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International audience; Bulk Metallic Glasses (BMG) have exceptional mechanical properties compared to their traditional crystalline counterparts. However, BMGs often exhibit scattered tribological behaviours. To better understand the underlying mechanisms governing the behaviour of BMGs, four different alloys (Zr52.5Ti5Cu17.9Ni14.6Al10, Cu47Zr46Al7, Cu60Zr33Ti7, and Ni62Nb33Zr5) were studied in pure sliding, reciprocating conditions. The study shows that the residual elements coming either from the counterpart or the surrounding environment plays a major role in theirtribological behaviour. The composition of the BMG have a lower impact in the process. Eventually, the study concludes that optimization of the tribological behaviour of the BMGs might not necessarily rely on optimizing BMG composition but might rather rely on optimizing the choice of counterparts as a function of contact conditions.

Tremendous properties of bulk metallic glasses (BMGs) have led to numerous potential applications in the past few years. An obvious interest has been given to their potential as structural material given their high mechanical properties and their ability to be produced in bulk parts of several millimeters in size. Unlike their crystalline counterparts, they also show very promising insulating properties due to the absence of a periodic lattice. In the present work, the interest of using bulk amorphous metallic alloys as materials combining good structural properties and thermal insulation is discussed. The study of Hf-rich BMGs enables to reduce the thermal losses through a higher σc/κ performance ratio and thus shows very good potential for future insulating and structural applications. Keywords: Bulk metallic glasses, Thermal properties, Structural properties

The fundamental plasticity mechanisms in thin freestanding Zr65Ni35 metallic glass films are investigated in order to unravel the origin of an outstanding strength/ductility balance. The deformation process is homogenous until fracture with no evidence of catastrophic shear banding. The creep/relaxation behaviour of the films was characterized by on-chip tensile testing, revealing an activation volume in the range 100–200 Å3. Advanced high-resolution transmission electron microscopy imaging and spectroscopy exhibit a very fine glassy nanostructure with well-defined dense Ni-rich clusters embedded in Zr-rich clusters of lower atomic density and a ~2–3 nm characteristic length scale. Nanobeam electron diffraction analysis reveals that the accumulation of plastic deformation at room-temperature correlates with monotonously increasing disruption of the local atomic order. These results provide experimental evidences of the dynamics of shear transformation zones activation in metallic glasses. The impact of the nanoscale structural heterogeneities on the mechanical properties including the rate dependent behaviour is discussed, shedding new light on the governing plasticity mechanisms in metallic glasses with initially heterogeneous atomic arrangement.

International audience; The development of biocompatible metallic glasses with enhanced ductility that are free of potentially toxic elements such as aluminium and nickel remains a challenge. Although Cu is associated with cytotoxicity levels above a safe threshold, the mechanical properties of Cu-rich metallic glasses are particularly attractive. Herein, the effect of minor additions of yttrium to the Cu–Zr–Ti system is studied. Rods of composition (Cu60Zr30Ti10)(100−x)Yx (with x = 0, 1, 2) were prepared, and the critical diameter was improved from 2 to 3.5 mm with the addition of 1 at% Y. The compressive fracture strength was greater than 2000 MPa for all of the compositions, and the plastic strain increased from 0.9% to 2% with the addition of 1 at% Y. Transmission electron microscopy analysis revealed the presence of yttria precipitates surrounded by nanocrystallized areas, suggesting that the precipitates improved the plasticity. The addition of yttrium also led to the formation of a protective layer, which enhanced both the thermal stability and corrosion resistance of the metallic glass. Finally, the biocompatibility improved with minor additions of yttrium. Our findings demonstrate the wide range of beneficial properties of Cu-based metallic glasses, which may open the door for their potential application as biomaterials.

International audience; In the last years new composite materials made of a metallic glass matrix with embedded nanocrystals have arisen as a promising alternative to the metallic glasses, due to their higher hardness. Although the effects of a partial nano-crystallization onto mechanical and magnetic properties have been widely investigated, nothing is known about thermal transport properties, interesting for recently proposed novel applications. Here we investigate how thermal transport is modified in presence of nanocrystalline inclusions in a Zr-based bulk metallic glass. By means of electric measurements and inelastic x ray scattering we are able to disentangle the effects of a partial nanocrystallization onto the two different contributions to heat transport, the electronic and the vibrational one. We show that no enhanced electrons or phonons scattering from the interfaces is observed, while the presence of crystalline nanoinclusions leads to an increase of both contributions, via an increased electric conductivity and transverse acoustic speed of sound. Surprisingly, while a gradual modification of the electric conductivity with the crystalline fraction is observed, even a low crystalline fraction is sufficient to modify elastic and vibrational properties. Our results indicate that low crystallinity composites are indeed promising alternative to both amorphous and crystalline alloys, thanks to their unique combination of high ductility, polycrystal-like vibrational properties and amorphous-like electric transport. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

International audience; Motivated by recent studies demonstrating a high strength- high ductility potential of nano-scale metallic glass samples, the mechanical response of freestanding Zr65Ni35 film with sub-micron thickness has been investigated by combining advanced on-chip tensile testing and electron microscopy. Large deformation up to 15% is found for specimen thicknesses below 500 nm with variations depending on specimen size and frame compliance. The deformation is homogenous until fracture, with no evidence of shear banding. The yield stress is doubled when decreasing the specimen cross-section, reaching 3 GPa for small cross-sections. The fracture strain variation is related to both the stability of the test device and to the specimen size. The study concludes on clear disconnect between the mechanisms controlling the onset of plasticity and the fracture process. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

17th IUMRS International Conference in Asia, Qingdao, PEOPLES R CHINA, OCT 20-24, 2016; International audience; The serrated plastic flow, microstructure and residual stress of a Zr-33 Cu-30 Ni-5 Al-10 bulk metallic glass (BMG) undergone surface mechanical attrition treatment (SMAT) have been investigated by a combination of compression tests with scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and the incremental hole-drilling strain-gage method. It is found that SMAT leads to various microstructural modifications and residual stress distribution in the surface layers of the Zr-based BMG due to the mechanically-induced nanocrystallization and generation of shear bands. As a result, the BMG alloy exhibits a remarkable work-hardening like behavior and significant increase of plastic strain from less than 1 % to 15 %, and its plastic deformation dynamics yields a power-law distribution of shear avalanches. Based upon the analysis of the experimental results, it is indicated that this can be connected to the SMAT-induced microstructural modifications and the resulting residual compressive stress in the Zr-based BMG.

The crystallization kinetics of Cu38Zr46Ag8Al8 bulk metallic glass in non-isothermal mode and isothermal mode are investigated by differential scanning calorimetry. In non-isothermal conditions, the average value of activation energy is determined by Kissinger equation, and the value is around 310 kJ/mol. The crystallization enthalpy is about 28.69 J/g. In addition, the local Avrami exponent is adopted to describe the crystallization process. In isothermal route, the average value of activation energy for crystallization is calculated by the Arrhenius equation, and the value is about 451 kJ/mol. The crystallization enthalpy is about 1.63 J/g. And the Avrami exponent n ranges from 4.10 to 4.74, which indicates that the crystallization mechanism is mainly governed by constant nucleation rate. The created phases in the two conditions are different which can be confirmed by the X-ray diffraction test, this result is in accordance with the different crystallization enthalpies, but it's different with other investigations.

International audience; Mechanical size effects in Zr65Ni35 Thin Film Metallic Glass (TFMG) are studied with an emphasis on a transition in failure mechanisms occurring below 500 nm. XRD and TEM analyses of Zr65Ni35 TFMG confirm the absence of a crystalline phase. The thickness effect on the mechanical properties of Zr65Ni35 TFMGs is investigated using nanoindentation, Brillouin spectroscopy, and fracture tests. Brillouin spectroscopy shows no variation of the elastic constants, while the hardness - measured by nanoindentation - presents a continuous increase with decreasing film thickness. The fracture surface of the TFMGs involves corrugations which disappear when the thickness is reduced below 500 nm.

Room-temperature plasticity in metallic glasses (MGs) is commonly associated with local structural heterogeneity; however, direct observation of the subtle structural change caused by plasticity is vitally important but the data are extremely scarce. Based on dynamic atomic force microscopy (DAFM), here we show that plasticity-induced structural evolution in a Zr-Ni MG can be revealed via nano-scale viscoelastic contacts between an AFM tip and plastically deformed MG surface layers. Our experimental results clearly show a spatial amplification of the nano-scale structural heterogeneity caused by the distributed plastic flow, which can be linked to the limited growth, reorientation and agglomeration of some nano-scale energy-absorbing regions, which are reminiscent of the behavior of the defect-like regions with non-affine deformation as conceived in many theories and models. Furthermore, we are able to experimentally extract the thermodynamic properties of these nano-scale regions, which possess an energy barrier of 0.3–0.5 eV, about half of that for a typical shear transformation event that usually occurs at the onset of plasticity. The outcome of our current work sheds quantitative insights into the correlation between plasticity and structural heterogeneity in MGs.

International audience; Unlike crystalline metals, which have a well-understood periodical structure, the amorphous structure of metallic glasses (MGs) is still poorly understood, particularly when such a structure rearranges itself at the nanoscale under external agitations. In this article, we provide compelling evidence obtained from a recently developed high-resolution atomic force microscopy (HRAFM) technique that reveals the nanoscale structural heterogeneity after thermal annealing in a Zr-Ni metallic glass. Through the HRAFM technique, we are able to uncover the annealing-induced fractal growth of the dense-packing phases in the binary MG thin film, which exhibits a fractal dimension of similar to 1.7, in line with a two-dimensional diffusion limited aggregation process. The current findings not only reveal the evolution process of atomic packing in the annealed MG thin film, but also shed light on the possible cooling rate effect on the atomic structure of MGs. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

International audience; A Metallic Glass/Alloy (MeGA) rod with a core in zirconium-based bulk metallic glass and a sleeve in aluminium alloy has been successfully elaborated by co-extrusion. SEM observations of the cross-section of the rod show that the interface between the glass and the alloy is defect-free. Compression tests are carried out at room temperature on the MeGA rods containing various glass volume fractions. The yield stress is well described by the rule of mixtures which combines the strength of the glass and that of the alloy, suggesting isostrain behaviour as could be expected. During compression, a good mechanical bonding is observed in the MeGA-rod even after the first fracture of the metallic glass. Finally, push-out tests are performed to evaluate the bonding quality between the two materials. Large values of the shear strength are measured which confirms that co-extrusion leads to good bonding between the glass and the aluminium alloy.

International audience; It is well known that nickel-free zirconium-based bulk metallic glasses are of interest for their possible good biocompatibility properties. In this framework, a Zr-Nb-Cu-Al-Fe bulk metallic glass was elaborated under the form of 2 mm diameter rods. The amorphous structure was confirmed by XRD and DSC and the associated reduced index were measured The mechanical properties of the glass were investigated at both room and high temperature. At room temperature, the glass exhibits some macroscopic plasticity The rheology at high temperature was studied by both strain rate jump tests and tests at constant strain rate. From these tests, It was difficult to identify an experimental window in which the amorphous structure could be maintained and in which viscoplastic forming could be carried out under low flow stresses. In the super-cooled liquid region, this behaviour is discussed in relation with the resistance to crystallization of the investigated glass and the nature of the formed crystallites was studied by XRD The mechanical properties of this glass are also compared to results previously obtained in the case of other zirconium-based bulk metallic glasses.

The effects on mechanical properties of partial crystallization of a zirconium based bulk amorphous alloy (Vit1) are investigated. Nanocomposites are produced by appropriate heat treatments at temperatures higher than the glass transition temperature. Mechanical properties at room temperature are investigated by compression tests and hardness measurements including nanoindentation. The variation of the fracture stress with the degree of crystallinity is related to the nature, the size and the dispersion of the crystals in the amorphous phase. The variations of microstructure are estimated thanks to differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. A significant connexion between crystals induces a decrease of the fracture stress whereas hardness continuously increases with crystallinity. From nanoindentation tests, Young's modulus and apparent yield stresses were roughly estimated and it is concluded that crystallization tends to increase the yield stress. Nevertheless, AFM observations of the imprints after indentation suggest that the mechanism of deformation can vary significantly with crystallization.

International audience; The fracture toughness of submicron Zr65Ni35 sputter-deposited films is thickness dependent. The failure mechanism involves corrugations for the thickest films and a transition-at 500 nm thickness-to a perfectly flat fracture surface. Both mechanisms are brittle-type, even though the elastic and plastic properties are independent of thickness and similar to the expected ductile bulk response. Finite-element simulations unravel the origin of the size effect and the transition of failure mode from the constraint on plastic dissipation.

International audience; The high temperature rheological behaviors of Zr46Cu38Ag8Al8 and Cu46Zr45Al7Y2 bulk metallic glasses in supercooled liquid region have been investigated by uniaxial strain rate jump compression tests. The results indicate that the high temperature rheological behaviors are sensitive to strain rate and temperature. The rheological behavior will transform from non-Newtonian flow to Newtonian flow with the decrease of strain rate as well as the increase of temperature, which can be explained in the framework of transition state theory. And the rheological behavior of Zr46Cu38Ag8Al8 bulk metallic glass in high temperature is better than that of Cu46Zr45Al7Y2 bulk metallic glass. In addition, the parameter S of both metallic glasses are in a middle level compared with typical bulk metallic glasses, which shown that the metallic glass has an excellent formability in its supercooled liquid region. The optimum domain for thermal workability of these alloys in supercooled liquid region have been roughly located by the deformation map and the power dissipation map, in which the power dissipation efficiency is larger than 0.8. The larger area of optimum domain in the power dissipation map of Zr46Cu38Ag8Al8 exhibited that this bulk metallic glass presents an excellent workability in its supercooled liquid region, but the workability of Cu46Zr45Al7Y2 is not as good as Zr46Cu38Ag8Al8. (C) 2014 Elsevier B.V. All rights reserved.

International audience; The deformation at high temperature of a La60Ni15Al25 bulk metallic glass has been investigated. In the experimental domain in which homogeneous deformation could be obtained, activation energies in agreement with what is usually measured for metallic glasses with similar glass transition temperatures were identified but particularly large values of activation volumes were measured. Such values of activation volumes were attributed to the pronounced secondary relaxation displayed by the investigated glass, suggesting the existence of structural heterogeneities in the microstructure. (C) 2013 Elsevier B.V. All rights reserved

International audience; Multi materials, associating zirconium based bulk metallic glasses and crystalline metallic alloys like magnesium alloys or copper are elaborated by co-deformation processing performed in the supercooled liquid regions (SLR) of the bulk metallic glasses. Two processes are investigated: co-extrusion and co-pressing. In the first case, filamentary composites with various designs can be produced whereas in the second case sandwich structures are obtained. The experimental window (temperature, time) in which processing can be carried out is directly related to the crystallisation resistance of the glass which requires getting information about the crystallisation conditions in the selected metallic glasses. Thermoforming windows are identified for the studied BMGs by thermal analysis and compression tests in their SLR. The mechanical properties of the produced multi materials are investigated thanks to specifically developed mechanical devices and the interfaces between the amorphous and the crystalline alloys are characterised.

International audience; Metallic glasses display interesting mechanical properties including high mechanical resistance and large capacity to store elastic energy. Since metallic glasses can now be produced in bulk conditions, they can be used for structural applications. In this new context, thermoforming of metallic glasses can be a particularly well adapted technique for production of small size components with good surface finish. As for other glasses, metallic glasses can be preferentially thermoformed in their supercooled liquid region. However, since metallic glasses are still emerging materials, the mechanisms of high temperature deformation remain not completely understood and intensive work is currently carried out to identify the elementary mechanisms of deformation. Moreover, since thermoforming is performed in the supercooled liquid region, the thermal stability of the glass to be formed is a key point for the success of the thermoforming process. Finally, it is also shown that thermoforming conditions can be used to produce multi materials associating metallic glasses and conventional metallic alloys.

Bulk metallic glasses are out of equilibrium materials and consequently they tend to crystallize when heated in the supercooled liquid region. Crystallization has been shown to change drastically the properties of the BMG. However, in order to quantify these changes due to crystallization, it is first necessary to get detailed information about the structure evolution and especially the size and volume fraction of the crystallites. Partial crystallization of the Vitreloy 1 metallic glass is carried out in the supercooled liquid region. The crystallization is quantified by DSC and XRD measurements and the obtained values are compared. An important difference is observed and the maximum volume fraction is compared to the value predicted by appropriate numerical simulations. It is shown that XRD measurements seem to be appropriate to calculate the crystallite volume fraction while care must be taken if using DSC measurements.

Due to their brittleness, bulk metallic glasses (BMG) are generally difficult to form at room temperature. Casting of BMG is one way to get components but an alternative route is to use the capacity to reach particularly large strains when the glasses are deformed in their supercooled liquid region (SLR). The experimental window (temperature, time) in which high temperature forming can be carried out is directly related to the glass resistance to crystallization. Such forming windows have been identified for various bulk metallic glasses (mainly zirconium and magnesium based BMG) thanks to compression tests in the supercooled liquid region. The effects of partial crystallization on the high temperature rheologies are also discussed. Finally, forming experiments were carried out in the selected windows.

International audience; High temperature deformation of a ZrTiCuNiBe bulk metallic glass (BMG) is investigated by compression tests in the supercooled liquid region. When temperature is decreased or strain rate is increased, the amorphous alloy exhibits the usual Newtonian – non Newtonian behavior transition. Owing to appropriate heat treatments, partially crystallized alloys are produced, the associated microstructures are characterized and the volume fractions of crystal are measured. The interaction between high temperature deformation and crystallization is investigated by appropriate mechanical tests. According to the mechanical measurements, partial crystallization is responsible for an important increase of the flow stress and a promotion of the non-Newtonian behaviour. The deformation does not change significantly the volume fraction of crystal, the crystal composition neither the size. The flow stress increase with crystallization is analyzed under different hypotheses. It comes out that the flow stress increase can not be interpreted through a composition change in the residual amorphous matrix neither by the reinforcement due to hard crystallites nor by connections between crystals. It finally seems that the effect is rather due to the nanometric size of the crystals.

International audience; The plastic behaviour of amorphous, partially and totally crystallized Zr41.25 Ti13.75 Cu12.5 Ni10 Be22.5 (at.%) Vitreloy1 samples is investigated using self similar instrumented indentation. Residual imprints are imaged using atomic force microscopy (AFM). Young's modulus and hardness are estimated using the value of the contact area derived from AFM imaging. Compressive tests and three point bending ultrasonic measurements are performed to probe the macroscopic properties. Despite a significant change in the shape of the imprint during the loading, we show that the amorphous alloy does not exhibit any size effect. Both for the amorphous and for the partially crystallized samples, the values of the Young modulus and the yield stress obtained by nanoindentation are consistent with those obtained by the macroscopic procedure. Inversely, the full crystallization samples display high brittleness on compressive tests while no cracking but only an increasing yield stress are observed using indentation. It is concluded that indentation is a well-suited test to probe the mechanical properties of the bulk metallic glasses.