Allotropic phase transformations induced by ultraviolet power laser irradiation of graphite

Optically induced allotropic phase transformations of carbon were studied. Under irradiation with a laser beam of 337.1 nm wavelength, at an energy density of 1.9 mJ per 0.1 mm², graphite transformed into -carbin, and amorphous carbon-film transformed into rhombohedral graphite with no evidence of high-temperature effects. The transformations differ from the changes occurring due to heating alone. We suggest that the results could be explained by the one-photon excitation and recombination of electrons.     

On modeling of phase transformations in ferroelectric materials

Summary This paper represents an effort to model phase transformations in ferroelectric materials induced by the combined influence of electric fields and thermo-mechanical loading. The implications of thermodynamics principles for the stability of phases exhibited by a ferroelectric material and for the corresponding phase transformations are studied. Some of the theoretical predictions concerned with the dependence of the ferroelectric-paraelectric phase transformation temperature on the applied electric fields and stresses are in qualitative agreement with experimental observation. A ferroelectric material is capable of exhibiting more than one phase for given stresses, applied electric fields and temperature, a transformation among these phases can occur only if it does not result in an increase of the Gibbs energy of the system. In studying polarization reversal, it is found that the difference of the Gibbs energy between the electric twins, i.e. the two ferroelectric phases which share the same polar axis, coincides formally with thedriving traction introduced by Abeyaratne and Knowles [6] in modeling phase transformations induced by thermomechanical loading. An expression for the generalized driving traction, acting on a surface of discontinuity within a continuum in the presence of electromagnetic fields, is presented in this paper.     

Spontaneous phase and morphology transformations of anodized titania nanotubes induced by water at room temperature

We report a spontaneous phase transformation of titania nanotubes induced by water at room temperature, which enables the as-anodized amorphous nanotubes to be crystallized into anatase mesoporous nanowires without any other post-treatments. These mesoporous TiO(2) nanomaterials have a markedly improved surface area, about 5.5 times than that of the as-anodized TiO(2) nanotubes, resulting in a pronounced enhanced photocatalytic activity. The present approach not only allows a flexible control over the morphology of TiO(2) nanostructures but can fundamentally eliminate the need for high temperature operations for crystallizing amorphous TiO(2).     

On the thermodynamic driving force for coherent phase transformations

Using the basic principles of continuum thermodynamics the notion of the thermodynamical driving force for coherent phase transformations is introduced. The new representations of thermodynamical driving force are derived. It is shown that thermodynamical driving force reaches an extremum at any dynamically admissible phase amplitude, provided that two phase solids exhibit instantaneous elasticity. The possible interpretation of macroscopic thermodynamical driving force for martensitic transformation is presented.     

Rapid phase transformations caused by thermodynamic instability in cryogens

The cause of rapid phase transformations observed by several investigators can be explained by the phenomenon of thermodynamic metastability and incipient instability. A liquid cryogen coming into intimate, sudden contact with a substantially warmer host liquid is heated and forms a thin layer of metastable, superheated liquid at the interface between the two. A shock wave is initiated in the metastable layer when the interface temperature is approximately 0.84 times the critical temperature of the cryogen. A heat transfer and thermodynamic model is given that enables us to predict the necessary host liquid temperature that will cause a shock wave for a given cryogen. Experimental results are presented that support the analytical contention, that for several water-cryogen combinations the water temperature necessary to produce a shock wave is about 1.10 times the critical temperature of the cryogen.     

Differential geometry of phase transformations

The foundations have been laid with respect to a generalized theory of phase transformations in solids. In particular, the methods of differential geometry have been employed and such important tensor quantities as distortion, metric, torsion, and anholonomic object have been developed with respect to such transformations. It is further shown that both Riemannian as well as non-Riemannian (dislocation) geometries are needed to describe these transformations properly.     

On pressure-shear plate impact for studying the kinetics of stress-induced phase transformations

Pressure-shear plate impact experiments are proposed for studying the kinetics of stress-induced phase transformations. The purpose of this paper is to determine loading conditions and specimen orientations which can be expected to activate a single habit plane variant parallel to the impact plane, thereby simplifying the study of the kinetics of the transformation through monitoring the wave profiles associated with the propagating phase boundary. The Wechsler-Lieberman-Read phenomenological theory was used to determine habit plane indices and directions of shape deformation for a Cu---Al---Ni shape memory alloy which undergoes a martensitic phase transformation under stress. Elastic waves generated by pressure-shear impact were analyzed for wave propagation in the direction of the normal to a habit plane. A critical resolved shear stress criterion was used to predict variants which are expected to be activated for a range of impact velocities and relative magnitudes of the normal and transverse components of the impact velocity.     

Thermally induced transformations of amorphous carbon nanostructures fabricated by electron beam induced deposition

We studied the thermally induced phase transformations of electron-beam-induced deposited (EBID) amorphous carbon nanostructures by correlating the changes in its morphology with internal microstructure by using combined atomic force microscopy (AFM) and high resolution confocal Raman microscopy. These carbon deposits can be used to create heterogeneous junctions in electronic devices commonly known as carbon-metal interconnects. We compared two basic shapes of EBID deposits: dots/pillars with widths from 50 to 600 nm and heights from 50 to 500 nm and lines with variable heights from 10 to 150 nm but having a constant length of 6 μm. We observed that during thermal annealing, the nanoscale amorphous deposits go through multistage transformation including dehydration and stress-relaxation around 150 °C, dehydrogenation within 150-300 °C, followed by graphitization (>350 °C) and formation of nanocrystalline, highly densified graphitic deposits around 450 °C. The later stage of transformation occurs well below commonly observed graphitization for bulk carbon (600-800 °C). It was observed that the shape of the deposits contribute significantly to the phase transformations. We suggested that this difference is controlled by different contributions from interfacial footprints area. Moreover, the rate of graphitization was different for deposits of different shapes with the lines showing a much stronger dependence of its structure on the density than the dots.     

Some characteristics of first-order phase transformations

The distinction between first-order and second-order phase transformations is brought out. The general characteristics of first-order transformations are described. The salient features of transformations controlled by long-range diffusion, short-range diffusion and by (diffusionless) shear mechanisms are discussed. Martensitic trasnformations, which belong to the shear category, occur in many metallic and non-metallic systems and deserve an intensive study.     

Zirconium phase transformations observed by “in-situ” XRD analysis

The high temperature phase transformations of zirconium were investigated by “in-situ” X-ray diffraction analysis. The experiments were carried out in a high temperature chamber Anton Paar HTK 1200N mounted on powder diffractometer Panalytical X’Pert Pro. The chamber was evacuated by a turbo-molecular pump creating a pressure about 3 × 10⁻⁴ Pa at the temperature 1200 °C before the measurement. The experimental samples were uniformly heated without thermal gradients up to 1000 °C or 1200 °C and then exposed for 180 min. At high temperature four or six diffraction patterns of each sample were recorded for the illustration of the phase changes in the material structure. Chemical composition of some samples after the exposure was determined by secondary ion mass spectrometry and compared with composition of the material in its initial state.     

SEM/EBSD based in situ studies of deformation induced phase transformations in supermartensitic stainless steels

Abstract

Recent year's equipment design has enabled the combination of in situ deformation tests with near real time electron backscatter diffraction (EBSD) mapping of the microstructure evolution in the scanning electron microscope (SEM). The present work involves studies of deformation induced phase transformations in supermartensitic steel containing ～40 vol.-% retained austenite at room temperature. The martensite formation was initiated already at low strains, and increased gradually with increasing plastic strains up to ～10%. It was observed that the martensite formed homogeneously within the microstructure, independent of the crystallographic orientations of the retained austenite. But no new martensite variants, besides those already present in the as received condition, did form during deformation. At the same time, the mutual distribution of these variants remained approximately constant throughout the deformation process.     

Crystallographic features of phase transformations in solids

The paper reviews the current knowledge and understanding of the crystallographic features of phase transformations in solid materials – metals, ceramics and alloys. It covers both of the broad classes of phase transformations in crystalline solids – martensitic or ‘displacive’ and ‘diffusional’ or ‘reconstructive’. The factors that govern the crystallographic features of these two classes of transformations are compared and contrasted. This provides an appropriate basis for examining the ‘diffusional–displacive’ transformations that appear to exhibit the characteristics of both classes. After a brief summary of the considerable body of experimental data available on the crystallographic characteristics of these various types of phase transformation, the different models/theories advanced to account for these observations are discussed. The main emphasis is on those models/theories that are capable of predicting, rather than just rationalising or explaining, these crystallographic features. The review purposely adopts a unifying approach and attempts to reconcile the controversy that has on occasions existed between the ‘displacive’ group and the ‘diffusional’ group – particularly in respect of the ‘diffusional–displacive’ transformation. Developing a comprehensive understanding of the crystallographic features of all classes of phase transformations is obviously the ultimate goal. The review concludes by assessing how close we are to this final achievement, identifies the gaps in current knowledge and suggests future work.     

Compatibility and accommodation in displacive phase transformations

The role of disconnections and lattice-invariant deformation in displacive phase transformations is reviewed, particularly the defect structure of equilibrium habit planes, the mechanism of transformation, and the deformation accompanying growth. This is extended to the 3-D topological modeling of a product embedded in its parent phase. Stress concentrations arise where the interface orientation deviates from the equilibrium habit, such as plate tips and edges. Various mechanisms are discussed for the amelioration and accommodation of these stresses. These include the well established mechanism of self-accommodating assemblies of variants. New mechanisms are proposed relating to the defect structure within individual plates and its interaction with crystal dislocations. Some supporting experimental observations are presented.     

The role of disconnections in phase transformations

The topological model of phase transformations is described in terms of disconnections. Disconnection motion is shown to produce a variety of phase transformations with accompanying plastic strain. These strains, together with elastic strains associated with equilibrium arrays of interface defects, define expected habit planes and orientation relationships. Non-equilbrium defect arrays resulting from kinetic constraints are discussed. Example applications of the topological model are presented for several types of martensitic transformations and several examples of diffusional transformations.     

Pressure-induced structural phase transformations in silicon nanowires

High-pressure structural behavior of silicon nanowires is investigated up to approximately 22 GPa using angle dispersive X-ray diffraction measurements. Silicon nanowires transform from the cubic to the beta-tin phase at 7.5-10.5 GPa, to the Imma phase at approximately 14 GPa, and to the primitive hexagonal structure at approximately 16.2 GPa. On complete release of pressure, it transforms to the metastable R8 phase. The observed sequence of phase transitions is the same as that of bulk silicon. Though the X-ray diffraction experiments do not reveal any size effect, the pressure dependence of Raman modes shows that the behavior of nanowires is in between that of the bulk crystal and porous Si.