Showing papers on "Microstructure published in 1989"

High current density in bulk YBa2Cu3Ox superconductor

Abstract: A liquid phase processing method for the fabrication of bulk YBa2Cu3Ox superconductors with large current carrying capacity has been developed. Slow cooling through the peritectic transformation (1030–980 °C) has been shown to control the microstructure of these superconductors. A cooling rate of 1 °C/h in this temperature range has yielded a microstructure with long plate type, thick grains oriented over a wide area. Current density up to 18 500 A/cm2 has been obtained by continuous direct current measurements and in excess of 62 000 A/cm2 with pulse current of 10 ms duration and 75 000 A/cm2 using 1 ms pulse. The strong magnetic field dependence observed in sintered bulk 1‐2‐3 superconductors is also minimized to a large extent where a current density in excess of 37 000 A/cm2 is obtained in a field of 6000 G.

Intermetallic alloys based on gamma titanium aluminide

Abstract: Titanium-aluminide alloys based on TiAl have an excellent potential to become one of the most important aerospace materials because of their low density, high melting temperature, good elevated-temperature strength and modulus retention, high resistance to oxidation and hydrogen absorption, and excellent creep properties. The chief roadblock to their application is poor ductility at low to intermediate temperatures that results in low fracture toughness and a fast fatigue-crack growth rate. During the last several years, a great deal of effort has been made to improve these ductile properties. These endeavors have met with some success through chemistry modification and microstructure control.

A review of microstructure and properties of plasma sprayed ceramic coatings

Abstract: The present understanding of the plasma-sprayed coating formation process is reviewed. The development of porosity, and the very fine voids formed by incomplete inter-splat contact is discussed. The strong relation between porosity and mechanical and thermal properties of ceramic coatings is reviewed. Some of the controlling factors for the grain size and crystal structure within the deposited splats are discussed, particularly in reference to zirconia coatings.

Effects of matrix microstructure and particle distribution on fracture of an aluminum metal matrix composite

Abstract: This paper presents the results of a study on the effects of matrix microstructure and particle distribution on the fracture of an aluminum alloy metal matrix composite containing 20% by volume SiC particulate. The matrix microstructure was systematically varied by heat treating to either an under- or over-aged condition of equivalent strength, and was characterized using a combination of techniques. Quantitative metallographic techniques were utilized to characterize the material with respect to size, size distribution, and particle clustering, while transmission electron microscopy was utilized to characterize the details of the matrix microstructure in addition to the effects of aging on the character of the particle/matrix interfaces. Fracture experiments were conducted on smooth tensile, notched bend, shortrod toughness, and on specimens designed to permit controlled crack propagation, in an attempt to determine the effects of matrix microstructure and clustered regions on the details of damage accumulation. Large effects of microstructure on the notched properties were obtained with little effect of microstructure on tensile ductility. It is shown that the micromechanisms of fracture are significantly affected by the details of the matrix microstructure, interface character, and degree of clustering in the material. Fracture of the SiC was predominant in the underaged materials, with a preference for failure in the matrix and near the interface in the overaged material. Metallographic and fractographic analyses revealed that clustered regions were preferred sites for damage initiation in both the aging conditions tested, while preliminary results additionally indicate that damage accumulation ahead of a propagating crack also tended to occur in clustered regions.

Dense single-phase β-sialon ceramics by glass-encapsulated hot isostatic pressing

Abstract: Single phaseβ-sialon ceramics, Si6−z Al z O z N8−z , have been prepared from carefully balanced powder mixtures, also taking account of any excess oxygen in the starting materials. Sintering powder compacts in a nitrogen atmosphere (0.1 MPa) at 1800° C or higher transforms the starting mixture into aβ-sialon solid solution atz-values up to about 4.3, but the sintered material has an open porosity. Addition of 1 wt% Y2O3 to the starting mix improved the sintering behaviour somewhat and the density of the sintered compacts reached 95% of the theoretical value. By glass-encapsulated hot isostatic pressing at 1825° C, however, sintered materials of virtually theoretical density could be obtained, with or without the 1 wt% Y2O3 addition. These latter samples have been studied by X-ray diffraction and electron microscopy, and their hardness and indentation fracture toughness have been measured. It was found that the maximum extension of theβ-sialon phase composition at 1825° C and 200 MPa pressure is slightly below 4,z∼ 3.85 and about 4.1 at atmospheric pressure, and that the hexagonal unit cell parameters are linear functions of thez-value. The single-phaseβ-sialon ceramics had no residual glassy grain-boundary phase. The grain shape was equi-axed and the grain size increased from about 1μm at lowz-values to 5μm at highz-values. At lowz-values the hardness at a 98 N load was 1700 and the fracture toughness 3, whereas an increase inz above 1 caused both the hardness and fracture toughness to decrease significantly. Addition of 1 wt % Y2O3 to the starting mix prior to the HIP-sintering gave rise to a small amount of amorphous intergranular phase, changes in grain size and shape, a clear increase in fracture toughness and a moderate decrease in hardness.

Microstructural aspects of aluminium-silicon carbide particulate composites produced by a casting method

Abstract: When metal matrix composites are produced by molten metal methods there are some unique factors which have to be considered. In this paper, the microstructure of SiC-reinforced aluminium alloys produced by this method are considered. It is shown that the stability of SiC in the melt is dependent on the matrix alloy involved and that only alloys with high silicon contents have a low reactivity with this reinforcement. With other alloy matrices, SiC reacts to form Al 4 C 3 , and the nature of this reaction and its kinetics are considered in this paper. Initially, the reaction rates are very rapid but almost saturate after about 1 h. It is also shown that the distribution of the reinforcing particles is dependent on the solidification rate because particles are rejected and pushed ahead of the meniscus. At low solidification rates, and hence for large cell sizes, the reinforcing particles are clustered and form a network which delineates the cell walls. Because the SiC particles are in the interdendritic regions they will be associated with any coarse intermetallic particles present and this can influence the fracture behaviour.

Effects of chemical inhomogeneities on grain growth and microstructure in Al2O3

Abstract: Effects of chemical inhomogeneities and single-crystal seeds on normal and discontinuous grain growth were investigated in both undoped and MgO-doped Al2O3. The chemical impurities in the samples were exsolved at a lower temperature than the sintering temperature and measured by SEM/EDS to determine the correlation between the distribution of impurities and the microstructure in Al2O3. A feature of this study was the use of clean-room processing and firing procedures to maintain sample composition at the levels initially present in the starting powders. As the local concentrations of chemical impurities (i.e., Si, Ca) increased, the grain boundary–grain boundary dihedral angle distribution widened, with many angles at 180°, the grain-size distribution widened, and pore–boundary separation was enhanced. Discontinuous grain growth was observed in regions of undoped Al2O3 containing the largest Ca and Si concentrations. It is suggested that doping with MgO solute reduces the effects of impurities on grain growth by increasing the bulk solubility and decreasing interfacial segregation of impurities, especially Si, and by narrowing the distribution of grain boundary–grain boundary dihedral angles.

Hot Isostatic Press Sintering and Properties of Silicon Nitride without Additives

Abstract: Fully densified silicon nitride without additives was fabricated by means of hot isostatic pressing. The sintering process of highly pure powder was investigated with special interest in the evolution of α–β phase transformation, densification, and microstructure development. It was observed that the transformation occurred without a liquid phase below 1730°C, which corresponds to the melting point of SiO2. Above 1730°C, the densification and β-grain elongation accelerated concurrently because of the appearance of liquid SiO2. However, full densification was attained at 1950°C together with marked grain growth. Flexural strength, microhardness, fracture toughness, and Young's modulus of sintered bodies were measured as a function of temperature. In the sintered body started from highly pure powder, excellent MOR behavior was found up to 1400°C. Impurity content of a few hundred ppm was found to be sufficient to make densification easy and to degrade high-temperature strength.

Formation and characterization of grain-oriented VO2 thin films

Abstract: Grain‐oriented films of VO2 have been prepared to characterize the effects of film orientation on electrical and optical switching performance. This oriented growth has been achieved by lattice matching to (0001) sapphire, and results in intergranular structures favorable to rapid propagation of the monoclinic‐tetragonal phase transformation. The electrical and optical property changes across this transition are strongly affected by film microstructure, with the oriented structures favoring rapid switching and narrow hysteresis.

Sintering of Ceramics

Abstract: The primary goal of sintering research is the controlled manipulation of microstructure. Out of the entire range of microstructures which are theoretically possible, each material system will be able to achieve only a subset of them, depending on the intrinsic material properties. Within these material constraints, the aim is to produce microstructures which enhance specific properties. Our understanding of the relationships among materials processing, microstructure, and properties is just beginning to emerge, and is producing unexpected results. For example, in a recent study of toughness in Al2O3 by Bennison and Lawn, microstructures with platy grains and a bimodal grain size distribution in undoped Al2O3 exhibited a greater resistance to crack propagation than did the more uniform microstructures in MgO-doped Al2O3 [1]. As a result of this emerging understanding, the focus of sintering science is changing from the modification of microstructures in incremental ways for correspondingly incremental improvement in properties to more effectual manipulation of microstructures to optimize properties. However, the production of the optimum microstructure will be dependent on both the material and the application and may require radically different processing routes for different materials. In this review paper, we have examined the research in sintering science over the past five years which has advanced the goal of microstructure manipulation.

Properties and microstructures of Lanxide Al2O3-Al ceramic composite materials

Abstract: A study has been made of four Al2O3-Al composite materials fabricated by the directed oxidation of molten aluminium alloys. Their microstructures are described and measurements of density, coefficient of thermal expansion, thermal conductivity, hardness, elastic constants, compressive strength, flexural strength, fracture toughness, work of fracture, and thermal shock resistance are reported. Compared to a typical dense sintered Al2O3, such as Durafrax® 1542, which is somewhat harder, stiffer, and stronger in compression, the new composites can be stronger in flexure, particularly at high temperatures, far tougher, and considerably more resistant to thermal shock. Attempts are made to relate their differences in properties to microstructure.

Cobalt-bound tungsten carbide metal matrix composites and cutting tools formed therefrom

Abstract: Cobalt-bound tungsten carbide metal matrix composites having a unique microstructure are produced by consolidating partially sintered greenware under high pressures, e.g., 120,000 psi, at temperatures less than those used for conventional liquid phase sintering in a relatively short time, e.g., from less than one minute to less than about one hour. The composites have a binder phase which contains less than about 80 weight percent of the tungsten found in a composite prepared from the same or similar compositions via liquid phase sintering. These composites provide cutting tools with both toughness and wear resistance which exceed that of cutting tools made from the same or similar compositions via liquid phase sintering.

Influence of ultra-high strains at elevated temperatures on the microstructure of aluminium. Part I

Abstract: A1 (purity, 99·7%) with 0·1 or 2 mm grains was torsionally deformed at 400°C and 0·2s-1 up to equivalent strains of 60. Tangential sections were examined by polarized light, transmission electron microscopy and scanning electron microscopy. The grains wound into helicoids with an axial thickness varying inversely with the strain. Subgrains persisted at a constant size (about 7 μm) and equiaxed throughout the straining from 0·5 to 60. Relative misorientations, of structural units were determined by scanning transmission electron microscopy channelling patterns, which confirmed the X-ray diffraction textures. The grains, with their boundaries strongly serrated, retained their distinctness up to strains of 10 for 0·1 mm grains and up to 60 for 2 mm grains, while their thickness was greater than the subgrain diameter. For the former at strains of 20-60, the microstructure consists of subgrains having a mixture of small- and large-angle boundaries; this development is called geometric dynamic recrysta...

Comparison of the microstructures and abrasive wear properties of stellite hardfacing alloys deposited by arc welding and laser cladding

Abstract: The microstructure of cobalt-based hardfacing alloys deposited by manual metal arc (MMA) welding, tungsten inert gas (TIG) welding, and laser cladding has been investigated as part of a study attempting to establish the relationship between microstructure and abrasive wear properties. For typical deposition conditions, the differences in freezing rates associated with the three processes are found to give rise to large differences in microstructure. The MMA process is found to lead to the largest degree of dilution of the hardfacing deposit; the TIG and laser deposits exhibited much lower levels of mixing with the base plate. For the deposition conditions used in this study and for the alloys examined, the scale of the microstructure decreases in the order MMA, TIG, and laser cladding, leading to an increase in the deposit hardness in the same order. It is found that with alumina as an abrasive, the wear rate persistently is higher with the MMA deposits (which have the coarsest microstructure with the lowest starting hardness), the weight loss being approximately linear with time. The laser and TIG deposits, which have more refined microstructures and slightly higher carbon concentrations, both are found to exhibit significantly lower wear rates. Initially, the TIG samples are the most resistant to abrasion, even though their microstructure compares with that of the laser samples; this is a consequence of their higher ductility associated with a lower rate of strain hardening. The laser samples, which contain a lower matrix iron concentration, strain harden more rapidly; consequently, they exhibit an initial decrease in wear rate. With the much harder silicon carbide abrasive, all samples show similar wear rates which do not decrease with time. The wear data are found to correlate with scanning and transmission electron microscopy observations, and it is possible to rationalize the interaction among microstructure, abrasive, and alloy deposition processes.

Mechanical Property-Microstructural Relationships in Abalone Shell

Abstract: The microstructure and mechanical properties of abalone shell were studied. It was found that fracture strength, αf, is 180 MPa, and fracture toughness, KIC, is 7 ± 3 MPa-m1/2; these values are comparable with or better than most “high technology” ceramic materials. The microarchitecture of the nacre section of the red abalone shell is similar to a “brick and mortar” structure, where CaCO3 is the brick and organic matter is the mortar, constituting 95% and 5% of the microstructure by volume, respectively. This impressive combination of af and KIc values is attributed to the laminated structure of the shell with hard and thick (0.25±0.5 μm) CaCO3 and superplastic and thin (20–30 nm) organic components. Although there are several toughening mechanisms operating in the shell, fractographic studies identified sliding of CaCO3 layers and bridging by the organic layers to be the most effective ones. These phases also have a strong interface. The results of our experiments are discussed in the context of using abalone shell as a model for the design of synthetic laminates such as cermet (ceramic-metal) and cerpoly (ceramic-polymer) composites.

Solidification of high-speed tool steels

Abstract: Gradient solidification and differential thermal analysis (DTA) experiments were used to study the process of solidification and the solidification microstructure of 11 alloys comprising the composition range of customary commercial high-speed steels (with the exception of cobalt-alloyed grades). Also included are a number of experimental high-speed steels alloyed with niobium. The results include the effects of alloy composition and cooling rate on the width of the solidification interval and on the sequence of the solidification reactions; the types of eutectics formed (austenite with M6C, M2C, or MC) and their volume fractions; the chemical compositions of the ledeburitic and primary carbides; and the relation between the chemistry of the carbides and that of the melt. Special attention is given to the formation and composition of heterogeneously nucleated primary MC particles and to the chemistry and stability of eutectic M2C, which is important as a precursor to MC and M6C in the microstructure of finished (hot-worked and heat-treated) material.

Synthesis and Microstructure of Highly Oriented Lead Titanate Thin Films Prepared by a Sol-Gel Method

Abstract: Thin films of PbTiO3 were deposited on fused silica, resistor-grade alumina, and single-crystal (100) MgO by a sol–gel processing method. Whereas the films deposited on silica and alumina substrates were randomly oriented and polycrystalline, highly {100} oriented PbTiO3 films were grown on the MgO single crystals. The perovskite-type structure was observed with films deposited on the single-crystal MgO and annealed at temperatures as low as 470°C, whereas a pyrochlore-type strcuture was observed with films on fused silica and alumina processed in a similar manner. All films heat-treated at temperatures in excess of 570°C showed significant formation of a second PbTi3O7 phase. The films were characterized by electron microscopy and glancing-incidence-angle X-ray diffraction.

Low-Temperature Sintering of Lead-Based Piezoelectric Ceramics

Abstract: The low-temperature sintering of lead-based piezoelectric ceramics has been studied. The sintering temperature of lead zirconate titanate (PZT) ceramics could be reduced from ∼ 1250° to ∼960°C by the addition of a small amount of the lower-melting frit, B2O3–Bi2O3—CdO. It exhibited the following dielectric and piezoelectric properties: Kp= 0.52 to 0.58, Qm= 1000, eT33/e0= 800 to 1000, tan δ= 50 × 10−4, ρ= 7.56 to 7.64 g/cm3. Ceramics with the aid of suitable dopants (CdO, SiO2, and excess PbO) in the Pb-(Ni1/3Nb2/3)O3—PZT family could be sintered at 860° to 900°C. For these materials, Kp= 0.56 to 0.61, Qm= 1000, eT33/e0= 1500 to 2000, tan δ≤ 50 × 10−4, ρ= 7.80 to 8.03 g/cm3. The microstructure, sintering mechanism, and the effects of various impure additions have been analyzed by means of scanning electron microscopy, scanning transmission electron microscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy.

Properties of conductive zinc oxide films for transparent electrode applications prepared by rf magnetron sputtering

Abstract: High‐quality zinc oxide (ZnO) films have been deposited by rf magnetron sputtering from a zinc oxide target. The material has been obtained with resistivities ranging from about 1 Ω cm to 4.6×10−4 Ω cm, whereas the average transmission was always greater than 90% between 400 and 800 nm. A relatively high deposition rate usually leads to low resistivity of films. The films with the lowest resistivity differ from higher‐resistivity films by their surface morphology, as observed by scanning electron microscopy and by their x‐ray diffraction pattern. In particular, good quality films have a diffraction peak at 2Θ≂30.5°. The best films have also been characterized by Hall‐effect measurements, Auger electron spectroscopy, and infrared transmission data. It is suggested that the film properties are dependent on the flux of energetic neutral oxygen atoms to the growing surface, which can be varied by changing the sputtering conditions, such as the target to substrate distance. These films have considerable potent...

Cryogenic toughness of commercial aluminum-lithium alloys: Role of delamination toughening

Abstract: Mechanisms influencing the plane-strain fracture toughness behavior of commercial aluminum-lithium alloys at cryogenic temperatures are investigated as a function of microstructure and plate orientation. It is confirmed that certain alloys show a markedincrease in tensile ductility and toughness withdecrease in temperature, although such behavior is not found in the short-transverse orientations, or for all alloys and aging conditions. Specifically at lower temperatures, the majority of Al-Li alloys, namely 2090-T8E41, 8091-T8X, 8090-T8X, and 2091-T351, show a significantincrease in fracture toughness in the in-plane orientations (L-T, T-L), without any apparent change in fracture mode. Such behavior is attributed primarily to loss of through-thickness constraint resulting from enhanced short-transverse delamination (termed crack-divider delamination toughening), consistent with observed reductions in plane-strain ductility and short-transverse (S-L, S-T) toughness. Conversely, in underaged microstructures of 8091, 8090, and peak-aged 2091, a decrease in toughness with decreasing temperature is found for both L-T and S-L orientations, behavior, which is associated conversely with a fracture-mode change from ductile void coalescence to brittle transgranular shear and integranular delamination at lower temperatures.

Stoichiometry effects on the deformation of binary TiAl alloys

Abstract: The room temperature deformation behavior and microstructure of Ti 48 Al 52 and Ti 52 Al 48 alloys are compared. The material was fabricated by rapid solidification melt spinning, and examined in both as-cast and consolidated forms. The Ti 52 Al 48 alloy exhibited enhanced strength and ductility in both forms in bend tests compared with the Ti 48 Al 52 alloy. The microstructure of the Ti 52 Al 48 alloy was two-phase γ–TiAl and α 2 –Ti 3 Al. The Ti 48 Al 52 alloy was single-phase γ–TiAl and had a larger grain size than the previous alloy. The microstructure of the Ti 52 Al 48 alloy after room temperature deformation consisted primarily of {111} mechanical twins and a /2〈110〉 perfect dislocations. The comparable Ti 48 Al 52 alloy microstructure contained fewer twins, and many more a 〈101〉 and a /2〈112〉 superdislocations were present in addition to a /2〈110〉 dislocations. The superdislocations had dissociated and formed sessile faulted dipoles. The possible reasons for the differences in microstructure and mechanical behavior between these two alloys are discussed.

The effect of Bi2O3 content on the microstructure and electrical properties of ZnO varistor materials

Abstract: The microstructures of three ZnO varistor materials with different Bi2O3 contents have been evaluated by analytical electron microscopy in combination with x‐ray diffractometry. The results have been correlated to microelectrode measurements, where breakdown voltages of individual ZnO junctions were measured, and also to current/voltage characteristics of bulk specimens. The volume fraction of the continuous intergranular network of Bi‐rich phases, which lies along the triple junctions of the ZnO grains, increases with increasing Bi2O3 content, The conductivity of this network is strongly influenced by its internal microstructure. It was found that increased volume fractions of δ‐Bi2O3 and less interpenetration between α‐Bi2O and δ‐Bi2O3 increases the conductivity of the network. Individual ZnO/ZnO grain boundaries exhibited breakdowns at 3.2 and 3.6 V, depending upon whether they contained segregated Bi atoms or thin Bi‐rich amorphous films. The current/voltage characteristics of heterojunctions between ...

Correlation of microstructure and creep stages in the 〈100〉 oriented superalloy SRR 99 at 1253 K

Abstract: The microstructure of creep specimens tested at 1253 K is analyzed in the three stages of the creep curve by TEM investigations. Stage I is characterized by the glide of screw dislocations in the matrix, while the transition to stage II is the beginning of γ′ rafting. Throughout stage II, the γ′ phase is not cut by dislocations. From these and other metallographic results, the mechanism of creep in the single-crystal alloy can be deduced. The misfit stress at the γ/γ′ interfaces proved to be of special importance for this type of alloy containing a high volume fraction of γ′.

The microstructure and mechanical properties of unidirectionally solidified Al−Si alloys

Abstract: Hypereutectic Al−Si alloys with minor additions of Sr were directionally solidified with a temperature gradient of 125°C cm−1 in the liquid. Silicon in the range 14–17 wt%, Sr in the range 0.0–0.5 wt% and specimen traction velocities between 1 and 1500 μm sec−1 were used. The relationship between hardness and traction velocity and spacing in eutectic silicon morphologies is defined and shown to be of the same form as that for yield stress. The possibility of using hardness measurements to be of the same form as that for yield stress. The possibility of using hardness measurements to indicate mechanical properties is discussed. The complex regular silicon structure makes a significant contribution to the hardness of hypereutectic alloys. This makes the relationship between hardness and traction velocity more complex adding difficulties to the use of hardness to measure mechanical properties.