Direct Observation Of Microstructure Research Articles

Compression cycling and magnetic response of single crystalline Ni-Mn-Ga particles/polymer composite: In-situ and ex-situ study

Ni-Mn-Ga particles/polymer composites are new magnetostrain active materials suitable for actuation and sensing. In the present work, we prepared mechanically anisotropic samples of the “30 vol% single crystalline Ni-Mn-Ga particles/silicone” composite material and systematically studied their cyclic stress-strain (S-S) evolutions along and perpendicular to the particle chains, while monitoring changes of magnetization curves. Four different compression cycling routes with the strain amplitude of 30% were explored. Magnetic measurements were served as the probes to indirectly detect microstructure changes of composite samples, whereas X-ray micro-CT images, alongside dimensions control of samples, were used for direct microstructural observations. The influences of the compression cycling routes on the parameters of S-S behaviors, such as an output stress, effective stiffness, and stress hysteresis, were revealed. It was found that the composite material after achieving equilibrium showed a rather good compression cycling stability of the mechanical and magnetic properties. It was revealed that composites being strongly deformed either along easy-magnetization axis (along the particle chains) or along hard-magnetization axis (perpendicularly to the particle chains) exhibited a magnetically harder behavior along the particle chains and magnetically easier behavior in the orthogonal direction, respectively. These unusual effects of a strong deformation of composite on its magnetization processes were discussed in terms of the particle rearrangements in the chains.

Time-resolved grain-scale 3D imaging of hydrofracturing in halite layers induced by gypsum dehydration and pore fluid pressure buildup

Fluid release from dehydration reactions is considered to have significant effects on the strength and dynamics of tectonic faults at convergent plate boundaries. It is classically assumed that the production of fluid leads to increased pore fluid pressures that perturb a fault's stress state and thereby facilitates and enhances deformation. This important assumption has never been supported by direct microstructural observations. Here, we investigate the role of gypsum dehydration in the deformation of evaporitic rocks using synchrotron-based time-resolved X-ray computed microtomography (4D) imaging. This approach enables the documentation of coupled chemical, hydraulic and mechanical processes on the grain scale. In our experiments with deforming halite-gypsum-halite sandwiches we observe that the fluid released by dehydrating gypsum accumulates at the gypsum-halite interface before a distributed hydraulic failure of the halite layer drains the fluid. From our observations we conclude that perceivedly impermeable halite layers in evaporites are unlikely to trap overpressured fluid, e.g., in thin-skinned tectonic detachment horizons. Moreover, as the hydraulic failure is diffuse and not localized, our experiments suggest that dehydration reactions alone may not explain intermediate depth seismicity in subduction zones. Our data demonstrate the significant potential that in-situ 4D imaging has for the grain-scale investigation of fundamental tectonic processes.

Clarification of the false liquation crack existed in 9% Cr/CrMoV dissimilar welded joint by comparative etching process and in situ laser scanning confocal microscope

Liquation cracks are generally regarded as thermal defects in various materials, e.g. in steels and nickel-based alloys. However, direct observation of microstructure with crack-like morphology that may be revealed by chemical etching or electrochemical etching methods, might lead to the misinterpretation of liquation cracks. To avoid the possible misinterpretation, comparative etching was employed to reveal the effect of various etching processes on the determination of liquation cracks. As a result, the formation process and mechanism of the false liquation crack in 9% Cr/CrMoV welded joint were clarified. The results obtained by optical microscope (OM) and scanning electron microscope (SEM) in the heat treatment zone of 9% Cr steel (9% Cr-HAZ) after chemical etching clearly demonstrate the existence of crack-like morphology, and the disappearance of element segregation zone after regrinding and re-polishing. Besides, laser scanning confocal microscope (LSCM) was applied to further exhibit the microstructure evolution experiencing high temperature to confirm the possibility of the liquation crack. The results showed that the element segregation and subsequent chemical etching process were the crucial factors to cause the formation of false liquation crack.

Measurements and understanding of the stiffness of an air plasma sprayed thermal barrier coating

In this work, the stiffness of an air plasma sprayed thermal barrier coating (APS TBC) was studied using the three-point bending technique. Various measurements over different time periods were made on detached TBC beams, prepared by acid etching, in both as-deposited states and after heat treatment. Monotonic and cyclic loading were conducted to understand the mechanical behaviour of the TBCs in detail. The TBC beams during bending show nonlinear behaviour, inelastic deformation and hysteresis loops. The derived global in-plane elastic modulus, at first increases and then decreases with thermal exposure, and is of the range 14–18 GPa. This behaviour is a result of competition between sintering and development of microstructural damage. Direct microstructural observations during In-situ mechanical testing show that intra-splat opening/closure and inter-splat sliding give rise to the relatively low elastic modulus of the APS TBC.

Thermally-induced cracks and their effects on natural and industrial geomaterials

Thermal stress can result in significant changes in the mechanical and transport properties of building materials, especially in terms of cracking. Three building materials were studied: two concretes, a siliceous and a calcareous one, and a natural calcareous rock, the Tuffeau. The samples were subjected to thermal shock, repetitive heating-cooling cycles, and high temperature heating in order to analyze the effects of maximum temperature, cooling rate, and repetitive heating on the three materials. The induced cracks were then characterized by physical and hydraulic measurements, namely elastic wave velocities, porosity and effective thermal conductivity. Elastic wave velocities were used to determine crack density while effective thermal conductivity was used to determine crack connectivity. Cracks were also quantitatively described through direct microstructural observations using scanning electron microscopy. Results show the effectiveness of the different protocols in inducing cracks. Unexpectedly, repetitive heating-cooling cycles caused the most significant sample damage, whatever the sample. A second main result is based on the comparison of the different materials. It was found that the behavior of the two concretes was very similar: the stronger the thermal treatment, the more the crack density and connectivity increased, albeit with a slight difference in that the siliceous concrete appeared to be less resistant to sharp thermal variations. This is interpreted as being linked to microstructural effects: in the siliceous concrete, we observed cracks that nucleated around and inside grains, but not in the calcareous concrete. Lastly, the behavior of the Tuffeau limestone was different from that of the concretes: when crack density increased, the crack connectivity and the porosity both decreased. This different behavior is interpreted in the light of microstructural observations of the crack apertures: the thermally induced cracks in Tuffeau are too small to influence the effective thermal connectivity measurement and to allow fluid flow during the porosity measurement, whereas in the concretes, cracks were observed to be much more open. As an outlook, we discuss a possible equivalent test to the normalized fire protocol, performed at high temperature, to test the fire resistance of materials.

Sintering dense nanocrystalline 3YSZ ceramics without grain growth by plastic deformation as dominating mechanism

It is difficult to obtain nanocrystalline ceramic bulks due to its high surface activity at high temperatures. In the study, in order to achieve both high density and ultrafine morphology, the plastic deformation was induced by an ultrahigh pressure at a deliberately selected temperature, which was much lower than the threshold temperature for rapid grain growth. According to the ultra-high pressure route, nanocrystalline 3YSZ ceramics without grain growth were fully densified at 900 °C under 1.5 GPa. Both direct microstructural observations and calculation results proved that the plastic deformation including high temperature yield and sliding played a dominant role in densification during the sintering process.

Tiny timekeepers witnessing high-rate exhumation processes

Tectonic forces and surface erosion lead to the exhumation of rocks from the Earth’s interior. Those rocks can be characterized by many variables including peak pressure and temperature, composition and exhumation duration. Among them, the duration of exhumation in different geological settings can vary by more than ten orders of magnitude (from hours to billion years). Constraining the duration is critical and often challenging in geological studies particularly for rapid magma ascent. Here, we show that the time information can be reconstructed using a simple combination of laser Raman spectroscopic data from mineral inclusions with mechanical solutions for viscous relaxation of the host. The application of our model to several representative geological settings yields best results for short events such as kimberlite magma ascent (less than ~4,500 hours) and a decompression lasting up to ~17 million years for high-pressure metamorphic rocks. This is the first precise time information obtained from direct microstructural observations applying a purely mechanical perspective. We show an unprecedented geological value of tiny mineral inclusions as timekeepers that contributes to a better understanding on the large-scale tectonic history and thus has significant implications for a new generation of geodynamic models.

MEMS based nanomechanical testing method with independent electronic sensing of stress and strain

We report significant improvements in the sensing scheme of a microelectromechanical system (MEMS) based nanomechanical tensile testing technique that has been previously demonstrated to allow direct microstructural observations inside a transmission electron microscope (TEM) at high magnifications while simultaneously measuring the stress and strain in the sample electronically. The particularity of the MEMS device is the presence of two capacitive sensors on either side of a specimen gap (across which nanostructures like nanowires and thin films can be manipulated and clamped), allowing independent measurement of applied load (stress) and crosshead displacement (strain). The improvement in the sensing technique lies in the independent, separate measurements of the signals from the two capacitive sensors (as opposed to the previous technique based on the differential measurement between the two sensors). The new technique (called technique 2 in this paper) provides independent electronic sensing of stress and strain without making any assumption about the material behavior and opens up the possibility of doing force controlled tests. The new sensing technique is demonstrated and compared to the previous one (called technique 1 in this paper) by performing ex-situ monotonic and stress relaxation tests on freestanding 100-nm-thick Au films. Important practical matters such as specimen clamping and signal drift are also discussed.

Development and Experimental Validation of Morphology Predictive Model for Compatibilized Ternary Polymer Blends I. Effect of Interfacial Tension

To evaluate the prediction reliability of conventional morphology predicting models, polypropylene (PP)/polyamide6 (PA6)/ethylene propylene diene monomer (EPDM) (70/15/15) ternary polymer blends compatibilized with Maleic-anhydride grafted EPDM (EPDM-g-MA) were prepared through melt blending using a twin screw extruder (TSE). Different EPDM/EPDM-g-MA ratios i.e. 100/0, 75/25, 50/50, 25/75 and 0/100 were used to prepare the ternery blend PP/(EPDM-g-MA + EPDM)/PA6 samples. The effects of compatibilizer content on the microstructures and consequently mechanical properties of prepared ternary blends were studied. Direct microstructural observations were compared to the predictions of conventional phenomenological models including spreading coefficient, minimum relative free energy, and dynamic interfacial energy. A comparison depicted the relative inaccuracy of the existing models in predicting the morphology of the present ternary system due to the ignorance of some effective parameters and/or discomfit of model assumptions. A novel predictive model was developed considering parameters ignored in conventional models. A thorough investigation of the model’s validation results showed a reasonable agreement between model predictions and direct microstructural observations.

Microbiological, physicochemical and rheological properties of fermented soymilk produced with exopolysaccharide (EPS) producing lactic acid bacteria strains

Growth of Lactobacillus plantarum 70810, Lactobacillus rhamnosus 6005 and a commercial yogurt starter culture in soymilk was investigated in the present study. It was found that the fermented soymilk using L. plantarum 70810 had significantly higher viable cell counts, water holding capacity (WHC, 88.27%), apparent viscosity (1840.35 mPa s) and exopolysaccharide (EPS) amount (832.15 mg/L) than the other two starter cultures in soymilk. Direct observation of microstructure in fermented soymilk indicated that the network structures of EPS-protein could improve the texture of fermented soymilk. Considering that the beneficial effects of L. plantarum 70810 in fermented soymilk, volatile compounds in fermented soymilk were further identified. Then the growth and fermentation characteristics of L. plantarum 70810 including changes in viable cell counts, pH, titratable acidity, apparent viscosity and EPS production during storage were investigated. In comparison to original soymilk base, the concentrations of the characteristic flavor compounds for fermented soymilk using L. plantarum 70810 increased, whereas hexanal, 2-pentylfuran and 2-pentanone in relation to beany flavor of soymilk decreased. In addition, fermented soymilk using L. plantarum 70810 maintained high viable cell count (>108 cfu/mL), apparent viscosity (966.43 mPa s) and amounts of EPS (635.49 mg/mL) during storage at 4 °C for 21 days.

Direct observations of microscale turbulence and thermohaline structure in the Kuroshio Front

Direct observations of microstructure near the Kuroshio Front were conducted in August 2008 and October 2009. These show negative potential vorticity (PV) in the mixed layer south of the front, where directly measured turbulent kinetic energy dissipation rates are an order magnitude larger than predicted by wind‐scaling. These elevated dissipation rates scale better with an empirical scaling, which considers local wind and Ekman buoyancy flux driven by downfront wind. Near‐zero PV in the thermocline under the Kuroshio mainstream is observed at 200–300 m depth, with dissipation exceeding open ocean thermocline values by factors of 10–100. Overall, the large turbulent dissipation rates measured in the Kuroshio can be categorized into two groups, one characterized by low Richardson number along the Kuroshio Front thermocline, and the other characterized by high stratification away from the Kuroshio mainstream. The former is attributed to mixing by unbalanced frontal ageostrophic flows, and the latter is attributed to internal wave breaking. On average, both groups appear in regions of large horizontal density gradients. Observed thermohaline structure shows low salinity tongues from the surface to over 300 m depth and deep cold tongues, extending upward from 500 to 100 m depth in a narrow (20 km) zone, suggesting down and upwelling driven by geostrophic straining, which is confirmed by Quasigeostrophic‐Omega equation solutions. This implies that adiabatic along isopycnal subduction and diabatic diapycnal turbulent mixing acting in tandem at the Kuroshio Front likely contribute to NPIW formation.

Microstructure characterization of ball-mill prepared ternary Ti 0.9Al 0.1C by X-ray diffraction and electron microscopy

Synthesis of ternary carbide Ti 0.9Al 0.1C by high energy ball-milling under argon atmosphere is investigated thoroughly. Microstructure characterization of ball-milled samples in terms of lattice imperfections and quantitative estimation of different phases present in the samples are done by using X-ray diffraction data employing Rietveld's powder structure refinement method. The results show that the formation of Ti 0.9Al 0.1C nano-carbide starts after 48 min of ball-milling from Ti–Al–C solid solution. Full formation of Ti 0.9Al 0.1C without any contaminated phase is achieved within 3 h of milling and to reduce particle size ball-milling is continued up to 10 h. Direct observation of microstructure of 10 h ball-milled sample under high resolution TEM reveals supportive evidence of structural and microstructural evaluation by indirect method using XRD data.

Preparation of ternary Ti 0.9Ni 0.1C cermets by mechanical alloying: Microstructure characterization by Rietveld method and electron microscopy

Synthesis of ternary Ti 0.9Ni 0.1C carbide by high energy ball-milling of the stoichiometric elemental powder mixture under argon atmosphere is investigated in detail. Microstructure characterization of ball-milled samples in terms of lattice imperfections and quantitative estimation of different phases present in the samples have been made by analyzing the X-ray diffraction data employing Rietveld's powder structure refinement method. The results reveal that the formation of Ti 0.9Ni 0.1C nano-carbide starts after 55 min of ball-milling from Ti–Ni–C solid solution. After 8 h of milling full formation of Ti 0.9Ni 0.1C is achieved without any contaminated phase. To study the effect of milling time on particle size, ball-milling is continued up to 12 h and the results indicate that particle size reaches its lowest value after 8 h of milling and then starts to agglomerate. Direct observation of microstructure of 12 h ball-milled sample under high resolution TEM reveals supportive evidence of structural and microstructural evolution by indirect method using XRD data.

Direct observations of ordered R 2CuSi 3 (R=Ce and Nd) cluster–glass compounds in real space by HRTEM

We present the direct observations of microstructure with clustering by electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM) on cluster–glass (CG) magnetic compounds R 2CuSi 3 (R=Ce and Nd) related to a hexagonal AlB 2-type structure. Observed diffraction patterns show superstructure reflections at 1 2 1 2 1 2 -typed positions, which are very similar to the results observed for the CG compound U 2IrSi 3. Fourier-filtered HRTEM images clearly exhibit a domain structure. The size of the domains with a base-centered ordering structure in Ce 2CuSi 3 is estimated to be 2–3 nm in much smaller than that in Nd 2 CuSi 3. These observations are consistent with the inferences based on the magnetic measurements, and can be considered as the direct evidence for the formation of magnetic cluster in Ce 2CuSi 3 and Nd 2CuSi 3.

Focused Ion Beam Study of Ni5Al Single Splat Microstructure

AbstractThermally sprayed (TS) coatings are widely used for surface engineering across a range of industries, including aerospace, infrastructure and biomedical. TS materials are formed via the successive impingement, rapid quenching and build-up of molten powder particles on a substrate. The impacted ‘splats’ are thus the fundamental microstructural constituents of the coatings, and their intrinsic properties, as well as intersplat bonding and morphology, dictate coating behavior. Beyond the obvious practical considerations, from a scientific standpoint, splats represent a fascinating template for study, due to the highly non-equilibrium processing conditions (rapid deceleration from sub-sonic velocities, million-degree/sec cooling rates). In the literature, many studies of isolated splats on substrates have been carried out, but these have focused on overall morphology (disc-shape vs fragmented). Direct observations of microstructure, in particular cross-section, are limited in the specimen preparation stage due to splat size (tens of microns in diameter, 1-2 microns in thickness). However, Focused Ion Beam (FIB) techniques have allowed this problem to be addressed in a robust manner; in this paper we will discuss such approaches to observe Ni5Al splats on stainless steel substrates. Cross-sections through the splat and the substrate were created by recourse to ion milling and the ion beam itself provided good channeling contrast for grain imaging. The typical splat microstructure with sub-micron Ni(Al) columnar grains, a chill zone at the bottom and a lift off area is observed in high detail. In addition, an amorphous aluminum oxide top layer of 100-200 nm is partially present on top of the Ni(Al) columnar grains. At the splat/substrate interface, defects such as micro- and nano-scale pores were characterized for the first time and will be discussed. These observations provide insights into splat and interface formation during the deposition process and may drastically improve our current understanding of Ni5Al splat properties.

Modelling of transient liquid phase bonding

Modelling of transient liquid phase (TLP) bonding is reviewed. The outputs produced during analytical and numerical modelling are discussed in detail and compared with actual experimental results produced when joining simple binary alloy systems. The effects of increased diffusivity at base material grain boundaries, of grain boundary motion, and of grain boundary grooving, on isothermal solidification during TLP bonding are described. There is a critical need for detailed research in which modelling output is closely related to direct microstructural observations during bonding of complex alloy systems.

Modelling of transient liquid phase bonding

Modelling of transient liquid phase (TLP) bonding is reviewed. The outputs produced during analytical and numerical modelling are discussed in detail and compared with actual experimental results produced when joining simple binary alloy systems. The effects of increased diffusivity at base material grain boundaries, of grain boundary motion, and of grain boundary grooving, on isothermal solidification during TLP bonding are described. There is a critical need for detailed research in which modelling output is closely related to direct microstructural observations during bonding of complex alloy systems.

Comparative study of supported catalyst particles by electron microscopy methods

High-resolution transmission electron microscopy, high-resolution scanning electron microscopy and high-angle annular dark-field imaging were used to study the size distribution and surface structures of Pt model catalysts on various oxide supports. The relative merits of different electron microscopy methods for catalyst research were evaluated by comparing images recorded with microscopes of different type. It was concluded that HRTEM profile imaging was the most effective technique for direct observation of microstructure, especially the surface structure of supported particles, while HRSEM and HAADF, respectively, were preferred for characterizing the surface topology of catalyst supports and the size distribution of supported particles. Using profile imaging, crystalline monolayers caused by high-temperature reduction on {111} surfaces of Pt/TiO 2 could be recorded with atomic resolution. These overlayers help explain the drop in chemisorption ability due to high-temperature reduction, a phenomenon usually referred to as strong metal-support interaction. HRSEM showed the presence of surface steps on model TiO 2 and a concentration of larger Pt particles on these steps.

Microstructure evolution of newly developed electroless ruthenium deposition on silicon observed by scanning transmission electron microscope

Autocatalytic Ruthenium thin films have been successfully deposited on silicon. The deposition reactions were proven to be electroless by the measurement of the electrochemical rest potentials of anodic and cathodic partial reactions and mixed potentials. A new method for direct observation of microstructure and formation at the initial stage of deposition has also been developed. The progressive formation and microstructure of ruthenium thin films during various growth stages have been investigated by scanning transmission electron microscopy. Deposition takes place immediately after the n-type silicon is immersed in the electroless plating solution. The film was identified to be amorphous by the selected area electron diffraction analysis. Cross-sectional transmission electron microscopy method was utilized to explore the uniformity and morphology of the film as well as the Ru/Si interface. Accordingly, the deposition rate was also measured with a high precision nanometer. The microstructure evolution and significance in applications are also discussed.