Showing papers on "Microstructure published in 1981"

Preparation, microstructure and mechanical properties of dense polycrystalline hydroxy apatite

Abstract: Hydroxy apatite ceramic blocks of varying density have been prepared from a commercial powder. The elastic properties, fracture toughness, strength and sub-critical crack growth of these materials have been investigated. Young's modulus for the nearly fully dense material is 112 GPa while the compressive strength is about 800 MPa. For the same material the strength and fracture toughness under dry conditions are 115 MPa and 1.0 MPa m1/2, respectively. Substantial slow crack growth was found under these conditions. Under wet conditions the values for strength and fracture toughness drop to about 75% of their “dry” values. In this case very serious slow crack growth is present.

The effects of microstructure on the electrocaloric properties of Pb(Zr,Sn,Ti)O3 ceramics

Abstract: When an electric field is applied to a dielectric material under adiabatic conditions there is a resultant change in temperature, a phenomenon known as the electrocaloric effect Compositions in the Pb(Zr,Sn,Ti)O3 system (PZST) have several phases of nearly equal free energy, allowing antiferroelectric (AFE) to ferroelectric (FE) and paraelectric (PE) to ferroelectric (FE) transitions to occur Data are presented for electrocaloric measurements on AFE materials in the vicinity of the Curie point The effect of microstructure is reported, and experimental values are related to dielectric properties through thermodynamic relations Culmination of this work resulted in the largest electrocaloric effect (26°K) measured to date for coarse grain material in the PZST system

Reaction Mechanisms in the Formation of PZT Solid Solutions

Abstract: The solid-state reactions occurring in the system PbO-TiO2-ZrO2 were investigated using constant heating rates up to 1000°C. DTA, dilatometric length changes, and XRD analysis were used for characterization. PbO and TiO2 reacted exothermally to form the product PbTiO3 with a large volume expansion between 450° and 600°C. Formation of PbZrO3 from PbO and ZrO2 occurred endothermally with a large volume expansion between 700° and 800°C. The expansion was due to reaction topology, differential molar volumes of products and reactants, and the pellet microstructure. In the formation of PZT from ternary powder mixtures, PT formed between 450° and 600°C, followed by PZT formation at >700°C with no measurable amounts of PbZrO3 formed as determined by XRD analysis. The analysis of the mechanisms indicates that the overall kinetics of homogeneous PZT solid-solution formation are determined by either the ionic transport within the perovskite lattice or the phase-boundary reactions leading to perovskite formation and not by the diffusion of Ti across PbO, which is relatively rapid.

Microstructure and properties of step aged rare earth alloy magnets

Abstract: Alloys with compositions Co‐25.5 wt % Sm‐8 wt % Cu‐15 wt % Fe‐3 wt % Zr and Co‐Sm‐Cu‐Fe‐1.5 wt % Zr have been step aged to produce magnets with coercive force (iHc) in the range of 10–25kOe, much higher than those reported so far in the literature for the Zr alloys. The high coercive force magnets are typically aged at 800–850 °C for 10–30 hours following the solution treatment at 1150 °C. Subsequently, these are step aged to produce materials with high coercivity. The microstructure in all these alloys has a two phase cellular morphology with 2:17 phase surrounded by a 1:5 boundary phase. The long aging treatments at 800–850 °C lead to coarsening of the two phase structure. The subsequent step aging does not change the morphology, but only changes the chemical composition of the two phases. Best properties are obtained in materials with a coherent microstructure of optimum boundary phase thickness and optimum chemical composition. The highest values of iHc obtained so far are ∼26kOe and ∼16kOe for the 3% Zr and 1.5% Zr alloys respectively. The best hard magnetic properties of (BH) max = 33 MGOe and iHc = 13kOe are for a 25% Sm‐20%Fe‐4 Cu‐2% Zr alloy.

Fabrication and characterization of SiC-AIN alloys

Abstract: SiC-AIN alloys were prepared by the carbothermal reduction of silica and alumina, derived from an intimate mixture of silica, aluminium chloride and starch. The resulting single-phase SiC-AIN powder was hot-pressed without additives to a high density. The dense bodies had a fine-grained uniform microstructure. The Young's elastic modulus, microhardness, fracture toughness, thermal expansion and thermal conductivity were measured as functions of composition. The creep behaviour of the SiC-AIN alloy was compared with that of silicon carbide.

The influence of carbon on the microstructure and mechanical properties of sintered boron carbide

Abstract: It was found that boron carbide can be sintered pressureless to ultrafine-grained bodies with densities of more than 95% of the theoretical density of B 4 C, if small additions of carbon are made to boron carbide powders of submicron particle size and if the compacts are heated in inert atmospheres to temperatures near 2150 °C. In particular the carbon acts as an inhibitor of surface-to-surface matter transport so that densification via grain boundary and/or lattice diffusion is enhanced at lower sintering temperatures. The mechanical properties of this new sintered material are summarized and compared with those of the commercial hot-pressed product.

The structure and properties of aluminum alloys produced by mechanical alloying: Powder processing and resultant powder structures

Abstract: Mechanical alloying of two aluminum alloy powders to form composite A1-A12O3 powders has been studied. Changes in powder microstructure with processing are reported and interpreted. Mechanical alloying proceeds by the continual cold welding and fracturing of the constituent powder particles when subjected to the large compressive forces of a high speed mill. A suitable organic surfactant must be added so that a balance between cold welding and fracturing is obtained. The organic surfactant is embedded and finely distributed in the powder particles during mechanical alloying and is converted to discrete A14C3 particles after hot pressing. The establishment of steady state processing conditions, characterized by equiaxed powder particles, a constant particle size distribution and a saturation hardness, is found to depend on the size distribution of the initial powders. The oxide particles formed and distributed during mechanical alloying are equiaxed, small (30 nm) and homogeneously distributed with a volumetric center to center distance of about 60 nm.

A Model for Electrochromic Tungstic Oxide Microstructure and Degradation

Abstract: Despite much investigation of the electrochromic (EC) coloration process in films, such displays have not yet become commercially viable because of limited useful device life. Device degradation occurs by film dissolution on the shelf and erosion during cycling. Water plays a crucial role in both efficient coloring/bleaching and in film degradation. To better understand the degradation process and the role of water, dissolution of EC films in aqueous media was studied. The results strongly suggest that EC films formed by evaporation are amorphous molecular solids consisting of trimeric molecules bound weakly to each other through water‐bridge, hydrogen, and van der Waal's bonding. The nature of this microstructure is responsible for the high solubility. Films subjected to ion bombardment show decreased dissolution rates as well as decreased electrochromism and, while amorphous, are believed to have a random network rather than molecular microstructure.

Microstructure and properties of rf‐sputtered amorphous hydrogenated silicon films

Abstract: The effect of both total gas pressure (Ar+H2) and partial pressure of H2 during the rf‐sputtering of amorphous hydrogenated silicon films was investigated. It is shown that decreasing the total gas pressure (PT) leads to a number of consistent and desirable film characteristics including no post‐deposition oxidation, essentially zero etch rate, no resolvable microstructure, predominant monohydride bonding, and low optical band gap. These lowest PT films contain ∼17 at. % H and 4 at. % Ar, have reasonable deposition rates (60–70 A/min) and are in a high state of compressive stress. The relations between the deposition variables and the more fundamental surface reaction processes are discussed. Specifically, we demonstrate that Ar ion bombardment effects, due to high negative substrate self‐biases, are dominant at low PT.

Studies of the hydrogen passivation of silicon grain boundaries

Abstract: A model is presented which predicts the current flow through and around grain boundaries which have been treated with atomic hydrogen to reduce their trap‐state densities. Measurements on hydrogenated silicon grain boundaries are shown to be in agreement with this model and quantitative estimates of hydrogen penetration depth are made. The dependence of this depth on sample temperature, surface preparation, hydrogen pressure, and geometry are systematically investigated. Maximum penetration is achieved in high‐pressure discharges for sample temperatures between 350 and 400 °C. The condition of the surface of the polycrystalline silicon is shown to be critically important for the in‐diffusion process.

Electrodes for Alkaline Water Electrolysis

Abstract: Electrodes for alkaline water electrolysis have been made by applying high specific surface area coatings of nickel or nickel‐iron alloy to steel or nickel substrates. The coatings are applied as polysilicate‐based paints containing particles of the desired metals. The coating is sintered into a porous structure which is bonded firmly to the substrate. The present electrode preparation method has been used to coat a variety of substrate forms, such as flat plates or wire screens, and is compatible with commercial alkaline electrolysis equipment. The resulting electrodes were found to be particularly effective as anodes for oxygen evolution. The efficiency of the electrodes was greatly influenced by coating microstructure. This microstructure, in turn, could be controlled by adjusting the sintering conditions. Electrochemical operating characteristics of the electrodes in at 80°C were determined. Comparable oxygen evolution efficiencies were obtained with coatings made from nickel powders, nickel flake, and nickel‐iron alloy powder.

On the Morphology of Melt-Crystallized Polyethylene. III. Spherulitic Organization

Abstract: The lamellar microstructure of a wide range of melt-crystallized polyethylenes has been examined in the context of spherulitic organization. Radial continuity is achieved through lamellar continuity. In the simplest cases ridged or planar sheets grow outwards along b, maintaining their identities over distances as long as tens of micrometres, subject to changes in relative dominance depending on whether or not they can continue to grow in the most favourable direction. The generalized 'fibres' are to be identified with dominant lamellae which lay down the framework of a spherulite's development. Banded spherulites have S-shaped dominant sheets which are multiply-connected allowing the average molecular orientation to spiral round the radius while individual lamellae are substantially untwisted. Space within spherulites is filled by branching, for which three mechanisms have been identified, namely the nucleation of new, misaligned lamellae, the incorporation of small-angle boundaries and spiral terraces associated with screw dislocations. Contrary to prediction, the width of dominant lamellae varies much less than the Keith & Padden parameter $\delta$, but remains within a factor of 3 of 3 $\mu m$. The scale of texture revealed by the segregation of 'impurities' does not appear to be determined solely by the scale of the diffusion field. It is suggested that the misfit energy between adjacent non-parallel 'fibrils', and geometrical factors, must also be taken into account.

Spatially delineated growth of metal films via photochemical prenucleation

Abstract: Photodissociation of a thin surface layer of metal‐alkyl molecules with a UV laser is shown to be an effective technique for predisposing the irradiated region to subsequent film growth. The efficacy of this technique, termed ’’prenucleation,’’ can be understood using a simple mathematical model for the early stages of thin‐film formation. The technique has applications in direct writing of microstructures.

Dispersion strengthened aluminum-4 pct magnesium alloy made by mechanical alloying

Abstract: Mechanical alloying is a unique high energy milling process for producing metal powders with a controlled microstructure When applied to aluminum based alloys, a uniform, equiaxed fine dispersion of oxygen- and carbon-based particles is obtained In addition, a very fine grain structure, pinned by the dispersoid, is generated Relatively low volume loadings of dispersoid may be employed to achieve attractive combinations of properties including ambient temperature tensile strength, corrosion resistance and stress corrosion cracking resistance The development of a dispersion strengthened aluminum-4 pct magnesium alloy is discussed

Optical properties of low‐pressure chemically vapor deposited silicon over the energy range 3.0–6.0 eV

Abstract: The optical properties of undoped and P‐doped silicon prepared by low‐pressure chemical vapor deposition were measured by spectroscopic ellipsometry over the energy range 3.0–6.0 eV. A marked effect of material microstructure is observed. Approximate values of the density deficit and of the volume fractions of crystalline and amorphous material are estimated as components of the microstructure by comparing measured spectra to those synthesized from constituent spectra in the Bruggeman effective‐medium approximation.

Thermal ageing effects in AISI type 316 stainless steel

Abstract: The effects of long-term thermal ageing on the microstructure of AlSl type 316 stainless steel are described. The microstructure of the aged steel is related to the reported embrittlement of the steel. The precipitation found is complex and the embrittlement appears to result, in part, from specimen preparation techniques interacting with the microstructure. The dominant creep deformation mechanism under service conditions appears to be diffusion creep.

Characterization and modeling for forging deformation of Ti-6Ai-2Sn-4Zr-2Mo-0.1 Si

Abstract: The deformation behavior during upset forging has been determined for Ti-6242 in both the (α + β) and β starting microstructures. For (α + β), flow softening attributed to deformation heating was observed. Deformation heating accounted for only a fraction of the extensive flow softening of the β microstructure. The dependence of log σ on 1/T was linear for (α + β) and bilinear for the β microstructure, with an approximate transition temperature of 930 °C. The two temperature regimes for β corresponded to distinct deformed microstructures which were manifestations of different softening mechanisms, all promoting the flow-induced transformation of metastable β microstructure to the equilibrium (α + β) microstructure. Based on the experimental data, flow stress equations for both microstructures, and empirical equations describing the flow softening behavior of β have been developed. WithT and ġe as the only input variables, these equations can accurately predict the σ - e relationships for process modeling of this alloy.

Effect of phase composition and microstructure on the thermal diffusivity of silicon nitride

Abstract: The effects of compositional and microstructural variables and processing conditions on the room temperature thermal diffusivity of hot-pressed and reaction-sintered silicon nitride were determined. The thermal diffusivity for hot-pressed silicon nitride increases withβ-content. Maximum thermal diffusivity is reached at about 3 wt % MgO. The higher thermal diffusivity of the β-phase is attributed to its higher purity level and the less distorted crystal structure compared to theα-phase. In reaction-sintered nitride the thermal diffusivity is strongly influenced by the relative amount and needle-like morphology of theα-phase. Correlations of the thermal diffusivity with mechanical properties are discussed.

Structure investigation of organosilicon polymers by silicon-29 NMR

Abstract: High resolution silicon-29 NMR is used to study the microstructure of organosiloxane chain copolymers and three-dimensional cross-linked silicone resins. The signal splittings of the Si-29 NMR spectra of methy1-pheny1 siloxane copolymers have been assigned to monomer sequences up to the pentad level and the quantitative microstructure parameters have been estimated. It is shown that Si-29 NMR offers valuable information on the polymer framework of silicone resins both in the liquid and in the solid state.

Precipitation sequences in heat-affected zone of 2-25Cr-1 Mo steel during stress-relief heat treatment

Abstract: The development of precipitation microstructure during stress-relief heat treatment of the bainitic heat-affected zone in 2.25Cr-1Mo steel has been studied using transmission electron microscopy, scanning transmission electron microscopy, and energy-dispersive X-ray analysis techniques on carbon extraction replicas and thin foils. Examinations have been made of four 2.25Cr-1Mo alloys (‘pure’, doped with 540 ppm phosphorous, doped with 500 ppm tin, and commercial purity) as part of an overall investigation into stress-relief cracking in 2.25Cr- 1Mo steel. The precipitation behaviour is discussed in terms of its relevance to fracto graphic observations and segregation characteristics which are reported in detail elsewhere.

Microstructure of SrTiO3 Boundary‐Layer Capacitor Material

Abstract: SiTiO3 capacitor material with indiffused Bi2O3 was studied using SEM, ESCA combined with Ar+ ion-etching, and TEM equipped with EDX. The apparent thickness of a second-phase layer observed with SEM was found to be influenced by in-depth effects. ESCA and TEM results show that only a 10- to 100-nm thick layer of second phase is present between the SrTiO3 grains. In addition, it was found with TEM that the outer part of each grain contained Bi (at most 2 at.%), representing a diffusion layer. These results have implications for the boundary-layer model proposed to explain the dielectric properties.