TEM study of metastable β-phase decomposition in rapidly solidified Ti-6Al-4V alloy

A new rationale is presented for various decomposition products obtained from the metastable -phase found in Ti-6A1-4V alloy produced by hot isostatic pressing comminuted melt-spun fibres and cooled to room temperature by furnace cooling. This alloy has an -matrix with about 8 vol% retained -phase, which is supersaturated with -stabilizers to such an extent that the martensitic transformation has been suppressed. The metastable -phase decomposes by different modes during continuous cooling, depending on the actual composition of individual -grains. Less enrichment of vanadium and iron favours the direct formation of the equilibrium -phase from the -matrix, while greater enrichment of vanadium and iron leads to a spinodal decomposition of the metastable -phase, resulting in a + two-phase structure. During further continuous cooling, the -phase which is lean in -stabilizers will transform into isothermal -phase. In addition, an unknown phase has also been observed in the -phase, which is typified by the appearance of 1/2{112} reflections in the SAD patterns.     

Observations on the formation of β-Al5FeSi phase in 319 type Al-Si alloys

In Al-Si alloys, the properties are influenced by the shape and distribution of the eutectic silicon particles in the matrix, as also by the iron intermetallics and copper phases that occur upon solidification. The -Al₅FeSi iron intermetallic phase, in particular, is known for its detrimental effect on the properties, and is controlled variously by the iron content and the melt/solidification conditions of the alloy. The formation of the iron intermetallics has been observed in commercial 319 alloy end-chilled castings, obtained from non-treated and treated melts, where the effects of cooling rate, strontium modification and grain refinement have been studied. The volume fraction of -phase formed was seen to increase with the decrease in cooling rate (i.e. with increasing distance from the chill) in the untreated alloy. Sympathetic (preferential) nucleation of the -iron needles was also observed, in which the branching of -needles from a parent needle resulted in the formation of large -needle entities that can cover distances of 1200 m across the matrix surface. The beneficial effect of modification, i.e. strontium addition to the melt, was manifested through its influence on the fragmentation and dissolution of the -needles. The strontium poisons the sites where sympathetic nucleation takes place. Dissolution was accelerated with increasing strontium content, the optimum level being 300 p.p.m. Grain refining, on the other hand, negated the beneficial effect of modification, in that the -needles underwent thickening and the sympathetic nucleation/branching also occurred. The modified alloy was found to possess the lowest volume fraction of -Al₅FeSi phase among the unmodified, modified, grainrefined, and modified/grain-refined alloys.     

Influence of stabilizers on Na-β′′-Al2O3 phase formation in Li2O(MgO)-Na2O-Al2O3 ternary systems

The influences of stabilizers on - and -Al2O3 phase formations in Li2O(MgO)-Na2O-Al2O3 systems were investigated. When stabilized with 4MgCO3Mg(OH)25H2O, most of the -Al2O3 phase formed below 1200°C and further - to -Al2O3 transformation with an increase of temperature was not observed. On the other hand, when stabilized with Li2CO3,-Al2O3 formation occurred by two steps. First, -Al2O3 was partly formed below 1200°C, and, second, noticeable transformation from -Al2O3 to -Al2O3 occurred at higher temperature ranges. It was shown that transient eutectic liquid in the Li2O-Na2O-Al2O3 system promoted the - to -Al2O3 transformation at higher temperatures. Uniform distribution of both Mg2+ and Li+ stabilizing ions enhanced -Al2O3 formation at low temperatures. In the Li-stabilized systems, however, homogeneous distribution of Li+ ions hindered both the formation of transient eutectic liquid and the second - to -Al2O3 phase transformation at high temperatures.     

Assessment of the role of cytokines in bone resorption in patients with total joint replacement

Periprosthetic osteolysis is known to be a consequence of a local chronic inflammatory reaction in the synovial tissue and the bone-implant interface membrane, and is mediated by macrophages (M) and foreign body multinucleated giant cells (MNGC) in these tissues. Activated M produce major classes of cytokines which have been documented in the regulation of bone cell formation, function and activity. In rheumatoid arthritis, inflammatory mediators released by M participate significantly in articular tissue destruction. In this study we have analysed the production and tissue distribution of 4 cytokines in the interface membranes obtained from patients with osteolysis associated aseptic loosening and in rheumatoid synovium to determine their role in the functional transformation of effector cells in these two conditions. The production of IL-1, GM-CSF, IL-6 and TNF- was assessed using immunohistochemistry on cryostat sections of the interface and the synovial tissues. IL-1 and GM-CSF were detected in significantly high numbers of the inflammatory M in both RA and aseptic loosening. A specific pattern of expression was noted in the interface. IL-1 production was sporadic throughout the sections, while GM-CSF was immunolocalized in a distinct subset of phagocytic macrophages on the implant side. IL-6 showed moderate expression in both conditions and was more widely produced at sites near the bone side in the interface. TNF- expression was absent or reduced in the interface but was more abundant in RA synovial M. The differential expression of cytokines indicates that bone lysis in these two pathological conditions is mediated by different mechanisms and regulated by different cytokines.     

Sintering and characterization of HA and TCP bioceramics with control of their strength and phase purity

HA and -TCP-based ceramics were prepared using commercial powders. Powder characteristics were defined and the processing parameters studied, aimed at the production of samples with improved microstructural and mechanical properties. The behaviour of HA powder subjected to various thermal treatments was investigated in order to control the formation of secondary phases (- and -TCP) during sintering. The optimal thermal treatment required to prepare pure -TCP powder from the precursors (HA and DCP) was determined and the sintering method required to prepare fully dense -TCP completely free from -form, was identified. Translucent hot-pressed -TCP ceramics with potential applications in aesthetic restorative prostheses were prepared and characterized. The interval of existence of -TCP and -TCP as secondary products was also defined. Crystallographic analysis was carried out on the imperfectly known low-temperature -TCP phase, and a proper monoclinic unit cell determined.     

Microstructure and fracture toughness of liquid-phase-sintered β-SiC containing β-SiC whiskers as seeds

The effect of seeding on microstructural development and fracture toughness of -SiC with an oxynitride glass was investigated by the use of morphologically rodlike -SiC whiskers. A self reinforced microstructure consisting of rodlike -SiC grains and equiaxed -SiC matrix grains was obtained by seeding 1–10 wt% SiC whiskers, owing to the epitaxial growth of -SiC from the seed whiskers. Further addition of seeds (20 wt%) or further annealing at higher temperatures led to a unimodal microstructure, owing to the impingement of growing seed grains. By seeding -SiC whiskers, fracture toughness of fine-grained materials was improved from 2.8 to 3.9–6.7 MPa · m^1/2, depending on the seed content.     

Haemocompatiblity of controlled release glass

There are many medical applications which benefit from the use of soluble biomaterials, including the sustained release of drugs over a precise period of time, or temporary conduits for controlling nerve regrowth. We have manufactured a series of phosphate-based controlled release glasses (CRGs) in which the solubility could be controlled by varying the concentration of CaO and Na2O. Fibres of the CRG containing iron and cerium were placed into direct contact with human neutrophils and macrophages in tissue culture for 2.5 and 24 h respectively and the responses analysed by scanning electron microscopy (SEM) and confocal microscopy. The supernatants were analysed for the cytokine IL-1 by enzyme-linked immunosorbent assay (ELISA). Disks of CRG of various compositions were placed in contact with whole blood for 30 min and platelet adhesion assessed by SEM. Activation of platelets, granulocytes and complement were quantified by ELISA for -thromboglobulin, elastase and iC3b. Intrinsic coagulation activation was measured by timing the clotting of recalcified plasma. Only the cerium fibre inhibited IL-1 release from macrophages. No platelet adhesion was observed to any disk composition. Three compositions containing MgO inhibited plasma clotting and showed an insignificant level of complement activation. This study has demonstrated the development of a number of compositions of CRG, which have great potential in a wide variety of biomedical applications.     

Thermodynamic investigations of liquid Bi-Na and Sn-Na alloys by coulometric titration using β″-alumina

The following double galvanic cell was assembled and the thermodynamic properties of liquid Bi-Na and Sn-Na alloys, and the ion selectivity of -alumina during coulometric titration, were investigated. Mo, Na(I)¦-alumina¦M-Na(I), Mo [I] M-Na(I)¦-alumina¦Au + Au₂Na, Mo [II] (M = Bi or Sn) where M-Na(1) and Au + Au₂Na were used as the common electrode and reference electrode, respectively. Sodium was coulometrically titrated through the -alumina electrolyte of cell I both ways, and the EMFs were measured. It was found that no ion-exchange reaction occurs between the liquid alloys and the -alumina, and only Na was transferred in the -alumina during coulometric titrations. The thermodynamic properties of liquid Sn-Na and Bi-Na alloys were found to be in agreement with the literature.     

Phase transformation and densification during pressureless sintering of Si3N4 with MgO and Y3Al5O12 additives

The - transformation of Si₃N₄ during liquid-phase sintering appears to be controlled by the growth of the -Si₃N₄ grains in the direction perpendicular to thec-axis in the case of MgO additive. The diffusion through the liquid is the rate-controlling step in the case of the Y₄Al₅O₁₂ additive. The density of the sintered body at the solid skeleton stage was influenced by the change in the - transformation rate and/or by a change of the transformation mechanism. The indirect proportionality between the -phase content in the starting powder and the density at the solid skeleton stage was found. The microstructure of the sintered body is influenced by both the -phase content in the starting powder and the chemical composition of the additive. Fine, uniform microstructure with a high aspect ratio of -grains is obtained, when the -phase content in the starting powder is as small as possible and when the - transformation is controlled by grain growth.     

Structural transformation during sintering and annealing of beta alumina

Changes in the phase content, microstructure and lattice parameter are observed in stabilized/ alumina specimens following extended sintering and annealing treatments. The resulting state is dependent on composition of the starting powder and on temperature and duration of heat treatment; the kinetics of transformation between and alumina are generally slow and certain/ ceramics remain in a metastable state even after a prolonged high temperature anneal. Following post-sinter heat treatment, splitting of X-ray diffraction peaks reveals a segregation of the phase into two components of differing lattice parameter. With sintering schedules of a long duration the splitting may even be present in the as-fired condition as recently reported by Harbach [1]. The splitting is attributed to a structural change resulting from the expulsion of Na₂O from supersaturated grains.     

Compressibility measurements of β- and β″ -alumina

The compressibilities of the a- and c-axes for sodium - and -aluminas were determined up to 10 GPa from the pressure dependence of powder X-ray diffraction measured at room temperature using synchrotron radiation as an X-ray source. Powders of sodium - and -aluminas which were prepared from grinding synthesized single crystals were used as the specimens for X-ray diffraction. The compressibilities of - and -aluminas are 1.5 ± 0.2 ×10^–12 and 1.7 ± 0.2 × 10^–12 Pa^–1 for the a-axis and 2.9 ± 0.2x10^–12 and 1.6 ± 0.2 ×10^–12 Pa^–1 for the c-axis, respectively. For the c-axis, the compressibility of -alumina is larger than that of -alumina. This experimental fact is explained by the different stacking of oxygen layers and the different content in sodium ion between - and -aluminas.     

A high-temperature solid-state potentiometric sensor for nitrogen based on ceramic LaAl12O18N

The nitrogen-sensing properties of LaAl12O18N are described for the first time. Positive e.m.f. measurements at high temperature across the cells -Nb (N), -Nb2N, -Al2O3, LaAl12O18NLaAl11O18 (or LaAl12O18N)LaAl12O18N, -Al2O3, -Nb2N, -Nb4N3, were in agreement with PN2(-Nb2N, -Nb4N3)>PN2(-Nb(N), -Nb2N) for the idealized stoichiometric cell reaction -Nb4N3+2-Nb(N)3-Nb2N, thus demonstrating the nitrogen-sensing property of these cells. The e.m.f. for a variety of cells with electrodes containing -V2N, -Nb2N, -Ta2N and Ti2N, were consistent with the predicted equilibrium nitrogen partial pressures.     

A new solution branch of the Falkner-Skan equation

Summary We present here results for a new solution branch of the Falkner-Skan equation with parameter . It is found that there are two turning points on this new branch which results in two solutions of the problem for 37.844<<, three solutions for =37.844, four solutions for 14.533<<37.844, three solutions for =14.533, and two solutions for 1<<14.533. This solution branch is found to end singularly at =1; its structure is analytically investigated and the principal characteristics described. The spatial stability of such solutions is also commented on.     

Analysis of the α to β phase transformation in Y-Si-Al-O-N ceramics

The silicon nitride- sialon phase transformation in the Y-Si-Al-O-N system was investigated by transmission and scanning electron microscopy and X-ray diffractometry using samples which contained up to 40% liquid-forming components at temperatures between 1500 and 1600 °C. Completely dense samples suitable for TEM analysis, in which the to transformation could be examined, were prepared using hold times as short as 5 min under a nominal uniaxial pressure followed by rapid cooling. Spheroidal, partially dissolved, silicon nitride grains, together with acicular grains of sialon, were observed in a glass phase containing a very low nitrogen content (undetectable by electron microprobe analysis). This absence of nitrogen build-up in the liquid phase between the dissolution and precipitation sites during the to transformation indicates that the diffusion of nitrogen through the liquid phase is extremely rapid. Nucleation of the sialon was almost entirely homogeneous and the unconstrained nature of the liquid environment resulted in growth of defect-free sialon grains with curved growth fronts perpendicular to the c-axis. The technique described allows direct observation of the effect of various additives on the to phase transformation.     

Precipitate strengthening mechanisms in magnesium zinc alloy single crystals

The mechanical behaviour of Mg-5.1 wt % Zn alloy single crystals was studied in the 4.2 to 300° K temperature range. Quenched crystals have activation energies and volumes best associated with the cutting of small clusters of Zn atoms by dislocations. Fully hardened crystals contain fine ₁ and occasional ₂ precipitates with an average ₁ interparticle spacing of 330 to 660 Å. Strengthening in these crystals is mainly ascribed to the cutting of ₁ particles by dislocations. In the overaged condition ₁, ₂ and equilibrium particles are present and lead to a considerable temperature-dependence unusual for an overaged condition. Analysis of this temperature-dependence suggests that below 77° K the relatively easy cutting of ₁ particles by dislocations takes place in addition to the cutting of ₂ and particles. Above 77° K the difficult cutting of ₂ and particles alone controls the deformation, ₁ being more easily cut with the aid of thermal fluctuations.     

The relative stability ofβ andβ″-phases in Na2O-Al2O3 beta alumina

To reveal the relative stability of and -phases in beta alumina, effects of Na₂0 content and calcination and annealing history on the relative content of and -phases have been studied for compositions ranging from Na₂O-5Al₂O₃ to Na₂O-9.5Al₂O₃. The relative stability appears to be dependent on the calcination or annealing history, except holding time such as annealing or re-annealing time, but independent of the Na₂O content and the holding time. From these results it can be demonstrated that the relative stability of and -phases is sensitive to the heat treatment path, except the holding time. The two stage model on the /. polytypic transition in silicon carbide has been quoted to explain the heat treatment pathdependent characteristics.     

Strain-induced β-α polymorphic transition in iPP as revealed by microhardness

The microhardness (H) technique was used for characterization of the - polymorphic transition in isotactic polypropylene (iPP). For this purpose the microhardness in the damage zone of a tensile loaded deeply edge-notched (DEN-T) -iPP specimen was mapped. Mapping of H was performed, both along the loading direction (central) and close to the shorter fracture edge. Around half-length of the plastic zone a sharp increase of the H values in both cases was observed. The H increase is related to the polymorphic transition. Microvoid formation in the central part results in lower H values. However for the edge zone close to the top of the fracture surface unusually high H values (around 200 MPa) are obtained. The latter are explained in terms of the formation of microfibrils due to crazing during deformation which are characterized by very high molecular orientation as reported from X-ray analysis.     

Nucleation and growth of β′magnesium sialons and their composition limits

Previous reports on the solubility of magnesium in -sialons have been conflicting. The present work shows conclusively that crystalline magnesium sialons with the silicon nitride structure do exist. They are formed by crystallization of magnesium sialon glasses at low temperatures. -magnesium sialon crystals nucleate on -sialon nuclei which are themselves formed by precipitation from Mg-Si-Al-O-N liquids at high temperatures. The current results suggest that -magnesium sialons exist only over a limited composition range within the 3M/4X plane of the Mg-Si-Al-O-N system, which is indicative of some form of ordering within the structure. Although the compositions investigated in the present study are unstable with respect to forsterite above about 1000° C, the possibility of producing -magnesium sialons which are stable at much higher temperatures cannot be fully discounted at present.     

Crystallization of some spodumene-lithium zinc orthosilicate glasses

By using thermal analysis, X-ray diffraction, polarizing and electron microscopy the effect of compositional variation, thermal treatment and nucleation catalysts TiO₂ and ZrO₂ on the nature, type and stability fields of the crystallizing phases, as well as the resulting microstructures, is described for some stoichiometric glass compositions within the system LiAlSi₂O₆-Li₂ZnSiO₄. Intense uniform volume crystallization was achieved. Transparent glass-ceramics with ultrafine microstructures could also be obtained at temperatures near 700 °C. Crystallization begins with the formation of a proto - and/or _II-Li₂ZnSiO₄ followed by, or concomitant with, -eucryptite ss. The proto -phase was formed over a narrow temperature range and rapidly transformed into its _II-modification which, although showing a wider stability range, ultimately transformed into the stable _o-modification. The metastable -eucryptite ss starts its transformation into -spodumene around 800 °C. By prolonged heating at temperatures as high as 1000–1040 °C, -spodumene and _o-Li₂ZnSiO₄ were the main stable end products. TiO₂ and ZrO₂ have contrasting effects on the stability of the LiZn orthosilicates and -eucryptite ss -spodumene transformations. The former exhibits a catalytic effect and the latter showed a retarding effect on these processes.     

Effects of amines on the formation of β-ferric oxide hydroxide

Colloidal particles of -ferric oxide hydroxide (-FeOOH) were prepared by ageing an FeCl₃ solution at 100°C in the presence of various amines with different numbers of nitrogen atoms. The particle and crystallite sizes of the formed -FeOOH decreased with increasing concentration of amines, but the particles produced contained no amine. The influence of triethanolamine on the growth of -FeOOH particles was more marked than those of ethylenediamine, diethylenetriamine and triaminotriethylamine. The adsorption isotherms of nitrogen and water on the formed particles of different size were measured and surface fractal analysis was performed.