Transformation Front Research Articles

Banded solidification microstructures

Banded microstructures are composed of alternate structures or phases which develop mostly parallel to the transformation front. At low growth velocities, bands of the same microstructure but with different scales form through periodic fluctuations of the solidification system. On the other hand, banding can occur as a transformation microstructure when the growth front becomes unstable to oscillations. This instability is either due, at low velocities, to nucleation of another phase (peritectics) or, at high velocities, to nonequilibrium effects at the interface which lead to periodic changes of the microstructure. In this article, the inherent banded patterns of low velocity peritectic solidification and high velocity nonequilibrium solidification will be presented and their origin will be discussed.

High-resolution andin-situtransmission electron microscopy investigation of the martensitic fcc ↔ dhcp phase transformations in Co-Fe single crystals

Abstract The thermally induced fcc↔dhcp martensitic phase transformations were studied in a Co-5.75 at.% Fe single crystal. Transmission electron microscopy (TEM) methods were applied including in-situ experiments; lattice fringe and high-resolution transmission electron microscopy (HRTEM) images were analysed. At room temperature the as-grown single crystal was in the dhcp low-temperature phase and dhcp lamellae with an average thickness of 10–11nm were observed that are separated by very thin fcc lamellae of retained austenite. The shape of the transformation fronts and the lack of long-range strains indicate that the Shockley Burgers vectors of the partial dislocations occurring at the fronts are compensating each other to a large extent. The reverse dhcph →fcc transformation was studied during in-situ heating. HRTEM images were used to analyse the partials located at the fronts of the transformation by drawing Burgers circuits around them. In a HRTEM image (with a [1210]dhcp ≡ [110]fcc beam direction)...

In Situ Tem Analysis of TiSi2 C49-C54 Transformations During Annealing

AbstractIn situ transmission electron microscope (TEM) observations of TiSi2 C49-to-C54 phase transformations were recorded on video tape and photographic negatives, and transformation front velocities (vTF) were measured from the data. The samples studied in this work include: C49-TiSi2 blanket films and 0.2 μm wide, 7 mm long lines on undoped poly-Si and 0.13 μm wide, 10 μm long lines on either B- or As-doped poly-Si. The in situ TEM analysis showed that blanket TiSi2 films on undoped poly-Si fully transformed at 830°C and had an average vTF of 0.5 ± 0.2 μm/s. The transformation occurred from a sparse nucleation density of ˜0.1 site/μm2. The 0.2 μm wide lines transformed at temperatures greater than 885°C, and the average vTF was 1.1 ± 0.2 μm/sec. Agglomeration started for both the blanket film and the 0.2 μm wide lines at temperatures above 900°C. In situ x-ray diffraction (XRD) analyses of C49-TiSi2 on B- and As-doped poly-Si showed that blanket films completely transformed to C54-TiSi2 at T = 835–843°C, but 0.13 μm wide, varying length lines did not fully transform, even when rapid thermal annealed to 1025°C. From in situ TEM analysis of 0.13 μm wide, 10 μm long C49-TiSi2 lines on B-doped poly-Si, a distinct transformation was not observed. Instead, the lines slowly agglomerated, and electron diffraction of the agglomerated regions showed that the film had transformed to C54-TiSi2. An individual line either 1) completely transformed and agglomerated or 2) remained as C49-TiSi2 and did not agglomerate. Approximately 85% of all lines transformed. In situ TEM analysis of 0.13 μm wide, 10 μm long lines of C49-TiSi2 on As-doped poly-Si also showed an indistinct transformation to the C54 phase along with agglomeration, and 100% of the lines transformed.

Singularities of the propagation of shot waves on the plane

The subject of this paper is the geometric optics of linear shot waves on the plane. We describe the transformation fronts and scattering of rays when the light hypersurface has conic singularities. They appear if the waves propagate in a non-homogeneous anisotropic medium subject to the hyperbolic variational principle.

Pressure-Induced Structural Transformation in Cristobalite Studied by Molecular Dynamic Simulations

ABSTRACTThe phase transformation between the α- and β-cristobalite modifications of SiO2 were studied using molecular dynamic simulations. The transformation was induced isothermally by control of the pressure within the structure, either through an externally applied constant stress or by changing the simulation box volume. The atomic scale mechanisms of the transformation can best be observed by means of a time-correlation function describing the spatial orientation of the planes containing the Si-O-Si bonds. These planes rotate by 90° over the course of the transition. Both, the bulk modulus, calculated for static structures, as well as the vibrational spectra of the Si-O-Si planes, reflect the softening of acoustic modes in the midst of the transition, which is characteristic of displacive phase transformations. Although the aperiodic shift of atomic positions upon passage of the transformation front could be responsible for a momentary softening of the structure, this is not the only reason, since this behavior persists when maintaining the structure at intermediate densities, which corresponds neither to α-, nor to β- cristobalite.

Metallography of soldered lead and lead alloys: The case of extruded alloys

The mixture of three parts acetic acid and one part hydrogen peroxide is probably the most widely used chemical polishing agent for lead and lead alloys. In certain conditions the structure is sufficiently revealed that additional attack is not necessary. However, lead alloys are subject to structural changes (such as aging and recrystallization) at room temperature. We have previously described a technique of electrolytic polishing at −50°C followed by repeated chemical etching with an appropriate reagent at 20°C. The applications of this technique are various: observation of the not-yet-transformed structure, determination of the transformation sites (intercrystalline or intracrystalline), movement of transformation fronts along specific crystallographic planes, measurement of the transformation front movement at different temperatures, and estimation of the transformed volume and of the activation energy associated with the transformation process. Classical soldering lines between two lead alloys of a different kind or between two alloys of the same kind can also be observed using this technique. However, the technique is not suitable in other cases, such as solderings obtained by extrusion. A new technique of deep dissolution has now been developed that allows us to reveal the soldered extruded zones. It consists of chemical dissolution with a mixture of one part acetic acid and three parts hydrogen peroxide.

Electrically induced uniform planar alignment of S*C liquid crystals on homeotropically treated plates

Chevron textures of SC* films are usually obtained with the conventional procedure where a planar alignment coating (e.g., buffed polyimide) is used. We describe a new technique which enables us to obtain uniform bookshelf textures which show bistability and can be switched with low fields (∼1 V/μm). This is achieved with a homeotropic surface coating. With a homeotropic coating the samples form a uniform texture with horizontal layers on cooling. The homeotropic texture becomes unstable under sufficiently strong electric fields and transforms to a birefringent texture with smectic layers oriented vertically. Without an additional bias the texture has domains of different layer orientations. By simultaneous application of a vertical field and horizontal shear stable uniform bookshelf textures can be obtained. Here we show that it is possible to obtain uniform bookshelf alignment by the simultaneous action of a horizontal field gradient and ac fields. The horizontal gradient leads to a more or less well-defined propagating front of the texture transformation. It results in a biased domain texture with vertical layers. The effect of the strong ac field (∼100 Hz) is to smooth out the domain texture.

Thermodynamic Analysis of Isothermal Transformations of Hypo-eutectoid Fe-C Austenites

The thermodynamics of decomposition reactions of undercooled austenite in Fe-C alloys were studied using a regular solution sublattice model for austenite and ferrite and taking cementite as a stoichiometric compound. From a Gibbs free energy point of view intermediate transformations, not leading to stable phases, are possible: carbide formation above the A 1 temperature, and carbon enrichment in austenite below the A 1 temperature. The differences in chemical potential of Fe and of C between both sides of the transformation front were calculated as a function of the degree of transformation and related to the kinetics of transformation interface progress and carbon rearrangement, respectively

Can sharp seismic discontinuities be caused by non-equilibrium phase transformations?

A long-posed problem of the seismic discontinuities at the depths of 410 and 660 km in Earth's mantle is whether these discontinuities are caused by phase transitions alone or together with chemical layering. Among several tests, the sharpness of the boundary is one of the most crucial. Recent data suggest that the transition region thickness is less than 4 km for both the 410 and the 660 km discontinuities. This may be smaller than predicted by phase equilibria. We suggest that if there is a sufficiently large nucleation barrier for phase transformation and if the transformation front cannot move faster than the convective flow, the transformation in rising or descending material does not take place until the metastable overshoot becomes equal to the nucleation barrier. An avalanche-like transformation following this overshoot occurs in a very narrow region and can be the cause of a sharp seismic discontinuity. In this case, the topography of the phase boundary is also substantially modified.

Quantitative Hrem of Electron-Irradiated Quartz

AbstractThe progress of heterogeneous amorphization of electron-irradiated quartz was studied in situ by high-resolution electron microscopy (HREM). The transformation front appears surprisingly sharp when viewed close to edge-on, but the apparent morphological evolution when aperiodic and periodic material overlap in projection is one in which gradual changes in image regularity are observed. Electron-microscopical contrast obtained in HREM irradiations was interpreted quantitatively, through the use of image contrast simulations. When the amorphized portion occupies less than about two-thirds of the total thickness, the HREM images continue to appear crystalline. Only when the amorphized thickness fraction exceeds about two-thirds does the resultant image contrast show mainly non-crystalline features.

Phase transformation and dehydration of calcium sulphate dihydrate in solution studied by SEM

The transformation of separate crystals of CaSO4·2H2O (DH) into CaSO4·O.5H2O (HH) in an aqueous solution of H3PO4 and H2SO4 at 90–97°C was studied using a JSM-35CF SEM. The source DH and acids were reagent or commercial (taken from the production of H3PO4 by a wet process). DH was allowed to convert into HH and periodic samples of the suspension were drawn, filtered, and investigated in the solid phase by SEM. It was shown that the dehydration of reagent DH specimens occurred through the recrystallization mechanism, whereas the transformation of commercial DH into HH was basically topochemical and developed by a relay race mechanism without any phase transformation front. The observed difference between commercial and reagent DH specimens may be explained by differences in their crystalline structure, initial particle size and by the presence of impurities in DH and acids. It is established that chemical impurities can change the mechanism of the phase transformation of the crystals DH into HH.

On the origin of recrystallization textures in aluminium

The electron back scattering pattern technique (EBSP) in SEM has been applied to follow in detail the orientation aspects of the nucleation and growth of recrystallization in cold rolled aluminium. The investigation includes both high purity metal and a commercial grade. The cube- and Goss-oriented recrystallized grains nucleate from transition bands. In the cube case these bands are found in the Cu- and ND-rotated-Cu deformation texture components while the Goss bands have been identified in the brass texture component. Shear bands in the S-deformation component have been identified as nucleation sites for recrystallization. The orientation inside the bands is also S (complementary to the matrix), causing the S-orientation to reappear as a recrystallization texture component. These types of deformation heterogeneities and others are frequently associated with a 40°C 〈111〉 orientation relationship to the surrounding matrix, making this specific orientation relationship primarily a property to be associated with the oriented nucleation concept. Isolated examples of a true 40° 〈111〉 growth selection due to the rapid growth of 〈111〉-tilt-boundaries have been observed. A characteristic aspect in these cases is a growth selection from a planar transformation front. From a careful literature review as well as from experimental observations no direct evidence has been found in support of the hypothesis that 40° 〈111〉 grains, when uniformly distributed in space, have a transformation rate potential exceeding that of grains belonging to other texture components. No growth selection has been found in the special case of the growth of cube oriented grains.

A transmission electron microscopy study of the β→α-phase transformation of tin

The β→α-phase transformation of pure tin has been studied using transmission electron microscopy. A preferred orientational relationship was found between the two phases, with the β[001] being parallel to the α[110]. The transformation front did not proceed into regions of foil below a critical thickness and this may have been the result of surface energy differences between the two phases. The general absence of a specific three-dimensional orientational relationship and observations regarding the nature of the interface suggest strongly that the transformation is massive in nature.

High Resolution Electron Microscopy Of Intercalated Phases In The Y-Ba-Cu-0 System

ABSTRACTHigh resolution transmission electron microscopy (HRTEM) is used to investigate the transformation mechanism responsible for the occurrence of the many stable layered phases in the pseudo-binary Y2Ba4Cu6+xO14+x system. In particular, HRTEM results are compared with microstructural configurations generated by an earlier theoretical study which modeled this transformation by means of an intercalation mechanism. HRTEM images of partially transformed Y-Ba-Cu-O reveal that the predominant structural change is limited to the CuO planes. The location of the transformation front, revealed in the micrographs by the presence of these partial dislocations, suggests that the transformation is nucleated at grain boundaries and internal surfaces within the material. The observation of local CuO layer ordering along the [001] direction is taken as evidence for the presence of longer range interactions. A two-dimensional approximation to the bulk Monte Carlo simulation incorporating long range screened Coulomb [001] interactions is used to investigate the possibility and nature of local, transient stage orderings.

The amorphous-to-gamma transformation in ion implanted Al 2O 3

Amorphous surface layers, free of implanted impurities, were produced on single crystal α-Al 2O 3 substrates by implantation at − 185 °C with 2 × 10 16 Al/cm 2 at 90 keV and 3 × 10 16 O/cm 2 at 55 keV. The response of these amorphous layers to post-implantation annealing in the temperature range from 700 to 800 ° C was investigated by in situ transmission electron microscopy (TEM) and conventional TEM of specimens annealed in bulk form. It was found that the amorphous Al 2O 3 transformed to the cubic transitional γ phase along an irregular front. The recrystallized material is columnar with the individual columns or domains being irregular in size and shape. The γ-Al 2O 3 is composed of two variants which are twin related and have the following orientation relationship with the substrate: 〈110〉) γ∥〈101̄0〉 α and {111} γ∥{0001} α. The temperature-dependent velocity of the amorphous-to-γ transformation front was measured by in situ TEM. Assuming a thermally activated process, the activation energy has been determined from an Arrhenius plot of velocity versus T −1. These results are discussed in terms of possible rate controlling processes of the transformation.

Signal transmission and covert prepattern in the metamorphosis of Hydractinia echinata (Hydrozoa).

Planulae are simply structured larvae lacking an overt longitudinal organization. In the course of a rapid metamorphosis, however, they transform into polyps, which display striking structural patterns. Metamorphosis takes place only in response to external stimuli. Surgical removal and transplantation of larval parts reveal that external stimuli, including artificial inducers such as cesium ions, tumor promoters and diacylglycerol, act on the anterior quarter of the larva where sensory cells containing Arg-Phe-amide-like peptides are located. The external stimuli initiate the release of an internal signal, which is transmitted to the posterior end causing the successive transformation of larval into adult tissue. The transformation front moves from the anterior to the posterior quarter in 60 min. The internal signal can be released or bypassed by a transitory lowering of the Mg2+ content of the seawater. By using this procedure, or by administering an extract containing the putative internal signal substance, each isolated part of the larva can be induced to metamorphose separately. Provided there is no time for regeneration after cutting before metamorphosis is initiated, the most anterior fragment forms only stolons, the most posterior fragment forms only a head. The overt pattern of the polyp is, therefore, generated under the influence of a covert anterior-posterior prepattern of the larva.

In situ observations of the amorphous-to-gamma transformation in ion-implanted Al2O3

Transmission electron microscopy (TEM) was used in an investigation of the recrystallization behavior of amorphous surface layers on single crystal [0001] α-Al2O3. Conventional imaging and diffraction techniques were used to characterize the microstructures produced by annealing bulk specimens in the temperature range from 800°C to 1200°C. It was found that the amorphous Al2O3 first transforms to γ-Al2O3, a cubic transitional form, which in turn transforms to α-Al2O3. The γ to α transformation begins at the original crystalline-amorphous interface and proceeds toward the specimen surface. By measuring the position of the γ-α interface as a function of annealing temperature and time it was possible to determine the temperature-dependent velocity of the γ-α transformation front. Assuming a thermally activated process, the data are consistent with an activation energy of 3.6 eV for the transformation. However, no amorphous material remained, after even the shortest anneals (15 min) at the lowest temperature (800°C). These results indicate that the kinetics of the amorphous to γ transformation are faster than those of the γ to α transformation. Therefore a series of in-situ experiments was carried out to investigate the transformation of amorphous Al2O3 to γ-Al2O3. Amorphous Al2O3 surface layers, free of implanted impurities, were produced by implantation at −185°C with 2 × 1016 Al/cm2 at 90 keV and 3 × 1016 O/cm2 at 55 keV. Cross-sectioned specimens were prepared following established techniques.2 The specimens were annealed in-situ in a Philips CM12 operating at 120 kV with a Gatan Model 628 single tilt heating holder. Progress of the transformations was monitored with a Gatan model 622 video system.

Investigation of the kinetics of laser heating of Fe–C alloys by a magnetic induction method

A new method was developed for investigating the kinetics of laser heating of ferromagnetic alloys based on determination of the change in the magnetic conductance in the heat treatment zone. The lifetime of the paramagnetic phase and the velocity of the ferromagnetic-paramagnetic transformation front were determined.

INVARIANT PROPERTIES OF ABSORPTION PROFILES IN SPORANGIOPHORES OF Phycomyces blakesleeanus UNDER BALANCING BILATERAL ILLUMINATION

Abstract— Absorption spectra and spectra of the refractive indices were separately measured for cytoplasm and vacuole of Phycomyces blakesleeanus in the wavelengths domains 300 to 750 nm and 250 to 750 nm respectively. These data were then used to simulate absorption profiles for different orientations of the photoreceptor for measured action spectra for phototropic balance. The photoreceptor was assumed to be a flavin. Using the reference wavelengths 394, 450 and 507 nm as an example it is shown that the absorption profiles have, independent of the photoreceptor orientation, invariant properties. Under bilateral balancing illumination the symmetry of the absorption profiles is conserved and the total amount of energy absorbed at proximal and distal sides is an invariant under a symmetry transformation front to the rear side.

Mathematical model of the thermal processing of steel ingots: Part II. Stress model

A mathematical model has been developed to predict the internal stresses generated in a steel ingot during thermal processing. The thermal history of the ingot has been predicted by a finite-element, heat-flow model, the subject of the first part of this two-part paper, which serves as input to the stress model. The stress model has been formulated for a two-dimensional transverse plane at mid-height of the ingot and is a transient, elasto-viscoplastic, finite-element analysis of the thermal stress field. Salient features of the model include the incorporation of time-temperature and temperature-dependent mechanical properties, and volume changes associated with nonequilibrium phase transformation. Model predictions demonstrate that the development of internal stresses in the ingot during thermal processing can be directly linked to the progress of the phase transformation front. Moreover, the low strain levels calculated indicate that metallurgical embrittlement must be very important to the formation of cracks in addition to the development of high tensile stresses.