Solid-solution structure and the weakly first-order displacive transformation in Fe-Pd alloys

The solid-solution structure of an Fe-31 at. pct Pd single crystal quenched from 1173 K was measured by combined X-ray diffraction (XRD) and atom-probe microanalysis, with analysis of diffuse X-ray scattering supplemented by computer simulation. Detailed comparison of composition-fluctuation amplitudes on the size scale of 50 atoms shows good agreement between diffraction and atom-probe results, revealing a nonrandom solution structure with a broadened composition distribution, consistent with the existence of a miscibility gap in the system. Also consistent with the L1₀-FePd ordering present in the Fe-Pd system, the diffuse scattering indicates some short-range order in the higher Pd regions. Computer simulation of the diffuse scattering indicates Fe atom displacements in 〈100〉 directions in Fe-rich regions forming microdomains oriented along {110} planes, consistent with previous electron microscopy observations of fine structures above the nominal transformation temperature. Combining the measured composition distribution with a magnetics-based model of the composition dependence of the parent-phase instability temperature predicts a spreading of the weakly first-order cubic-tetragonal phase transformation, whereby tetragonal domains are locally stabilized at temperatures 300 K above the temperature where macroscopic plate products first appear. Composition fluctuations can thus introduce a significant component of continuous character to a weakly first-order displacive transformation.     

The stimulus-evoked release of glutamate and GABA from brain subregions following transient forebrain ischemia in the rat

The release of glutamate and GABA in response to K+ depolarization was determined for tissue prisms prepared from brain subregions removed from rats following 30 min of forebrain ischemia or recirculation periods up to 24 h. There were statistically significant effects of this treatment on release of both amino acids from samples of the dorsolateral striatum, an area developing selective neuronal degeneration. However, for at least the first 3 h of recirculation the calcium-dependent and calcium-independent release of both amino acids in this region were similar to pre-ischemic values. Differences were observed under some conditions at longer recirculation times. In particular there was a decrease in calcium-dependent GABA release at 24 h of recirculation and a trend towards increased release of glutamate at 6 h of recirculation and beyond. No statistically significant differences were seen in samples from the paramedian neocortex, a region resistant to post-ischemic damage. These results suggest that changes in the ability to release glutamate and GABA in response to stimulation are not necessary for the development of neurodegeneration in the striatum but rather that release of these amino acids may be modified as a result of the degenerative process.     

Arginine vasopressin release inhibitor RU51599 attenuates brain oedema following transient forebrain ischaemia in rats

RU51599 is an arginine vasopressin (AVP) release inhibitor and a selective kappa opioid agonist which has a pure water diuresis effect without associated electrolyte excretion. The effect of RU51599 on brain oedema following transient forebrain ischaemia in rats was examined. Under microscopy, the visible vertebral arteries at the second vertebra could be easily electrocauterized and completely cut by microscissors to yield complete cessation of circulation of both vertebral arteries. Transient forebrain ischaemia was induced by this improved highly reproducible technique of four-vessel occlusion model. Forty-three male Wistar rats were separated into six groups; saline-treated (1 ml/kg) normal rats (n = 10), RU51599-treated (1 mg/kg) normal rats (n = 4), saline-treated (1 ml/kg) rats with complete occlusion of both vertebral arteries (n = 5), RU51599-treated (1 mg/kg) rats with complete occlusion of both vertebral arteries (n = 5), saline-treated (1 ml/kg) rats with both complete occlusion of both vertebral arteries and carotid occlusion bilaterally during 45 minutes followed by 60 minutes of reperfusion (n = 11), RU51599-treated (1 mg/kg) rats with both complete occlusion of both vertebral arteries and carotid occlusion bilaterally during 45 minutes followed by 60 minutes of reperfusion (n = 8). The brain water content was determined by the dry-wet weight method. Cerebral blood flow was monitored during ischaemia and reperfusion was performed by laser Doppler flowmetry to make sure to obtain reversible forebrain ischaemia. Effects of RU51599 on concentration of glutamate released from the hippocampal CA1 of rats subjected to 5 minutes four-vessel occlusion and 60 minutes of reperfusion were also investigated by the microdialysis method. This modified four-vessel occlusion method produced reversible forebrain ischaemia with a high level of success. Bilateral carotid occlusion followed by 60 minutes reperfusion caused a significant increase in brain water content (P < 0.01), which was significantly attenuated by RU51599 (P < 0.01). These findings indicate that the AVP-release inhibitor RU51599 reduced brain oedema following transient forebrain ischaemia in rats.     

On the rule of additivity in phase transformation kinetics

It is commonly held that a sufficient condition for the rule of additivity to be valid is that the transformation rate depend only on temperature and volume fraction. This is not true in general.     

Submillisecond kinetics of glutamate release from a sensory synapse

Exocytosis-mediated glutamate release from ribbon-type synaptic terminals of retinal bipolar cells was studied using AMPA receptors and simultaneous membrane capacitance measurements. Release onset (delay <0.8 ms) and offset were closely tied to Ca2+ channel opening and closing. Asynchronous release was not copious and we estimate that there are approximately 5 Ca2+ channels per docked synaptic vesicle. Depending on Ca2+ current amplitude, release occurred in a single fast bout or in two successive bouts with fast and slow onset kinetics. The second, slower bout may reflect a mobilization rate of reserve vesicles toward fusion sites that is accelerated by increasing Ca2+ influx. Bipolar cell synaptic ribbons thus are remarkably versatile signal transducers, capable of transmitting rapidly changing sensory input, as well as sustained stimuli, due to their large pool of releasable vesicles.     

Growth kinetics and mechanism of the massive transformation

During the composition invariant massive transformation, the propagation of a reaction interface is motivated by the free energy change due to a change in crystal structure and results in either single or duplex product morphologies. A common feature underlying the growth of massive product phases is a thermally activated control by boundary diffusion. Microstructural and kinetics information indicate that massive growth involves the migration of mobile, incoherent interfaces through a lateral growth process that may evolve into continuous growth at the highest velocity. The high driving free energy for massive growth is reflected by a relative insensitivity to microstructural obstacles and by a rapid transformation in the absence of local interfacial equilibrium. For the limiting conditions under which massive growth can prevail at the transition into lattice shear or solute partitioning, an incomplete dissipation of driving free energy is indicated by product structures of metastable supersaturated solid solutions. This paper is based upon a presentation made at a symposium on The Massive Transformation, held at the Pittsburgh meeting of The Metallurgical Society of AIME and the Materials Science Division of ASM, October 9, 1980, under the sponsorship of the MSD Phase Transformations Committee.     

The kinetics and mechanism of the pyrite-to-pyrrhotite transformation

The kinetics of the transformation of pyrite to pyrrhotite have been investigated. The study was performed using thermogravimetric analysis over the temperature range of 620 to 973 K in atmospheres of H₂, He, Ar, and in vacuo over a wide range of pressures: 0.20 Pa to 4.24 MPa. Based on the kinetic results, a mechanistic picture of the various steps exerting control over the transformation is proposed. The thermal decomposition proceeds via a two-step, consecutive process. The rate-controlling step is the desorption of sulfur vapor from the surface. The presence of H₂ introduces different rate-controlling steps into the sequence, providing the H₂ exists at a pressure sufficiently high to suppress the rate of thermal decomposition. Rates at which the H₂ reduction occurs with pyrite samples from different sources depends upon the samples’ impurity level and the extent to which various crystallographic faces are exposed.     

The kinetics of transformation in Zn-Ai superplastic alloys

We report herein on the kinetics of transformation of a eutectoid Zn-AI alloy containing additions of Cu, Mg and Ca. The alloy possesses excellent superplasticity at elevated temperatures, and it has a relatively high strength at ambient temperature (∼345 MPa). TTT curves for the alloy are presented, and the corresponding microstructures obtained at the various transformation temperatures are reported. Also, the results of Jominy endquenched tests are reported and the corresponding continuous cooling kinetics are compared to the isothermal kinetics. The alloy was observed to decompose by two distinct mechanisms, depending upon the degree of supercooling. At temperatures just below the eutectoid, it decomposes into a lamellar microstructure, whereas at larger undercooling it decomposes into a coherent two-phase mixture. The interlamellar spacing and colony size are reported as a function of transformation temperature, and shown to follow expected trends. Since neither the lamellar nor coherent microstructure is superplastic, thermomechanical methods of producing a superplastic structure are discussed.     

An assessment of the additivity principle in predicting continuous-cooling austenite-to-pearlite transformation kinetics using isothermal transformation data

The limits of applicability of the Additivity Principle, necessary for the prediction of continuous-cooling transformation kinetics from isothermal transformation data, are clarified based on an analysis of recently measured austenite-to-pearlite transformation kinetics in a eutectoid, plain-carbon steel. It has been found that additivity holds for the transformation event, exclusive of the incubation period, in this steel. But the isokinetic condition defined by Avrami, and the early site-saturation criteria postulated by Cahn as sufficient conditions for additivity are not satisfied. Thus a new condition, termed “effective site saturation”, is proposed in which the growth of pearlite nucleated early in the transformation dominates the overall kinetics of austenite decomposition. A criterion for effective site saturation has been established.     

Study of the austempering transformation kinetics in compacted graphite cast irons

In this work, the phases involved in the transformation austenite→ferrite+high-carbon austenite in cast irons were assessed by means of Mössbauer spectroscopy and hardness measurements for samples austempered at five different temperatures between 573 and 673 K with two Mn contents. The C content in the high-carbon austenite was found to have a dependence of t ^0.40±0.05 on the austempering time t, which evidences that diffusion of carbon has taken place. The kinetic parameter k determined using the Johnson-Mehl-Avrami equation has a maximum of 3.9×10^?3 (s^?1) at ≈623 K and corroborates that Mn slows the transformation rate.     

Effect of transient cerebral ischaemia on acetylcholine release in the gerbil hippocampus

To clarify the relationship between presynaptic cholinergic dysfunction and postsynaptic cell death in the hippocampus, extracellular levels of acetylcholine (ACh) were assayed and CA1 pyramidal cells were histologically investigated in gerbils which had undergone 2, 5 and 10 min ischaemia. It was found that the KCl- and atropine-induced release of ACh, an index of the functioning cholinergic system at the presynaptic terminals, was significantly lower in the ischaemic groups than in control groups. The hippocampal CA1 pyramidal cell area of the 5 and 10 min ischaemic animals was also significantly decreased, but the 2 min ischaemia caused no cell damage. These findings indicate that the presynaptic terminals of the cholinergic neurone are vulnerable to ischaemic insult and that cholinergic dysfunction precedes postsynaptic CA1 pyramidal cell death in the hippocampus.     

Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys

The kinetics and morphology of isothermal transformation in the vicinity of the time-temperaturetransformation (TTT) diagram bay have been investigated with optical and transmission electron microscopy (TEM) in 19 Fe-C-Mo alloys at three levels of carbon concentration (approximately 0.15, 0.20, and 0.25 wt pct) and at Mo concentrations from 2.3 to 4.3 wt pct, essentially always at temperatures above or at that of the bay,T _b . Quantitative metallography yielded no evidence for incomplete transformation (stasis) in any of these alloys atT > T _b . Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs (invariably containing either interphase boundary or fibrous Mo₂C) demonstrated four different patterns of behavior. The customary parabolic time law for allotriomorph thickening in Fe-C and in many Fe-C-X systems was obtained only at higher temperatures and in the more dilute Fe-C-Mo alloys studied. With decreasing temperature and increasing solute concentrations, a two-stage and then two successive variants of a three-stage thickening process are found. In the most concentrated alloys and at temperatures nearest the bay, the second stage of the three-stage thickening process corresponds to “growth stasis”—the cessation of allotriomorph thickening. Sufficient prolongation of growth stasis presumably leads to “transformation stasis.” A number of models for growth of the carbide-containing allotriomorphs were investigated during attempts to explain the observed kinetics. It was concluded that their growth is controlled by carbon diffusion in austenite but with a driving force drastically reduced by a very strong solute drag-like effect (SDLE) induced by Mo segregation at disordered-type austenite: ferrite boundaries. Carbide growth in the fibrous structure appears to be fed by diffusion of Mo along austenite: ferrite boundaries, whereas carbides in the interphase boundary structure grow primarily by volume diffusion of Mo through austenite. Formerly Republic Steel Corporation Fellow, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI, and Visiting Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly Professor, Michigan Technological University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

A fractal analysis of external diffusion limited first-order kinetics for the binding of antigen by immobilized antibody

A fractal analysis of external diffusion limited first-order kinetics for the binding of antigen in solution by immobilized antibody on a fibre-optic biosensor indicates that as the fractal parameter (measure of "disorder" on the surface) increases the rate of binding and the amount of antigen bound to the antibody on the surface decreases. The fractal analysis and exponential type binding rate coefficients are used to analyze the influence of time-dependent binding rate coefficients on external diffusion limited kinetics. A decrease in the binding rate coefficients with time decreases the Damkohler number (decrease in the mass transfer limitations) leading to an increase in the rate of binding and the amount of antigen bound to the antibody on the surface, as expected. An increase in the (exponential) binding rate coefficient with time leads to unusual shapes of the binding curves. The time-dependent binding rate coefficients provide a more realistic picture of the binding of antigen in solution to the antibody covalently attached to the surface, and should assist in the control and manipulation of these interactions at the surface. A value of the fractal dimension of the surface of 2.96 to 2.97 obtained for our system characterizes the anomalies in the reaction-diffusion system and the heterogeneity of the surface.     

The effect of transformation stresses on the devitrification kinetics of a minory phase in liquid phase sintered ceramics

The crystallization of minory amorphous constituents in liquid phase sintered ceramics, for example in Si₃N₄, is usually accompanied by a volume change. The resulting mismatch between crystallizing second phase inclusions and the surrounding matrix of the primary phase leads to the formation of transformation stresses. The strain energy stored in the stress field reduces the thermodynamic driving force of crystallization. The coupling of crystallization, stress formation and relaxation is modelled. The extended duration of the crystallization process due to an intermediate stress induced decrease of the crystallization rate is assessed. The properties of amorphous grain boundary films are discussed with respect to stress relaxation and creep resistance at high temperatures.