Characteristics of the continuous coarsening and discontinuous coarsening of spinodally decomposed Cu-Ni-Fe alloy

The spinodal decomposition and coarsening reaction of a 45Cu-30Ni-25Fe alloy aged with and without prior 75% cold rolling have been studied by Vickers hardness tester, X-ray diffraction, optical microscope, and scanning and transmission electron microscopes. Spinodal decomposition took place in the alloys aged in the temperature range of 600–950°C without prior deformation, and then spinodal structure in these alloys would coarsen only continuously. Spinodal decomposition and continuous coarsening reaction of spinodal structure took place in the alloys aged at lower temperature with prior deformation. This process was corresponding to recovery of spinodal structure in the deformed alloys. Discontinuous coarsening reaction of spinodal structure would also take place in the alloys aged at higher temperature with prior deformation. This process was corresponding to recrystallization of spinodal structure in the deformed alloys.     

Objects that make objects: the population dynamics of structural complexity.

To analyse the evolutionary emergence of structural complexity in physical processes, we introduce a general, but tractable, model of objects that interact to produce new objects. Since the objects--epsilon-machines--have well-defined structural properties, we demonstrate that complexity in the resulting population dynamical system emerges on several distinct organizational scales during evolution--from individuals to nested levels of mutually self-sustaining interaction. The evolution to increased organization is dominated by the spontaneous creation of structural hierarchies and this, in turn, is facilitated by the innovation and maintenance of relatively low-complexity, but general individuals.     

Anisotropy driven interrupted coarsening of the Asaro-Tiller-Grinfeld instability

We investigate the effects of surface energy anisotropy on the coarsening dynamics of the Asaro-Tiller-Grinfeld instability at stake in thin semiconductor films. We consider a continuum model which accounts for wetting interactions between the film and its substrate, elasticity driven mass currents and surface energy anisotropy. We derive an explicit non-linear, non-local evolution equation for the film height that we solve numerically. Anisotropy, which dictates the island shapes, impacts the growth kinetics by weakening the possible elastic relaxation, which can lead during annealing to an interruption of Ostwald ripening. The resulting stationary state is characterized by square-based pyramids separated by a wetting layer. It is found that the instability onset is delayed when the film thickness decreases above the critical thickness for the instability to occur. We characterize the influence of the growing flux used for the film deposition on the stationary state reached during subsequent annealing, and find that the density of the resulting self-organized islands increases with the flux.     

Coerced mechanical coarsening of nanoparticle assemblies

Coarsening is a ubiquitous phenomenon that underpins countless processes in nature, including epitaxial growth, the phase separation of alloys, polymers and binary fluids, the growth of bubbles in foams, and pattern formation in biomembranes. Here we show, in the first real-time experimental study of the evolution of an adsorbed colloidal nanoparticle array, that tapping-mode atomic force microscopy (TM-AFM) can drive the coarsening of Au nanoparticle assemblies on silicon surfaces. Although the growth exponent has a strong dependence on the initial sample morphology, our observations are largely consistent with modified Ostwald ripening processes. To date, ripening processes have been exclusively considered to be thermally activated, but we show that nanoparticle assemblies can be mechanically coerced towards equilibrium, representing a new approach to directed coarsening. This strategy enables precise control over the evolution of micro- and nanostructures.     

Discontinuous and continuous coarsening of lamellar precipitates in Cu-Be alloys

A study has been conducted of the coarsening reaction which occurs after discontinuous precipitation in Cu-Be alloys. When the discontinuous precipitation is nearly complete, a secondary cellular reaction with much coarser interlamellar spacing forms at the original grain boundaries and advances into the primary cells. The growth rate decreases and the interlamellar spacing increases with the cell growth for the secondary reaction. These phenomena are attributed to the continuous coarsening reaction which simultaneously occurs in the primary cells. The phase in the cells produced by the first reaction is supersaturated at the initial cell growth, but attains its equilibrium concentration prior to the appearance of the secondary cells. Cold work does not influence the cell growth of the secondary reaction. The driving force for the secondary reaction is the reduction in total interlamellar surface energy.     

Dendrite coarsening during solidification of hypo- and hyper-eutectic Al-Cu alloys

Dendrite coarsening during cooling at a constant rate was compared at various stages of solidification with that during isothermal holding for Al-Cu alloys of hypo- and hypereutectic compositions. For each specimen, the undercooling for the initial dendrite formation and the time elapsed after it were measured directly. The dendrite arm spacing was shown to be determined solely by the latter, and the dendrite structure was therefore coarsening-controlled from the early stage of solidification. The rate of coarsening in terms of the dendrite arm spacing during solidification at a constant cooling rate was same as that during isothermal holding in all the alloys tested. Numerical values of the fractional rate of solidification were evaluated for the hypo-eutectic compositions and the results show that the rate of dendrite coarsening does not depend on the fractional rate of solidification. Aluminium dendrites show structural coarsening with progressive solidification in the same way as during isothermal holding. CuAl₂ dendrites show curved boundaries after isothermal holding whereas those cooled at a constant rate are faceted.     

The effect of titanium and nitrogen contents on the austenite grain coarsening temperature

Three series of titanium steels with different nitrogen contents were used to evaluate the effect of titanium and nitrogen contents on the grain coarsening behaviour of austenite during reheating. Quantitative metallography and transmission electron microscopy were employed to measure the austenite grain size and TiN particle size after being reheated and quenched from austenitization temperatures. The results show that for steels with the same nitrogen content the highest grain coarsening temperature occurs around the Ti/N stoichiometric ratio, i.e. 3.42 and an increase in grain coarsening temperatures with increasing nitrogen content also occurs at Ti/N around 3.42. Higher titanium content beyond the stoichiometric ratio decreases the grain coarsening temperature as a result of TiN particle growth.     

Modelling the evolution of particle size distribution during nucleation,growth and coarsening

Abstract

A model, based on the numerical framework of Kampmann and Wagner, has been developed to predict the evolution of particle size distribution (PSD) during the decomposition of supersaturated solid solutions by nucleation, growth, and coarsening. During the early stages of transformation, where nucleation and growth are dominant, the PSD shape is predicted to be constantly evolving. Only during the latter stages of transformation, when coarsening becomes dominant, does the PSD tend towards a steady state shape, which closely matches that expected from classical coarsening theory. It is also predicted that as the PSD evolves, a transient double peak forms and then decays. The physical basis of this double peak has been investigated and the effect of supersaturation on its formation has been predicted.     

Devitrification and microstructural coarsening of a fluoride-containing barium aluminosilicate glass

Barium aluminosilicate (BAS) glass-ceramics have the potential to be used in the production of cast prostheses for biomedical applications because of their radiopacity and increased strength compared with traditional feldspathic porcelains. It is essential to understand the crystallization kinetics of these materials in order to fabricate products with increased fracture resistance rapidly. It was hypothesized that the addition of fluoride (F) to the composition of BAS glass would reduce the necessary processing time and temperatures by obviating the need for a separate crystal nucleation treatment. BASF glass was subjected to both linear non-isothermal and one-stage isothermal crystallization treatments, and the resulting glass-ceramics were characterized using x-ray diffraction, differential thermal analysis, and stereology. BASF glass had a low energy barrier to crystallization (397 kJ/mol) and transformed to 76 ± 2% crystallinity within 30 min at 975°C. A fine-grained microstructure was produced by bulk crystallization without the need for a separate crystal nucleation stage. After the initial crystal precipitation, the mean crystal size and mean free path between crystals increased over time at elevated temperature by a diffusion rate-limited coarsening mechanism.     

Microstructural observations and mechanisms of lamellar coarsening in iron meteorites

In many iron meteorites, a lamellar structure is found which consists of ferrite () and another phase which is probably austenite (γ). In some of these regions, this lamellar structure has decomposed to a coarser structure of and another phase which may be austenite. Microstructural evidence is presented that the coarsening occurs by movement of a high-angle boundary separating the two lamellar structures. Examination of diffusion coefficients in the Fe-Ni system shows that the coarse structure could not have formed by volume diffusion, but the lamellar spacing is consistent with formation by diffusion along the interface. Diffusion calculations are made which support the argument that high-angle interface diffusion allows the coarsening to occur at temperatures considerably below those where volume diffusion is too low.     

Isothermal coarsening of liquid inclusions in aluminium-copper alloys

An isothermal coarsening model was introduced for liquid inclusions in homogeneous aluminium-copper alloys. Discrepancies between experimentally measured and analytically predicted variations of the solid-liquid interface area per unit volume of liquid,Sv, the average inclusion diameter,¯D, and the number of inclusions per unit volume of solid,Nv, with isothermal coarsening time were attributed to experimental evaluation errors, to the inaccuracy of some assumptions made or values adopted for certain system variables and to the effect of melting and solidification kinetics. At constant fraction liquid, isothermal coarsening slows down with increasing copper concentration.     

Dual effect of Cu on the Al_3Sc nanoprecipitate coarsening

It is generally considered that the Al_3Sc nanoprecipitates are highly thermal stable, mainly due to quite slow Sc diffusion in the α-Al matrix. In this paper, we demonstrate in an Al-Cu-Sc alloy that the Cu atoms have dual effect on the coarsening of Al_3Sc nanoprecipitates. On the one hand, the Cu atoms with high diffusivity tend to accelerate the Al_3Sc coarsening, which results from the Cu-promoted Sc diffusion.On the other hand, some Cu atoms will segregate at the Al_3Sc/matrix interface, which further stabilizes the Al_3Sc nanoprecipitates by reducing the interfacial energy. Competition between these two effects is tailored by temperature, which rationalizes the experimental findings that the coarsening kinetics of Al_3Sc nanoprecipitate is greatly boosted at 300℃-overaging while significantly suppressed at 400℃-overaging.     

Dissolution and coarsening of large niobium carbonitrides in a microalloyed steel

Observations and measurements of the kinetics of coarsening and dissolution of large cuboidal niobium carbonitrides during solution treatments of a high nitrogen niobium microalloyed steel are reported. At temperatures between 1473 and 1573 K a competitive coarsening and dissolution process was established where the larger niobium carbonitrides grew at the expense of the smaller, or employing niobium and nitrogen which remained in solid solution. In this temperature range growth or dissolution rates and critical sizes could be determined from the analysis of the evolution of particle size distribution. At higher temperatures (1623–1723 K), only a dissolution process existed, where the dissolution rates as a function of particle size was found to increase with increasing temperature.     

Gaussian and non-Gaussian speckle fluctuations in the diffusing-wave spectroscopy signal of a coarsening foam

All previous applications of diffusing-wave spectroscopy to aqueous foams have relied on the assumption that the electric field of the detected light is a Gaussian random variable and hence that the Siegert relation applies. We test this crucial assumption by simultaneous measurement of both second- and third-order temporal intensity correlations. We find that the electric field is Gaussian for typical experimental geometries equivalent to illumination and detection with a plane wave, both for backscattering and transmission through an optically thick slab. However, we find that the Gaussian character breaks down for point-in-point-out backscattering geometries in which the illumination spot size is not sufficiently large in comparison with the size of the intermittent rearrangement events.     

Model for growth and coarsening of two phase systems under diffusional control

Abstract

A model is described which is capable of simulating the two-dimensional evolution of microstructure in two phase systems undergoing diffusion controlled growth and surface tension driven coarsening. To solve the diffusion equation in the matrix phase, an integral equation method is employed. Thus, although it is necessary to describe the shapes of the second phase particles using a number of elements, it is not necessary to discretise the matrix phase as the particles evolve. This allows the computation times to be kept within reasonable limits. The boundary condition at the interface and the variation of the interfacial concentration with interface curvature are accounted for in a rigorous fashion. It is shown that the method can handle the ‘soft’ impingement of overlapping diffusion fields. A treatment of the ‘hard'’ physical impingement of particles is developed. To demonstrate the accuracy and stability of the method, the results from the model are compared with the exact solution for a spherical particle growing in a circular domain; it is shown that the agreement is reasonable. The results from a number of example computations are presented, which include (a) growth of a single particle in a finite domain, (b) soft and hard impingement of two particles in a finite domain, (c) coarsening of a significant number of particles at constant volume fraction, and (d) simultaneous nucleation, growth, and coarsening of second phase particles. Where appropriate, the results are compared with those from other models which have been published in the literature. The advantages and disadvantages of the present model are discussed.     

Topological characterization of the Bi-Ge-Te phase diagram

The available phase-diagram data for the Bi-Ge-Te, Ge-Te, Bi-Te, and Ge-Bi systems are critically evaluated. A topological image of the Bi-Ge-Te phase diagram is constructed in the form of a graph, with the nodes representing distinct phases and edges representing two-phase mixtures, ordered with respect to temperature. The proposed topological approach is used to construct isothermal sections, the liquidus surface of the ternary system, andT-x sections.