A micro-mechanical fracture testing method has been developed that uses a bowtie-shaped micro-beam specimen with a chevron notch. This clamped-clamped specimen can produce stable crack growth in ...brittle materials. Cyclic loading causes progressive crack extension, thereby producing multiple fracture toughness results in one experiment. The symmetric geometry eliminates the mixed mode fracture that exists in single-ended cantilevers. A 3D finite element analysis model was used to relate the crack length to the beam compliance, and then to the fracture toughness. The results of tests using fused quartz and glass-ceramic materials match very well with published fracture toughness values.
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The well-known thickness-dependent (111)-to-(100) texture transformation in thin FCC films is usually attributed to a competition between interface and strain energies. In this model, thin films ...retain their (111) texture due to the lower energy of the (111) interface, while thick films transform to (100) due to the lower stiffness and thus strain energy of a (100) film. However, recent work has called this model into question, suggesting that neither the stress nor the interface energy play a dominant role in texture transformation. We investigated the driving forces involved in this transformation by using a bulge test apparatus to induce different stresses in thin Ag films under identical annealing conditions. In situ synchrotron XRD measurements show the change in texture during annealing, and reveal that applied stresses have no effect on the transformation. Stress analysis shows that differences in driving forces for texture transformation due to applied bulge pressure were significant (≈200 kJ/m3), suggesting that a different, much larger driving force must be responsible. Reduction in defect energy has been proposed as an alternative. However, vacancy and dislocation densities must be exceptionally high to significantly exceed the strain energy and do not provide obvious orientation selection mechanisms. Nanotwins in reported densities are shown to provide greater driving force (≈1000 kJ/m3) and may account for orientation selection. The large difference between the calculated strain and defect energies and the driving force for grain growth (21,100 kJ/m3) casts doubt on the applicability of a simple thermodynamic model of texture transformation.
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Materials in single crystal form are often sought after because the absence of grain boundaries can result in unique properties relative to the polycrystal, but producing these materials is typically ...a slow and complex process. In this work, pseudo single crystals of the pseudobrookite compound CoTi2O5 were synthesized by solid‐state reaction from a duplex grain mixture of CoTiO3 and TiO2. The size of the crystallites was >250 µm. The transformation and subsequent microstructural evolution of the CoTi2O5 was studied by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and X‐ray diffraction (XRD). A novel growth mechanism was identified whereby a single crystal CoTi2O5 front advances simultaneously along multiple CoTiO3/TiO2 diphasic boundaries. The single crystal domains were composed of subgrains approximately 5 µm in diameter; differences in the subgrain size and misorientation were related to the growth mechanism and the initial grain size of the duplex CoTiO3–TiO2 mixture. CoTi2O5 is a little characterized compound, and this study represents the most significant microstructural study of CoTi2O5 to date. The findings may be applied to similar pseudobrookite compounds such as MgTi2O5 and Al2TiO5.
Metalorganic vapor phase epitaxy (MOVPE) nucleation studies of GaN on planar sapphire and nano-patterned AGOG (Deposition of
Aluminum,
Growth of
Oxide, and
Grain growth) sapphire substrates were ...conducted. The use of abbreviated GaN growth mode, which utilizes a process of using 15
nm low-temperature GaN buffer and bypassing etch-back and recovery processes during epitaxy, enables the growth of high-quality GaN template on nano-patterned AGOG sapphire. The GaN template grown on nano-patterned AGOG sapphire by employing abbreviated growth mode has two orders of magnitude lower threading dislocation density than that of conventional GaN template grown on planar sapphire. The use of abbreviated growth mode also leads to significant reduction in cost of the epitaxy. The growths and characteristics of InGaN quantum wells (QWs) light-emitting diodes (LEDs) on both templates were compared. The InGaN QWs LEDs grown on the nano-patterned AGOG sapphire demonstrated a 24% enhancement of output power enhancement over that of LEDs grown on conventional GaN templates.
Precipitation hardening copper alloy C70250 was selectively laser melted to successfully produce components around 98 pct dense with high mechanical strength and electrical conductivity. Aging heat ...treatments were carried out at 723 K (450 °C) directly on as-printed samples up to 128 hours. Mechanical testing found that peak yield strength of around 590 MPa could be attained with an electrical conductivity of 34.2 pct IACS after 8 hours of aging. Conductivity continues to increase with further aging while the peak strength appears to be less sensitive to aging time exhibiting a broad range of time where near-peak properties exist. After aging for 128 hours, there is a drop in yield strength to 546 MPa with an increase in conductivity to 43.2 pct IACS. Electron microscopy analysis revealed nanometer-scale silicon-rich oxide particles throughout the material that persist during aging. Deformation twinning is observed in the peak-age condition after tensile testing and several strengthening mechanisms appear to be active to varying degrees throughout aging which account for the broad range of aging time where nearly the peak mechanical properties exist.
Metal–ceramic composites exhibit desirable combinations of materials properties, but are limited by the complexity of processing, particularly for metal–ceramic nanocomposites. The in situ partial ...reduction technique is a simple processing method that can be used to produce tailorable metal–ceramic composite microstructures. In this work, in situ partial reduction was utilized to generate novel Co–Ti
x
O
y
composites, including nanocomposites, through the reduction of CoTiO
3
. By modifying the temperature (800–1400 °C) and time (1–8 h) of reduction, composites with varying cobalt particle size and cobalt grain size were fabricated. Differences in the cobalt crystal structure and nature of the titanium oxide phase were also observed. The lowest-temperature heat treatments resulted in metal–ceramic nanocomposites. The Co–Ti
x
O
y
composites were characterized through scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. The effect of processing variables on the properties of the composites was evaluated through nanoindentation of the embedded cobalt particles, and it was found that cobalt particle hardness is strongly correlated with grain size. The many useful properties of cobalt and titanium oxide, in conjunction with the range of controllable microstructures, demonstrate that the in situ partial reduction technique has excellent potential for metal–ceramic composite production.
Ni-23at.%W thin films were ion-beam machined and tested in bending. Microstructures were characterized with aberration-corrected electron microscopy and the fracture surfaces were inspected after ...each test with SEM. Nominally amorphous films were fractured along oxide streaks that formed during deposition. Heat treating at 700°C crystallized the samples to a grain size of 30nm, and the corresponding fracture toughness increased from 2.1 to 2.5MPam. This work demonstrates that mesoscale features and impurities influence amorphous and nanocrystalline Ni–W fracture.
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•An approach to characterize complexion transition kinetics is presented.•The fundamentals of bulk phase and complexion TTT diagrams are compared.•Avrami-type analysis quantifies the ...time dependence of complexion transitions.•Newly constructed complexion TTT diagrams enable engineered microstructures.•Future challenges and recommendations for complexion TTT diagrams are discussed.
Grain boundaries and other interfaces can undergo complexion transitions from one thermodynamic state to another, resulting in discontinuous changes in interface properties such as diffusivity, mobility, and cohesive strength. The kinetics of such complexion transitions has been largely overlooked until recently. Just as with bulk phase transformations, complexion transition kinetics can be represented on time-temperature-transformation (TTT) diagrams. An experimental complexion TTT diagram is presented here for polycrystalline Eu-doped spinel annealed at 1400–1800°C. This material developed a microstructure with a bimodal grain size distribution, indicating that a complexion transition occurs within this temperature range. The time and temperature dependence of this complexion transition was analyzed and used to produce a grain-boundary complexion TTT diagram for this system. Complexion TTT diagrams have the potential to be remarkably useful tools for manipulating the properties of internal interfaces in polycrystalline metals and ceramics. The development of experimental complexion TTT diagrams is likely to have an important impact on the field of grain-boundary engineering, and hence the development of these experimental diagrams should be an intense area of focus in the coming years.
Metal thin films are used as major components in microelectromechanical systems (MEMS). However, problems with long-term reliability that set limits on the lifetime of MEMS applications have been ...observed. The more efficiently the thin films resist stress relaxation, the longer the lifetime of the MEMS device. In particular, Al thin films have been used as capacitance switches, but are shown to have a low resistance to stress relaxation, thus hindering overall performance and leading to shorter lifetime. Using bulge testing, this study investigates the viscoelastic behavior of pure Al thin film in comparison with thin Al alloy films with 12% and 16% Mg. The results show that the addition of Mg to Al films significantly decreases the relaxation behavior and increases the strengthening mechanism of such thin films. The greater the Mg content in an Al film, the greater the resistance of the film. The normalized modulus decreases less with a greater Mg content. Al–Mg thin films have better relaxation resistance than pure Al thin films and thus serve as a better material for MEMS capacitance switches.
•Use of bulge test to measure viscoelasticity in thin Al and Al–Mg films•Strong Mg alloying dependence of relaxation resistance in thin Al and Al–Mg films•Alloying affects grain sizes and boundaries, which greatly impact the viscoelastic properties.
Wire-based directed energy deposition additive manufacturing techniques (AM) permit the rapid production of large-scale structural components which are not currently possible using the more common ...powder bed fusion (PBF) AM methods. However, due to larger melt pool widths and higher energy inputs than PBF methods, local thermal history effects produce significant location-dependent microstructure, porosity, and mechanical behavior that necessitates thorough quantification of this emergent technology. Wire + Arc Additive Manufacturing (WAAM) was used to produce austenitic stainless-steel single bead walls in order to statistically quantify the variation of critical material properties within the build. Individual grain geometric properties evaluated using electron back scatter diffraction at different points in the build were well fit by a three-parameter Weibull cumulative distribution function, yet sufficiently different from averaged values. X-ray diffraction for each location disclosed a strong wire texture in the build direction, leading to anisotropic elastic moduli values that were well described by directionally-dependent modulus predictions obtained from diffraction peak analysis. Location-dependent mechanical behavior was examined and accurately captured by an elasto-viscoplastic model based on the Fast-Fourier Transforms (EvpFFT) using the local microstructure orientation data as input. Overall, a high-quality build was realized, with minimal porosity of less than 0.32%, and median yield and tensile strength values of approximately 320.4 ± 8.0 MPa and 531.6 ± 8.2 MPa, respectively. In conclusion, mean values for mechanical behavior within the wall build were found to closely resemble single pass weldments, with statistical variation between individual locations mainly occurring in the weld direction due to local thermal history effects.
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