Exothermic Transformation Research Articles

Microstructural characterisation and thermal stability of a metastable Mg-8.6 wt.% Zr alloy produced by physical vapour deposition

Abstract The thermal stability of a Mg-8.6 wt.% Zr alloy processed by physical vapour deposition has been studied using differential scanning calorimetry and transmission electron microscopy. The alloy, in the as-deposited condition, is a solid solution of Zr in the Mg matrix. The microstructure is characterised by elongated grains oriented with their [0001] direction parallel to the growth direction. After DSC experiments, three exothermic transformations have been observed. The first one takes place above the deposition temperature and is related to the recovery phenomena due to stress relaxation generated during the growth process. The second transformation corresponds to the breakdown of the solid solution by the precipitation of Zr in the Mg matrix. This reaction takes place in two stages: (i) Zr plates are formed in the basal plane and (ii) these initial precipitates grow along the [0001] direction. An epitaxial relationship between those Zr precipitates and the Mg matrix of [0001]Zr || [0001]Mg and...

Influence of fiber on the phase transformations in the starch-water system.

High-sensitivity, temperature-controlled DSC measurements at a low heating rate and creation of differential DSC traces scaled with respect to the reference material (completely dehydrated starch or completely dehydrated fiber, or their respective blends) permitted investigation of the influence of fiber on phase transformations in the wheat-starch-water system in the course of thermal gelatinization. Thermal effects associated with water interactions over the temperature range from 283 to 384 K under atmospheric pressure were determined. These thermal effects and previous structural studies permit us to make the following observations: (1) The main endothermic transition associated with melting of the crystalline part of the starch granule followed by a helix-coil transition in amylopectin occurs over the temperature range 319-333 K independent of the water and fiber contents. Adding fiber causes that transition to disappear both in the native blends and in water suspensions at low water contents. After adding more water and heating, recrystallization is observed and the transition reappears. (2) The fiber content has practically no influence on the slow exothermic transformation, which follows melting and helix-coil transition in amylopectin, proving that the slow transformation has a specific chemical character. In this reaction, the free ends of the unwound helices of amylopectin reassociate with parts of amylopectin molecules other than their original helix duplex partner, forming physical junctions and creating more general amorphous hydrogen bonded associations. (3) The high-temperature transition and small, but reproducible, distortions on the peaks of the main endothermic transition for water contents near 70-80 wt % are associated with smectic and nematic transitions, respectively. These are significantly influenced by the fiber content; higher fiber content causes an almost complete disappearance of these transitions. (4) The slow exothermic effect appearing almost from the very beginning of the heating in the starch-water system, associated with softening and uptake of water in the amorphous growth rings of the starch granule, is significantly hindered by added fiber.

Crystallization processes of Ag–Ge–Se superionic glasses

The interest in superionic systems has increased in recent years because of the potential application of these materials as solid electrolytes. In this field, amorphous materials present important advantages when compared to the crystalline solids: larger conductivity, isotropy and absence of grain boundaries. In this work, amorphous alloys of compositions (Ge 25Se 75) 100− y Ag y with y=10, 15, 20 and 25 at.% have been studied. Amorphous samples in bulk were obtained from the liquid by water quenching (melt-quenching technique). The crystallization kinetics of the amorphous alloys have been studied under continuous heating and isothermal conditions by means of differential scanning calorimetry. A glass transition and two exothermic transformations were observed in all the samples. The quenched samples and the crystallization products have been characterized by X-ray diffraction. The primary crystallization of the ternary phase Ag 8GeSe 6 and the secondary phase GeSe 2 was observed. The glass and crystallization temperatures, the activation energy and the crystallization enthalpy are reported. The first step of the crystallization of the Ag 8GeSe 6 phase in all the (Ge 25Se 75) 100− y Ag y samples is modelled taking into account the Johnson–Mehl–Avrami–Kolmogorov theory and considering that the changes in the composition only modify the viscosity of the undercooled liquid. The transformation diagrams (TTT and THRT) are calculated and the glass forming ability is analyzed. The experimental results are discussed and correlated with the structures proposed for the glass. The present results and conclusions are also compared with those reported by other authors.

Crystallisation of spray-dried amorphous precursors in the SrO–Al2O3 system: a DSC study

Abstract The crystallisation of amorphous precursors in the SrO–Al 2 O 3 system has been studied by DSC and XRD. Amorphous precursors were obtained for compositions ranging from 12SrO.7Al 2 O 3 to Al 2 O 3 by spray-drying aqueous solutions of strontium and aluminium nitrates followed by heating the powders to decompose the nitrates. Of the stable compounds SrA, SrA 2 and SrA 6 , only SrA crystallises directly at a usual heating rate of 5 °C/min (in this notation Sr=SrO and A=Al 2 O 3 ). All the alumina-richer compositions are characterised by the first and strong crystallisation of metastable solid solutions of SrA in its hexagonal form, stabilised by excess aluminium cations, or γ-Al 2 O 3 stabilised by strontium cations, or even the two solid solutions, depending on the composition. The hexaaluminate SrA 6 is formed as single phase by an exothermic transformation of the γ-Al 2 O 3 solid solution at 1183 °C. The formation of SrA 2 is complicated by the easy and competitive formation of Sr 4 A 7 , of close composition. The latter compound is stable only up to 1500 °C and is decomposed at higher temperatures. However SrA 2 , which is characterised by a slow kinetics of formation from the amorphous state as well as from the high temperature liquid state, may be obtained pure by slow heating or annealing at 900–1000 °C.

Caracterización microestructural y estabilidad térmica de la aleación Mg-23 %wt Zr sintetizada mediante Deposición Física en Fase Vapor (PVD)W

The objective of this work has been the study of the microstructure and the thermal stability of the alloy Mg-2.3 %Zr grown by Physical Vapour Deposition. This study has been carried out by Differential Scanning Calorimetry and Transmission Electron Microscopy. The alloy in the as-deposited condition consists of a solid solution of zirconium atoms in the magnesium matrix. The microstructure of the alloy is characterised by elongated grains oriented in the growth direction. These grains grow with the [0001] direction parallel to the deposit growth direction. During a continuous heating, the deposit shows three exothermal transformations which have been associated with a recovery process, the zirconium precipitation in the magnesium matrix and oxidation at high temperatures. The activation energy for the precipitation reaction calculated from Kissinger analysis is 139 kjmol -1 .

Nanometer-scale solute clustering in aluminum–nickel–ytterbium metallic glasses

The transformation sequence of melt-spun Al–5.2at.% Ni–6.8at.% Yb is characterized by X-ray diffraction, differential scanning calorimetry (DSC), and atom-probe field-ion microscopy. An isochronal DSC thermogram detects three major exothermic transformation steps, with onset temperatures of 180, 292, and 327 °C, respectively. Atom-probe analyses reveal the evolution of compositional inhomogeneities on an atomic scale. In addition to Ni–clusters present in the as-quenched alloy, Ni–Yb co-clusters form during isochronal heating to 188 °C. Al–rich precipitates are detected after heating to 250 °C, with the Ni and Ni–Yb clusters still present, constituting possible precursors of intermetallic phases formed during advanced transformation stages.

Thermal and spectral properties of some anhydrous and hydrated rare earth hydrazinecarboxylates

This study focuses on the thermal and structural characteristics of some Brazilian kaolinitic clays. X-ray diffraction analysis have shown that the clay samples consist mainly of kaolinite, and quartz and gibbsite as principal impurities. On heating three endothermic transformations were identified as water loss from TG-MS data within the 60-64, 268-276 and 499-503°C ranges, followed by an exothermic transformation with maximum within the 950-959°C range. The endothermic valleys can be mainly interpreted as the release of free moisture, dehydration of gibbsite and dehydroxylation of kaolinite, respectively. The exothermic peak is associated with crystallization of new phases from metakaolinite. The quantification of kaolinite (59.6-74.2%) and gibbsite (5.9-10.2%) was taken by the thermogravimetric measurements. In addition, it was also demonstrated from the thermal curves that the character of the clays from South-Eastern Brazil (Campos-RJ) is predominantly kaolinic.

“Burst-like” Characteristics of the δ/α′ Phase Transformation in Pu-Ga Alloys

ABSTRACTThe δ to α′ phase transformation in Pu-Ga alloys is intriguing for both scientific and technological reasons. On cooling, the ductile fcc δ-phase transforms martensitically to the brittle monoclinic α′-phase at approximately −120°C (depending on composition). This exothermic transformation involves a 20% volume contraction and a significant increase in resistivity. The reversion of α′ to δ involves a large temperature hysteresis and begins just above room temperature. In an attempt to better understand the underlying thermodynamics and kinetics responsible for these unusual features, we are examining the δ/α' transformations in a Pu-0.6 wt% Ga alloy using differential scanning calorimetry (DSC) and resistometry. Both techniques indicate that the martensite start temperature is −120°C and the austenite start temperature is 35°C. The heat of transformation is approximately 3 kJ/mole. During the α ′ → δ reversion, “spikes” and “steps” are observed in DSC and resistometry scans, respectively. These spikes and steps are periodic, and their periodicity with respect to temperature does not vary with heating rate. With an appropriate annealing cycle, including a “rest” at room temperature, these spikes and steps can be reproduced through many thermal cycles of a single sample.

ZrNbCuNiAl bulk metallic glass matrix composites containing dendritic bcc phase precipitates

We report on phase formation of a multicomponent Zr66.4Nb6.4Cu10.5Ni8.7Al8 glass-forming alloy upon copper mold casting. A bcc phase embedded in a glassy matrix forms for moldcast bulk samples yielding an in-situ bulk metallic glass matrix composite upon slow cooling from the melt. Upon annealing, the first exothermic transformation of the material is related to precipitation of an icosahedral phase from the glassy matrix. The formation of the bcc phase-containing metallic glass composite is strongly governed by the alloy composition and the actual cooling rate during solidification. Room-temperature compression tests reveal significant yielding and plastic deformation before failure.

Precipitation study on Mg-12 wt% Ti alloy obtained by physical vapour deposition

Abstract The thermal stability of Mg-12 wt%Ti alloy processed by physical vapour deposition has been studied using differential scanning calorimetry (DSC) and transmission electron microscopy. The alloy in the as-deposited condition is a solid solution of titanium in magnesium. During DSC experiments, three exothermal transformations have been observed. The first at 430 K is related to the stress relaxation of defects introduced during the growth process. The second transformation is associated with the precipitation of titanium in the magnesium matrix, which occurs in two stages. In the first stage, small coherent precipitates around 1 nm in size are present in the basal plane. The second stage has been associated with the growth of the titanium precipitates along the [0001] direction. The activation energies for these transformations are 144 and 155kJmol−1 respectively. At higher temperatures, above 703 K, an increase in heat flow takes place that is related to the strong oxidation of the magnesium matrix.

Estimation of residual stresses induced by shot-peening. Measurement of the thermal dissipation with an infrared camera

The plasticization induced by the shot-peening treatment is an exothermic transformation. One can imagine to link the induced residual stress level to the thermal behaviour of the sample during the treatment or, at least, to propose some means of process monitoring control via temperature measurements. Steel samples were submitted to shot-peening under various conditions, both with a compressed air machine and a turbine machine. The temperature of the sample rear face was measured by IR thermography during the treatment. The thermal behaviour of the samples during the treatment have been compared to Almen intensity measurements and to residual stress measurements by the hole-drilling method. We conclude to the feasibility of process monitoring by temperature measurements. More academic experiments are being set up in order to derive quantitative relations between the thermal dissipation and the stress level.

Nonstationary Regimes of Transformation of Multilayered Heterogeneous Systems

A nonstationary mathematical model of thermal propagation of flame in a layered heterogeneous system is proposed. The structure and dynamics of the frontal exothermal transformation in quasihomogeneous, transitional, and relay-race regimes are studied. Averaged characteristics of the front and dynamics of transformation of individual elements of a “discrete” combustion wave are analyzed using the model proposed. A correlation is established between the combustion of a model medium and real heterogeneous compositions. It is shown that the maximum combustion velocity is reached at an intermediate level of medium dispersion in a transitional parametric region. Key words: combustion waves, heterogeneous systems, multilayered nonstationary regimes, modeling.

Atomic ordering and magnetic properties in Nd57Fe20B8Co5Al10 solids

Nd 57 Fe 20 B 8 Co 5 Al 10 bulk cylinders and ribbons were quenched from the melt at different cooling rates using copper mold casting and melt spinning, respectively. Both the melt-spun ribbons and the cast cylinders display x-ray diffraction patterns without obvious crystalline peaks. However, the ribbons show soft magnetic properties, the cylinders are hard magnetic, and the crystallized alloys are not ferromagnetic at all. On the other hand, the cylinders show different exothermic transformation behavior than the ribbons upon heating to elevated temperatures. While the ribbons are regarded as amorphous, the structure of the cylinders prepared at slower cooling rate is considered to consist of metastable ordered clusters. This structure forms in the undercooled melt most likely due to intensive generation of ordered nuclei and slow growth kinetics. The magnetic coherence length, which is equivalent to the dimension of the ordered clusters, may be larger than the exchange length. Therefore, the cylinders shows much larger coercivity than the ribbons.

Condensed-phase modifications in magnesium particle combustion in air

Free-falling individual magnesium particles were produced and ignited using a pulsed micro-arc and burned in air. Particle combustion temperatures were monitored in real time using a three-color pyrometer. Black-body and MgO radiation histories were separated using interference filters and compared with each other. Particles were rapidly quenched at different combustion times and their elemental compositions were studied using energy and wavelength dispersive spectroscopy techniques. Particle combustion times and radiation histories were consistent with those reported previously. Measurements of the elemental compositions of quenched particles showed substantial amounts of dissolved oxygen, consistent with recent metal particle combustion studies using the same experimental technique. An estimate based on the rate constants reported in the literature for the magnesium vapor-phase burning, showed that the reaction rate is insufficiently fast to prevent oxygen gas diffusion through the vapor-phase stand-off flame to the particle surface. A combustion mechanism is discussed in which oxygen approaching the surface of a burning particle is dissolved in magnesium producing a new liquid-phase Mg-O solution. The heterogeneous reaction of oxygen dissolution begins simultaneously with the vapor-phase combustion and later on the heterogeneous reaction becomes the primary combustion mechanism. After the solution becomes saturated, an exothermic phase transformation, Mg-O solution → Mg + MgO, occurs that contributes to the production of large amounts of oxide detected on the surfaces of partially burned particles and causes the observed oscillations and peaks in particle radiation.

Multiple seismic discontinuities near the base of the transition zone in the Earth's mantle

The seismologically defined boundary between the transition zone in the Earth's mantle (410-660 km depth) and the underlying lower mantle is generally interpreted to result from the breakdown of the gamma-spinel phase of olivine to magnesium-perovskite and magnesiowustite. Laboratory measurements of these transformations of olivine have determined that the phase boundary has a negative Clapeyron slope and does indeed occur near pressures corresponding to the base of the transition zone. But a computational study has indicated that, because of the presence of garnet minerals, multiple seismic discontinuities might exist near a depth of 660 km (ref. 4), which would alter the simple negative correlation of changes in temperature with changes in the depth of the phase boundary. In particular, garnet minerals undergo exothermic transformations near this depth, acting to complicate the phase relations and possibly effecting mantle convection processes in some regions. Here we present seismic evidence that supports the existence of such multiple transitions near a depth of 660 km beneath southern California. The observations are consistent with having been generated by garnet transformations coupling with the dissociation of the gamma-spinel phase of olivine. Temperature anomalies calculated from the imaged discontinuity depths--using Clapeyron slopes determined for the various transformations--generally match those predicted from an independent P-wave velocity model of the region.

A comparison of the various thermal treatments of chemically deposited bismuth sulfide thin films and the effect on the structural and electrical properties

The physicochemical transformations of chemically precipitated bismuth sulfide powder have been studied by differential scanning calorimetry (DSC) and X-ray diffraction analysis (XRD). The structural changes have been correlated to the electrical properties of chemically deposited bismuth sulfide thin films annealed at different temperatures ( T) and pressures ( P t). Two irreversible exothermic transformations are observed at 210 and 253°C in the powder, which correspond to the crystallization of Bi 2S 3, and to the crystallization and decomposition of local defects and their reaction to form Bi 2S 3. As in the case of the precipitate, the conductivity of chemically deposited bismuth sulfide thin films upon annealing is due to the formation of multicomponent phases assisted by further sulfur loss. The complex mechanism that enhances the film conductivity shows a compensating effect between P t and T. The temperature and pressure regime at which conductive films were obtained with Bi 2S 3 as the only component of the crystalline phase were: 190°C under vacuum, and 250°C in the P t range of 10–1000 mbar. Under these conditions, the dark conductivity and photoconductivity of the films are in the range of 10 −2 to 3 Ω −1 cm −1, mainly determined by the charge transport through Bi 2S 3 crystals.

A note on latent heat release from disequilibrium phase transformations and deep seismogenesis

Latent heat release by equilibrium mineralogical transformations in an adiabatically subducting slab reversibly perturbs temperatures and pressures so as to conserve entropy. However, latent heats of metastable transformations in such a slab yield irreversible isobaric temperature changes which increase entropy despite adiabatic constraints. As a result, latent heat release by metastable exothermic transformations can yield local superheating above the background adiabat, with the degree of potential superheating increasing with extent of metastable overstep. In real slabs, however, regions of metastably persisting low-pressure phases should undergo conductive warming from surrounding transformed material. Such warming should proceed more rapidly than warming of the bulk slab from the surrounding mantle, and the resulting decrease in metastable transition pressures will slightly decrease the degree of local superheating. Nonetheless, such local temperature increases may trigger seismic release of accumulated strain energy via a number of proposed mechanisms of shear instability. Adiabatic instability, in the form of shear localization in material with temperature-dependent rheology, is one mechanism which may be triggered by such latent heat release in metastable regions yet produce rupture that extends beyond the boundaries of such regions.

On exothermic transformation from metastable gel phase to stable crystalline phase of fully hydrated dimyristoylphosphatidylethanolamine in heating scan

When a fully hydrated dimyristoylphosphatidylethanolamine (DMPE) is heated from −50°C with a slow scanning rate (∼0.1°C/min), an exothermic transition peak is observed at around 47–48°C by differential scanning calorimetry (DSC). This paper deals with the structural change associated with this exothermic transition in a slow heating scan (0.2°C/min). Simultaneous X-ray diffraction and DSC measurements showed that at the exothermic transition, the fully hydrated DMPE cooled to −50°C converts directly from a metastable gel phase to a stable crystalline phase without the appearance of any intermediate phases. This result suggests that a following process occurs: (1) nuclei emerge partly on cooling to −50°C; (2) the nuclei remain unchanged during slow heating until about 43°C; and (3) the nucleation growth becomes active above ∼43°C.

Metastable Nickel Sulfides with Composition Close to Ni7S6—Stability and Structural Properties

Four well-defined metastable phases with composition close to Ni7S6are described. These phases are structurally closely related to the high-temperature phase Ni7±xS6, which is formed eutectoidally from Ni9S8and Ni3S2at ∼675 K. The relative stability and a preliminary structural description of the metastable phases have been obtained from X-ray diffraction, transmission electron microscopy, calorimetry, and thermal analysis. Conditions for preparation of the different metastable phases are provided. On rapid quenching of Ni7±xS6in ice + water + NaCl, an as-quenched state is obtained. This state is rather unstable and transforms on storing at room temperature to the metastable phase denoted αivand subsequently to αiii. The actual phase sequence observed during heating of these modifications depends strongly on the thermal history of the sample and especially on the heating rate used in the experiments. Either of the two quasi-reversible phase sequences αiv⇔ αii⇔ αi⇔ Ni7±xS6or αiii⇔ αi⇔ Ni7±xS6is observed, where αi, αii, αiii, and αivdenote the four well-defined metastable phases. The as-quenched samples appear rather similar to αi. Furthermore, an exothermal phase transformation is observed for αiii. It has not, however, been possible to relate this transformation to structural changes.

The Effect of Heat Generation on the Development of Vermicular α-Al 2O3 in Alumina Films

AbstractThe transformation of transition aluminas (i.e. θ-A12O3) to α-A12O3 results in an interconnected, porous microstructure that is commonly described as ‘wormy’ or‘vermicular’. The effect of heat generated during the exothermic transformation at the θ-Al2O3/α-Al2O3 interface on the development of the vermicular α-A12O3 morphology was analyzed in boehmite-derived thin films. The vermicular structure may arise in an analogous manner to the thermal dendrites formed in metals during solidification. However, the analysis shows that heat generation and transfer at the interface does not play a significant role in the development of α-A12O3 dendrites.