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Microstructural Evolution (microstructural + evolution)
Selected AbstractsLocalization Events and Microstructural Evolution in Ultra-Fine Grained NiTi Shape Memory Alloys during Thermo-Mechanical Loading,ADVANCED ENGINEERING MATERIALS, Issue 6 2010Andreas Schaefer Subjecting a thin NiTi specimen to uniaxial tension often leads to a localized martensitic transformation: macroscopic transformation bands form and propagate through the specimen, separating it into regions of fully transformed martensite and original austenite. In the present study, the alternating current potential drop (ACPD) technique is used to analyze the change in electrical resistance of ultra-fine grained NiTi wires subjected to a broad range of thermo-mechanical load cases: (i) uniaxial tensile straining at constant temperatures (pseudoelastic deformation); (ii) cooling and heating through the transformation range at constant load (actuator load case); (iii) a combination of mechanical and thermal loading. We monitor the ACPD signals in several zones along the gauge length of specimens, and we demonstrate that a localized type of transformation is a generic feature of pseudoelastic as well as of shape memory deformation. Moreover, the ACPD signals allow to differentiate between temperature-induced martensite (formed during cooling at no or relatively small loads), stress-induced martensite, and reoriented martensite (formed under load at low temperatures). [source] Microstructural Evolution in Some Silicate Glass,Ceramics: A ReviewJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2008Linda R. Pinckney Just as the microstructures in glass,ceramics encompass the range from nanocrystalline transparent materials to microcrystalline tough materials, so the paths of microstructural evolution in glass,ceramics vary widely. Evolution can proceed in numerous ways, their genesis being a perturbation of some type, including the surface nucleation used in glass frit processing, crystallization of the primary phase or phases upon distinct crystalline nuclei, and nucleation promoted by nano- or microscale amorphous phase separation in the parent glass. Examples of the crystallization history of several glass,ceramic materials are described, with emphasis on how their microstructural evolution influences their ultimate physical and optical properties. [source] Effect of Interface Structure on the Microstructural Evolution of CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2006Wook Jo The interface atomic structure was proposed to have a critical effect on microstructure evolution during sintering of ceramic materials. In liquid-phase sintering, spherical grains show normal grain growth behavior without exception, while angular grains often grow abnormally. The coarsening process of spherical grains with a disordered or rough interface atomic structure is diffusion-controlled, because there is little energy barrier for atomic attachments. On the other hand, kink-generating sources such as screw dislocations or two-dimensional (2-D) nuclei are required for angular grains having an ordered or singular interface structure. Coarsening of angular grains based on a 2-D nucleation mechanism could explain the abnormal grain growth behavior. It was also proposed that a densification process is closely related to the interface atomic structure. Enhanced densification by carefully chosen additives during solid state sintering was explained in terms of the grain-boundary structural transition from an ordered to a disordered open structure. [source] Microstructural Evolution of Calcium-Doped ,-AluminaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2003Arzu Altay Effect of different calcium doping levels on the microstructure of high-purity ,-alumina was studied as a function of sintering time and temperature using scanning electron microscopy (SEM). Microstructural evolution was related to hypothetical calcium excess at the grain boundaries (,Ca) that was calculated assuming zero solubility of calcium in bulk ,-alumina. Under all sintering conditions, grains were uniform in size and equiaxed for low calcium concentrations (<3 Ca atoms/nm2). The grain morphology became elongated when the calcium concentration at the grain boundaries reached calcium excess of ,Ca= 3,3.5 Ca atoms/nm2 in all samples. The average grain sizes of undoped samples were ,10% larger than the average grain sizes of low-calcium-doped samples. This decrease is believed to be due to solute drag effect of segregated Ca impurities on the grain boundary mobility. For the samples that were sintered at 1500° and 1600°C, slablike abnormally grown grains appeared for critical calcium excess concentrations of ,Ca= 4.5,8 Ca atoms/nm2. With abnormally grown grains a dramatic increase in average grain size was observed. However, when the calcium concentration was increased further, above certain calcium excess concentration depending on sintering temperature, a significant decrease in grain size was observed. In contrast to samples sintered at 1500° and 1600°C, when the samples sintered at 1400°C, although the hypothetical calcium coverage exceeded ,Ca= 11 Ca atoms/nm2, only few grains grew abnormally without significantly affecting the average grain size. Observations clearly indicated that calcium impurities caused elongated (slablike) grain morphology when their excess concentrations reached a critical level at the grain boundaries. [source] Effect of Oxygen Partial Pressure During Liquid-Phase Sintering on the Dielectric Properties of 0.9MgTiO3,0.1CaTiO3JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2008Hee-Kyun Shin Microstructural evolution and microwave dielectric properties of liquid-phase-sintered 0.9MgTiO3,0.1CaTiO3 dielectric ceramic material have been investigated as a function of oxygen partial pressure () during sintering. Sintering in a weakly reducing atmosphere (=10,14 atm) generally increased the density, permittivity, quality factor (Q×f), and the temperature coefficient of resonance frequency (,f), but further reducing atmosphere down toof 10,14 atm generally decreased Q×f and ,f. When the 5 wt% lithium borosilicate glass-added specimen was sintered at 950°C and=10,14 atm, it demonstrated a permittivity of 18.8, Q×f of 19 000 GHz, and ,f of 10 × 10,6 K,1. [source] Microstructural Evolution of Calcium-Doped ,-AluminaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2003Arzu Altay Effect of different calcium doping levels on the microstructure of high-purity ,-alumina was studied as a function of sintering time and temperature using scanning electron microscopy (SEM). Microstructural evolution was related to hypothetical calcium excess at the grain boundaries (,Ca) that was calculated assuming zero solubility of calcium in bulk ,-alumina. Under all sintering conditions, grains were uniform in size and equiaxed for low calcium concentrations (<3 Ca atoms/nm2). The grain morphology became elongated when the calcium concentration at the grain boundaries reached calcium excess of ,Ca= 3,3.5 Ca atoms/nm2 in all samples. The average grain sizes of undoped samples were ,10% larger than the average grain sizes of low-calcium-doped samples. This decrease is believed to be due to solute drag effect of segregated Ca impurities on the grain boundary mobility. For the samples that were sintered at 1500° and 1600°C, slablike abnormally grown grains appeared for critical calcium excess concentrations of ,Ca= 4.5,8 Ca atoms/nm2. With abnormally grown grains a dramatic increase in average grain size was observed. However, when the calcium concentration was increased further, above certain calcium excess concentration depending on sintering temperature, a significant decrease in grain size was observed. In contrast to samples sintered at 1500° and 1600°C, when the samples sintered at 1400°C, although the hypothetical calcium coverage exceeded ,Ca= 11 Ca atoms/nm2, only few grains grew abnormally without significantly affecting the average grain size. Observations clearly indicated that calcium impurities caused elongated (slablike) grain morphology when their excess concentrations reached a critical level at the grain boundaries. [source] Microstructure Development in Unsupported Thin FilmsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2002Brian P. Gorman To better understand the role of the substrate in the microstructural evolution of thin films, unsupported nanocrystalline yttrium-stabilized zirconia (ZrO2:16%Y or YSZ) films were examined as a function of temperature and annealing time. Grain growth, texturing, and pinhole formation were measured using transmission electron microscopy (TEM) and electron diffraction. Films were produced and subsequently annealed on metallic grids using a previously developed technique that results in near full density films at low annealing temperatures. Microstructural evolution in these films was unique compared with constrained films. Grains were found to spheroidize much more readily, ultimately resulting in the formation of porosity and pinholes. Grain growth was found to stagnate at a size particular to each annealing temperature, presumably due to the effects of Zener pinning. It is proposed that the lack of substrate strain and confinement effects allows for the dominance of surface energetics with respect to microstructural evolution. [source] Where Does the Lithium Go?ADVANCED ENGINEERING MATERIALS, Issue 4 2010A Study of the Precipitates in the Stir Zone of a Friction Stir Weld in a Li-containing 2xxx Series Al Alloy The main strengthening precipitates of aluminum alloy 2198-T8, which are of the T1 phase, dissolve during friction stir welding, sending many Li atoms into solid solution. The stir zone precipitates are characterized using high-resolution transmission electron microscopy, energy dispersive spectroscopy, and selected area diffraction techniques to begin answering questions about the microstructural evolution and the relationship between microstructure and mechanical properties in friction stir welding of the next generation of lightweight Li-containing Al alloys. [source] Quantitative Phase Field Modeling of Precipitation Processes,ADVANCED ENGINEERING MATERIALS, Issue 12 2006Q. Bronchard Phase Field modelling of microstructural evolution in alloys has already a long and successful history. One of the basics of the theory is the introduction of continuous fields (concentration, long-range order parameters) that describe the local state of the alloy. These fields have a meaning only at a mesoscopic scale. One consequence is that we can treat much larger systems than with microscopic methods such as Monte Carlo or molecular dynamics simulations. The aim of this work is to precisely analyse the status of the mesoscopic free energy densities that are used in Phase Field theories and, simultaneously, to clarify the form that the Phase Field equations should adopt. [source] A Screening Design Approach for the Understanding of Spark Plasma Sintering Parameters: A Case of Translucent Polycrystalline Undoped AluminaINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 5 2010Yann Aman An experimental screening design was used to evaluate the effects of spark plasma sintering (SPS) parameters such as heating rate, sintering temperature, dwell duration, and green-shaping processing on the relative density, grain size, and the optical properties of polycrystalline alumina (PCA). It is shown that heating rate and sintering temperature are the most critical factors for the densification of PCA during SPS. Green-shaping processing could prevent grain growth at low SPS sintering temperatures. No predominant SPS parameters are observed on the optical properties. Hence, the optical properties of PCA are controlled by microstructural evolution during the SPS process. [source] Petrogenetic modelling of strongly residual metapelitic xenoliths within the southern Platreef, Bushveld Complex, South AfricaJOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2010T. E. JOHNSON Abstract Xenoliths of quartz-absent Fe-rich aluminous metapelite are common within the platinum group element-rich mafic/ultramafic magmatic rocks of the Platreef. Relative to well-characterized protoliths, the xenoliths are strongly depleted in K2O and H2O, and have lost a substantial amount of melt (>50 vol.%). Mineral equilibria calculations in the NCKFMASHTO system yield results that are consistent with observations in natural samples. Lower-grade rocks that lack staurolite constrain peak pressures to ,2.5 kbar in the southern Platreef. Smaller xenoliths and the margins of larger xenoliths comprise micro-diatexite rich in coarse acicular corundum and spinel, which record evidence for the metastable persistence of lower-grade hydrous phases and rapid melting consequent on a temperature overstep of several hundred degrees following their incorporation in the mafic/ultramafic magmas. In the cores of larger xenoliths, temperatures increased more slowly enabling progressive metamorphism by continuous prograde equilibration and the loss of H2O by subsolidus dehydration; the H2O migrated to xenolith margins where it may have promoted increased melting. According to variations in the original compositional layering, layers became aluminosilicate- and/or cordierite-rich, commonly with spinel but only rarely with corundum. The differing mineralogical and microstructural evolution of the xenoliths depends on heating rates (governed by their size and, therefore, proximity to the Platreef magmas) and the pre-intrusive metamorphic grade of the protoliths. The presence or absence of certain phases, particularly corundum, is strongly influenced by the degree of metastable retention of lower-grade hydrates in otherwise identical protolith bulk compositions. The preservation of fine-scale compositional layering that is inferred to be relict bedding in xenolith cores implies that melt loss by compaction was extremely efficient. [source] Influence of Ca content and oxygen partial pressure on microstructural evolution of (Co,Ca)O at elevated temperaturesJOURNAL OF MICROSCOPY, Issue 1 2006J. KUSINSKI Summary Ca-doped (1, 1.7, 5 and 10 mol% CaO) cobalt oxide single-crystal samples, with an [001] orientation, were annealed at elevated temperatures of 1000,1200 °C for different times and at different oxygen partial pressures. The microstructure was examined by means of transmission light and electron microscopy. High-temperature X-ray diffractometry was used, with the aim of determining the temperature of the CoO , Co3O4 transition in these materials. Extensive precipitation of Ca-free Co3O4 spinel crystals was observed with increasing Ca content and oxygen activity. It is suggested that the electrical conductivity changes in this material may be related to this precipitation, because it changes the electronic state of cobalt cations. [source] Microstructure evolution in age-hardenable aluminium alloy during processing by hydrostatic extrusionJOURNAL OF MICROSCOPY, Issue 1 2006M. LEWANDOWSKA Summary In the present work, scanning and transmission electron microscopy were used to investigate the microstructural evolution occurring during the hydrostatic extrusion of an age-hardenable aluminium alloy. It was shown that processing by hydrostatic extrusion leads to grain refinement to 95 nm in equivalent diameter. Hydrostatic extrusion also influences the geometrical parameters of two different types of particle: intermetallic inclusions and precipitates. The intermetallic inclusions slightly decrease in mean equivalent diameter, but their size remains at the micrometre level. The precipitates are fragmented to nanoscale spherical particles, and their evolution delays the process of grain refinement. [source] EBSD and TEM investigation of the hot deformation substructure characteristics of a type 316L austenitic stainless steelJOURNAL OF MICROSCOPY, Issue 3 2004P. Cizek Summary The evolution of crystallographic texture and deformation substructure was studied in a type 316L austenitic stainless steel, deformed in rolling at 900 °C to true strain levels of about 0.3 and 0.7. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used in the investigation and a comparison of the substructural characteristics obtained by these techniques was made. At the lower strain level, the deformation substructure observed by EBSD appeared to be rather poorly developed. There was considerable evidence of a rotation of the pre-existing twin boundaries from their original orientation relationship, as well as the formation of highly distorted grain boundary regions. In TEM, at this strain level, the substructure was more clearly revealed, although it appeared rather inhomogeneously developed from grain to grain. The subgrains were frequently elongated and their boundaries often approximated to traces of {111} slip planes. The corresponding misorientations were small and largely displayed a non-cumulative character. At the larger strain, the substructure within most grains became well developed and the corresponding misorientations increased. This resulted in better detection of sub-boundaries by EBSD, although the percentage of indexing slightly decreased. TEM revealed splitting of some sub-boundaries to form fine microbands, as well as the localized formation of microshear bands. The substructural characteristics observed by EBSD, in particular at the larger strain, generally appeared to compare well with those obtained using TEM. With increased strain level, the mean subgrain size became finer, the corresponding mean misorientation angle increased and both these characteristics became less dependent on a particular grain orientation. The statistically representative data obtained will assist in the development of physically based models of microstructural evolution during thermomechanical processing of austenitic stainless steels. [source] Microstructural Evolution in Some Silicate Glass,Ceramics: A ReviewJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2008Linda R. Pinckney Just as the microstructures in glass,ceramics encompass the range from nanocrystalline transparent materials to microcrystalline tough materials, so the paths of microstructural evolution in glass,ceramics vary widely. Evolution can proceed in numerous ways, their genesis being a perturbation of some type, including the surface nucleation used in glass frit processing, crystallization of the primary phase or phases upon distinct crystalline nuclei, and nucleation promoted by nano- or microscale amorphous phase separation in the parent glass. Examples of the crystallization history of several glass,ceramic materials are described, with emphasis on how their microstructural evolution influences their ultimate physical and optical properties. [source] Processing and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride: (II) Effects of Magnesium Compound and Yttria AdditivesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2007Xinwen Zhu The effects of the magnesium compound and yttria additives on the processing, microstructure, and thermal conductivity of sintered reaction-bonded silicon (Si) nitride (SRBSN) were investigated using two additive compositions of Y2O3,MgO and Y2O3,MgSiN2, and a high-purity coarse Si powder as the starting powder. The replacement of MgO by MgSiN2 leads to the different characteristics in RBSN after complete nitridation at 1400°C for 8 h, such as a higher ,-Si3N4 content but finer ,-Si3N4 grains with a rod-like shape, different crystalline secondary phases, lower nitrided density, and coarser porous structure. The densification, ,,, phase transformation, crystalline secondary phase, and microstructure during the post-sintering were investigated in detail. For both cases, the similar microstructure observed suggests that the ,-Si3N4 nuclei in RBSN may play a dominant role in the microstructural evolution of SRBSN rather than the intergranular glassy chemistry during post-sintering. It is found that the SRBSN materials exhibit an increase in the thermal conductivity from ,110 to ,133 (Wm·K),1 for both cases with the increased time from 6 to 24 h at 1900°C, but there is almost no difference in the thermal conductivity between them, which can be explained by the similar microstructure. The present investigation reveals that as second additives, the MgO is as effective as the MgSiN2 for enhancing the thermal conductivity of SRBSN. [source] Multi-Scale Study of Sintering: A ReviewJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2006Eugene A. Olevsky An integrated approach, combining the continuum theory of sintering with a kinetic Monte-Carlo (KMC) model-based mesostructure evolution simulation is reviewed. The effective sintering stress and the normalized bulk viscosity are derived from mesoscale simulations. A KMC model is presented to simulate microstructural evolution during sintering of complex microstructures taking into consideration grain growth, pore migration, and densification. The results of these simulations are used to generate sintering stress and normalized bulk viscosity for use in continuum level simulation of sintering. The advantage of these simulations is that they can be employed to generate more accurate constitutive parameters based on most general assumptions regarding mesostructure geometry and transport mechanisms of sintering. These constitutive parameters are used as input data for the continuum simulation of the sintering of powder bilayers. Two types of bilayered structures are considered: layers of the same particle material but with different initial porosity, and layers of two different materials. The simulation results are verified by comparing them with shrinkage and warping during the sintering of bilayer ZnO powder compacts. [source] Microstructure Development in Unsupported Thin FilmsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2002Brian P. Gorman To better understand the role of the substrate in the microstructural evolution of thin films, unsupported nanocrystalline yttrium-stabilized zirconia (ZrO2:16%Y or YSZ) films were examined as a function of temperature and annealing time. Grain growth, texturing, and pinhole formation were measured using transmission electron microscopy (TEM) and electron diffraction. Films were produced and subsequently annealed on metallic grids using a previously developed technique that results in near full density films at low annealing temperatures. Microstructural evolution in these films was unique compared with constrained films. Grains were found to spheroidize much more readily, ultimately resulting in the formation of porosity and pinholes. Grain growth was found to stagnate at a size particular to each annealing temperature, presumably due to the effects of Zener pinning. It is proposed that the lack of substrate strain and confinement effects allows for the dominance of surface energetics with respect to microstructural evolution. [source] High-Temperature Oxidation Behavior of High-Purity ,-, ,-, and Mixed Silicon Nitride CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2002M. Backhaus-Ricoult High-temperature oxidation behavior, microstructural evolution, and oxidation kinetics of additive-free ,-, ,-, and mixed silicon nitride ceramics is investigated. The oxidation rate of the ceramics depends on the allotropic ratio; best oxidation resistance is achieved for ceramics rich in ,-phase. Variations in the oxidation kinetics are directly related to average grain size and glass distribution in the oxidation scale. The oxygen contents incorporated into the Si3N4 phase before its dissolution at the oxidation front affects the local glass composition and thereby yields nucleation and growth rates of SiO2 crystallites within the glass phase and a final oxidation scale microstructure, which depend on the incorporated oxygen contents. For the ,-polymorph, the dynamic oxygen solubility is found to remain negligible; therefore, a nitrogen-rich glass forms at the oxidation front, which promotes devitrification and yields a scale with small grain size and thin intergranular glass films. ,-Si3N4 is observed to form oxygen-rich solid solutions on oxidation, which are in contact with silicon oxynitride or oxygen-rich glass. Nucleation of cristobalite in the latter is sluggish, yielding coarse-grained oxidation scales with thick intergranular glass film. [source] | ||||||||||