Transition in plastic deformation of nanolayered thin films: Role of interfaces and temperature

Insights into the parameters governing the plasticity of immiscible, nanocrystalline metals stacked in the form of layers are pivotal both from scientific and applications’ perspectives. An outstanding case consists of the contact metallurgy of pure copper used ubiquitously as metallic interconnects in electronic devices. Diffusion barrier layers such W or TiN are necessary to prevent undesirable diffusion of Cu into the Si-based device during synthesis and service. Also, supersaturated Cu-Cr alloys are desirable for improving the strength, while retaining optimal functional properties required for the application. The scientific curiosity lies in understanding the effects of reducing microstructural length scales on the mechanical properties of both of these materials at elevated temperatures. In addition, alternate layering with an immiscible element forms a viable solution to the difficultly in synthesis and application of pure nanocrystalline materials due to their poor microstructural stability. The mechanical behavior of several nanolayered thin films consisting of soft and relatively hard metals or brittle ceramics have been extensively studied at ambient conditions [1-3] by using various models predicting strength as function of grain size or layer thickness. But, few have investigated the elevated temperature mechanical response [4] of similar systems and have been restricted to a specific metal (Al) – ceramic (SiC) combination [5]. This presentation attempts to highlight the role of interfaces and diffusion in plastic flow and failure of mutually immiscible, nanolayered systems at elevated temperatures. The nanolayered thin films consist of mainly sub-100 nm thick layers of pure Cu sandwiched by layers of pure metals of Cr and W and a pure ceramic of TiN, which were grown on Si(100) substrates to thickness of 2-5 μm by using direct current magnetron sputtering. The mechanical response at elevated temperatures of the films was studied by compressing micropillars, which were fabricated using a focused Ga+ beam, in situ SEM using an AlemnisÒ indenter modified for high temperature testing. Lateral flow of Cu promoted by stress-assisted diffusion at homologous temperatures as low as 0.35 occurred in all three systems in contrast to interfacial shear-dominated flow at lower temperatures (Fig. 1). Predictions of discrete dislocation and continuum plasticity models were used to evaluate the change in the yield strengths of the films with respect to the layer thicknesses of Cu in the different systems

A constitutive model for unsaturated cemented soils under cyclic loading

On the basis of plastic bounding surface model, the damage theory for structured soils and unsaturated soil mechanics, an elastoplastic model for unsaturated loessic soils under cyclic loading has been elaborated. Firstly, the description of bond degradation in a damage framework is given, linking the damage of soil's structure to the accumulated strain. The Barcelona Basic Model (BBM) was considered for the suction effects. The elastoplastic model is then integrated into a bounding surface plasticity framework in order to model strain accumulation along cyclic loading, even under small stress levels. The validation of the proposed model is conducted by comparing its predictions with the experimental results from multi-level cyclic triaxial tests performed on a natural loess sampled beside the Northern French railway for high speed train and about 140 km far from Paris. The comparisons show the capabilities of the model to describe the behaviour of unsaturated cemented soils under cyclic loading

Reconstructing the trajectory of the August 1680 hurricane from contemporary records

This paper draws on a range of contemporary documentary evidence from the New and Old Worlds as well as from the oceanic regions to reconstruct the trajectory and intensity of an Atlantic hurricane from August 1680. In doing so, it offers the example of one of the earliest and most comprehensive hurricane reconstructions thus far attempted. The source material includes evidence from land-based observers and some of the earliest examples of instrumental barometric data from the Caribbean and from Europe; importantly, it also calls on the written accounts offered in ships' logbooks from various parts of the Atlantic. The latter provide the opportunity of tracking the system across the otherwise data-deficient areas of the North Atlantic as it recurved toward Europe. The findings are of intrinsic interest in documenting a notable historical event. They also offer a methodological model of how such a variety of documentary sources can be drawn together and used to identify, track, and reconstruct such events from the distant past and thereby improve the chronology of hurricanes and make more reliable our interpretation of their changing frequencies

A new dynamic module for in-situ nanomechanical testing at high strain rate

In-situ nanomechanical testing is commonly used to probe surface mechanical properties of bulk materials or thin films, like hardness, Young’s modulus, Yield stress…Actually most of the instruments can measure these properties only statically, i.e. a low frequency, leading to property measurement only at low strain rate (usually 10-1s-1 by nanoindentation). This is mainly caused by the low resonance frequency of the system, preventing making tests at higher speed. Performing high dynamic measurements could bring new information on materials properties like deformation mechanism at high strain rate, or high dynamic fatigue properties. A new high dynamic module usable for in-situ mechanical testing has been developed. It is composed of a small piezotube attached directly behind the tip. Because of the small dimensions of the module, his resonance frequency is very high (higher than 50kHz) in comparison to classical nanomechanical testers, permitting to perform and measure precisely the signals at very high frequency. Moreover, it can be used as a sensor and as an actuator, in x, y and z directions which gives to this module a very large range of measurements. Firstly, the characteristics, the performances and the limits of the new high dynamic module will be presented. Secondly some indentations experiments performed at high strain rate on nanocrystalline nickel with the in-situ nanomechanical tester (Alemnis Gmbh) equipped with the high dynamic will be presented and discussed (Fig. 1). Finally, some micropillar compression at high strain rate on the same material will be described and discussed

Some recent advances in nanomechanical testing: High strain rates, variable temperatures, fatigue and stress relaxation, combinatorial experimentation

In the first part of the talk, I will present two recently developed platforms for high temperature nanomechanical testing. The first platform allows for variable temperature and variable strain rate testing of micropillars in situ in the scanning electron microscope. By utilizing an intrinsically displacement-controlled micro-compression setup, which applies displacement using a miniaturized piezo-actuator, we’ve recently extended the attainable range of strain rates to up to~ 103 s−1, and enabled cyclic loading up to 107 cycles and load relaxation tests. Stable, variable temperature indentation/micro-compression in the range of -45°C to 600°C is achieved through independent heating and temperature monitoring of both the indenter tip and sample and by cooling the instrument frame. A second system allows for measurements at lower loads ex-situ in a dedicated vacuum chamber in the range of -150 °C to 700 °C. The cryo temperature is achieved by means of a liquid nitrogen line, while the high temperature is generated by three independent heat sources for the sample and the two tips of the differential displacement measurement system, establishing an infrared bath in the measurement area. In the second part several case studies will be presented. Using these new capabilities, we examine the plasticity of electrodeposited nanocrystalline Nickel, of combinatorial thin film libraries, of hard nanocrystalline ceramic thin films. Activation parameters such as activation volume and activation energy were determined and discussed in view of the most probable deformation mechanism. High strain rates and cyclic fatigue tests were performed on nanocrystalline Ni. The strain rate sensitivity seems to increase for strain rates higher than 10 s-1 suggesting a change in deformation mechanism with increasing strain rate. Cyclic fatigue tests up to 1 million cycles were performed on nanocrystalline Ni microbeams and compared with existing data from literature. Combinatorial libraries of bulk metallic glasses were synthesized by a combination of gradient sputtering and evaporation. Hardness and Young’s modulus was mapped as a function of temperate, strain rate and composition. The results are discussed in the light of shear band kinetics. Finally, a wide range of chromium nitride-based hard coatings was investigated using in situ micro-cantilever bending and compression testing. This allowed the first direct measurement of the high temperature compressive strength and fracture toughness

Anomalous yielding in the complex metallic alloy Al13Co4

The single crystal deformation behaviour of orthorhombic Al13Co4 hasbeen studied below the brittle-ductile transition temperature observedin bulk material from room temperature to 600 degrees C, usingindentation, microcompression and transmission electron microscopy. Atroom temperature, slip occurred most easily by dislocation motion on the(0 0 1)[0 1 0] slip system, as observed in the ductile regime at hightemperatures. However, as the temperature was increased towards 600degrees C, the slip pattern changed to one consisting of linear defectsrunning perpendicular to the loading axis. Serrated flow was observed atall temperatures, although at 600 degrees C the magnitude of theserrations decreased. Anomalous yielding behaviour was also observedabove 226 degrees C, where both the yield and the 2\% flow stressincreased with temperature, almost doubling between 226 and 600 degreesC. It has been suggested that this might arise due to the increasingstability of orthorhombic Al13Co4 with respect to the monoclinic formwith increasing temperature. This is shown to be consistent with thetheoretical predictions that exist

The Efficacy of Three Objective Systems for Identifying Beef Cuts That Can Be Guaranteed Tender

The objective of this study was to determine the accuracy of three objective systems (prototype BeefCam, colorimeter, and slice shear force) for identifying guaranteed tender beef. In Phase I, 308 carcasses (105 Top Choice, 101 Low Choice, and 102 Select) from two commercial plants were tested. In Phase II, 400 carcasses (200 rolled USDA Select and 200 rolled USDA Choice) from one commercial plant were tested. The three systems were evaluated based on progressive certification of the longissimus as “tender” in 10% increments (the best 10, 20, 30%, etc., certified as “tender” by each technology; 100% certification would mean no sorting for tenderness). In Phase I, the error (percentage of carcasses certified as tender that had Warner- Bratzler shear force of ≥ 5 kg at 14 d postmortem) for 100% certification using all carcasses was 14.1%. All certification levels up to 80% (slice shear force) and up to 70% (colorimeter) had less error (P \u3c 0.05) than 100% certification. Errors in all levels of certification by prototype BeefCam (13.8 to 9.7%) were not different (P \u3e 0.05) from 100% certification. In Phase I, the error for 100% certification for USDA Select carcasses was 30.7%. For Select carcasses, all slice shear force certification levels up to 60% (0 to 14.8%) had less error (P \u3c 0.05) than 100% certification. For Select carcasses, errors in all levels of certification by colorimeter (20.0 to 29.6%) and by BeefCam (27.5 to 31.4%) were not different (P \u3e 0.05) from 100% certification. In Phase II, the error for 100% certification for all carcasses was 9.3%. For all levels of slice shear force certification less than 90% (for all carcasses) or less than 80% (Select carcasses), errors in tenderness certification were less than (P \u3c 0.05) for 100% certification. In Phase II, for all carcasses or Select carcasses, colorimeter and prototype BeefCam certifications did not significantly reduce errors (P \u3e 0.05) compared to 100% certification. Thus, the direct measure of tenderness provided by slice shear force results in more accurate identification of “tender” beef carcasses than either of the indirect technologies, prototype BeefCam, or colorimeter, particularly for USDA Select carcasses. As tested in this study, slice shear force, but not the prototype BeefCam or colorimeter systems, accurately identified “tender” beef

Surface and capillary transitions in an associating binary mixture model

We investigate the phase diagram of a two-component associating fluid mixture in the presence of selectively adsorbing substrates. The mixture is characterized by a bulk phase diagram which displays peculiar features such as closed loops of immiscibility. The presence of the substrates may interfere the physical mechanism involved in the appearance of these phase diagrams, leading to an enhanced tendency to phase separate below the lower critical solution point. Three different cases are considered: a planar solid surface in contact with a bulk fluid, while the other two represent two models of porous systems, namely a slit and an array on infinitely long parallel cylinders. We confirm that surface transitions, as well as capillary transitions for a large area/volume ratio, are stabilized in the one-phase region. Applicability of our results to experiments reported in the literature is discussed.Comment: 12 two-column pages, 12 figures, accepted for publication in Physical Review E; corrected versio

Entropy of Lovelock Black Holes

A general formula for the entropy of stationary black holes in Lovelock gravity theories is obtained by integrating the first law of black hole mechanics, which is derived by Hamiltonian methods. The entropy is not simply one quarter of the surface area of the horizon, but also includes a sum of intrinsic curvature invariants integrated over a cross section of the horizon.Comment: 15 pages, plain Latex, NSF-ITP-93-4

Correlation between muonic levels and nuclear structure in muonic atoms

A method that deals with the nucleons and the muon unitedly is employed to investigate the muonic lead, with which the correlation between the muon and nucleus can be studied distinctly. A "kink" appears in the muonic isotope shift at a neutron magic number where the nuclear shell structure plays a key role. This behavior may have very important implications for the experimentally probing the shell structure of the nuclei far away from the $\beta$-stable line. We investigate the variations of the nuclear structure due to the interaction with the muon in the muonic atom and find that the nuclear structure remains basically unaltered. Therefore, the muon is a clean and reliable probe for studying the nuclear structure. In addition, a correction that the muon-induced slight change in the proton density distribution in turn shifts the muonic levels is investigated. This correction to muonic level is as important as the Lamb shift and high order vacuum polarization correction, but is larger than anomalous magnetic moment and electron shielding correction.Comment: 2 figure