Mechanical properties of tailored nanostructured alloys produced by electrodeposition

The studies presented in this thesis aim to extend the current knowledge and understanding of the mechanical behavior of nanocrystalline materials with respect to temperature- and time dependence. Free standing electrodeposited nanocrystalline nickel specimen with mean grain sizes in the order of 40nm are synthesized and investigated. A miniaturized in situ test rig is developed, capable of performing uniaxial tensile tests on dedicated miniaturized specimen. A novel displacement controlled in situ high temperature indentation system is presented, capable of performing sharp tip indentation and uniaxial micropillar compression experiments up to 600°C. Elevated temperature micropillar strain rate jump tests are performed to extract the strain rate sensitivity factor, apparent activation volume and activation energy. The results suggest grain boundary diffusion to be the rate controlling deformation mechanism. Post deformation imaging also indicates the activity of dislocation based mechansims. To compare the results to established techniques, micro specimen are tested under three different load cases: uniaxial tension, uniaxial compression and hydrostatic pressure. The results compare well to the previous, suggesting the rate controlling mechanism to be the same for all three load cases. A transient load relaxation tests is developed to assess time dependent plasticity on short and intermediate time scales. The results suggest grain boundary mediated processes to be rate controlling. To enhance the identification of specific deformation mechanisms, elevated temperature micropillar strain rate jump- and load relaxation tests are also conducted on an inert gas condensated nanocrystalline palladium-gold model alloy. Distinct features of this material are extremely high chemical purity and small, homogeneous grain sizes in the order of ~10nm. Shear transformation mediated plasticity is observed, similar to the deformation of metallic glasses. The demonstrated methods are relevant for the Swiss watch industry. A series of mechanical tests on standard watch materials are compared. An approach for the electrodeposition of a nickel-tungsten alloy for watch applications with improved thermal stability and less pronounced time dependent plastic behavior is presented

Diffusion-based deformation in elevated temperature micropillar compression of Mg-Nb multilayers

multilayer, elevated-temperature, micropillar compressio

Comparison of in situ micromechanical time dependent plasticity techniques: micropillar compression, nanoindentation and micro-tensile tests

Nanocrystalline metals exhibit strongly time-dependent plastic deformation. This results in a high degree of strain-rate sensitivity and susceptibility to creep and relaxation, even at room temperature. With the advent of thin film processing techniques like sputtering and electrodeposition for fabrication of controlled microstructures of nanocrystalline materials, nanoindentation and microcompression techniques are increasingly used to extract time dependent plasticity parameters from stress relaxation, creep and strain rate sensitivity measurements [1, 2]. However, no systematic comparison of the micromechanical experimental techniques (nanoindentation and microcompression) has been performed on the same material to establish the relative merits and consistency of these results with their counterpart “bulk” tests. This poster will present experimental data from micro-tensile, micropillar compression and nanoindentation tests on nanocrystalline nickel at room temperature to directly compare and validate the test results for stress relaxation and strain rate sensitivity measurements. Microcompression and nanoindentation tests were performed on the non-deformed gripper section of the tensile bar to ensure that the same material is interrogated so as to rule out sample-to-sample variations. The extracted time dependent plasticity exponents and apparent activation volumes will be compared for all three test types and the possible rate controlling deformation mechanism(s) will be discussed. It is hoped that this study will conclusively bridge the gap between uniaxial bulk, uniaxial microcompression and triaxial nanoindentation tests

In situ micromechanical testing inside the scanning electron microscope at subambient temperatures

In material science, the measurement of mechanical material properties as a function of temperature is of great interest, as it allows determining the activation parameters of the underlying deformation mechanism. In the case of nanostructured materials or MEMS devices, it is interesting to probe local properties by means of micromechanical experiments. However, in the case of nanograined metals, testing at elevated temperatures is not possible due to heat induced grain growth and thus changes in the microstructure during testing. We developed a device that allows performing micromechanical tests at low temperatures down to -150°C inside a scanning electron microscope. A cold finger connected to the sample and tip holders by copper braids is cooled by circulating nitrogen. Independent thermal management of the indenter and the sample allows minimizing temperature differences and thereby drift. Local cooling, thermal isolation of the cold regions, and a closed loop frame temperature control reduce frame drift, noise, and the need for a temperature-dependent calibration. A symmetric design of the cooling bodies was chosen in order to minimize bending moments on the indenter and the sample, which increases the accuracy of the measurements. The high vacuum environment minimizes condensation of water vapor and hydrocarbons on the indenter and sample. Positioning as well as in situ observation is made possible by the use inside a scanning electron microscope. For validation of the system, indentations in Cu were performed down to -150°C. Please click Additional Files below to see the full abstract

High-temperature small-scale fracture mechanics and plasticity of a hardcoating system

Forging and cutting tools for high-temperature applications are often protected using hard nanostructured ceramic coatings. While a moderate amount of knowledge exists for material properties at room temperatures, significantly less is known about the system constituents at the elevated temperatures generated during service. For rational engineering design of such systems, it is therefore important to have methodologies for testing these materials to understand their properties under such conditions. Additionally, small-scale mechanical testing is of inherent importance for thin-films systems and materials subject to surface modification or treatment as for plasma nitrided steels. In this work, we present results on both the hard ceramic coating and the nitrided steel substrate using in situ micro-mechanical measurements at temperatures to 500 °C. The fracture and plastic yield behavior of FIB milled micro-pillars of plasma-nitrided tool steel was first investigated using in situ compression experiments. It was found that the yield strength of nitrided steel is particularly sensitive to temperatures within the service range. Elevated temperature led to significant softening of the nitrided steel and transition from slip-based to more ductile plastic flow. A 70% reduction in yield stress was observed when transitioning from room-temperature to 500 °C, which was then recovered upon cooling back to RT indicating a mechanistic activation at high temperature. The fracture toughness behavior of a hard CrN coating was then investigated using various micro-geometries and notching parameters. Toughness measurements at high temperatures highlighted the profound effect of the notching ion during small-scale fracture measurements. It was found that gallium ion implantation led to significant toughening of CrN, based on gallium dosage experiments and alternative notching using both xenon and helium sources. The effect of different notching ions was additionally understood through Monte Carlo simulations of energetic ion interactions in a dense ceramic matrix

The brittle-ductile transition of tungsten single crystals at the micro-scale

The process zone or plastic zone around a loaded crack tip can significantly influence the fracture behavior of a material. Especially in micro-scale specimens, the plastic zone size may make out a large share of the sample volume and lead to a different fracture behavior than the one usually observed for macroscopic samples of the same material. Furthermore, the theoretical description of the plastic zone according to Irwin is not valid for single crystals. Therefore, a characteristic elastic-plastic fracture behavior is observed depending on crystallographic sample orientation and slip system activation. It is the aim of the study to understand the fracture process and behavior in micro-scale specimens in the presence of crack tip plasticity. Notched micro-cantilevers were prepared by focused ion beam (FIB) milling in a tungsten single crystal. This material has nearly perfect elastic isotropy, a limited amount of activated slip systems and detailed knowledge of the macroscopic fracture behavior is available [1]. The cantilevers have dimensions of 25 µm in length, 5-7 µm in thickness and crack length to thickness ratios a/w of ca. 0.4. Loading rate and temperature are known to influence the fracture behavior decisively in bcc metals. Therefore displacement-controlled fracture tests were performed inside a scanning electron microscope in the temperature range between -150°C and 500°C. Applying the recently presented J-Integral technique [2] to plot continuous crack resistance curves, the fracture toughness and brittle-to-ductile transition (BDT) temperatures, which depend on the applied loading rate, were determined. This allows a thorough investigation of the activation energy of the BDT at the micro-scale. Crack tip plasticity before and during crack growth was investigated by high-resolution electron backscatter diffraction measurements (HR-EBSD) on FIB cross-sections of the micro-cantilevers after mechanical testing. Plastic zones, which are strongly depending on the activated slip systems, and plastic strain gradients in terms of geometrically necessary dislocations were quantified and linked with the observed BDT behavior. Transmission electron microscopy was used to confirm the EBSD results and to provide dislocation analysis. [1] P. Gumbsch, J. Riedle, A. Hartmaier, H.F. Fischmeister, Controlling Factors for the Brittle-to-Ductile Transition in Tungsten Single Crystals, Science. 282 (1998) 1293–1295. [2] J. Ast, B. Merle, K. Durst, M. Göken, Fracture toughness evaluation of NiAl single crystals by microcantilevers - a new continuous J-integral method, Journal of Materials Research. 31 (2016) 3786–3794

Elevated temperature microcompression transient testing of nanocrystalline materials: Creep, stress relaxation and strain rate jump tests

Traditionally, time-dependent properties of nanocrystalline metals have been measured on bulk samples. With the advent of thin film deposition techniques like sputtering and electrodeposition for fabricating nanocrystalline materials, it has become necessary to adapt bulk mechanical testing for thin films. Nanoindentation has been extensively applied for this purpose, particularly on thin films where conventional testing is difficult or impossible, and has been demonstrated to successfully extract strain rate exponents [1]. However, the interpretation of the indentation results can be difficult due to the complex stress state, and the nearly instantaneous onset of large-strain plasticity. Microcompression, on the other hand, is advantageous due to the relatively simple, well understood uniaxial stress state. In this talk, micro-compression creep, stress relaxation and strain rate sensitivity [2] testing performed on nanocrystalline Ni at elevated temperatures (25-125 °C) will be described. All tests were performed on the same sample to remove sample-to-sample variation and allow direct comparison to help understand the correlation between these three time dependent tests. The observed stress relaxation and creep behaviors were found to be significant at stresses even below the 0.2% offset yield strength. Strain rate jump and creep tests yielded strain rate sensitivity and creep stress exponents as a function of temperature. Elevated temperature studies permit the extraction of activation parameters (activation volume and activation energies) that provide an initial estimate of the footprint of the dominant deformation mechanisms. The activation parameters were compared for all the three tests. Based on the results from these studies, possible rate controlling deformation mechanism(s) will be discussed. Overall, this study aims to bridge the gap between the three time-dependent tests and provides useful insights into developing similar indentation based tests, for creep and stress relaxation measurements in particular

Identification of in situ lignin strength based on micropillar compression and micromechanical modeling of wood cell walls

Many biological materials feature a hierarchical architecture with remarkable mechanical properties combining low weight with both toughness and strength. In order to better understand the mechanisms leading to this unusual combination of traits, structure-property relationships have to be assessed on all length scales. Wood is such a hierarchical material. Its cell walls feature semi-crystalline cellulose fibrils embedded in an amorphous polymer network that are aligned at an angle to the cell main axis resembling a fiber reinforced composite. Continuum micromechanics can predict mechanical behavior on a higher length scale based on the composition, microstructure, and properties of the individual phases. However, the experimental data for yield properties at the microscale is sparse making an identification of phase properties and validation of yield predictions difficult. Specifically, the lignin shear strength in wood remains to be measured, which proves to be difficult due to the intermixed nature of the polymer network and the small length scales involved. Inverse determination of phase properties from experiments on a higher length scale is possible using continuum micromechanics, if composition, microstructure, and boundary conditions are sufficiently well understood. An experimental setup for micromechanical testing with well-defined boundary conditions is micropillar compression. Micron sized pillars are eroded from bulk cell wall material using a focused ion beam and compressed uniaxially using a flat punch indenter. Due to the mostly homogeneous and uniaxial loading conditions, the experimental data may be combined with micromechanical modeling to access phase properties at a lower length scale. The aim of this work was to perform micromechanical tests leading to homogeneous and uniaxial stress fields on a single cell wall layer for normal (NW) and compression wood (CW) of Norway spruce. Additionally, the chemical composition was determined by wet chemical analysis and the cellulose fibril angle distribution was measured using wide angle XRD. Subsequently, a continuum micromechanics model for elastic limit states was used to explain the measured properties and to relate them to species-independent phase properties on a lower length scale, more specifically the lignin yield stress. The study demonstrates a novel approach for measuring phase properties of inhomogeneous materials by a combination of continuum micromechanical modeling and micropillar compression experiments inside a scanning electron microscope under controlled conditions. The mostly homogeneous and uniaxial stress state in this experimental setup allows to identify yield stresses at the microscale and to assess phase properties on a lower length scale with high accuracy and reproducibility if the microstructure and the inelastic deformation mechanisms of the tested material are well understood. This could be an interesting approach for validating multiscale models or identifying phase properties for other nanostructured materials in the future

Anisotropic deformation of ZrB2 ceramic grains during in-situ micropillar compression up to 500°C

Deformation behaviour of ZrB2 micropillars was investigated under ex-situ and in-situ micro compressions at room and elevated temperatures up to 500°C. Pillars were fabricated by focused ion beam (FIB) in large grains nearly basal and prismatic orientations which were selected by means of electron backscatter diffraction (EBSD). Micro-compressions were carried out by both ex-situ and in-situ nanoindenter machines equipped with flat punch diamond tips. In the latter case, the deformation process was simultaneously captured by a scanning electron microscope (SEM). At room temperature, ex-situ measurements revealed considerable anisotropy showing ~80% higher yield () and rupture () stress values for the basal oriented pillars (, ) compared to the prismatic pillars (, ). Careful analysis of the compressed pillars revealed the activation of the type slip system in prismatic orientation both in the form of single- and multiple-slip which has not been reported so far in the relevant literature. In-situ micro-compressions at elevated temperatures revealed that the rupture stress of basal pillars was comparable with that was measured at room temperature showing their catastrophic collapse with the presence of slight crystal plasticity (Fig.1). In case of prismatic oriented pillars, the scenario of slip activation controlled plastic deformation was acquired showing an increase deformation without failure with increasing temperature (Fig.1). Additionally, the yield and rupture stress values were significantly reduced down to at 500°C. These findings suggest that the slip activation in ZrB2 ceramic grains strongly depends on the temperature. Please click Additional Files below to see the full abstract

High-Temperature In situ Deformation of GaAs Micro-pillars: Lithography Versus FIB Machining

The plasticity of silicon-doped GaAs was investigated between 25°C and 400°C using microcompression to prevent premature failure by cracking. Micropillars with diameters of ~2.5 μm were fabricated on a ⟨100⟩-oriented GaAs single crystal by means of both conventional lithographic etching techniques and focused ion beam machining and then compressed in situ in the scanning electron microscope (SEM). A transition in deformation mechanisms from partial dislocations to perfect dislocations was found at around 100°C. At lower temperatures, the residual surface layer from lithographic processing was found to provide sufficient constraint to prevent crack opening, which resulted in a significant increase in ductility over FIB-machined pillars. Measured apparent activation energies were found to be significantly lower than previous bulk measurements, which is mostly attributed to the silicon dopant and to a lesser extent to the size effect.ISSN:1047-4838ISSN:1543-185