Propagation and ignition of fast gasless detonation waves of phase or chemical transformation in condensed matter

Fast self sustained waves of chemical or phase transformations, observed in several contexts in condensed matter effectively result in “gasless detonation". The phenomenon is modelled by coupling the reaction diffusion equation, describing chemical or phase transformations, and the wave equation, describing elastic perturbations. The coupling considered in this work involves (i) a dependence of the sound velocity on the chemical (phase) field, and (ii) the destruction of the initial chemical equilibrium when the strain exceeds a critical value (strain induced phase transition). Both the case of an initially unstable state (first order kinetics) and metastable state (second order kinetics) are considered. An exhaustive analytic and numerical study of travelling waves reveals the existence of supersonic modes of transformations. The practically important problem of ignition of fast waves by mechanical perturbation is investigated. With the present model, the critical strain necessary to ignite gasless detonation by local perturbations is determined. Received 18 November 1999     

Phase transformations of nanoparticles exposed to hydrostatic pressure

A thermodynamic analysis of phase transformations of nanoparticles under hydrostatic pressure has revealed important differences between phase transformations under isothermal or adiabatic conditions. This presuppositionless analysis fully explains a hysteresis with respect to the phase fraction and the pressure observed experimentally. It is important to mention that the results of this analysis may be transferred to the role of any external volumetric field acting on phase transforming nanoparticles. Typical examples are phase transformations of ferromagnetic intermetallics subjected to the influence of magnetic fields.     

Thermodynamic analysis of ageing-induced multiple-stage transformation behaviour of NiTi

Near-equiatomic NiTi shape memory alloys normally exhibit three martensitic transformations among three phases: the B2 phase, the monoclinic (M) phase and the rhombohedral (R) phase. Some recent work, however, has revealed complex transformation behaviour involving multiple-stage martensitic transformations and multiple-stage R-phase transformations. This paper presents an analysis of these complex transformation behaviours based on thermodynamic concepts of reversible and irreversible energies associated with the transformations. The analysis is successful in identifying all observed transformations and in defining relative positions of various stages of transformations on a temperature scale. It also defines positions of thermodynamically prohibited transformations as well as permitted transformations that have not been experimentally measured. Such identifications enable the determination of actual transformation hystereses that are not directly measured experimentally. Based on the thermodynamic principles adopted, the analysis also renders it possible to identify the possible causes that contribute to the complex multiple-stage transformation behaviour.     

Doppler Effect Reconsidered

Doppler effect, as currently formulated. uses numerical phase invariance and any linear coordinate transformations with identity transformations of nonzero spatial coordinates transverse to a rectilinear uniform relative motion; this is shown to subtly deny presumed motion, angular aberration effects, and Doppler effect. A critical review of past Doppler effect experiments confirms (10⁻³ to 10⁻⁷) a classical Doppler effect formulation that does not involve an ether concept, numerical phase invariance, or the use of any coordinate transformations.     

Two scenarios for phase-transformation in disordered media

The reasons for the existence of various scenarios for structural transformations in disordered condensed media, such as liquids and amorphous substances, where both smeared transformations and sharp first-order transitions may occur, were analyzed. The ratio between the spatial scale of structural correlations and the size of the smallest possible region occupied by a new phase in the matrix of initial modification is the key parameter determining the scenario for equilibrium phase transformations in liquids. In amorphous substances, the experimentally observed transformations occur far from equilibrium, and the possible size of the region occupied by the new phase corresponds to the minimal nucleus size. For some amorphous solids, quantitative analysis of the transformation width was carried out and the main classes of covalent substances, in which the smeared or sharp transitions occur, were revealed. Specific features of the interparticle interactions determining various transformation scenarios are discussed.     

Coupling of the phase-field and CALPHAD methods for predicting multicomponent,solid-state phase transformations

The development of an effective microstructure design method for multicomponent alloys is of considerable importance for improving both the design of alloys and the design of processes for producing alloys with unique properties. The coupling of the phase-field method and the calculation of phase diagrams (CALPHAD) method can be used for predicting the evolution of microstructures in multicomponent alloys. Such predictions make use of CALPHAD thermodynamic information with the chemical free energy function in the phase-field method. This article reviews several of these coupling methods, focusing on solid-state phase transformations in multicomponent systems, such as phase separation and disordered or ordered phase precipitation from a matrix. When calculating disordered phase transformations, the Gibbs energy function derived from the CALPHAD database can be used directly in the phase-field method. On the other hand, when dealing with an order/disorder transition, the degrees of freedom of the element site fraction for an ordered phase in the CALPHAD method can be reduced using the Gibbs energy single formalism for constituent phases, by using a database that stores the Gibbs energy and chemical equilibrium conditions, or by obtaining the driving force calculated using the Thermo-Calc software. The current status and future directions for further development of these coupled methods are discussed.     

Influence of size factors on the character of phase transformations in light actinides

The transformations of crystalline phases of light actinides have been analyzed as the processes of formation and growth of nanoparticles. The influence of size factors on the character of martensitic phase transitions and reverse phase transformations has been investigated using plutonium as an example.     

Spectroscopic study of phase transformations between orthorhombic and tetragonal C60 polymers

We present a detailed study of transformations between the orthorhombic and tetragonal polymeric states of C₆₀. The transformations are characterised by Raman spectroscopy and X-ray diffraction. We show that the transformation from the orthorhombic (O) phase to the tetragonal (T) phase is very fast and our results indicate that the transformation goes via an intermediate dimer (D) state in a two-stage process, O↦D and, D↦T transformations, where the second process is slower than the first. On the other hand, the transformation from the tetragonal to the orthorhombic phase is significantly slower, indicating a high-energy threshold to break the polymer bonds at the temperatures used. The results also support earlier suggestions that the tetragonal phase contains disordered dimers that can be viewed as lattice defects in the formation of higher polymers.     

Study of phase transformations in CMSX-6 and CMSX-8 superalloys

Nickel-based superalloys are extensively used mainly in the aircraft and aeronautic industry, particularly in the hottest parts of engines or turbo-reactors. The phase reactions occurring in these heat-resistant materials play a crucial role in many aspects of the processing and service of the highly alloyed materials. Cast Ni-based superalloys are obtained in a complex way and their structure is complicated. Differential scanning calorimetry (DSC) technique was applied for determination of temperature ranges of the phase transformations occurring in the CMSX-6 and CMSX-8 superalloys during heating/cooling processes. Thermophysical properties, including temperatures of the phase transformation, are the critical input parameters in mathematical models of solidification and casting of metallic materials. The literature data concerning phase transformations and performance of the heat treatment for CMSX-6 and CMSX-8 are incomplete and ambiguous. DSC results accompanied by scanning electron microscopy characterization of microstructure of CMSX-6 and CMSX-8 superalloy was applied. The present study will improve the understanding of the fundamental mechanisms of phase transformations of single-crystal nickel-based superalloys.     

Local internal supersymmetry of the free Maxwell theory

The free Maxwell theory is shown to possess an extended gauge invariance consisting of local internal supersymmetry transformations in addition to the usual local phase transformations. The Maxwell lagrangian is derived as a particular gauge choice in the extended theory.     

Phase formation and structural changes in the chromium-silicon system exposed to compressed plasma fluxes

The results of the study of structural and phase transformations in the silicon substrate-chromium coating system exposed to compression plasma fluxes with power densities of 0.3–1.2 GW/m² are discussed. The formation of hexagonal chromium disilicide and an amorphous phase, the growth of silicon dendrites, and the appearance of a chromium-enriched near-surface layer are revealed effects. The mechanisms of structural and phase transformations caused by rapid cooling of a mixed melt and concentration overcooling during solidification are analyzed.     

Growth by Ledges

The prediction and observation of growth ledges at various moving interphase boundaries in solid-solid phase transformations has become increasingly widespread since first proposed in the early 1960's by Aaronson. The role of steps at the growth interfaces in several phase transformations is reviewed. For precipitate plates emphasis is placed on rationalizing experimental growth kinetics with mathematical models developed for the migration rate of a ledge. More complex transformations are employed to examine the possible role of growth ledges to account for microstructure development. In all cases, the growth ledge is shown to be necessary to fully understand solid-solid phase transformations.     

改性锆钛酸铅温度诱导相变的热释电性

测量结果显示,Pb_0.99Nb_0.02［(Zr_1-xSn_x)_1-yTi_y］_0.98O₃在温度诱导相变时伴随有正的和负的热释电电流峰.电流的方向与相变类型有如下关系：铁电相向反铁电或顺电相转变时形成正的电流峰,反铁电相向铁电或顺电相转变时形成负的电流峰.按照铁电相与反铁电和顺电相之间存在静电势差的观点,可以很好地解释热释电电流的方向与 关键词： 改性锆钛酸铅化合物 温度诱导相变 热释电性     

Phase transformation of quasicrystals in aluminium-transition metal alloys

Quasicrystals in Al–Mn, Al–Cu–TM (TM = Fe, Cr, Mn and Ru) and Al–Cu–Fe–Cr alloys can undergo two different modes of phase transformation. Discontinuous transformations of quasicrystals are characterized by the existence of a definite reaction front separating the quasicrystalline phase from the resulting crystalline one; the kinetics are controlled by the migration of the reaction front. Continuous transitions, on the other hand, proceed by structural evolution such as modulation or chemical ordering inside the quasicrystalline phase without creating any high-energy interfaces. Both types of transformations are thermally activated and need atomic diffusion.     

Study of ageing effects of a low-interstitial 18Cr-2Mo ferritic steel

The phase transformations which occur in the 18Cr-2Mo ferritic steel by annealing in the temperature range 603–1093 K have been studied using Moessbauer spectroscopy and X-ray diffraction. It is found that the 748 embrittlement is to be ascribed to Cr redistribution and/or to the formation of an intermetallic R-type phase. Precipitation of an h.c.p. Laves phase Fe (Mo,Nb) of MgZn₂-type takes up at about 850 K and Nb to Mo substitution goes on by increasing the annealing temperature.     

Evolution of dynamic spiral domains over a period of an alternating magnetic field

The stroboscopic method was applied to study the evolution of dynamic spiral domains in garnet ferrite films in an ac magnetic field. The spiral-domain shape was shown to change significantly within a field period; the basic shape transformations take place in the phase range ?π/4 to +π/4 with respect to the polarity inversion time. During the spiral-domain formation or decay, the area and shape of a hysteresis loop of the film region containing the domain gradually change. The upper boundary of the frequency range in which spiral domains form was established to be associated with transformations of the domain wall structure.     

Stochastic physical origin of the quantum operator algebra and phase space interpretation of the Hilbert space formalism: The relativistic spin zero case

Based on a recent association of quantum observable algebra with stochastic processes in the frame of the causal stochastic interpretation of quantum mechanics, a relativistic Hilbert space is defined for the Klein-Gordon case. It is demonstrated that unitary transformations in Hilbert space reflect canonical transformations in the associated phase space, manifesting thus an underlying symplectic structure.     

Superfield formulation of the phase space path integral

We give a superfield formulation of the path integral on an arbitrary curved phase space, with or without first class constraints. Canonical tranformations and BRST transformations enter in a unified manner. The superpartners of the original phase space variables precisely conspire to produce the correct path integral measure, as Pfaffian ghosts. When extended to the case of second-class constraints, the correct path integral measure is again reproduced after integrating over the superpartners. These results suggest that the superfield formulation is of first-principle nature.     

Athermal resistance to interface motion in the phase-field theory of microstructure evolution

A method of introducing an athermal resistance to interface propagation for the Ginzburg-Landau (GL) approach to the first-order phase transformations (PTs) is developed. It consists of introducing oscillating fields of stresses (due to various defects or a Peierls barrier) or a jump in chemical energy. It removes some essential drawbacks in GL modeling: it arrests experimentally observed microstructures that otherwise converge to a single phase, and it reproduces rate-independent stress hysteresis. A similar approach can be applied for twinning, dislocations, and other PTs (e.g., electric and magnetic).     

Finite Nilpotent BRST Transformations in Hamiltonian Formulation

We consider the finite field dependent BRST (FFBRST) transformations in the context of Hamiltonian formulation using Batalin-Fradkin-Vilkovisky method. The non-trivial Jacobian of such transformations is calculated in extended phase space. The contribution from Jacobian can be written as exponential of some local functional of fields which can be added to the effective Hamiltonian of the system. Thus, FFBRST in Hamiltonian formulation with extended phase space also connects different effective theories. We establish this result with the help of two explicit examples. We also show that the FFBRST transformations is similar to the canonical transformations in the sector of Lagrange multiplier and its corresponding momenta.