Function Of Crystallographic Orientation Research Articles

Compositional effects on etching of fossil confined fission tracks in apatite

Abstract Fission-track analysis is a thermochronologic method for dating rocks and reconstructing their low-temperature thermal histories. We investigate the influence of the apatite composition on the etching of fossil confined fission tracks, and its consequences for the fission-track method. We conducted step-etch experiments with 5.5 M HNO3 at 21 °C on samples with etch pit diameters (Dpar) spanning most of the range for natural apatites (Panasqueira: 1.60 µm, Slyudyanka: 2.44 µm, Brazil: 3.92 µm, and Bamble: 4.60 µm) to determine their apatite etch rates vR (the rate at which each lattice plane is displaced parallel to itself) as a function of crystallographic orientation (ϕ’). Our measurements revealed significant differences between the four samples. We fitted three-parameter functions, vR = a(Dpar) ϕ’ eb(Dpar)ϕ’ + c, describing vR as a function of the angle to the apatite c-axis for our hexagonal samples (excluding Bamble) and Durango apatite. The parameters a and b both exhibit a linear correlation with Dpar, whereas the constant c is small (~0.1 µm.min-1) and its between-sample variation negligible at the resolution of our measurements. Bamble exhibits a different, bimodal relationship between vR and ϕ’, which we fitted with a sum of two sine functions. In all cases, including Bamble, there is a striking correlation between the angular frequencies of horizontal confined tracks and the magnitude of the apatite etch rate vR perpendicular to the track axes. This shows that the sample of confined tracks selected for measurement and modeling is to a much greater degree determined by the etching properties of the apatite sample than by geometric or subjective biases. The track etch rate vT is constant along most of the track length but varies from track to track. The mean vT correlates with Dpar, so that tracks etch to their full lengths in a shorter time in faster etching apatites. The mean rate of length increase between etch steps, vL, also correlates with Dpar. The length increments of individual tracks are however irregular. This points to an intermittent structure at the ends of the tracks.

In Situ Fracture Behavior of Single Crystal LiNi0.8Mn0.1Co0.1O2 (NMC811)

AbstractSingle crystal particle morphologies have become highly desirable for next generation cathode materials, removing grain boundary fracture and thereby reducing the surface area exposed to electrolyte. The intrinsic mechanical behavior of single crystal layered oxides, however, is poorly understood. Here, faceted single crystal LiNi0.8Mn0.1Co0.1O2 (NMC811) particles are compressed in situ in a scanning electron microscope (SEM), to determine mechanical deformation mechanisms as a function of crystallographic orientation. In situ, the dynamical deformation sequence observed is initial cracking at the compression zone, followed by accelerated transparticle crack propagation and concurrent (0001) slip band formation. The greatest loads and contact pressure at fracture, non‐basal cracking, and activation of multiple basal slip systems in larger (>3 μm) particles, occur for compression normal to the (0001) layered structure. Loading on {012} preferentially activates basal fracture and slip at lower loads. Regardless of particle orientation, non‐basal slip systems are not observed, and non‐basal cracking and particle rotation occur during compression to compensate for this inability to activate dislocations in 3‐dimensions. Crystallographic dependent mechanical behaviour of single crystal NMC811 means that particle texture in cathodes should be monitored, and sources of localised surface stress in cathodes, e. g. particle‐to‐particle asperity contacts during electrode manufacture, should be minimised.

A unified model for yield strength and plastic behavior of nanovoid evolution in tungsten based on molecular dynamics simulations

Void evolution is closely related to the damage and failure of materials under dynamic conditions, such as high strain rate. The rearrangement of atoms on the entire void surface results in microscopic plastic behaviors like dislocation emission and twinning on the surface and further macroscopic plastic flow nearby. In this work, molecular dynamics simulations are conducted to study the compression flow stress and the corresponding plastic behavior of nanovoid inside monocrystal tungsten as a function of crystallographic orientation, void size, and strain rate. Colorful plastic deformation mechanisms, like shear loops, prismatic dislocation loop, twinning, amorphization, etc., are observed at different strain rates and for compression along various crystallographic orientations. A Kelvin tetrakaidecahedron, formed before yielding, is utilized to determine the critical shear stress for initiation of plastic deformation on void surface within the framework of yield criterion and facture mechanics. A unified model is thus proposed based on the macroscopic hardening effect raised by plastic strain and strain rate, and the microscopic plastic behavior of dislocation emission related to the crystallographic orientation and nanovoid size. The newly proposed unified equation gives a good recount on the yield strength and plastic behaviors obtained from molecular dynamics simulations, but also shed lights on the future works of crack members, inclusions, voids, and porous materials at extremely high strain rate.

Small-scale mechanical response at intermediate/high temperature of 3D printed WC-Co

The present work is focused on WC-Co dry-electropolished. A systematic nanomechanical study of a 3D printed WC-Co grade is investigated. In doing so, nanoindentation technique is implemented and the main deformation/damage mechanisms induced at the submicrometric length scale are investigated at different temperatures, from room up to 600 ºC. In general, three different approaches are followed to accomplish this research: (1) assessment of intrinsic hardness values as a function of crystallographic orientation from a room temperature up to 600 ºC and (2) the determination of effective hardness and flow stress through the Tabor’s equation of the metal cobalt binder. Finally, the elastic strain to break was also determined for the main crystallographic orientations for the WC particles as a function of the temperature. The preliminary results highlight that the strength reduction with increasing temperature is attributed to metallic binder softening. On the other hand, the WC particles presents an isotropic behavior when the testing temperature is over 500 ºC because, to inside these particles, the dislocations and the stacking faults are the main deformation mechanisms induced at intermediate/high testing temperature.

Atomistic study of the effect of crystallographic orientation on the twinning and detwinning behavior of NiTi shape memory alloys

Understanding the effect of crystallographic orientation on the twinnin/detwinning mechanisms in NiTi shape memory alloys at an atomistic scale can help to control and tune the mechanical properties and failure behavior of such materials. In this work, we employed classical molecular dynamics (MD) and density functional theory (DFT) computational methods to better understand how twinning and detwinning occurs through a combination of slip, twin, and shuffle on 〈010〉, 〈110〉, and 〈111〉 crystallographic orientations under uniaxial tensile test. Elastic constants including Young's Modulus (E), Bulk modulus (B), Poisson’s ratio (ν), and Shear Modulus (G) are obtained and computed for resultant stress-induced martensite variants as a function of crystallographic orientation using DFT calculations. In addition, computational nanoindentation tests are carried out using MD simulations to evaluate the effect of crystallographic orientation on the twinning and detwinning characteristics in martensite in NiTi alloys under sphere indenter, both qualitatively and quantitatively. Based on a careful polyhedral template matching (PTM) and dislocation analysis (DXA) by taking into account the textures, it is determined that the microscopic stress-strain and load-displacement responses strongly depend on the crystallographic orientation. Our findings reveal that the size of twinned and detwinned zones in martensite increases in the order of 〈111〉 < 〈010〉 < 〈110〉. Based on DFT results, against 〈111〉 direction, abrupt changes in the free energy-strain curves occurs at 4% strain in 〈001〉, and 8% strain in 〈110〉 directions. The twinning and detwinning mechanisms are controlled by monoclinic martensite (B19′) → orthorhombic martensite (B19) phase transformation in 〈110〉 orientation and by body-centered orthorhombic martensite (BCO) → an intermediate structure (B19″) → monoclinic martensite (B19′) phase transformation in 〈001〉 orientation. Finally, the predicted orientation-dependent critical energy release rate is analyzed to examine the effect of the twinning and detwinning process on the fracture toughness of the material. Our results show that reducing the density of twins results in increasing the critical energy release rate. Therefore, the fracture stress intensity increases in the order of 〈001〉 < 〈110〉 < 〈111〉.

Orientation-dependent plastic deformation mechanisms and competition with stress-induced phase transformation in microscale NiTi

Understanding the orientation-dependent deformation behavior of NiTi shape-memory alloys at small length scales is of importance for designing nano- and micro-electromechanical systems. However, a complete understanding of the orientation- and size-dependent competition between the various modes of slip, deformation twinning, and martensitic transformation in NiTi shape-memory alloys is still lacking, especially in micron-scale specimens. In the present study, we perform micro-compression tests on [001]- and [112]-oriented micro-pillars of a solutionized Ti-49.9at.% Ni alloy. Post-mortem TEM analysis of the deformed pillars reveal that the operating plastic deformation modes are {011}<100> slip and {114}<221¯> deformation twinning, which compete with the martensitic transformation, depending on the crystal orientation. Furthermore, in both experiments and molecular dynamics simulations, we consistently find residual B19′ martensite in a herringbone microstructure composed of finely spaced (001)B19′ compound twins instead of the generally assumed [011]B19′ type II twins common in bulk samples, suggesting that the operative martensitic transformation mode may be size-dependent. Schmid factors in compression are calculated for all commonly reported slip, deformation twinning, and martensitic transformation modes as a function of crystallographic orientation, which rationalize the orientation-dependent competition between these deformation modes.

Breaking of inversion symmetry in NdGaO3

$\mathrm{NdGa}{\mathrm{O}}_{3}$ (NGO) is an important substrate material often used in depositing functional oxide thin films. We have carried out temperature-dependent polarized Raman and infrared spectroscopy studies along with theoretical calculations on single-crystal NGO, which revealed a structural phase transition from centrosymmetric space group Pbnm to noncentrosymmetric space group $Pbn{2}_{1}$ at \ensuremath{\sim}200 K. The microscopic origin of the phase transition is extracted using a unique protocol involving polarized Raman spectroscopy (PRS) studies carried out as a function of crystallographic orientation. Here, we demonstrate the power of PRS in probing directional atomic rearrangements across the phase transition.

Crystallographic orientation-dependent strain hardening in a precipitation-strengthened Al-Cu alloy

While the strengthening of Al-Cu alloys due to precipitation has been extensively studied, the effect of crystallographic orientation of the matrix and precipitates, as well as precipitate morphology, on the strain hardening behavior is not well understood. Here we investigate this effect with in situ neutron diffraction during deformation of an Al-Cu alloy (206) after multiple aging treatments. Precipitate-dislocation interactions were found to change from precipitate shearing for microstructures predominantly containing GPI and θ′′ precipitates to Orowan looping for microstructures with primarily θ′ and θ precipitates. Notably, significant anisotropy in strain hardening behavior was observed when θ′ precipitates were present, which was attributed to crystallographic orientation dependent load transfer from the Al matrix to the θ′ precipitates. The anisotropic load transfer is hypothesized to be caused by the extent of rotation of high aspect-ratio θ′ precipitates, owing to dislocations looping around them during plastic deformation of the matrix. Predictions from an analytical model describing the anisotropic magnitude of load transfer from precipitate rotation agree well with experimental results, successfully validating the precipitate rotation hypothesis and explaining the anisotropic strain hardening behavior. This model allows for the prediction of stresses separately in the precipitate and matrix phases as a function of crystallographic orientation, only given the bulk mechanical properties.

Shear anisotropy-driven crystallographic orientation imaging in flexible hexagonal two-dimensional atomic crystals

Hexagonal two-dimensional (2D) atomic crystals commonly reveal intrinsically isotropic elastic properties, but stretching or bending deformation can lead to their mechanical symmetry breaking. So far, little work has been done on strain-induced in-plane anisotropic shear behaviors of such 2D atomic crystals. Here, in theory, we predict the appearance of in-plane shear stiffness anisotropy under uniaxial normal tension strain in monolayer molybdenum disulfide. We verify experimentally such a shear characteristic based on friction-driven stretch deformation during a contact scan by employing transverse shear microscopy, and we demonstrate the visualization of anisotropic shear deformation as a function of crystallographic orientation. The present work provides deep insights into flexibility governed interactions among friction, deformation, and in-plane elastic characteristics in 2D atomic crystals.

Crystallographic orientation effects of hydrogen diffusion in α-uranium from DFT: Interpreting variations in experimental data

Experimental values of hydrogen diffusion coefficients in bulk α-uranium obtained at elevated temperatures have significant scatter, leading to dramatic differences in extrapolated diffusion coefficients at room temperature. Previous density functional theory (DFT) calculations predicted higher diffusion barriers than experiments, suggesting hydrogen diffusion through undefected bulk is not possible. In this manuscript DFT calculations of hydrogen diffusion coefficients as a function of crystallographic orientation in α-uranium are reported. The findings explain previous discrepancies, show that diffusion of hydrogen through bulk uranium is consistent with experiments, and suggest a reasonable value for room temperature diffusion coefficient based on proposed pathway.

The isotope effect for Mg-Fe interdiffusion in olivine and its dependence on crystal orientation, composition and temperature

Isotopic fractionation associated with diffusion in crystals is the most reliable means of understanding the origin of mineral zoning in igneous and metamorphic rocks. We have experimentally determined the relative diffusivities of iron isotopes in olivine as a function of crystallographic orientation, composition, and temperature. For two isotopes i and j of an element, the isotope effect for diffusion is parameterized as Di/Dj = (mj/mi)β, where β is a dimensionless parameter, and D and m stand for diffusivity and mass, respectively. A series of single crystal diffusion couple experiments were conducted at an oxygen fugacity of QFM – 1.5 at temperatures of 1200, 1300, and 1400 °C. For the Fo83.4-Fo88.8 composition pair, βFe is isotropic and a value of 0.16 ± 0.09 can be used to describe diffusion along all major crystallographic axes in olivine. Based on our experiments and previously reported coupled Mg-Fe isotopic data, we also estimate βMg = 0.09 ± 0.05 for this range of olivine composition. For the Fo88.8-Fo100 composition pair, βFe becomes anisotropic with βFe [100] = 0.11 ± 0.03, βFe [010] = 0.14 ± 0.03 (both within error of the value measured for the Fo83.4-Fo88.8 pair), and βFe [001] = 0.03 ± 0.03. For Fo# between 83.4 and 100, βFe [100] and βFe [010] are thus independent of composition. The reason why βFe [001] transitions from ∼0.16 to ∼0.03 close to the Mg-endmember is unclear. Over the temperature range studied, a dependence of βFe on temperature was not resolved. General analytical expressions are introduced to calculate isotopic fractionation as a function of distance, time, β, and the concentration contrast between the diffusing media for spherical, cylindrical, and planar geometries.

Atomic-Scale Insights into the Oxidation of Aluminum.

The surface oxidation of aluminum is still poorly understood despite its vital role as an insulator in electronics, in aluminum-air batteries, and in protecting the metal against corrosion. Here we use atomic resolution imaging in an environmental transmission electron microscope (TEM) to investigate the mechanism of aluminum oxide formation. Harnessing electron beam sputtering we prepare a pristine, oxide-free metal surface in the TEM. This allows us to study, as a function of crystallographic orientation and oxygen gas pressure, the full oxide growth regime from the first oxide nucleation to a complete saturated, few-nanometers-thick surface film.

Angle-dependent electron spin resonance of [formula omitted] measured with planar microwave resonators and in-situ rotation

We present a new experimental approach to investigate the magnetic properties of the anisotropic heavy-fermion system YbRh2Si2 as a function of crystallographic orientation. Angle-dependent electron spin resonance (ESR) measurements are performed at a low temperature of 1.6K and at an ESR frequency of 4.4GHz utilizing a superconducting planar microwave resonator in a 4He-cryostat in combination with in-situ sample rotation. The obtained ESR g-factor of YbRh2Si2 as a function of the crystallographic angle is consistent with results of previous measurements using conventional ESR spectrometers at higher frequencies and fields. Perspectives to implement this experimental approach into a dilution refrigerator and to reach the magnetically ordered phase of YbRh2Si2 are discussed.

Substitution and diffusion of Cr2+ and Cr3+ in synthetic forsterite and natural olivine at 1200–1500 °C and 1 bar

The diffusion and substitution mechanisms of Cr in forsterite were studied as a function of crystallographic orientation and the chemical potentials of all four components in the system MgO-SiO2-Cr-O. Oxygen fugacity (fO2) was varied over 15.4 log units at 1400 °C and was fixed at the iron-wüstite equilibrium for a temperature series (1200–1500 °C). The valence state changes of Cr along some diffusion profiles was also investigated using X-ray absorption near edge structure spectroscopy.Cr diffusion was found to be anisotropic (fastest along the c axis), and considerably faster in the presence of protoenstatite (high silica activity, aSiO2) than in the presence of periclase (low aSiO2). Cr diffusion profiles were longer at lower fO2 with more extreme diffusive anisotropy at higher fO2. Determined Cr diffusion coefficients were similar to those of Fe-Mg inter-diffusion in olivine at equivalent conditions.The diffusivity of Cr was found to be a function of its own concentration, but, as an approximation, Cr diffusion (in m2/s) along the c axis of pure forsterite, at an fO2 corresponding to iron-wüstite at 200 µg/g Cr could be described using:logDCr=-4.37±0.26m2/s+-316±8kJ/mol2.303RTwhere R is the gas constant in kJ/Kmol and T is the temperature in Kelvin.In forsterite, the valence state ratio of chromium (Cr2+/ΣCr, where ΣCr= Cr2++Cr3+) changed systematically along diffusion profiles. The equilibrium Cr2+/ΣCr ratio imposed by the fO2 of the experiment was observed at the diffusion interface, and this changed to a constant value in the crystal interior.Cr3+ always substitutes onto an octahedral site in olivine. At high aSiO2, Cr3+ uses M site vacancies ([vac]) for charge balance (i.e. Cr4/33+[vac]2/3SiO4), whereas at low aSiO2 a new mechanism was observed with charge balance achieved by Mg2+ on the tetrahedral site producing a ‘spinel type’ substitution, Cr23+MgO4. Cr2+ always substitutes directly for Mg2+, forming the Cr22+SiO4 substitution.Comparative experiments using San Carlos olivine showed slightly faster Cr diffusion, a lesser concentration dependence of diffusion and no Cr valence state change along diffusion profiles.

Pulse/Pulse-Reverse Electrodeposition of Copper Electrocatalysts for CO2 Reduction to Ethylene

There have been many attempts to find efficient approaches to reduce CO2 to various organic compounds due to the industrial need for a carbon source and the general need to mitigate the large amounts of CO2 generated by human activities. However, a recent DOE report indicates that major scientific challenges still exist to realize development of an efficient, inexpensive, and durable catalytic system that efficiently facilitates complex, multi-electron and atom/ion transfer events [[1]]. Modern carbon emission mitigation efforts have focused primarily on carbon capture and sequestration (CCS) [[2]]. Despite challenges extant in developing technologies for CO2 conversion to value-added products, such capabilities are anticipated to reduce risk and offset the cost of CCS development and thus encourage broader industrial adoption of CO2 mitigation processes. Recent work toward development of efficient, selective, and active copper electrocatalysts for reduction of CO2 to ethylene is presented. Copper is well known as a unique CO2 reduction electrocatalyst, capable of forming alcohol (e.g., ethanol) and hydrocarbon (e.g., methane, ethylene) products in addition to aldehydes, carboxylic acids, carbon monoxide and hydrogen [[3],[4]]. Ethylene is of specific interest, as its role as a platform feedstock for the chemicals and plastics industries affords it an appreciably higher market value than many other potential products [[5]]. However, the catalytic activity and selectivity achieved to date with Cu catalysts and CO2-saturated aqueous carbonate solutions have not been sufficient to enable development of an industrially viable process. Faraday and MIT are currently investigating pulse/pulse-reverse electrodeposition methods as a means to fabricate copper catalysts with greater activity and ethylene selectivity in order to enable large-scale electrocatalytic conversion of CO2. The microstructure of metallic catalysts is known to influence various properties including selectivity and activity; for copper in particular, a strong effect on the selectivity for ethylene versus methane as a function of crystallographic orientation has been reported [[6],[7]]. Given that pulsed electrodeposition is known to have a significant effect on the microstructure of the resulting metal films [[8],[9]], the technology is a natural candidate for fabrication of novel, high-performance copper CO2 reduction catalysts. In this work, catalyst activity and selectivity were further enhanced through the use of a modified literature activation protocol involving aerobic thermal oxidation followed by electrochemical reduction [[10]]. Photographs of a representative catalyst coupon at each stage of fabrication are presented in Figure 1. This talk will present data confirming that catalytic properties can be enhanced by tuning both the pulsed waveform used for deposition as well as the conditions used in the oxidation/reduction activation protocol. We are focusing development toward the use of aqueous solutions containing CO2-solubilizing additives such as room-temperature ionic liquids and cyclic amines. These materials have been extensively studied as a liquid-phase capture media for removing CO2 from post-combustion exhaust streams (e.g., power plants) [[11],[12]], and have the potential to increase the effective CO2 concentration at catalyst active sites and thus enhance the faradaic efficiency of the CO2 reduction reaction relative to H2 formation from water electrolysis. There are advantages to developing an advanced technology for electrocatalytic conversion of CO2 using capture media already in large-scale industrial use, since it would dramatically reduce the logistical and economic costs of integration into existing facilities. The data gathered to date provide encouraging evidence that enhanced catalytic performance can be realized through the use of such CO2-solubilizing additives in tandem with pulse-deposited copper electrocatalysts. The authors acknowledge the financial support of NASA Contract NNX14CC53P and US DOE Contract DE-SC0015812. References [[1]] Basic Research Needs: Catalysis for Energy, 8/6-8/2007. www.sc.doe.gov/bes/reports/list.html [[2]] Carbon Dioxide Capture and Sequestration. http://www3.epa.gov/climatechange/ccs/ [[3]] Y. Hori. “Electrochemical CO2 reduction on metal electrodes.” Modern Aspects of Electrochemistry, 42, C. Vayenas, Ed. New York: Springer (2008). [[4]] Chaplin, R., Wragg, A.A.; J. Appl. Electrochem.(2003), 33, 1107. [[5]] S. Verma et al. ChemSusChem 9: 1972 (2016). [[6]] Y. Hori et al. J Phys Chem B 106: 15 (2002). [[7]] K.W. Frese, Jr. “Electrochemical Reduction of CO2 at Solid Electrodes.” Electrochemical and Electrocatalytic Reactions of Carbon Dioxide, B.P. Sullivan et al., eds. Amsterdam: Elsevier, pp. 145 (1993). [[8]] E.J. Taylor, J.J. Sun. U.S. Patent 8,603,315, 12/10/2013. [[9]] A.C. Mishra et al. J Mater Sci 44(13): 3520 (2009). [[10]] C.W. Li, M.W. Kanan. J Am Chem Soc 134: 7231 (2012). [[11]] J.D. Figueroa et al. Int J Greenhouse Gas Control 2(1): 9 (2008). [[12]] J. Anthony, J. Carroll, DOE NETL, Aug 2016. https://www.netl.doe.gov/File%20Library/Events/2016/c02%20cap%20review/1-Monday/J-Anthony-SouthernCo-Testing-at-National-Carbon-Capture-Center.pdf Figure 1

Atom probe study of B2 order and A2 disorder of the FeCo matrix in an Fe-Co-Mo-alloy

The physical and mechanical properties of intermetallic alloys can be tailored by controlling the degree of order of the solid solution by means of heat treatments. FeCo alloys with an appropriate composition exhibit an A2-disorder↔B2-order transition during continuous cooling from the disordered bcc region. The study of atomic order in intermetallic alloys by diffraction and its influence on the material properties is well established, however, investigating magnetic FeCo-based alloys by conventional methods such as X-ray diffraction is quite challenging. Thus, the imaging of ordered FeCo-nanostructures needs to be done with high resolution techniques. Transmission electron microscopy investigations of ordered FeCo domains are difficult, due to the chemical and physical similarity of Fe and Co atoms and the ferromagnetism of the samples. In this work it will be demonstrated, that the local atomic arrangement of ordered and disordered regions in an industrial Fe-Co-Mo alloy can be successfully imaged by atom probe measurements supported by field ion microscopy and transmission Kikuchi diffraction. Furthermore, a thorough atom probe parameter study will be presented and field evaporation artefacts as a function of crystallographic orientation in Fe-Co-samples will be discussed.

Investigation of piezoelectric property and nanodomain structures for PIN–PZ–PMN–PT single crystals as a function of crystallographic orientation and temperature

The anisotropy of nanodomain structures and the domain switching in the Pb(In1/2Nb1/2)O3–PbZrO3–Pb(Mg1/3Nb2/3)O3–PbTiO3 system were investigated systematically.

Effect of Crystallographic Orientation on the Corrosion of Magnesium: Comparison of Film Forming and Bare Crystal Facets using Electrochemical Impedance and Raman Spectroscopy

The corrosion rate of Mg indicates a strong crystallographic dependence in chloride-containing, alkaline environments that correlates inversely with oxide film thickness. In contrast, a different crystallographic orientation dependency is observed initially during open circuit corrosion in non-chloride containing, near neutral pH buffered, chelating environments such as Tris(hydroxymethyl)aminomethane (TRIS) and Ethylenediaminetetraacetic (EDTA) which minimize air-formed MgO oxides. The origins of the differences in the rates of the coupled corrosion processes as a function of crystallographic orientation were investigated utilizing electrochemical impedance spectroscopy (EIS) and Raman vibrational spectroscopy. High frequency constant phase elements (CPEs) were exploited to determine oxide thicknesses as a function of crystal orientation. In unbuffered NaCl, open circuit corrosion was faster on basal planes while lower corrosion rates were observed on low index, prismatic and pyramidal planes. This variation in rate with crystallographic orientation in 0.6M NaCl was interpreted to depend on the MgO and Mg(OH)2 film thicknesses as a function of orientation. In particular, crystal planes with a lower MgO/Mg(OH)2 thickness corroded at higher rates as assessed in-situ by EIS and suggested by Raman spectroscopy. The crystallographic orientation dependence of corrosion seen initially upon exposure in the buffered neutral pH environments corresponded with crystal facet surface energy for bare Mg but this trend disappeared with exposure time and Mg(OH)2 films were then detected.

Orientation dependence in superelastic Cu-Al-Mn-Ni micropillars

The superelastic behavior of single crystal Cu-Al-Mn-Ni shape memory alloy micro-pillars was studied under compression as a function of crystallographic orientation. Cylindrical pillars of about 2 μm diameter were micro-machined from targeted crystal orientations. While pillars oriented close to the [001] direction showed the largest total transformation strain (∼7%), plastic deformation dominated the compressive response in the pillars milled close to the [111] direction due to their high elastic anisotropy combined with the large stresses required to induce the transformation. Shape strain contour plots were constructed for γ′ and β′ martensites, and the martensite start stress was calculated using the Clausius-Clapeyron equation. The same general trends are observed in both the experimental and calculated results, with some exceptions: larger transformation stresses and lower transformation strains are observed in the microsized pillars.

In situ multiple sulfur isotope analysis by SIMS of pyrite, chalcopyrite, pyrrhotite, and pentlandite to refine magmatic ore genetic models

With growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems, we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic, magmatic-hydrothermal, and hydrothermal ore systems. These materials include Sierra pyrite (FeS2), Nifty-b chalcopyrite (CuFeS2), Alexo pyrrhotite (Fe(1−x)S), and VMSO pentlandite ((Fe,Ni)9S8) that have been chemically characterized by electron microprobe analysis, isotopically characterized for δ33S, δ34S, and δ36S by fluorination gas-source mass spectrometry, and tested for homogeneity at the micro-scale by secondary ion mass spectrometry. Beam-sample interaction as a function of crystallographic orientation is determined to have no effect on δ34S and Δ33S isotopic measurements of pentlandite. These new findings provided the basis for a case study on the genesis of the Long-Victor nickel-sulfide deposit located in the world class Kambalda nickel camp in the southern Kalgoorlie Terrane of Western Australia. Results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite, pyrrhotite and chalcopyrite can better constrain genetic models related to ore-forming processes. Data indicate that pentlandite, pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar Δ33S values +0.2‰. This isotopic equilibrium unequivocally fingerprints the isotopic signature of the magmatic assemblage. The three sulfide phases show slightly variable δ34S values (δ34Schalcopyrite=2.9±0.3‰, δ34Spentlandite=3.1±0.2‰, and δ34Spyrrhotite=3.9±0.5‰), which are indicative of natural fractionation. Careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage, which may help resolve the nature of the ore-forming process. Hence, this SIMS-based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic- and alteration-related sulfides, which may not be resolved during bulk rock fluorination analysis. The results indicate that, unlike the giant dunite-hosted komatiite systems that thermo-mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative Δ33S values, the Kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales.