar 12](E)//[0001](eta)//[011](Al), (01 (1) over bar0 )(eta)//(220)(E)//(34 (4) over bar)(Al). The eta phase is preferential to nucleate along the {111}(E) or the {220}(E) planes, depending on its OR. The heterogeneous nucleation of B phase on the E particle could stabilize the E/Al interface by introducing a coherent E/eta interface, which increases the drive force of heterogeneous precipitation. The reorientation of eta phase and mutual diffusion of solute atoms could assist the coherency of the E/eta interface. The present results suggest that increasing the coherency of the E/Al interface is a promising method to suppress the heterogeneous precipitation of the eta phase. (C) 2020 Elsevier B.V. All rights reserved.

The determination of the mechanical properties of serpentinites is essential toward the understanding of the mechanics of faulting and subduction. Here we present the first in situ tensile tests on antigorite in a transmission electron microscope. A push‐to‐pull deformation device is used to perform quantitative tensile tests, during which force and displacement are measured, while the evolving microstructure is imaged with the microscope. The experiments have been performed at room temperature on 2 × 1 × 0.2 μm3 beams prepared by focused ion beam. The specimens are not single crystals despite their small sizes. Orientation mapping indicated that several grains were well oriented for plastic slip. However, no dislocation activity has been observed even though the engineering tensile stress went up to 700 MPa. We show also that antigorite does not exhibit a purely elastic‐brittle behavior since, despite the presence of defects, the specimens accumulate permanent deformation and did not fail within the elastic regime. Instead, we observe that strain localizes at grain boundaries. All observations concur to show that under these experimental conditions, grain boundary sliding is the dominant deformation mechanism. This study sheds a new light on the mechanical properties of antigorite and calls for further studies on the structure and properties of grain boundaries in antigorite and more generally in phyllosilicates., Key Points No dislocation activity has been observed although tensile stress reached 700 MPaThe behavior of antigorite is not purely elastic‐brittle despite the presence of defects in the specimens (pores and damage)The main deformation mechanism is by grain boundary sliding

International audience

International audience

Aluminum oxide (Alumina, Al2O3) and silicon dioxide (silica, SiO2) freestanding thin films are extensively used in a large number of electronic applications as protective layers, dielectric materials and sacrificial release layers during a device micromachining stage. Al2O3 films were deposited by electron beam-evaporation technique while SiO2 were thermally grown. The fracture toughness and environmentally assisted subcritical crack growth have been investigated using a new on chip fracture mechanics test. This technique consists of two long actuators beams pulling on a notched specimen thanks to the release of the residual stress inside the actuators after etching the material beneath. A crack is initiated from the notch tip, propagates and finally stops when the energy release rate has decreased down to its critical value [1]. Both freestanding films are susceptible to stress corrosion especially, in the presence of moisture. The mechanisms responsible for this slow cracking behavior are investigated as well as the subcritical crack growth rate in laboratory air and dry environments under various temperature conditions. The combined experimental and finite element studies provide an insight on the variation of the crack propagation rate (v) as a function of the stress intensity factor (K) in different environments. This variation follows a power law exhibiting a higher cracking rate under high K loading configurations. The fracture toughness value is around 1 MPa√𝑚 for both films. This toughness value is compared to the value obtained by push-to-pull technique (PTP) inside transmission electron microscopy (TEM) using Al2O3 and SiO2 specimens that contain a real sharp pre-crack generated during the crack-on-a-chip test.

Intermetallics (θ and η phases) with high aluminum content (>50 at.% Al) formed at the interface of Fe/Al welded joints significantly reduce the mechanical strength of those joints. Seeking to modify the intermetallics (IM) thus constitutes an opportunity to minimize the risk of brittle failure of the bimetallic joints. In this work, we proposed a method to modify the IM layers by a second diffusion of segregated elements. Firstly, thermomechanical simulation performed using DICTRA® allows estimating the segregation of alloying elements near the interface during friction melt bonding (FMB) of Al/Fe joints. For the experimental part, FMB welded joints composed of DP600 steel and AA6061 were used in this study. FMBed samples with the existing thin θ and η phases were reheated to verify the proposed method via second diffusion. The study revealed that the segregated Si at the interface contributes in a forward diffusion, while other elements (e.g. Mg and Cu) lead to backward diffusion. The experimental observations support that, the joint forms relatively thick θ phase compared to η phase due to the forward diffusion of Si. Moreover, the second heating facilitates the formation of a ternary IM (τ3 - FeAl2Si) at the interface. Using a second diffusion heat treatment may provide an alternate route to change the nature of the intermetallics or the compositions of the IM layer.

International audience

International audience

During the service life of an aluminum part, damage initiates on large iron-rich intermetallic particles and leads to final failure by growth of these cavities and their coalescence. This damage leads to significant economic loss for their replacement. A first step towards an increase in fracture strain is to decrease the size of these intermetallic particles. This has been shown to be highly efficient when 6xxx series Al alloys are processed by Friction Stir Processing (FSP) [1]. A second step, which is the aim of the present work and related to the ERC-ALUFIX project, is to integrate particles in the aluminum alloy by the very same process. These newly integrated particles can either favor crack deviation in order to increase toughness or be used for damage healing. Crack deviation can be favored by the integration of NiTi particles surrounded by controlled internal stresses. The internal stresses are introduced via pre-straining and shape recovery of the embedded NiTi particles. In addition, the NiTi particles enhance the mechanical strength of the composite. A homogenous NiTi distribution and a good bonding at the Al/NiTi interface is the key to a successful crack deviation. This is a challenge in a 7075 Al matrix while easily achievable in a 1050 Al matrix. An alternative is to introduce healing particles in the Al matrix and perform a healing treatment to heal micro-damage. Here the damage healing is assessed trough in-situ heating in the TEM and X-Ray nano holotomograph. It was found that small damage (up to 1 µm size) can be healed at 400°C within 10 min of heat treatment.

During the service life of an aluminum part, damage occurs due to the presence of large intermetallic particles and it leads to significant economic loss for their replacement. However, healing strategies could be developed to extend their lifetime. In the present work, a new class of healable 6XXX series based Al alloys is processed using Friction Stir Processing (FSP). The processed Al alloy shows a healing ability during heat treatment due to the presence of well-distributed healing particles in the matrix. FSP has already proven as a perfect method for redistribution and size reduction of Fe-rich intermetallic particles [1]. In situ tensile Scanning Electron Microscopy (SEM) tests have shown that the damage mechanism in the produced healable Al alloy is changed as new healing particles are damaged first, while Fe-rich intermetallics are mostly remaining intact. The pre-damaged samples produced by a micro-tensile machine were further investigated by in situ heating transmission electron microscopy (TEM) including EDX analysis in order to track healing evolution of damaged particles and reveal healing mechanism. In situ X-Ray nano holotomography experiment carried out at ESRF on the beamline ID16B with a resolution of 35 nm provided additional statistical data on the healing ability of the bulk material. The tests were performed using a dedicated furnace with a quartz chamber available at the beamline [2]. The samples were annealed at various temperatures for 8 hours in total and scanned in between each heat treatment step. It was found that small damage (up to 1 µm size) can be healed at 400°C within 10 min of heat treatment. [1] F. Hannard et al. Acta Materialia 130 (2017) 121-136 [2] J. Villanova et al. Materials Today 20 (2017) 354-359

n/a

Aluminum matrix composites (AMCs) are promising and alternative materials for the aerospace and automotive industry due to their superior mechanical properties. Shape memory alloys (SMA), e.g., nitinol, can be used as reinforcement agent to enhance the mechanical properties of the aluminum alloys. In literature, friction stir processing (FSP) was found to be the optimum manufacturing process for particulate metal matrix composites due to the absence or minimal presence of intermetallic formation at the particle-matrix interface. However, previous work on AMCs, involving FSP on high strength alloys, especially 7xxx series, have all shown the formation of severe defects, such as clusters and cavities, due to very low material flow. In this work, we present an innovative strategy to manufacture a nearly defect free Al7075/NiTi composite, using FSP. The reasonable particle dispersion, the bonded Al/NiTi interface, as confirmed by experimental examination, ensure the tailoring of the material’s mechanical properties through the shape memory performance of the embedded NiTi particles.

Bulk metallic glasses (BMGs) exhibit some outstanding mechanical properties involving high fracture strength, and high elastic strains arising from the liquid‐like atomic structure with no grain boundaries, dislocations, and phase segregations, contrary to crystalline materials. However, the early occurrence at room temperature (RT) of shear band instabilities during plastic deformation of BMGs leads to a lack of ductility, thus drastically undercutting potential uses in structural applications. Recent reports have also shown that the brittle‐like behaviour is mitigated when the sample size is reduced down to the sub‐micron scale with the suppression of catastrophic shear banding. This discovery has opened avenues to study mechanical size effects in thin metallic glass films (TMGFs) with potential impact on a variety of applications in micro‐electro‐mechanical‐systems (MEMS) technology or surface coatings in harsh environments. However, despite extensive research over recent years, the origin of the mechanical size effect in TMGFs is not fully unravelled yet and it can result either from geometric confinement or from a change of atomic arrangement in the films. In the present work, the nanoscale mechanisms controlling the plastic deformation are investigated in CuZr TMGFs exhibiting well‐controlled density/chemical heterogeneities at the nanoscale and a high strength/ductility balance. The Local structure of the films is analysed using a new spatially resolved fluctuation electron microscopy (FEM) technique as well as aberration corrected imaging and spectroscopy in TEM. The deformation mechanism are directly analysed by combining FEM and quantitative in‐situ nanotensile testing using the Bruker PI 95 TEM PicoIndenter.

Aluminium alloys are commonly used in studies related to Friction Stir Welding (FSW) and Processing (FSP), due to their wide variety of applications. FSP provides grain size reduction; intermetallic rounding, size reduction and redistribution; and an overall increase in ductility. Moreover, FSP becomes a popular method for Metal Matrix Composites (MMCs) production, owing to the homogeneous distribution of reinforcement particles and their good interface with the initial matrix. In this work, a healable Al alloy from a base of Al 6063 is produced using FSP. FSP causes a uniform healing particle precipitation (with a size up to 1 μm), as well as the breakage of Fe-rich intermetallics in the nugget zone. This changes damage mechanism, meaning the classical failure source of Al 6063 - Fe-rich intermetallics - remain intact while the healing particles are broken. Further, at the healing stage, a heat treatment is applied to heal the damage.

n/a

Damage of Al components plays a significant role in their service life. That is why self-healing strategies are being developed in order to prevent or delay critical failure. However, mass transfer in metals is a complex question related to many phenomena, such as diffusion mechanisms, precipitation growth and stability, etc. To investigate healing ability and mechanism, in-situ heating experiments have been achieved inside a transmission electron microscope (TEM). The results showed that healing starts after just one minute of heating at 400°C. Further heating leads to complete healing of damaged particles, and subsequently their dissolution and growth. A healing time of 10 minutes yielded the optimum quantity of healed particles and mechanical properties of the alloy.

Recently developed beta-metastable Ti grades take advantage of the simultaneous activation of TRIP and TWIP effects for enhancing their work hardening rate. However, the role of each plasticity mechanism on the macroscopic mechanical response is still unclear. In this work, the nucleation mechanism of the first activated plasticity mechanism, namely {112} < 111 > twinning, was investigated. Firstly, post-mortem TEM analysis showed that twins nucleate on pre-existing microstructural defects such as thermal jogs with the zonal dislocation mechanism. The precipitation of the omega phase on twin boundaries has been observed, as well as the emission of numerous dislocations from super-jogs present in these twin boundaries. It is also shown that {112} < 111 > twins act as effective dislocation sources for the subsequent plasticity mechanisms such as beta -> alpha '' martensitic transformation and {332} < 111 > twinning. Secondly, in situ TEM tensile testing of the investigated Ti grade highlighted the primary role of the initial defect configuration present in the microstructure. It is shown that twins cannot nucleate without the presence of specific defects allowing the triggering of the dislocation decomposition needed for the twinning mechanism highlighted in investigated bulk samples.

β-Ti alloys with plasticity driven by dislocation glide suffer from a lack of work hard-ening, which is most of the time a major drawback. Recently, several β-metastable Ti grades have been designed to exhibit simultaneous TRIP and TWIP effects in order to improve mechanical properties of as-quenched β phase. Indeed, work hardening rates never reached before by Ti alloys seem to result from the activation of α” stress-induced martensitic (SIM) phase as well as from both {332} and {112} twinning systems. Hardly any result has been reported about the activation of both twinning systems. The aim of the present study is to identify the formation mechanism as well as the role of {112}⟨111⟩ mechanical twins in the global mechanical behaviour of TRIP/TWIP titanium alloys. TEM revealed the dislocations involved in the twinning process as well as their interactions with the freshly formed mechanical twins. Also, quantitative in situ TEM tensile testing of FIB-milled samples have been performed to understand the very first steps of twin nucleation.

n/a

International audience

The fracture of coarse and elongated iron-rich intermetallic particles or interface decohesion are the main sources of microvoid nucleation in 6xxx series aluminum alloys. Friction Stir Processing (FSP) has proven an excellent method to break, round and homogenize these iron-rich particles, as well as reduce the grain size and porosity. This delays damage nucleation and final coalescence improving the fracture strain of the alloy As a result, FSP increases the fracture strain of the Al alloy while making it also more isotropic [1]. Yet another approach to postpone failure is rising: make healable materials. In polymeric materials, this strategy is already applied at the industrial level. The material then consists of capsules containing a healing agent embedded into the polymeric matrix. During crack propagation the agent is released leading to crack closure [2]. Extension of this strategy to metals requires solving problems such as phase formation, the poor interface between the healing agent and the matrix and the anisotropy in healing potential [3]. In the present work, the issues of the healing agent distribution and its interface with the matrix are solved by FSP. Fine Mg particles are embedded by FSP in an Al6063 matrix. In-situ tensile Scanning Electron Microscopy (SEM) tests showed that, in the presence of these particles, the damage mechanism is changed and under the load have revealed a change of the main void nucleation mechanism, i.e., the new healing particles are broken first while the iron-rich intermetallics remain intact. Then, after a heat treatment corresponding to the healing step no damage could any longer be observed by SEM. References [1] Hannard, F., Castin, S., Maire, E., Mokso, R., Pardoen, T., Simar, A., “Ductilization of aluminium alloy 6056 by friction stir processing”, Acta Materialia, Vol. 130, page 121-136 (2017). [2] White, S.R., Sottos, N.R., Geubelle, P.H., Moore, J.S., Ressler, M.R., Sriram, S.R., Brown, E.N., Viswanathan, S., “Autonomic healing of polymer composites”, Nature, Vol. 409, page 794-979 (2001). [3] Rohatgi, P.K., “Al-shape memory alloy self-healing metal matrix composite”, Materials Science and Engineering A, Vol 619, page 73-76 (2014).

n/a

Thin films ("thin" meaning thinner than 1μm) are nowadays present everywhere such as in MEMS. The mechanical integrity of the films is the key for the reliability of the devices under interest. Compared to their bulk counterparts, thin films involve, owing to the very fine microstructure, high strength, long elasto-plastic transition and moderate to high strain rate sensitivity, which can lead to detrimental creep. A dislocation-based crystal plasticity model is developed in order to study the mechanical and creep/relaxation behaviour of polycrystalline metallic thin films. The model accounts for the confinement of plasticity due to grain boundaries and for the anisotropy of individual grains, as well as for the significant viscoplastic effects associated to dislocation dominated thermally activated mechanisms. Numerical predictions are assessed based on experimental tensile test followed by relaxation on freestanding Pd films, gathered using an on-chip test technique. The dislocation-based mechanism assumption captures all the experimental trends, including the stress-strain response, the relaxation behaviour and the dislocation density evolution, confirming the dominance of a dislocation driven deformation mechanism for the present Pd films with high defects density. The scale transition based on the ALAMEL model [3] is for the first time successfully applied to thin films with columnar grains. The model has also been used to address some original experimental evidences involving back stresses, Bauschinger effect, backward creep and strain recovery.

Thin films ("thin" meaning thinner than μ1m) are nowadays present everywhere such as in MEMS, solar cells and protective or functional coatings. The mechanical integrity of the films is the key for the reliability of the devices under interest. Compared to their bulk counterparts, thin films involve, owing to the very fine microstructure, moderate to high strain rate sensitivity, which can lead to detrimental creep.....

Crystal plasticity based finite element modelling (CPFEM) is used to study strain hardening and creep in freestanding nanocrystalline Pd films. The model accounts for the confinement of plasticity due to grain boundaries and to the presence of growth twins as well as for the significant viscoplastic effects associated to thermally activated mechanisms. The model microstructure is a 2D periodic Voronoï tessellation that reproduces the grain size distribution. The finite element mesh conforms to the grain and twin boundaries. Periodic boundary conditions reproduce the initial loading and the subsequent slow relaxation occurring in the experimental test. Numerical predictions are assessed based on experimental data gathered during a previously reported experimental campaign of tests on freestanding submicron Pd films using an on-chip technique. The Pd films were deposited by e-beam evaporation which induces a microstructure exhibiting a columnar morphological texture with an ~30 nm in plane grain size. The films involve a large strain hardening capacity and a significant increase of the strength with decreasing thickness. Transmission electron microscopy has revealed the presence of a moderate density of growth nanotwins interacting with dislocations. Intense dislocation activity after deformation was detected and an increased twin density has been observed to enhance the ductility of the films. As expected, the large apparent strain hardening at the onset of plasticity results from a gradual elastic-plastic transition. The largest grains yield much earlier than the softest ones and this effect is captured by the CPFEM approach. The model allows also the study of the creep behaviour of the Pd films and of the influence of the twin density on strain hardening.

Various modes of transmission electron microscopy including aberration corrected imaging were used in order to unravel the fundamental mechanisms controlling the formation of growth twins and the evolution of twin boundaries under mechanical and hydrogen loading modes in nanocrystalline (nc) palladium thin films. The latter were produced by electron-beam evaporation and sputter deposition and subjected to uniaxial tensile deformation as well as hydriding/dehydriding cycles. The results show that the twins form by dissociation of grain boundaries. The coherency of Σ3{111} coherent twin boundaries considerably decreases with deformation due to dislocation/twin boundary interactions while Σ3{112} incoherent twin boundaries dissociate under hydrogen cycling into two-phase boundaries bounding a new and unstable 9R phase. The effect of these elementary mechanisms on the macroscopic behavior of the palladium films is discussed and compared to recent experimental and simulation works in the literature. The results provide insightful information to guide the production of well-controlled population of growth twins in high stacking fault energy nc metallic thin films. The results also indicate directions for further enhancement of the mechanical properties of palladium films as needed for instance in palladium-based membranes in hydrogen applications.