Stages Of Phase Transformation Research Articles

Ageing characteristics of cast Zn-Al based alloy (ZnAl7Cu3)

Microstructure and ageing characteristics of a cast Zn-Al based alloy (ZnAl7Cu3) were studied using X-ray diffraction, electron scanning microscopy and back-scattered diffraction techniques. Two stages of phase transformation, i.e., decomposition of zinc rich η phase and four phase transformation, α + e → T′ + η, were detected during ageing at 150°C. Electron back-scattered diffraction technique was applied in distinguishing both zinc rich η and e phases.

Kinetics of Solvent-Induced Crystallization of Poly(ethylene terephthalate) at the Final Stage

The quantitative analysis of Fourier-transform infrared (FTIR) spectra was employed to elucidate the kinetics of solvent-induced crystallization (SINC) in poly(ethylene terephthalate) (PET). A previous study on the isothermal crystallization of PET by Koenig et al. showed that the crystallization process could be divided into two stages, the first followed a sigmoidal increase in crystallinity (or the primary crystallization) and the second exhibiting a linear increase in crystallinity (or the secondary crystallization). In the present study, multiple stages of phase transformation were found in the solvent-induced crystallization. The kinetics of the secondary crystallization was studied in detail. Finally, the variation of morphology of PET during crystallization was also discussed in terms of structure observed by an atomic force microscope.

Kinetics of L10-type and L12-type orderings in alloys atearly stages of phase transformations

We study the kinetics of the L10 ordering in face-centred alloysat early stages of phase transformations, when the tetragonaldistortion of the L10 phase is still insignificant as regards themicrostructural evolution, and compare it with the kinetics of theL12 ordering. We use the earlier-described kinetic cluster-fieldmethod to simulate A1→L10 and A1→L12transformations after a quench of an alloy from the disordered A1phase to the single-phase L10 or L12 state for a number ofalloy models with both short-range and extended-range interactions.The simulations reveal a number of peculiar features in bothtransient microstructures and transformation kinetics - inparticular, a sharp dependence of the microstructural evolution andthe structure of antiphase boundaries under the L10 ordering onthe composition of the alloy; the importance for both the L10 andL12 ordering of a peculiar mechanism of domain growth via thefusion of different in-phase domains; the presence in transientmicrostructures under the L10 ordering of some specificlong-living configurations, quadruple junctions of four differentantiphase domains, as well as of peculiar kinetic processes of`splitting' of certain antiphase boundaries into two others; andother features.

Energy consideration for variant selection in precipitation on dislocations

Preferential precipitation on dislocations has been discussed by considering interaction energy between transformation strains to change the crystal structure and stress fields around dislocations. For quantitative calculation, precipitation of the hcp a phase from the bcc b matrix in a Ti alloy is analyzed. A previous experiment shows that a certain variant of precipitates among crystallographically equivalent ones was formed preferentially on a given dislocation. The present analysis is successful in explaining such variant selection. Comparison with other studies has revealed that the stress always plays an important role in the early stage of phase transformation to change the crystal structure.

Phase Transformations in Iron Fischer–Tropsch Catalysts during Temperature-Programmed Reduction

Temperature-programmed reduction using both carbon monoxide (CO-TPR) and hydrogen (H2-TPR) was used to study the phase transformations in iron catalysts. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) allowed us to follow the phase transformations in these iron catalysts during TPR. Two catalysts were used, a model supported catalyst with a nonporous silica support and a precipitated catalyst with no support. Under identical CO-TPR conditions, the supported and unsupported iron catalysts behaved very differently. For the supported catalyst, three stages of phase transformation could be identified: hematite to magnetite, magnetite to iron carbide, and in the third stage significant carbon deposition accompanying further carburization. No carbide formation or significant carbon deposition was observed in the unsupported Fe catalyst, due to the presence of S impurities in the unsupported catalyst. Most importantly, the results show that carbon deposition occurs in parallel with carbide formation, no carbon being seen in the catalyst that was not carbided. In these catalysts, copper facilitates the reduction of iron oxide, especially the transformation of hematite to magnetite. Segregation of copper from iron oxide causes the hematite to magnetite transformation to shift to higher temperatures. No interfacial Fe–SiO2 phases could be detected during both CO-TPR and H2-TPR. The major role of the silica support is to prevent the sintering of the Fe phases.

Effect of α-Si3N4 initial powder size on the microstructural evolution and phase transformation during sintering of Si3N4 ceramics

By using the Si3N4 ceramic specimens prepared with fine and coarse α-Si3N4 powders, respectively, the phase transformation from α- to β-Si3N4 and concurrent microstructural evolution during sintering were monitored. For the compact prepared with fine powder, the α/β transformation was completed much earlier than the coarse powder. The higher fraction of pre-existing β-grains in fine powder and its higher reactivity compared to those of coarse one are likely to cause a rapid phase transformation. The growth rate of β-Si3N4 grains at the expense of α-Si3N4 during phase transformation stage was quite significant while that after they impinge each other was very limited. As a result, the specimens prepared with coarse and fine initial α-Si3N4 powders resulted in coarse and fine grained β-Si3N4 ceramics, respectively. The specimen prepared with mixture of fine and coarse α-Si3N4 powders exhibited the microstructure containing a few elongated large grains and showed an increased value of fracture toughness.

Early stages of phase transformation in quenched zinc–aluminum based alloys

X-ray diffraction, optical and electron microscopic techniques were used to investigate the early stages of ageing in supersaturated Zn–Al based quaternary alloys. Discontinuous precipitation was observed to occur by means of a cellular reaction in both monotectoid and eutectoid Zn–Al based alloys. The cellular reaction was correlated with an equilibrium phase transformation that took place at about 276°C.

Effect of heat treatment on the magnetic compensation state of amorphous Gd–Co and layered Gd/Co films

Abstract The influence of heat treatment on the magnetic properties of amorphous Gd 22 Co 78 and multilayered {Gd(75 A)/Cu(L Cu )/Co(30 A)/Cu(L Cu )} 20 films close to the magnetic compensation state has been investigated. It is found that annealing of amorphous films at temperatures up to 250°C results in the disappearance of perpendicular anisotropy and magnetic compensation. The heat treatment of the multilayered films leads to an increase of the compensation temperature, which is less pronounced in the samples with Cu layers. These changes are attributed to initial stages of phase transformations in amorphous films and to interlayer diffusion in the multilayered films.

A self-consistent model for the stress–strain behavior of shape-memory alloy polycrystals

In spite of the recent progress in the development of constitutive theory for shape-memory alloys (SMAs), no micromechanical model has been developed to calculate the stress–strain behavior of a SMA polycrystal from the behavior of its constituent grains. A self-consistent model is developed for such a purpose. Using a recently developed micromechanical theory to describe the behavior of single crystals (Lu and Weng, J. Mech. Phys. Solids, 1997, 45, 1905), the model provides a self-consistent relation to connect the stress and phase transformation strain on the grain level with those on the polycrystal level. This relation allows the phase transformation strain in each grain to be calculated in accordance with its stress, and then, by an orientational averaging process, the overall phase transformation strain of the polycrystal is determined. To further test the validity of the single crystal theory, a micromechanical, crystallographic procedure is developed to calculate the anisotropic phase transformation strains of a Ni–Ti crystal under various directions of tensile loading; the computed results were found to be in general accord with the experimentally measured values. The self-consistent model is then applied to derive the stress–strain behavior of a Ni–Ti polycrystal at three different levels of temperature, first above A f, then between A f and A s, and finally between A s and M s, to illustrate its pseudoelastic behavior, shape-memory effect and ferroelasticity, respectively. In each case the theoretical phase transformation strain of the polycrystal was in very good agreement with experiments. The “work-hardening” nature of the stress–strain curve at the later stage of phase transformation is also vividly displayed by both theory and experiment.

Mechanism of solid–solid resolution of pantolactone

The enantioselective solid–solid clathration of (S)-pantolactone (1) into (R,R)-trans-2,3-bis(diphenylhydroxymethyl)-1,4-dioxaspiro[4·4]nonane (2) and -[4·5]decane (3) was studied preparatively and mechanistically using atomic force microscopy (AFM) measurements and crystal packing data. Short-distance solid-to-solid sublimation mechanisms occur in both cases with initial formation of epitaxial floes along the b-axis in 2 and random craters in 3. The bulk control in the phase rebuilding stage is understood from the particular crystal packings. Bulk control was largely lost in the phase transformation stage when passivation of the surfaces occurred in both cases. The host lattices of 2 and 3 exhibit closed molecular double layers with all hydroxyl groups inside and lacking access for intercalation of 1. The reaction has to start at isolated surface defects. Once started, new doors for further entrance of guest molecules (S)-1 are opened in the double layer. For practical use, both the crystallographic difficulties and the passivation have to be overcome. A slurry technique involving addition of stoichiometric amounts of water to the solids provided useful conditions for the preparative resolution of (R/S)-1 or further racemates exhibiting passivation, whereas heating alone speeded up the resolution of 2-methylpiperazine with 2. © 1996 John Wiley & Sons, Ltd.

Mechanism of solid-solid resolution of pantolactone

The enantioselective solid–solid clathration of (S)-pantolactone (1) into (R,R)-trans-2,3-bis(diphenylhydroxymethyl)-1,4-dioxaspiro[4·4]nonane (2) and -[4·5]decane (3) was studied preparatively and mechanistically using atomic force microscopy (AFM) measurements and crystal packing data. Short-distance solid-to-solid sublimation mechanisms occur in both cases with initial formation of epitaxial floes along the b-axis in 2 and random craters in 3. The bulk control in the phase rebuilding stage is understood from the particular crystal packings. Bulk control was largely lost in the phase transformation stage when passivation of the surfaces occurred in both cases. The host lattices of 2 and 3 exhibit closed molecular double layers with all hydroxyl groups inside and lacking access for intercalation of 1. The reaction has to start at isolated surface defects. Once started, new doors for further entrance of guest molecules (S)-1 are opened in the double layer. For practical use, both the crystallographic difficulties and the passivation have to be overcome. A slurry technique involving addition of stoichiometric amounts of water to the solids provided useful conditions for the preparative resolution of (R/S)-1 or further racemates exhibiting passivation, whereas heating alone speeded up the resolution of 2-methylpiperazine with 2. © 1996 John Wiley & Sons, Ltd.

New dimensions in atom-probe analysis

A new class of atom-probe instruments has recently been developed which combine single atom sensitivity mass spectrometry with position sensing, and have the potential of reconstructing nanometre-scale composition variations in three-dimensions. These techniques will be very powerful in the study of the early stages of phase transformations, and surface and interface segregation. Some recent results obtained with the first of these instruments, the position-sensitive atom probe, are described. Difficulties arise in this instrument when more than one ion is evaporated from the analysis area on a single field evaporation pulse, which can limit the accuracy of analysis performed on the resultant data. Other three-dimensional atom-probe instruments currently under development are described, and their expected performance in this respect is discussed. The significance of defining composition on the sub-nanometre scale is addressed, and the importance of atomic-scale modelling of the phenomena under investigation is indicated.

Formation of Palladium Silicide and Germanide in the Pd/a-Si/Si., Pd/a-Ge/Si, and Pd/a-GeSi/Si Systems During Thermal Treatment

AbstractX-ray diffraction and transmission electron microscopy have been used to study the kinetics of phase transformations and the structure of Pd/a-Si, Pd/a-Ge and Pd/a-GeSi thin films deposited on Si substrates. Different kinds of amorphouis structures were used: a-Si:H:D, a-Si.:F, a-Ge:H:D, and a-GeSi:H:D. The first stage of phase transformation during heat treatment was palladium silicide (Pd2Si) and palladium germanide (Pd2Ge) formation at temperatures above 200°C. Annealing studies demonstrated that the presence of F in a-Si promotes the Pd2Si formation. The study of the Pd2Si crystallization process showed that: a) when the Pd layer and the a-Si layer are thin, then c-PdSi grows in a fractal-]ike form; b) when the Pd and a-Si both are thick, then c-Pd2Si grows in a globular structure; c) in both above mentioned cases a well-oriented [0011 texture forms. The growth of the silicide and germanide layers in the temperature range of 200-300°C was found to be controlled by a diffusion limited process. It was found that c-Pd2Ge transforms to c-PdGe above 200°C. The a-Ge,.,Si,. 5 alloy behaved similarly to a-Si forming only [001] textured c-Pd2(Ge,Si).

The Disordering Phase Transformation in (Ni70Fe30)3(V98-xAlxTi2) Alloys with 0 < × < 80

ABSTRACTThe sequence of disordering transformation processes in the A3B type alloy series (Ni70Fe30)3(V98-xAlxTi2), currently under development for high-temperature structural applications, was studied by differential scanning calorimetry (DSC), x-ray diffraction, optical microscopy, and transmission electron microscopy (TEM). Results of DSC show that in all alloys there are two endothermic stages of phase transformation from the ordered to the disordered state. With increasing x, the disordering transition temperature, Tc, reaches a maximum ∼1000°C at x ≈ 50, and then decreases. Interrupted heating, followed by water quenching, was used to characterize the crystal structure and the microstructure of the intermediate phases. For the x + 20 alloy, TEM observations showed ordered regions of DO22 phase in a matrix of disordered fcc (Al) phase at intermediate temperatures. The ordered domains transformed morphologically into cuboid like regions at higher temperatures. From a combined study by all the techniques, we conclude that in alloys with x between 0 and 20, the sequence of phase transformations upon heating is: DO22 → DO22 + Al → Al, whereas in alloys with x > 40, the major sequence is Ll2 + B2 → Ll2 + Al → Al.

Metastable Au-Si alloy formation induced by ion-beam interface mixing

Abstract An amorphous Au-Si alloy with composition Au-28·5 at. % Si (Au5Si2) has been formed by inplanting energetic Xe, Ar or Ne ions through a thin layer of Au deposited on a Si substrate. The ion-induced amorphous Au-Si alloy is uniform in composition and in thickness as indicated by 4He+ backscattering measurement. The Au-Si alloys so obtained were studied by resistivity measurements and by transmission electron microscopy and diffraction. Two distinct stages of phase transformation are observed as revealed by a sudden change of resistivity with increasing annealing temperature. In the first stage, a transformation from amorphous to a metastable crystalline Au5Si2 phase occurs in a narrow temperature range around 100°C. The second stage, which appears broader near 180°C, involves the transformation to the equilibrium two-phase state of Au and Si. The kinetics of the amorphous to metastable crystalline transformation has been determined by isothermal annealing over the temperature interval 85-101°C. An...

Lattice Image Studies of Ordered Alloys

Lattice imaging is becoming a viable technique for investigating the structure, defects and early stages of phase transformations in ceramics and metals. Our interest lies in its application to the study of ordering in alloys. Conventional images yield little information on the local degree of order in the specimen, the image width of defects may be as great as 50Å, and the interpretation of diffaction patterns and superlattice dark field micrographs is sometimes ambiguous. The advantage of the lattice image is its ability to provide data about atomic environment (from the nature of the lattice fringe profile) and to reveal atomic arrangements near defects.