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Zr-based bulk metallic glasses (BMGs) are amorphous alloys that can exhibit excellent mechanical properties, including high yield strength and fracture toughness. These properties are linked to local microstructural heterogeneities. Whether via microscopy-based techniques, synchrotron techniques, or calorimetric approaches, the amorphous structure of BMGs makes the characterisation of the details of these local structural and chemical heterogeneities extremely challenging. Our focus here is on atom probe tomography (APT), where considerable uncertainty remains in terms of how and when to apply this otherwise powerful technique to amorphous materials. This work reports a systematic evaluation of the experimental parameter space. We report results of BMG composition acquired against various APT operating parameters for Zr63.96Cu13.36Ni10.29Al11.04Nb1.25 (at. %). We demonstrate that a customised peak-based ranging approach yields satisfactory compositional accuracy with absolute errors of

The anisotropic mechanical properties of ultrasound freeze cast epoxy-ceramic composite materials were studied by measuring flexural strength and fracture resistance curves (R-curves) using both unnotched and notched three-point beam bending experiments, respectively, cut in three different orientations relative to the directional freezing axis. Three ultrasound frequencies of 0.699, 1.39 and 2.097 MHz were used in order to introduce different length scales into the microstructure, with 0 MHz used as the control samples. For all cases, the composites showed higher strength and fracture resistance when the crack plane cut across the direction of ice growth (denoted as the YX orientation). The anisotropic properties were more evident for the material produced without ultrasound (0 MHz) where the flexural strength was approximately 160% higher for the YX orientation compared to two orthogonal orientations. Most of the fracture resistance increase was found to occur within a crack extension, Δa, of ∼0.5 mm. Comparing the fracture resistance at Δa = 0.5 mm for the highly anisotropic 0 MHz samples showed that the YX orientation was approximately 86% tougher than the two orthogonal orientations. When the ultrasound operation frequencies of 0.699, 1.39 and 2.097 MHz were applied, the amount of anisotropy in the strength and fracture resistance gradually decreased as the operating frequency increased. The high strength and fracture resistance for the YX orientation was attributed to the alignment of the ceramic particles along the freeze front direction creating a barrier for crack propagation. Ultrasound modifies the material microstructure, introducing relatively dense ceramic layers perpendicular to the freezing front direction that act as an additional, orthogonal barrier to crack propagation. The addition of the denser layers acts to improve the mechanical properties in the weaker orientations and reduce the overall anisotropy.

Anticipating an increasing demand for hybrid double network (DN) hydrogels in biomedicine and biotechnology, this study evaluated the effects of each network on the mechanical and biological properties. Polyethylene glycol (PEG) (meth)acrylate hydrogels with varied monomer molecular weights and architectures (linear vs. 4-arm) were produced with and without an added ionically bonded alginate network and their mechanical properties were characterized using compression testing. The results showed that while some mechanical properties of PEG single network (SN) hydrogels decreased or changed negligibly with increasing molecular weight, the compressive modulus, strength, strain to failure, and toughness of DN hydrogels all significantly increased with increased PEG monomer molecular weight. At a fixed molecular weight (10 kDa), 4-arm PEG SN hydrogels exhibited better overall mechanical performance; however, this benefit was diminished for the corresponding DN hydrogels with comparable strength and toughness and lower strain to failure for the 4-arm case. Regardless of the PEG monomer structure, the alginate network made a relatively larger contribution to the overall DN mechanical properties when the covalent PEG network was looser with a larger mesh size (e.g., for larger monomer molecular weight and/or linear architecture) which presumably enabled more ionic crosslinking. Considering the biological performance, adipose derived stem cell cultures demonstrated monotonically increasing cell area and Yes-associated protein related mechanosensing with increasing amounts of alginate from 0 to 2 wt.%, demonstrating the possibility for using DN hydrogels in guiding musculoskeletal differentiation. These findings will be useful to design suitable hydrogels with controllable mechanical and biological properties for mechanically demanding applications. STATEMENT OF SIGNIFICANCE: Hydrogels are widely used in commercial applications, and recently developed hybrid double network hydrogels have enhanced strength and toughness that will enable further expansion into more mechanically demanding applications (e.g., medical implants, etc.). The significance of this work is that it uncovers some key principles regarding monomer molecular weight, architecture, and concentration for developing strong and tough hybrid double network hydrogels that would not be predicted from their single network counterparts or a linear combination of the two networks. Additionally, novel insight is given into the biological performance of hybrid double network hydrogels in the presence of adipose derived stem cell cultures which suggests new scope for using double network hydrogels in guiding musculoskeletal differentiation.

AlSi10Mg alloy modified with 4 wt.% cerium was fabricated by selective laser melting and subjected to a T6 solutionizing (2 h at 535°C) and ageing (10 h at 160°C) heat treatment. Microstructure, pr...

Both uniform and graded gyroid cellular structures (GCSs) of NiTi have been successfully fabricated by laser powder bed fusion (LPBF) additive manufacturing. NiTi GCSs were produced with gradients along two different directions, and the surface morphology and mechanical properties of different GCSs were systematically studied using micro-x-ray computed tomography, monotonic compression, and compression–compression fatigue testing. The results showed that all the LPBF NiTi structures exhibited similar nominal compressive elastic modulus (5–7 GPa) to human bones. By altering the density distribution of the novel GCSs, different deformation and fatigue properties were achieved. GCSs with graded density parallel to each layer (Y-GCSs) exhibited a similar shear compression failure behavior and slightly better fatigue behavior compared to the uniform structure. In contrast, GCSs with graded density parallel to the building direction (Z-GCSs) exhibited a layer-by-layer deformation and collapse behavior under compression, while the fatigue life was worse than the uniform structure.

Bulk metallic glasses (BMGs) are materials with outstanding strength and elastic properties that make them tantalizing for engineering applications, yet our poor understanding of how their amorphous atomic arrangements control their broader mechanical properties (hardness, wear, fracture, etc.) impedes our ability to apply materials science principles in their design. In this work, we uncover the hierarchical structure that exists in BMGs across the nano- to microscale by using nanobeam electron diffraction experiments. Our findings reveal that local hardness of microscale domains decreases with increasing size and volume fraction of atomic clusters with higher local medium range order (MRO). Furthermore, we propose a model of ductile phase softening that will enable the future design of BMGs by tuning the MRO size and distribution in the nanostructure.

Porous Fe3O4 scaffolds were fabricated while subject to a low (7.8 mT) magnetic field applied in helical and Bouligand motions using a custom-built tri-axial nested Helmholtz coils-based freeze-casting setup. This setup allowed for the control of a dynamic, uniform low-strength magnetic field to align particles during the freezing process, resulting in the majority of lamellar walls aligning within ± 30° of the magnetic field direction and a decrease in porosity by up to 42%. Similar to how helical and Bouligand structures in nature promote impact resistance, these magnetic field motions produced structures with improved high strain rate mechanical properties. Strain at failure was increased by up to 2 times as cracks deflected to match the applied angles of rotation of the magnetic field.

Refractory high-entropy alloys (RHEAs) are potential candidate materials for use in next-generation nuclear reactors due to their excellent mechanical performance at high temperatures. Here, we investigate the microstructure and mechanical properties of the nanocrystalline RHEA TiZrNbHfTa before and after irradiation with He2+ ions to determine radiation-induced property changes. Using nanoindentation and in situ microtensile testing we find only small changes in hardness after irradiation but a significant increase in yield and ultimate tensile strength without loss in ductility. This is associated with radiation hardening and a shift from shear localization failure with smooth fracture surfaces to a fracture morphology consisting of fine dimples and intergranular failure characteristics. Overall, the material shows excellent damage-tolerant properties with good combinations of strength and ductility both prior to and after ion irradiation.

Objectives Understand how cooling protocols control the microstructure and mechanical properties of veneering porcelains. Methods Two porcelain powders were selected, one used to veneer metallic frameworks (VM13) and one for zirconia frameworks (VM9). After the last firing cycle, the monolithic specimens were subjected to two cooling protocols: slow and fast. Flexural strength (FS) was evaluated by three-point beam bending and fracture toughness (KIC) was evaluated by the single-edge V-notch beam (SEVNB) method. Scanning electron microscopy (SEM) was performed to determine the leucite crystal volume fraction (%), particle size, and matrix microcrack density. The results were compared by analysis of variances (ANOVA) and Tukey’s multiple comparison test. Results The mechanical properties were significantly (p Significance Controlled crystallization using slow cooling can be applied as a means of strengthening dental porcelains. However, the benefits of slow cooling may be partially offset by increasing the microcrack density in the glass matrix. To achieve the maximum benefit of slow cooling, it is recommending to develop heat treatments to produce porcelain with fine-grained and homogenously dispersed leucite crystals to achieve minimal glass matrix microcracking.

In this work, temperature dependent mechanical properties of conventional and additive manufactured Inconel 718 (IN718) after solution and aging (SA) heat treatment were measured at room temperature, 350 °C, and 650 °C. Nanoindentation technique was used to obtain material properties including hardness, reduced modulus, creep rate, creep stress exponent, and thermal activation volume. The indentation size effect (ISE) was analyzed at each temperature. The reduced modulus was measured to decrease by 26% for the conventional samples and by 22% for AM samples as temperature increases from room temperature to 650 °C. The nanoindentation hardness decreases by 19% for the conventional samples and 16% for the AM samples. During the creep tests at 650 °C, the creep stress exponent was found in the range of 8–18, indicating that the deformation at the examined nano/micro scale is controlled by the dislocation climb mechanism.

In this study, as-cast and cold-rolled Zr63.78Cu14.72Ni10Al10Nb1.5 bulk metallic glass (BMG) samples were subjected to 20, 70, 120, and 170 cryogenic thermal cycles prior to fracture toughness testing. Thermal cycling raised the fracture toughness by promoting plastic deformation and stable crack growth, with the most significant increase occurring after the first 20 thermal cycles. Thermally cycled samples showed more tortuous crack paths and stable crack propagation left periodic blunting marks spaced at ∼57.5 μm on the fracture surface corresponding to each increment of crack advance. Microhardness mapping revealed a microstructure of hard and soft domains (∼63 μm × 105 μm), and thermal cycling heterogeneously softened the hard domains while the soft domains remained apparently unchanged. In addition to the observed softening, large increases in the relaxation enthalpy, and thus the average free volume, were found over the first ∼70 thermal cycles. While cold rolling the samples prior to the thermal cycling did not raise the mean fracture toughness values, the scatter was reduced compared to as-cast thermal cycled samples. This was attributed to the introduction of shear bands giving a more repeatable initial microstructure than casting.

Fatigue crack growth is analyzed for steady state and overload conditions. The model combines an elastic-plastic strain gradient hardening material and an irreversible cohesive zone model. Plastic strain gradients are found not to affect steady-state fatigue crack growth but play a key role in the overload response. Plastic strain gradient hardening lowers plastic strain magnitudes and alters the spatial distribution of plastic strain. This leads to reduced crack closure and higher crack growth rates during and after the overload. Using classical plasticity alone to describe fatigue crack growth following an overload is thus nonconservative.

Crack growth mechanisms for Alloy 617 at 800 °C were investigated in air with specific emphasis on the transition from cycle-dependent to time-dependent crack growth mechanisms in the creep-fatigue regime. Crack growth studies were conducted using compact tension samples, a load ratio of 0.5, and triangular 5 Hz, 0.33 Hz, and 0.05 Hz waveforms, a trapezoidal 0.05 Hz waveform with 17 s hold time, and sustained loading. Fatigue crack growth rates were relatively insensitive to changes in frequency and hold times in air up to ∆K ≈ 11.5 MPa√m for R = 0.5, i.e, Kmax = 23 MPa√m. Above this threshold, the onset of time dependent crack growth was observed via a creep void nucleation and coalescence mechanism for triangular and trapezoidal waveforms with a loading frequency of 0.05 Hz, and during sustained loading. An estimate of the threshold for stress assisted grain boundary oxidation (SAGBO) crack growth was calculated to be 23 MPa√m, and oxidized grain boundaries observed near the crack tip were mostly uncracked, suggesting the SAGBO threshold was not reached before the onset of the void nucleation mechanism. A comparison of results across all available studies suggests that a threshold-based transition from cycle- to time-dependent crack growth at 800 °C likely exists. However, the stress intensity factor does not maintain similitude to accurately define a threshold across studies. Thus, gaining an understanding of the crack tip stress states that define the various time dependent mechanisms should be considered in future work.

Journal of materials research 36(11), 2325 - 2336 (2021). doi:10.1557/s43578-021-00187-5, High strain rate micromechanical testing can assist researchers in elucidating complex deformation mechanisms in advanced material systems. In this work, the interactions of atomic-scale chemistry and strain rate in affecting the deformation response of a Zr-based metallic glass was studied by varying the concentration of oxygen dissolved into the local structure. Compression of micropillars over six decades of strain rate uncovered a remarkable reversal of the strain rate sensitivity from negative to positive above ~ 5 s$^{���1}$ due to a delocalisation of shear transformation events within the pre-yield linear regime for both samples, while a higher oxygen content was found to generally decrease the strain rate sensitivity effect. It was also identified that the shear band propagation speed increases with the actuation speed, leading to a transition in the deformation behaviour from serrated to apparent non-serrated plastic flow at ~ 5 s$^{���1}$., Published by Cambridge Univ. Press, Cambridge [u.a.]

Metallic glasses (MGs) with superior catalytic performance have recently been recognized as attractive candidates for wastewater treatment. However, further improving their performance will require knowledge of how to precisely regulate their electronic structures via compositional control. Here, two Fe-based MGs (Fe

Objectives Understand how discontinuous short glass fibers and braided long fibers can be effectively used to reinforce a resin modified glass ionomer cement (RMGIC) for carious lesion restorations. Methods Two control groups (powder/liquid kit and capsule) were prepared from a light cured RMGIC. Either discontinuous short glass fibers or braided polyethylene fiber ribbons were used as a reinforcement both with and without pre-impregnation with resin. For the former case, the matrix was the powder/liquid kit RMGIC, and for the latter case the matrix was the capsule form. Flexural strength was evaluated by three-point beam bending and fracture toughness was evaluated by the single-edge V-notch beam method. Compressive strength tests were performed on cylindrical samples. Results were compared by analysis of variances and Tukey’s post-hoc test. Flexural strength data were analyzed using Weibull statistical analysis. Results The short fiber reinforced RMGIC both with and without pre-impregnation showed a significant increase of ∼50% in the mean flexural strength and 160–220% higher fracture toughness compared with the powder/liquid RMGIC control. Reinforcement with continuous braided fibers gave more than a 150% increase in flexural strength, and pre-impregnation of the braided fibers with resin resulted in a significant flexural strength increase of more than 300% relative to the capsule control. However, for the short fiber reinforced RMGIC there was no significant benefit of resin pre-impregnation of the fibers. The Weibull modulus for the flexural strength approximately doubled for the fiber reinforced groups compared to the control groups. Finally, compressive strength was similar for all the groups tested. Significance By using a RMGIC as a matrix, higher flexural strength was achieved compared to reported values for short fiber reinforced GICs. Additionally, the short fibers provided effective toughening of the RMGIC matrix by a fiber bridging mechanism. Finally, continuous braided polyethylene fibers gave much higher flexural strength than discontinuous glass fibers, and their effectiveness was enhanced by pre-impregnation of the fibers with resin.

The mechanical performance of a Ni–Mo–Cr (GH3535) alloy weldment, produced using a matching filler metal, was assessed and compared to the surrounding parent metal. Ambient-temperature mechanical characterisation included hardness testing, small punch testing and uniaxial tensile testing, while a crystal plasticity finite element model was used to assess the impact of crystallographic texture on the mechanical properties. Despite the similar chemical composition, the weld metal exhibited superior strength and ductility to that of the parent metal. The higher strength was primarily attributed to the high dislocation density in the weld metal imbued by the welding-induced thermo-mechanical loading. In contrast, the ductility difference was attributed to M6C carbide stringers in the parent metal that initiated fracture at lower strains than for the weld metal, with the latter containing much finer, well-dispersed M6C carbides.

In this study, a machine-learning approach based on Gaussian process regression was developed to identify the optimized processing window for laser powder bed fusion (LPBF). Using this method, we found a new and much larger optimized LPBF processing window than was known before for manufacturing fully dense AlSi10Mg samples (i.e., relative density ≥ 99%). The newly determined optimized processing parameters (e.g., laser power and scan speed) made it possible to achieve previously unattainable combinations of high strength and ductility. The results showed that although the AlSi10Mg specimens exhibited similar Al-Si eutectic microstructures (e.g., cell structures in fine and coarse grains), they displayed large difference in their mechanical properties including hardness (118 - 137 HV 10), ultimate tensile strength (297 - 389 MPa), elongation to failure (6.3 - 10.3%), and fracture toughness (9.9 - 12.7 kJ/m2). The underlying reason was attributed to the subtle microstructural differences that were further revealed using two newly defined morphology indices (i.e., dimensional-scale index Id and shape index Is) based on several key microstructural features obtained from scanning electron microscopy images. It was found that in addition to grain structure, the sub-grain cell size and cell boundary morphology of the LPBF fabricated AlSi10Mg also strongly affected the mechanical properties of the material. The method established in this study can be readily applied to the LPBF process optimization and mechanical properties manipulation of other widely used metals and alloys or newly designed materials.

A Zr-based bulk metallic glass produced using selective laser melting (SLM) was compared to the same alloy fabricated using traditional suction-casting. Analysis of the fracture toughness through single edge notched beam bending experiments showed a significantly reduced damage tolerance for the laser-processed material (KQ ~ 138.0 ± 13.1 → 28.7 ± 3.7 MPa √m), even though X-ray diffraction and microhardness responses were identical. Using uniaxial quasistatic micro-pillar compression, it was found that as-cast samples more readily underwent shear transformations (evidenced through discrete load drops) below the nominal 0.2% yield stress, which was connected to the higher macroscopic toughness. Differential scanning calorimetry demonstrated that the increased barrier to shear transformation for the SLM material could not be explained by the relative relaxation states. Rather, it was attributed to the greater dissolved oxygen concentration in the laser-processed material, which is postulated to decrease atomic mobility in the structure and thereby increase the activation energy required to initiate shear transformations.

A Gaussian process regression-based machine learning approach was used to establish a processing window optimized for high density additive manufacturing of a 2 vol% TiCN reinforced AlSi10Mg composite by laser powder bed fusion. The optimized window for TiCN reinforced AlSi10Mg was found to be smaller than for AlSi10Mg. Within the optimized window, it was found that the Si eutectic cell size can be increased by raising the laser power/scanning speed at the constant energy density of 50 J/mm3 to control the tensile properties of the fabricated composites.

Novel Zr63.78Cu14.72Ni10-xCoxAl10Nb1.5 bulk metallic glasses (BMGs) were prepared with progressively increasing amounts of Co substituted for Ni. Critical casting thicknesses were in the range of 4–5 mm, indicating a good glass forming ability. Transmission electron microscopy (TEM) of the as-cast amorphous structure revealed primary glassy phase separated regions a few dozens of nanometers in diameter. High yield strength (∼1495 MPa), fracture toughness (∼55 MPa√m), and strong strain hardening behavior were observed. Deformation induced secondary phase separation (

A high strength Al-5024 alloy containing Sc and Zr with a bi-modal microstructure consisting of fine equiaxed and coarse columnar grains was successfully fabricated by laser powder bed fusion (LPBF) additive manufacturing. The formation of the bi-modal microstructure was mainly due to both the formation of primary Al3Sc precipitates that act as nucleation sites and the steep temperature gradient during LPBF. By simulating the thermal field of a single melt pool, the formation mechanism of the bi-modal microstructure was explained. It was found by simulation that a solidification interface velocity less than 110 mm/s was beneficial to the nucleation of Al3Sc precipitates and, hence, facilitated the formation of a fine grain microstructure. Applying different heat treatments revealed a trade-off trend between yield strength and ductility as a function of the heat treatment time, and a correlation in fatigue life and yield strength was observed, both of which were closely related to the status of the secondary Al3Sc precipitates. The highest ultimate tensile strength of 450 MPa and corresponding 107 cycle fatigue strength of 105 MPa were achieved after hot isostatic pressing for 4 h at 325 °C with 100 MPa pressure. Dynamic strain aging was found to occur in both as-built and some heat treated samples, which was related to magnesium (Mg) solute atom clustering attributed to: (i) the formation of a diffuse “Mg wall” due to the repetitive melting and rapid cooling in LPBF, and (ii) the growth of intragranular (Al3Sc) and intergranular precipitates (Fe-, Mn-rich) during subsequent heat treatment, thereby leading to an increasing number of misfit dislocations that promote the formation of Mg atom clusters.

Size effects occur in non-uniform plastically deformed metals confined in a volume on the scale of micrometer or sub-micrometer. Such problems have been well studied using strain gradient rate-independent plasticity theories. Yet, plasticity theories describing the time-dependent behavior of metals in the presence of size effects are presently limited, and there is no consensus about how the size effects vary with strain rates or whether there is an interaction between them. This paper introduces a constitutive model which enables the analysis of complex load scenarios, including loading rate sensitivity, creep, relaxation and interactions thereof under the consideration of plastic strain gradient effects. A strain gradient viscoplasticity constitutive model based on the Kocks–Mecking theory of dislocation evolution, namely the strain gradient Kocks–Mecking (SG-KM) model, is established and allows one to capture both rate and size effects, and their interaction. A formulation of the model in the finite element analysis framework is derived. Numerical examples are presented. In a special virtual creep test with the presence of plastic strain gradients, creep rates are found to diminish with the specimen size, and are also found to depend on the loading rate in an initial ramp loading step. Stress relaxation in a solid medium containing cylindrical microvoids is predicted to increase with decreasing void radius and strain rate in a prior ramp loading step.

OBJECTIVE To determine whether cellophane banding secured with locking polymer clips on cadaveric splenic veins would cause less CT imaging artifact and achieve equivalent mechanical strength, compared with cellophane banding secured with metal vascular clips. ANIMALS 10 canine cadavers. PROCEDURES Clips of each material were applied to each cadaver in a crossover design study. Triple-layer cellophane bands secured with 4 medium-large or large polymer or metal clips were placed on cadaveric splenic veins and evaluated by use of CT. Beam-hardening artifact was assessed by artifact length, attenuation, and a subjective grading scale ranging from 1 to 3 for mild to severe imaging artifacts. Secured cellophane bands were mechanically tested to determine force-deformation curves and yield forces. Findings for clip methods were compared with a 1-way ANOVA with a Tukey post-test. RESULTS For metal clips, beam-hardening artifact lengths and subjective artifact grades were significantly higher, whereas attenuation values were significantly lower, than findings for polymer clips. Polymer clips were significantly lower in strength than metal clips with mean ± SD yield loads of 1.9 ± 0.6 N (medium-large polymer clips), 2.8 ± 1.3 N (large polymer clips), 6.0 ± 1.9 N (medium-large metal clips), and 8.4 ± 2.7 N (large metal clips). CONCLUSIONS AND CLINICAL RELEVANCE Use of locking polymer clips to secure cellophane banding resulted in less CT imaging artifact and mechanical strength, compared with use of metal vascular clips. Use of locking polymer clips may allow improved assessment of postoperative CT imaging in dogs with extrahepatic portosystemic shunts, which warrants in vivo clinical evaluation.

The influence of strain rate across three orders of magnitude (1.70 × 10−5/s to 1.43 × 10−2/s) along with the effect of the plastic strain accumulation (up to 10%) on the serrated plastic flow were investigated in the nickel-chromium (Ni-Cr) solid solution alloy Nimonic 75 by performing constant-strain-rate tension testing at 600 °C. As the strain rate decreased, the critical strain for the onset of serrations transitioned from normal behavior to inverse behavior. The serrated flow was characterized as Type A+B serration at high strain rate (1.43 × 10−2/s). In the intermediate strain-rate regime (1.43 × 10−3/s and 1.45 × 10−4/s), Type B serrations were observed and followed by a transformation to Type C+B serrations. At the low strain rate (1.70 × 10−5/s), the plastic flow immediately displayed Type C serrations, which later evolved into Type C+B serrations. Regardless of the strain rate, plastic strain, or dislocation density, a critical dislocation velocity falling in the range of 1.2 × 10−6 – 2.2 × 10−6 m/s was identified to signify the onset of Type C serration, whereby the mobile dislocations break free from the solute cloud for short bursts of deformation. Finally, a novel model by solute rearrangement across dislocation cores was used to understand how the critical dislocation velocity is quantitatively determined by the rate at which solute atoms are able to hop across the glide plane as a partial dislocation core moves through the lattice.

Single phase high-entropy alloys (HEAs) are often assumed to be random solid solutions with homogeneous elemental distributions. This may not always be true and atomic arrangements and chemical homogeneity depend on the processing history of the material. In this study, electron probe microanalysis (EPMA) and atom-probe tomography (APT) were performed on nominally equiatomic CrMnFeCoNi HEAs with different processing histories. Our EPMA analysis revealed that the different processing routes can produce both compositionally homogenous and compositionally heterogeneous materials. The homogenous material was used for an APT parameter optimization study that was conducted in both voltage and laser-pulsed mode. The data acquisition parameters, i.e., specimen base temperature, detection rate, pulse fraction/pulse energy and pulse rate were varied, and results such as detector hit events, mass-spectra, signal-to-noise ratio, and local chemical composition analyzed. A set of optimized parameters was then used to examine micro and nanoscale compositional heterogeneities in the second HEA. Results of the parameter study indicate that while the laser-mode APT data yielded marginally better data quality, a wide range of APT parameters in either mode can be used to produce a compositionally accurate analysis. The APT data of the heterogeneous HEA revealed that the material contains nanoscale compositional heterogeneities in addition to the microscale heterogeneities revealed by EPMA. The combination of both methods provides a complementary set of characterization techniques to reliably identify chemical heterogeneities across nanometer to micrometer length scales that may affect local deformation mechanisms.

The effects of build orientation and post heat treatments on the tensile and fatigue crack growth (FCG) behavior of commercially pure titanium (CP-Ti) manufactured by laser powder bed fusion (LPBF) using grade 2 powder were examined. Two orthogonal build orientations were used in conjunction with hot isostatic pressing (HIP) both above (950 °C) and below (730 °C) the β-transus temperature and property comparisons were also made to commercially available wrought material. The HIP treatments coarsened the α grain structure, reduced the tensile strength, and increased the fatigue crack growth threshold. The LPBF materials were generally stronger and more fatigue resistant than the wrought material due higher interstitial oxygen and nitrogen content. Additionally, higher tensile strength values were found for one build orientation with higher nitrogen content that was attributed to the different thermal histories during LPBF. However, the build orientation effect was not observed for the FCG behavior of the LPBF material and the FCG resistance at low growth rates were mainly controlled by the grain size. This was in sharp contrast to the wrought material which showed strong anisotropy in the microstructure sensitive fatigue crack growth regime due to strong crystallographic texture. At higher growth rates, FCG became microstructure insensitive when the cyclic plastic zone size became of similar order of magnitude to the grain size.

The combination of refined microstructures (induced by rapid cooling) and melt pool-induced mesostructures in AlSi10Mg fabricated using laser powder bed fusion (LPBF) – a widely used additive manufacturing technique – impart high strength and fracture toughness. Further exploitation of such property combinations requires a detailed understanding of how the processing conditions control the micro- and mesostructures and, in turn, the mechanical performance, especially regarding fracture resistance. Towards this end, the crack resistance curve (R-curve) behavior in different orientations of LPBF-fabricated AlSi10Mg alloys processed with different layer thickness, hatch spacing, and scan strategies was evaluated and correlated with micro- and mesostructural features such as grain size and grain orientation, texture, cell morphology, and melt pool arrangement. Results show a strong anisotropy in both tensile stress-strain behavior and fracture toughness with layer thickness and hatch spacing controlling strength and scan strategy dictating fracture resistance. In terms of tensile stress-strain behavior, the arrangement of melt pool boundaries with respect to loading direction results in anisotropy in ductility whereas strength is controlled by grain size and cellular structure. In case of fracture toughness, measurements show that failure is dominated primarily by melt pool morphology and hence the mesostructure that is controlled by scan strategy. They furthermore reveal, that, despite the pronounced anisotropy in the R-curve behavior the presence of such mesostructure enables a level of damage-tolerance in AlSi10Mg that cannot be achieved in a cast alloy.

The mechanical performance of a Ni–Mo–Cr (GH3535) alloy weldment, produced using a matching filler metal, was assessed and compared to the surrounding parent metal. Ambient-temperature mechanical characterisation included hardness testing, small punch testing and uniaxial tensile testing, while a crystal plasticity finite element model was used to assess the impact of crystallographic texture on the mechanical properties. Despite the similar chemical composition, the weld metal exhibited superior strength and ductility to that of the parent metal. The higher strength was primarily attributed to the high dislocation density in the weld metal imbued by the welding-induced thermo-mechanical loading. In contrast, the ductility difference was attributed to M6C carbide stringers in the parent metal that initiated fracture at lower strains when compared to the weld metal, with the latter containing finer, well-dispersed M6C carbides.

Inconel 617 (IN-617) mainly contains nickel (Ni), chromium (Cr), cobalt (Co) and molybdenum (Mo). IN-617 is widely used in applications that require high temperature operation due to its high temperature stability and strength as well as its strong resistance to oxidation and carburization. The current work focuses on the measurement of temperature dependent mechanical properties of IN-617 from room temperature (around 25 °C) up to 800 °C. The properties measured are reduced modulus, elastic modulus, hardness, indentation creep rate, indentation creep exponent, and thermal activation volume. The indentation size effect is analyzed as a function of temperature. Using a combination of optical microscopy and scanning electron microscopy (SEM) imaging, the effect of precipitate distribution and oxidation on the measured properties is found to be negligible beyond a critical indentation depth. The mean hardness value ranged from 3.1 GPa at room temperature to 1.6 GPa at 800 °C. A relation between indentation depth and hardness as a function of temperature change was used to extract strain gradient plasticity associated length scales with values changing from 1.0 μm at room temperature to 1.8 μm at 400 °C and to 1.6 μm at 800 °C.

Several criteria have been proposed to predict whether a crack will penetrate through, or deflect along, an interface between two linear elastic materials. Moreover, the different criteria predict different crack penetration/deflection behavior creating uncertainty about which method to choose. To help remove some of this uncertainty, the present study presents experimental results on the quasi-static penetration/deflection behavior of cracks incident to interfaces in polymethyl methacrylate (PMMA) with various incident angles. Interfaces were created by bonding PMMA sheets using two different solvents. By varying the incident angles of the cracks on the interfaces, the transition angle for the transition from crack penetration to deflection was determined to be ɸ tran ≈ 80° for the stronger and tougher interfaces and ɸ tran ≈ 85° for the weaker and less tough interfaces. Using an energy-based criterion based solely on the toughness ratio, much lower transition angles ( ɸ tran = 47° and 57°) were predicted for the interfaces than were experimentally observed. In contrast, using a cohesive zone method (CZM) approach that incorporates both the strength and toughness ratios gave predicted transition angles much closer to those experimentally observed for both the stronger ( ɸ tran = 73°) and weaker ( ɸ tran = 80°) interfaces. Finally, an approach that only considers the normal strength ratio was examined and poor agreement was found between predictions and experiments for 90° indent angle samples. Overall, it was found that the CZM approach makes predictions of the crack penetration/deflection behavior that were closest to the experimental results of this study.

Failure of metallic materials due to plastic and/or creep deformation occur by the emergence of necking, microvoids, and cracks at heterogeneities in the material microstructure. While many traditional deformation modeling approaches have difficulty capturing these emergent phenomena, the discrete element method (DEM) has proven effective for the simulation of materials whose properties and response vary over multiple spatial scales, e.g., bulk granular materials. The DEM framework inherently provides a mesoscale simulation approach that can be used to model macroscopic response of a microscopically diverse system. DEM naturally captures the heterogeneity and geometric frustration inherent to deformation processes. While DEM has recently been adapted successfully for modeling the fracture of brittle solids, to date it has not been used for simulating metal deformation. In this paper, we present our progress in reformulating DEM to model the key elastic and plastic deformation characteristics of FCC polycrystals to create an entirely new crystal plasticity modeling methodology well-suited for the incorporation of heterogeneities and simulation of emergent phenomena.

Mode I fracture tests were conducted on both as-cast and cold-rolled Zr-Cu-Ni-Al-Nb bulk metallic glass (BMG) single edge notch bend samples. The results show a 54% improved fracture toughness ( K J = 55.2 vs. 85.0 MPa√m) is obtained by cold rolling to ∼2.5% true strain. The precrack patterns and fractography were also analyzed to evaluate the influence of cold rolling on the fracture process and also the validity of the mode I fracture toughness results. Cold rolling was found to promote numerous shear bands along the main crack, crack bifurcation, and a tortuous crack path, all of which are thought to contribute to the improved toughness. The influence of various precrack morphologies on the fracture toughness results are discussed. It was found that it is difficult to produce perfect standardized precracks in BMGs with heterogeneous glassy structures induced by cold rolling and this issue needs further evaluation if fracture toughness testing standards for BMGs are to be established.

The incorporation of antimicrobial agents in restorative dental composites has the potential to slow the development of carious lesions.The objectives of the present study were to develop experimental composite resins with chitosan or chitosan loaded with dibasic calcium phosphate anhydrous (DCPA) particles and to demonstrate their antimicrobial potential without loss of mechanical properties or biocompatibility.Chitosan and chitosan/DCPA particles were synthetized by the electrospray method. Experimental composites were formulated by adding 0, 0.5, or 1.0 wt% particles into a resin matrix along with 60 wt% barium glass. The degree of conversion and mechanical properties were measured after 1 and 90 days of aging in water after photoactivation. Cytotoxicity and genotoxicity were evaluated using fibroblasts from dental pulp in conditioned medium. The antimicrobial activity against Streptococcus mutans was assessed by crystal violet biofilm assay.The experimental restorative composites were not found to be cytotoxic or genotoxic, with cell viability of 93.1 ± 8.0% (p = 0.328) and 3.0 ± 0.8% micronucleus per group (p = 0.1078), respectively. The antimicrobial results showed that all composites with approximately 20% less biofilm (p0.001) relative to the control. No chitosan release was detected from the composites, suggesting direct contact of the bacteria with exposed chitosan particles on the surface was responsible for the observed antimicrobial effect. The addition of the chitosan and chitosan/DCPA submicrometer (250 nm average diameter) particles to restorative composites did not change the degree of conversion, flexural strength, elastic modulus and fracture toughness compared to the control group after 90 days aging in water.It can be concluded that the addition of chitosan or chitosan/DCPA particles in the restorative composites induced antimicrobial activity without compromising the mechanical properties or biocompatibility of the composites.

This short communication presents 2–4 μm sized TiCN reinforced AlSi10Mg composites (TiCN/AlSi10Mg) fabricated by laser powder bed fusion (LPBF) with refined Al-Si eutectic microstructure consisting of equiaxed bi-modal α-Al grains and enhanced elevated temperature tensile strength. The formation mechanism of reported bi-modal structure is related to the modified temperature gradients and induced heterogeneous nucleation in LPBF. The enhancement in elevated temperature tensile strength is mainly attributed to refined bi-modal Al-Si microstructure and thermal stability of TiCN particles.

In this study, two sets of optimized laser powder bed fusion (LPBF) additive manufacturing parameters with similar energy density but different laser powers (HP: high laser power with high scanning speed, LP: low laser power with low scanning speed) were used to produce fully dense and crack-free NiTi samples. The microstructure, phase transformation and mechanical properties of the LPBF fabricated HP and LP NiTi were investigated using scanning electron microscopy (SEM), differential scanning calorimeter (DSC), X-ray diffraction (XRD) and dynamic mechanical analysis (DMA). The results showed that the HP and LP NiTi samples had different microstructures, phase transformation temperatures and mechanical properties. It was found that the HP NiTi samples predominantly contained austenite at room temperature and exhibited lower phase transformation temperatures. In contrast, the LP NiTi samples contained a large amount of martensite and had larger thermal memory recovery and better damping capacity. Additionally, the microstructure, phase transformation temperatures and mechanical properties were found to vary at different locations along the building direction in both HP and LP NiTi. This study implies that by manipulating the LPBF processing parameters, in particular the laser power, the phase transformation, microstructure and dynamic mechanical properties of the LPBF fabricated NiTi can be controlled to target different applications.

This study aims to assess the accuracy of various modelling methodologies in predicting creep-fatigue damage ( d c f ) and cycles-to-crack-initiation ( N i ) of Alloy 617 at 850 °C and 950 °C. In the decoupled methodologies, four creep damage models were employed: (i) the time fraction (TF), (ii) ductility exhaustion (DE), (iii) stress-modified ductility exhaustion (SMDE), and (iv) strain-energy density (SED), to capture the creep damage ( d c ) contribution to the overall creep-fatigue damage ( d c f ), while fatigue damage ( d f ) contribution was always calculated using the simple “average” approach. Furthermore, we employed Holmstrom's integrated ϕ model, which combines the d c f into a single parameter without a need of separate assessment of creep damage ( d c ) and fatigue damage ( d f ). The results show that there is no significant difference between the integrated ϕ model and decoupled creep-fatigue methodologies when the creep damage models are calibrated to creep-fatigue (CF) data. However, it is shown that the accuracy of the creep-fatigue damage ( d c f ) predictions using the decoupled approach gets significantly worse when the creep damage models are calibrated against independent creep-rupture (CR) data. It is further shown that when using DE, SMDE, SED creep damage models, the d c f predictions are arranged by strain range, suggesting that these models are better at capturing strain range variations rather than temperature changes. Based on the obtained results it is recommended that any of the employed creep damage models are all suitable for coupling with the “average” fatigue damage model when capturing the creep-fatigue behaviour of Alloy 617, as long as the employed creep damage model is calibrated directly to the experimental creep-fatigue (CF) data.

A first order visco-plastic strain gradient constitutive model and a cohesive zone formulation are embedded within a modified boundary layer (MBL) model for the analysis of crack tip fields and crack growth. The MBL model is loaded using a mode I asymptotic crack tip solution, with the boundary displacement calculated from the stress intensity factor. The influence of the rate-dependent constitutive parameters and the intrinsic material length on fracture relevant quantities are investigated in parametric study. Two scenarios are considered: (1) a stationary crack under constant loading and (2) a crack advance under monotonic loading. Finite element model analyzes are performed. For stationary cracks it was found that the effects of the intrinsic lengthscale of the strain gradient plasticity model are more prominent for large visco-plastic power exponents, and increase with hold time. The results of computations of crack advances under monotonic loading suggest that plastic strain gradients reduce crack growth resistance and crack initiation toughness, especially for a large visco-plastic power exponent. For small values of intrinsic material length the dependence of the initiation toughness and tearing modulus on the intrinsic length is strong, but then saturates for large values of intrinsic material length. Loading rate effects are found to be more pronounced for cases with a small value of the intrinsic lengthscale.

Bulk metallic glasses (BMGs) can exhibit excellent combinations of strength and fracture toughness that cannot be achieved by traditional metals, making them attractive for load bearing, mechanical engineering applications. Furthermore, recent research on BMGs has shown that many early perceived shortcomings, such as apparent brittleness or poor fatigue resistance, are not as big of a problem as once thought. The purpose of this paper is to review some of the unique mechanical characteristics of BMGs along with some of the strategies that may be used to improve or exploit their characteristics so that BMGs may be used as structural materials in engineering applications.

Surgeons can choose from a wide selection of commercially available suture brands, which come at a range of prices. There is currently limited evidence in the literature to guide this selection process. This investigation examined the breaking force, stress, and elongation of a variety of commercially available nylon sutures compared to their relative prices. Seven 5-0, nonabsorbable, nylon suture brands were tensile tested in straight, knotted and knot-security configurations according to the procedures outlined by the United States Pharmacopeia for the tensile testing of sutures. Covidien, the cheapest brand tested, had the highest failure load of straight and knot-security tests. Dafilon was found to have the highest breaking force and percent elongation of knot-pull tests. J&J Ethicon and Supramid had the highest percent elongation to failure for straight-pull and knot-security tests, respectively. This study was limited to specific in vitro tensile properties of nylon suture. Other factors affecting suture quality and price, such as needle properties, were not investigated. The data presented in the study provide information for guiding the selection and purchase of sutures according to tensile properties. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 815-819, 2017.

Objective Bioactive glass (BAG) is known to possess antimicrobial and remineralizing properties; however, the use of BAG as a filler for resin based composite restorations to slow recurrent caries has not been studied. Accordingly, the objective of this study was to investigate the effect of adding 15 wt% BAG to a resin composite on bacterial biofilms penetrating into marginal gaps of simulated tooth fillings in vitro during cyclic mechanical loading. Methods Human molars were machined into approximately 3 mm thick disks of dentin and 1.5–2 mm deep composite restorations were placed. A narrow 15–20 micrometer wide dentin-composite gap was allowed to form along half of the margin by not applying dental adhesive to that region. Two different 72 wt% filled composites were used, one with 15 wt% BAG filler (15BAG) and the balance silanated strontium glass and one filled with aerosol silica and silanated strontium glass without BAG (0BAG—control). Samples of both groups had Streptococcus mutans biofilms grown on the surface and were tested inside a bioreactor for two weeks while subjected to periods of cyclic mechanical loading. After post-test biofilm viability was confirmed, each specimen was fixed in glutaraldehyde, gram positive stained, mounted in resin and cross-sectioned to reveal the gap profile. Depth of biofilm penetration for 0BAG and 15BAG was quantified as the fraction of gap depth. The data were compared using a Student's t -test. Results The average depth of bacterial penetration into the marginal gap for the 15BAG samples was significantly smaller (∼61%) in comparison to 0BAG, where 100% penetration was observed for all samples with the biofilm penetrating underneath of the restoration in some cases. Significance BAG containing resin dental composites reduce biofilm penetration into marginal gaps of simulated tooth restorations. This suggests BAG containing composites may have the potential to slow the development and propagation of secondary tooth decay at restoration margins.

In order to understand the fracture toughness anisotropy of avian eggshells, we have investigated eggshells of the emu (Dromaius novaehollandiae) whereby the large size (~13 cm × 9.5 cm) enabled the fabrication of beam samples in various orientations. The emu eggshell was found to have a hierarchical microstructure similar to chicken eggshell, with the only significant difference being the absence of a continuous cuticle layer. Emu eggshell was found to have significantly lower strength when samples were tested in the outwards direction (i.e., a crack initiates on the inside of the shell and propagates towards the outer surface) as compared to the inwards testing direction. Furthermore, samples that were oriented parallel to the egg axis (i.e., the longitudinal direction) and tested inwards showed higher strength, ~24 MPa, compared to the samples that were made from the latitudinal orientation, ~20 MPa. Independent of orientation, the outwards testing direction resulted in strength values of ~15 MPa. The fracture toughness of the emu eggshell for cracking in the circumferential direction was ~0.3 MPa√m, independent of sample orientation, and this value was comparable to the fracture toughness of chicken eggshell tested in the same orientation. In the radial outwards direction, however, the fracture toughness was ~80% lower (~0.06 MPa√m) than in the circumferential direction. The low fracture toughness for this orientation was associated with the separation of the highly oriented calcite crystals in the mammillary cone layer of the eggshell structure which is easier compared to calcite crystal fracture. The large anisotropy in fracture toughness is thought to allow for easy escape of the chick while simultaneously protecting the embryo during development.

An irreversible cohesive zone model was used to understand how plastic strain gradients and yield strength mismatches affect fatigue crack growth across bi-material and multilayer interfaces for a range of interface strengths. Parametric studies identified regimes of penetration through the interface, deflection into the interface, and a transition where cracks extend on both paths. Plastic strain gradients strongly influence the transition regime, and a yield strength mismatch is necessary to cause deflection. For crack growth across an interlayer, interactions between the interfaces occur when the crack-tip plastic zone spans the interlayer. Plastic strain gradients promote penetration through the interlayer.

Some natural materials, such as the dactyl club of the mantis shrimp, have impressive mechanical properties (e.g. strength) due to their microstructure that consists of periodic layers of high and low density material, which prevent crack propagation. Although such layered structures have the potential to increase the strength of engineered epoxy-ceramic composites relative to their constituents, synthetically replicating this class of layered structures in engineered materials has been challenging to date. To overcome this challenge, ultrasound freeze casting (UFC) was used to manufacture macroscale specimens of epoxy-ceramic composite materials with periodic layers of high and low density that mimic the structure of natural materials. The critical operating parameter of the UFC technique, the ultrasound operating frequency, was related to the resulting hardness, porosity, and flexural strength of the resultant epoxy-ceramic composite materials. Scanning electron microscopy and micro X-ray CT was used to visualize the microstructure of the specimens and connect it to the mechanical properties. The ultrasound operating frequency controlled the spacing of the layers as well as the local hardness of the epoxy-ceramic composite, which increased by up to 18%. The flexural strength of the epoxy-ceramic composite was also related to the ultrasound operating frequency, with a maximum increase of 52%.

Shear bands were introduced into a Zr–Ti–Cu–Ni–Al bulk metallic glass (BMG) by one-directional cold rolling and the effect on fracture toughness anisotropy was examined. Two different rolling and shear band orientations relative to the crack plane were used, with the rolling force oriented along either the width (CR-W) or thickness (CR-T) direction of single edge notched bend specimens. The results showed the CR-W samples demonstrated a 50% reduction in KQ relative to the as-cast material while the CR-T samples demonstrated the highest fracture toughness. The low fracture toughness of the CR-W samples was attributed to easy crack propagation paths formed by the shear bands. In contrast, the higher fracture toughness of the CR-T samples was attributed to the difficulty of crack twisting out of the mode I precrack plane onto the shear band planes and the high energy absorption of mode III tearing between the parallel crack planes. Overall, when cold rolling BMGs for structural applications, care must be taken to design the shear band morphologies to achieve the desired fracture toughness properties rather than solely focusing on increasing the average relaxation enthalpy/free volume.