Showing papers on "Microstructure published in 1993"
TL;DR: In this article, a new approach to the modelling of grain structure formation in solidification processes is proposed based upon a two-dimensional cellular automata technique, the model includes the mechanisms of heterogeneous nucleation and of grain growth.
Abstract: A new approach to the modelling of grain structure formation in solidification processes is proposed. Based upon a two-dimensional cellular automata technique, the model includes the mechanisms of heterogeneous nucleation and of grain growth. Nucleation occurring at the mould wall as well as in the liquid metal are treated by using two distributions of nucleation sites. The location and the crystallographic orientation of the grains are chosen randomly among a large number of cells and a certain number of orientation classes, respectively. The growth kinetics of the dendrite tip and the preferential 〈100〉 growth directions of cubic metals are taken into account. The model is then applied to small specimens of uniform temperature. The columnar-to-equiaxed transition, the selection and extension of columnar grains which occur in the columnar zone and the impingement of equiaxed grains are clearly shown by this technique. The calculated effect of the alloy concentration and cooling rate upon the resultant microstructure agree with experimental observations.
785 citations
TL;DR: In this article, the authors studied micron size indentations of Mo and W single crystals using the scanning tunneling microscopy (STM) and discussed the mechanism of plastic flow and the indentation size effect.
Abstract: Indentations of micron size into the (100), (110) and (111) surfaces of Mo and W single crystals have been studied using the scanning tunneling microscopy (STM). Results on geometrical parameters of indents are presented and discussed in connection with the mechanism of plastic flow and the indentation size effect.
636 citations
TL;DR: Spectroscopic analysis strongly suggests that the photogeneration of carriers occurs in the c-Si core, whose band gap is modified by the quantum-confinement effect, while the strong PL comes from the near-surface region of small crystallites.
Abstract: We have studied the microstructure and optical properties of free-standing porous Si thin films fabricated by electrochemical anodization. Raman-spectroscopy and transmission-electron-microscopy examinations show that Si crystallite spheres with diameters of several nanometers are dispersed in the amorphous phase. The blueshift of the optical-absorption spectrum is observed for decreasing average diameter of the Si crystallites. However, there is no clear size dependence of the peak energy of the broad photoluminescence (PL) spectrum. Spectroscopic analysis strongly suggests that the photogeneration of carriers occurs in the c-Si core, whose band gap is modified by the quantum-confinement effect, while the strong PL comes from the near-surface region of small crystallites.
378 citations
TL;DR: In this article, the evolution of mechanical properties in austenitic stainless steels during irradiation is reviewed and the effects of irradiation on fatigue appear to be modest except at high temperature where helium embrittlement becomes important.
362 citations
TL;DR: In this paper, the α to γ transformation in ODS martensitic steel has been successfully achieved by introducing the α-to-γ transformation in 13Cr-3W ODS ferritic steels.
288 citations
TL;DR: In this article, the basic principles involved in the synthesis of nanostructured materials are discussed in terms of the special properties sought using selected examples from particular synthesis and processing methodologies.
280 citations
TL;DR: In this article, the microstructure and microchemistry of silicon carbide sintered with yttrium-aluminum garnet (YAG) was characterized using plasma etching in conjunction with analytical transmission electron microscopy.
Abstract: Plasma etching in conjunction with scanning and analytical transmission electron microscopy was used to characterize the microstructure and microchemistry of silicon carbide sintered with yttrium-aluminum garnet (YAG). The SiC grains comprise a core/rim structure with small amounts of excess yttrium, aluminum, and oxygen being present in the rim while these elements are missing in the core. The core/rim interface was found to be coherent, and both the core and the rim are composed of the same polytype, predominantly the 6H structure. These results suggest that Ostwald ripening by solution and reprecipitation controls the sintering mechanism in this system.
266 citations
TL;DR: In this article, the development of the microstructure of high supersaturated nanocrystalline FexCu100−x alloys was investigated by x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy.
Abstract: Highly supersaturated nanocrystalline FexCu100−x alloys (10≤x≤95) have been prepared by mechanical alloying of elemental crystalline powders The development of the microstructure is investigated by x‐ray diffraction, differential scanning calorimetry, and transmission electron microscopy The results are compared with data for ball‐milled elemental Fe and Cu powders, samples prepared by inert gas condensation, and sputtered films The deformation during milling reduces the grain size of the alloys to 6–20 nm The final grain size of the powders depends on the composition of the material Single‐phase fcc alloys with x≤60 and single‐phase bcc alloys with x≥80 are formed even though the Fe‐Cu system exhibits vanishingly small solid solubilities under equilibrium conditions For 60≤x≤80, fcc and bcc solid solutions coexist The alloy formation is discussed with respect to the thermodynamic conditions of the material The role of the large volume fraction of grain boundaries between the nanometer‐sized cryst
265 citations
TL;DR: In this paper, a two-step seeding process was developed to lower the transformation temperature and modify the grain structure of ferroelectric lead zirconate titanate (PZT) thin films with high Zr/Ti ratio.
Abstract: A two-step seeding process has been developed to lower the transformation temperature and modify the grain structure of ferroelectric lead zirconate titanate (PZT) thin films with high Zr/Ti ratio. Previous study has shown that nucleation is the rate-limiting step for the perovskite formation. Therefore, any process that enhances the kinetics of nucleation is likely to decrease the transformation temperature. In this process, a very thin (45 nm) seeding layer of PbTiO3, which has a low effective activation energy for perovskite formation, was used to provide nucleation sites needed for the low temperature perovskite formation. In this study, we have shown that the pyrochlore-to-perovskite phase transformation temperature of PbZrxTi1−xO3 films of high Zr/Ti ratio (e.g., x = 53/47) can be lowered by as much as 100 °C. The grain size of these films can also be substantially modified by this two-step approach.
257 citations
TL;DR: In this article, compressive biaxial stress fields in thin film boron nitride are shown to provide a means of accessing the region of the phase diagram in which cubic BN (c-BN) is the stable phase.
239 citations
TL;DR: The surface modification of AISI 316 stainless steel by plasma immersion ion implantation (PI3) has been investigated over a range of treatment temperatures as discussed by the authors, and the results are similar to those obtained by conventional ion beam implantation of nitrogen, but the depth of nitrogen penetration increases dramatically with temperature.
Abstract: The surface modification of AISI 316 stainless steel by plasma immersion ion implantation (PI3) has been investigated over a range of treatment temperatures. Below 250°C the results are similar to those obtained by conventional ion beam implantation of nitrogen, but the depth of nitrogen penetration increases dramatically with temperature. Up to 450 °C a nitrogen-expanded austenite phase is formed which is shown to have improved corrosion performance over the untreated material. At 520 °C chromium nitride is precipated and the expanded austenite transforms to martensite, leading to a reduction in corrosion resistance. Pin-on-disc testing indicates improved wear resistance at all temperatures, with reduction in the wear volume by factors of several hundred at high loads. This can be attributed to the formation of an oxide layer which prevents the initiation of severe metallic wear.
TL;DR: In this article, it was shown that there is a common mechanism for solid-state amorphization of silicates in static and shock high-pressure experiments, meteorite impact, and deformation by tectonic processes.
Abstract: Solid-state amorphization is a transformation that has been observed in a growing number of materials. Microscopic observations indicate that amorphization of a-quartz begins with formation of crystallographically controlled planar defects and is followed by growth of amorphous silicon dioxide at these defect sites. Similar transformation microstructures are found in quartz upon quasihydrostatic and nonhydrostatic compression in a diamond-anvil cell to 40 gigapascals and from simple comminution. The results suggest that there is a common mechanism for solid-state amorphization of silicates in static and shock high-pressure experiments, meteorite impact, and deformation by tectonic processes. In general, these results are consistent with recently proposed shear instability models of amorphization.
TL;DR: In this paper, the thermal stability of supersaturated nanocrystalline FexCu100−x alloys (10 ~ 80) was studied using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry.
Abstract: X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry were used to study the thermal stability of highly supersaturated nanocrystalline FexCu100−x alloys (10 ~80. For 60<=x<=80 fcc and bcc phases coexist. Heating to elevated temperatures leads to structural relaxation, phase separation, and grain growth of the metastable nanocrystalline solid solutions. Single-phase fcc and bcc alloys undergo significant strain release but no appreciable grain growth prior to phase separation. After phase separation pronounced grain growth sets in. In contrast, samples in the two-phase region show some grain growth and significant chemical redistribution even at low temperatures. The phase separation of single-phase fcc and bcc alloys proceeds via different mechanisms: fcc solid solutions decompose by forming small Fe precipitates, while demixing in bcc alloys starts by segregation of Cu atoms to bcc grain boundaries before nucleation of Cu precipitates. These results show that the stability and grain growth behavior of nanocrystalline alloys is strongly affected by the microstructure of the material.
TL;DR: In this paper, the authors present five themes for fundamental research and design of high-temperature structural ceramics: chemical and environmental stability, grain-boundary sliding and cavitation, single-crystal micro-structure design, room-time temperature mechanical properties, and thermal shock.
Abstract: Structural ceramics for high-temperature applications should embody the following properties: oxidation resistance, chemical stability, low volatility, resistance to creep deformation, resistance to creep cavitation at interfaces, sufficient toughness at ambient temperature, and thermalshock resistance. These criteria lead to five themes for fundamental research and design of high-temperature structural ceramics: chemical and environmental stability, grain-boundary sliding and cavitation, single-crystal microstructure design, room-temperature mechanical properties, and thermal shock. It is recommended that research that is confined to any one of these five areas takes into consideration the broader implications of research results. For example, microstructure designs that require weak interfaces for obtaining toughness at room temperature directly or indirectly conflict with creep and cavitation resistance needed for long-term service at high temperatures. New research should be directed at mechanisms that can simultaneously achieve good mechanical properties over a wide range of temperatures. This paper addresses the following recommendations: (i) although non-oxide systems can be viable for structural applications below 1500°C, oxidebased ceramics are necessary for service above 1500°C; (ii) microstructure designs based on acicular grain morphologies and/or single-crystal fiber reinforcements have the potential for meeting the mechanical property requirements from room temperature up to very high temperatures; (iii) for fundamental studies of mechanical properties at high temperatures, simple uniaxial tension experiments should be used in tandem with four-point bending and uniaxial compression experiments; (iv) the study of the reinforcement phase should center on very pure, highly stoichiometric materials in the case of non-oxides, and on mixed and alloyed single crystals of cubic symmetry, or crystals having isotropic properties, and large unit cells in the case of oxides; (v) the study of interfaces in non-oxides should focus on the chemistry of the intergranular glass phase, particularly the control of the oxygen content and the crystallization of this phase for improvement of high-temperature properties; (vi) the study of interfaces in oxides is best directed at the relationship between interface structure, defect chemistry, and interfacial mechanical properties over a wide range of temperature; (vii) the understanding of the micromechanisms of thermal-shock failure and the application of this understanding for designing graded interfaces that may be able to cope with thermal-expansion stresses without leading to microfracture and cavitation is important in all classes of ceramic materials, and is of critical importance in the development of oxides for very-high-temperature applications; and (viii) research in processing science should emphasize the study of basic mechanisms that lead to in-situ growth of acicular and fibrous microstructures.
TL;DR: In this paper, the stability of the various hydrate phases in the electron microscope is discussed, although all are subject to damage in varying degrees, even the least stable phase, AFt, can be recognized in relict form in the TEM.
Abstract: Hardened ordinary Portland cement pastes of various ages have been examined by analytical transmission electron microscopy (TEM) and electron microprobe analysis (EMPA). The stability of the various hydrate phases in the electron microscope is discussed. Although all are subject to damage in varying degrees, even the least stable phase, AFt, can be recognized in relict form in the TEM. The basic framework of the microstructure and the differentiation into inner and outer hydration product are well-established at 24 h hydration. Although the dominant inner product formed within the boundaries of the original anhydrous grains is C-S-H gel, particles of AFt, AFm, Ca(OH)2, a magnesium-rich phase and an iron-rich phase are occasionally observed within the inner product. The Ca∶Si ratio of the C-S-H gel determined by TEM shows significant variation from one region to another in a given paste. There is no relationship between the average Ca∶Si ratio of the C-S-H and the maturity of the paste, although young pastes appear to show a bimodal distribution. Microanalysis by EMPA gives Ca∶Si ratios in substantial agreement with those found by TEM but it is essentially impossible to obtain by EMPA analyses of outer product C-S-H without admixture of other phases, particularly sulphoaluminate phases. Despite the presence of small amounts of embedded phases as revealed by TEM, single-phase inner product C-S-H can be analysed by EMPA. The compositions of AFt and AFm phases have been obtained by TEM and the results do not require the substitution of silicon in the formulae.
TL;DR: In this article, plan-view transmission electron microscopy revealed an open columnar structure with a relatively constant average grain size of ∼ 65 nm, and the amount of intercolumnar pores or voids gradually decreased.
TL;DR: In this article, the problems faced by the quality of cast products and their relation to the process variables and characteristics of a given process, constitute the main theme of the review and draw attention to the potential for application of cast composites in different industrial components and underlines the necessity of research in certain related fields so that industrial applications of cast metal-matrix composites will soon become a reality.
Abstract: The present review begins by briefly tracing history in the early days of development of cast metal-matrix composites and also outlines different casting routes for their synthesis. The problems faced by the quality of cast products and their relation to the process variables and characteristics of a given process, constitute the main theme of the review. The development of microstructure has been discussed in view of nucleation behaviour anticipated on the basis of estimated interface energies. The solidification around dispersoids and in regions away from it has been highlighted. Porosity in cast composites (its origin and control in cast components by suitable mould design) has engaged attention because of damage to mechanical properties due to porosity. The chemical reactions at the interface between dispersoid and matrix during processing of certain important systems of composites, have been described and the means of controlling these reactions have been indicated. The review concludes by drawing attention to the potential for application of cast composites in different industrial components and underlines the necessity of research in certain related fields so that industrial application of cast metal-matrix composites will soon become a reality.
TL;DR: In this paper, a relatively small addition of Zn significantly improves the mechanical strength of Sn•3.5% Ag eutectic solders while maintaining the same level of ductility, which is attributed to a substantial refinement of the precipitates in the solidification microstructure.
Abstract: New high‐strength Pb‐free solder alloys, based on the Sn‐Ag‐Zn system, have been developed. A relatively small addition of Zn significantly improves the mechanical strength of Sn‐3.5% Ag eutectic solders while maintaining the same level of ductility. The observed increase in strength is as much as 48% over that of the Zn‐free alloy. This strengthening from the Zn additions is attributed to a substantial refinement of the precipitates in the solidification microstructure. The problems of nonuniformity in solidification dendrite structure and solder surface roughness often observed in the Sn‐Ag binary alloys are also alleviated by the Zn addition. It is found that essentially all of the added Zn resides in the more corrosion‐resistant, Ag‐based, intermetallic precipitates, leaving the Sn‐rich matrix primarily free of Zn in solid solution. High‐temperature creep tests indicate that the new Zn‐containing alloys exhibit a remarkably improved creep resistance of more than an order of magnitude.
TL;DR: The design of an aluminum alloy having good strength while maintaining a high resistance to fracture is discussed in this article, where the desired microstructure consists of a small volume fraction of an ultra-fine dispersion of hard particles.
Abstract: The design of an aluminum alloy having good strength while maintaining a high resistance to fracture is discussed. Theory suggests that the desired microstructure consists of a small volume fraction of an ultra-fine dispersion of hard particles. In addition to conventional heat treatments, dispersion hardened aluminum alloys have been recently produced by rapid solidification or mechanically alloying and powder metallurgy consolidation. Alloys which can serve as models for mechanistic studies of nucleation of non-coherent phases as well as the basis for a new class of engineering aluminum alloys are identified.
TL;DR: In this paper, the principles, advantages and limitations of various TiN coating processes are summarized, the microstructures and mechanical properties of TiN coatings on tool steel substrates are reviewed and new developments in the property design of TiNs coatings are presented.
TL;DR: In this paper, a submicrometer-grained (SMG) Al−3% Mg solid solution alloy, with an initial grain size of ∼0.2 μm, was produced by intense plastic straining.
Abstract: A submicrometer-grained (SMG) Al−3% Mg solid solution alloy, with an initial grain size of ∼0.2 μm, was produced by intense plastic straining. Experiments show that tensile specimens of the SMG alloy exhibit high elongations to failure at low testing strain rates at the relatively low temperature of 403 K. The stress exponent is high (∼7–8) and calculations show deformation is within the region of power-law breakdown. The initial microstructure of the alloy consists of diffuse boundaries between highly deformed grains. At strain rates of ∼10−4 s−1 and lower, plastic deformation leads to dynamic recrystallization and the formation of highly nonequilibrium grain boundaries that gradually evolve into a more equilibrated configuration.
TL;DR: In this paper, the first direct observation of porous SiC formation from single-crystal SiC wafers has been reported, using TEM reveals pores of sizes 10-30 nm with interpore spacings ranging from roughly 5 to 150 nm.
Abstract: A process for forming porous SiC from single-crystal SiC wafers has been demonstrated. Porous SiC can be fabricated by anodizing n-type 6H-SiC in HF under UV illumination. TEM reveals pores of sizes 10-30 nm with interpore spacings ranging from roughly 5 to 150 nm. This is the first reported direct observation of porous SiC formation.
TL;DR: In this article, the plausibility of electrical double layers acting to stabilize an equilibrium thickness of intergranular glass films in polycrystalline ceramics is explored and estimates of the screening length, surface potential, and surface charge required to provide a repulsive force sufficiently large to balance the attractive van der Waals and capillary forces for observable thicknesses.
Abstract: The plausibility of the entropic repulsion of electrical double layers acting to stabilize an equilibrium thickness of intergranular glass films in polycrystalline ceramics is explored. Estimates of the screening length, surface potential, and surface charge required to provide a repulsive force sufficiently large to balance the attractive van der Waals and capillary forces for observable thicknesses of intergranular film are calculated and do not appear to be beyond possibility. However, it has yet to be established whether crystalline particles in a liquid-phase sintering medium possess an electrical double layer at high temperatures. If they do, such a surface charge layer may well have important consequences not only for liquid-phase sintering but also for high-frequency electrical properties and microwave sintering of ceramics containing a liquid phase.
TL;DR: Using pulsed laser deposition, YBa2Cu3O7−δ (YBCO) films ranging in thickness from 0.065 to 6.4 μm have been deposited on yttria-stabilized zirconia substrates with an intermediate layer of CeO2 as mentioned in this paper.
Abstract: Using pulsed laser deposition, YBa2Cu3O7−δ (YBCO) films ranging in thickness from 0.065 to 6.4 μm have been deposited on yttria‐stabilized zirconia substrates with an intermediate layer of CeO2. The thinnest films have critical current densities of over 5 MA/cm2 at 75 K with zero applied field; as film thickness is increased, Jc decreases asymptotically to 1 MA/cm2. X‐ray analysis of a 2.2‐μm‐thick film shows that the YBCO is predominantly c‐axis oriented and textured in‐plane, while a Rutherford backscattering spectrometry minimum channeling yield of ≊75% indicates that the film contains disordered material at this thickness.
TL;DR: In this article, a crystallization treatment of amorphous (Nd,Dy)xFe80.5−x−yCoyB18.5M1 alloys for 3≤x≤5 and 0≤y ≥5, where M represents the additives Al, Si, Cu, Ga, Ag, and Au. The results showed that adding Co and M results in a significant reduction of the crystal grain size and leads to significant improvements both in the intrinsic coercivity and in the energy product.
Abstract: High‐coercivity, high‐remanence permanent magnet materials have been obtained via a crystallization treatment of amorphous (Nd,Dy)xFe80.5−x−yCoyB18.5M1 alloys for 3≤x≤5 and 0≤y≤5, where M represent the additives Al, Si, Cu, Ga, Ag, and Au. It has been found that addition of Co and M results in a significant reduction of the crystal grain size and leads to significant improvements both in the intrinsic coercivity and in the energy product. The materials are superior to Nd2Fe14B‐based rapidly solidified materials in the remanence Br, the temperature coefficient of Br, and magnetizability. The typical values of temperature coefficients of Br and HcJ are, respectively, −0.05%/K and −0.35% K. The magnetizing force required to obtain substantial remanence is approximately 0.8 MA/m. Compaction‐molded resin‐bonded magnets produced from these materials have magnetic performance (Br, HcJ) in the range between 0.89 T, 0.29 MA/m and 0.80 T, 0.35 MA/m.
TL;DR: In this paper, the effects of incident ion/metal flux ratio Ji/JMe and ion energy Ei on the microstructure, texture, and phase composition of polycrystalline metastable Ti0.5Al 0.5N films produced by reactive magnetron sputtering were investigated using x-ray diffraction (XRD), plan-view and cross-sectional transmission electron microscopy, and Rutherford backscattering spectroscopy.
Abstract: The effects of incident ion/metal flux ratio Ji/JMe and ion energy Ei on the microstructure, texture, and phase composition of polycrystalline metastable Ti0.5Al0.5N films produced by reactive magnetron sputtering have been investigated using x‐ray diffraction (XRD), plan‐view and cross‐sectional transmission electron microscopy, and Rutherford backscattering spectroscopy. The films, typically ≂1 μm thick, were deposited at a pressure of 20 mTorr (2.67 Pa) in pure N2 on thermally oxidized Si(001) substrates at 250±25 °C. The N2+ ion flux to the substrate was controlled by means of a variable axial magnetic field superimposed on the permanent magnetic field of the magnetron. Films deposited at Ei=20 eV (≂10 eV per incident accelerated N) with Ji/JMe=1 exhibited a complete (111) texture with a porous columnar microstructure and an average column size of ≂30 nm. Increasing Ei from 20 to 85 eV, while maintaining Ji/JMe constant at 1, resulted in a small change in texture as the XRD intensity ratio I002/(I111+...
TL;DR: The microstructure of the network as well as the strands of particulate β-lactoglobulin gels formed at pH 53 have been characterized by microscopy as discussed by the authors.
TL;DR: In this paper, the microstructure of the nanocrystalline powders, the contribution of the stored enthalphy of cold work, and the nucleation of disorder/melting at grain boundaries/particle interfaces are discussed.
TL;DR: Experimental data available for the thermophysical properties of stainless steels have been searched, compiled, critically evaluated, analyzed, and correlated as mentioned in this paper, and particular attention was given to material characteristics such as alloy composition, microstructure, and conditioning treatments.
TL;DR: In this article, the strength and toughness of a wrought Nb-1.65 at. pct Si alloy with near-eutectic composition, at temperatures from 25 C to 1500 C, was determined.
Abstract: Recent studies have shown that within the Nb-Si system, the two-phase Nb/Nb[sub 5]Si[sub 3] microstructures exhibit excellent thermochemical stability and resistance to coarsening up to 1500 C. In addition, these Nb/Nb[sub 5]Si[sub 3] 'in situ composites' exhibit a good balance in mechanical properties, having a reasonably high strength up to 1500 C and high fracture toughness at room temperature. These combined properties of the composites provide an attractive basis for developing high-temperature structural materials. Studies are currently underway on creep mechanisms and alloying to enhance the oxidation resistance of these composites. Most of the published results on this system pertain to the wrought Nb-10 at. pct Si alloy in which the large primary Nb particles provide toughening while the intermetallic Nb[sub 5]Si[sub 3] phase in the eutectoid microconstituent provides high-temperature strength. The present work was undertaken to determine the strength and toughness of a wrought alloy with near-eutectic composition, Nb-1.65 at. pct Si, at temperatures from 25 C to 1500 C. The increased Si content is known to decrease the volume fraction of the primary Nb particles and increase the overall volume fraction of the Nb[sub 5]Si[sub 3] intermetallic phase.