Nucleation Of Phase Transformations Research Articles

Recent developments in computational modelling of nucleation in phase transformations

AbstractNucleation is one of the most common physical phenomena in physical, chemical, biological and materials sciences. Owing to the complex multiscale nature of various nucleation events and the difficulties in their direct experimental observation, development of effective computational methods and modeling approaches has become very important and is bringing new light to the study of this challenging subject. Our discussions in this manuscript provide a sampler of some newly developed numerical algorithms that are widely applicable to many nucleation and phase transformation problems. We first describe some recent progress on the design of efficient numerical methods for computing saddle points and minimum energy paths, and then illustrate their applications to the study of nucleation events associated with several different physical systems.

Optimization-based Shrinking Dimer Method for Finding Transition States

We present a new and efficient optimization-based shrinking dimer (OSD) algorithm for finding transition states associated with a given energy landscape. The transition states are given by index-one saddle points of the energy surface. By constructing the rotation and translation steps in the classical dimer method under an optimization framework, we are able to take advantage of more powerful optimization methods to substantially speed up the computation of transition states. Specifically, the Barzilai--Borwein gradient method is proposed as an effective implementation of OSD. We show that the OSD method is the generalized formulation of the original shrinking dimer dynamics (SDD) and enjoys superlinear convergence. We test various numerical examples, including some standard lower dimensional test examples, a cluster of seven particles, seven-atom island on the (111) surface of an FCC crystal, and nucleation in phase transformations. The results demonstrate that the OSD method with the proposed Barzilai-...

Nucleation Laws of Supercooled Austenite Transformation

It is significant theoretically to study the nucleation laws of the phase transformation products of the supercooled austenite. The different iron and steel materials were used to study the nucleation of pearlite, bainite and martensite by QUANTA-400 environmental scanning electron microscope and JEM-2100 transmission electron microscope. The results show that, from the high-temperature zone to the low-temperature zone, the nucleation of the transformation products of the supercooled austenite is a gradually evolutionary process from two-phase nucleation to single-phase nucleation. Pearlite nucleates in the austenitic grain boundary, bainite nucleates preferentially in the grain boundary and sometimes in the grain interior and martensite nucleates preferentially in the interface and generally in the grain interior. The preferential nucleation sites are the interface, and with the decrease of the transformation temperature, they gradually shift to the defects in the grain interiors, which accord with the general rules of the phase transformation nucleation.

Cellular Automaton Modeling of the Isothermal Hot Compression Phase Transformation in TA15 Titanium Alloy

A two-dimensional cellular automaton model is developed to simulate the isothermal hot compression phase transformation of β→α in TA15 titanium alloy. The model, based on the data of hot compression deformation and some assumption, the phase transformation nucleation and growth function of the phase β→α which is utilized by the automaton in a probabilistic fashion. The probability of nucleation and growth of α phase is confirmed and described by the existing theories of dislocation density. The competition between phase transformation and growth of transitional phase is described by the changes of dislocation density in this cellular automaton lattice. Based on the results of cellular automaton, the transformation fraction of α phase is calculated with the number of lattice and isothermal phase transformation kinetic equation is also calculated with data of transformation fraction, and the effect of hot compression deformation parameters on phase transformation is also discussed.

形状記憶合金に生じる不均一変形挙動の計測

Shape memory alloy (SMA) has been paid attention as smart material revealing shape recovery by heating. Many researchers have investigated its basic properties and applications. However, many problems have existed in usage of SMA such as change of properties (transformation temperature, Young's modulus) by process of fabrication, shape memory treatment, condition of phase and so on. Therefore, it has been pointing in recent years that expressing these phenomena using conventional constitutive relation of SMA has been difficult. In the present study, phenomena needed to construct novel constitutive relation of SMA, such as nucleation and propagation of phase transformation and so on, were measured using digital image correlation method. Here, tensile loading and unloading tests were performed for NiTi alloy plate in two temperature regions where the SMA shows the shape memory effect (26°C) and pseudoelasticity (60°C). The obtained results were as follows; (1) The propagation of the phase transformation during the stress-induced martensitic transformation and austenite transformation progressed from the both ends to the center of the specimen. (2) Staying phenomena of deformation was confirmed at nucleation point of phase transformation. Deformation behavior of SMA can be classified broadly into three categories, such as “Nucleation of phase transformation”, “Process between nucleation and propagation” and “Propagation of phase transformation”. (3) The transformation had a fixed angle along the longitudinal direction of the specimen, propagating from the both ends to the center of the specimen.

Influence of oxygen content on the reduction of the ordering temperature of L1/sub 0/ FePtCu alloy

The influence of the residual oxygen on the ordering (transformation from fcc to fct) temperature of sputtered Fe-Pt-Cu films was investigated. Secondary ion mass spectroscopy analysis revealed that high oxygen content increases the onset temperature of the ordering process. The oxygen is brought into an Fe-Pt-Cu film during the deposition process and forms stable Fe-O compound. This might increase the ordering temperature as well as prevent the coalescence of grains, that is, the nucleation of phase transformation. A hot-pressed Fe-Pt composite target which contains oxygen of 50 ppm was developed. This target is also effective for reducing the ordering temperature of Fe-Pt films.

Fatigue crack nucleation assisted by thermal activation

Abstract Experimental evidence is presented that very early in the fatigue lifetime of Sn–Pb eutectic solder the fatigue damage is governed by microcrack nucleation. A model for thermally assisted fatigue crack initiation is presented using standard equations for nucleation in phase transformations. Predictions of the model are in good agreement with nucleation data for this solder.

Nucleation of phase transformations at intragranular inclusions in steel

The control of phase transformation nucleation in steel by inclusions has attracted considerable attention for a few decades. The potency as a transformation catalyst appears to depend on the species of ~nclusions and so, they may not simply act as an inert heterogeneous nucleation site. The formation of a solute depletion zone in surrounding austenite and the provision of nucleation sites which can form low energy nucleuslinclusion interfaces are considered to be a principal mechanism. These are discussed on the basis of simple models and available experimental data to relate nucleation potency with thermodynamic and other material properties of inclusions.

Effects of Magnetic Field on Nucleation in Phase Transformations

Effects of Magnetic Field on Nucleation in Phase Transformations

Indirect nucleation in phase transformations with symmetry reduction

Abstract Non-classical nucleation in a first-order phase transition with a symmetry reduction and multicomponent long-range order parameter is investigated. A particular case of cubic → tetragonal transformation is considered. Based on the stochastic field equation, a computational method of finding the atomic configuration of a critical nucleus without any a priori constraints on the nucleation path is proposed. Two-dimensional computer simulations are performed. It is demonstrated that a transformation need not go directly to the equilibrium state during nucleation. Under certain conditions the system prefers an ‘indirect’ nucleation developing along a path that has a lower nucleation barrier. The critical nucleus formed along this path has a transient non-equilibrium structure, which transforms into the equilibrium structure during the growth process. The relation between indirect nucleation and the structural mechanism of the interfacial energy optimization in the case of multicomponent long-range ord...

Finite impurity size effects on heterogeneous nucleation occurring in diblock copolymers

The role played by heterogeneous nucleation in phase transformations occurring in asymmetric, diblock styrene-butadiene is examined using the experimental technique of time-resolved small-angle x-ray scattering. Transient nucleation effects are clearly identifiable through comparison of the early time transformation kinetics seen with samples containing varying impurity sizes. These results are interpreted in the context of a simple model of transient nucleation rate incorporated into classical theories of isothermal transformation kinetics.

The Role of the Lead Zirconate Titanate (PZT)-Pt Interface in the Anomalous Phase Transformation Kinetics in Sputtered Pzt Thin Film Capacitors at Sub-2000 Å Thicknesses

AbstractThe Pt-Lead Zirconate Titanate (PZT) thin film interface plays a key role in determining the electrical properties and phase transformation kinetics of Pt-PZT-Pt thin film capacitor structures. The results of the electrical and material properties of PZT (65/35) films that vary in thickness between 500 Å to 4000 Å deposited by DC-magnetron sputtering under identical deposition conditions, and subjected to the same post-deposition thermal processing conditions shows that the Pt-PZT interface dominates thin film properties at low thicknesses (≦ 2000 Å). The charge storage density (Qc') and maximum polarization (Pmax) shows an anomalous variation with thickness, showing an initial increase followed by a drastic decrease as the film thickness is scaled down to 500Å. Significant interdiffusion at the PZT film-Pt electrode retards the pyrochlore-to-perovskite phase transformation nucleation and growth rate in PZT films of thickness 2000Å and lower. Gate polarity dependence of the time-tobreakdown and the degradation field is observed for all PZT film thicknesses (including 4000Å). This indicates that the ferroelectric film-electrode interface has an important role to play in determining the electrical reliability properties even in the 4000Å thick PZT film, although Qc' and Pmax are not adversely affected at these thicknesses.

Theory of nucleation in binary systems

A method of calculating the rate of nucleation in phase transformations in binary mixtures is proposed.