Abnormal grain growth

July 17, 2009

Abnormal grain growth in Al–3.5Cu

J Dennis et al

Significant abnormal grain growth has been observed in an Al–3.5 wt.% Cu alloy at temperatures where the volume fraction of small CuAl2 particles was less than about 0.01. The initial fine-grained material had a weak crystallographic texture and there was no indication that any special boundaries were involved in the abnormal growth. Island grains isolated within the abnormal grains also showed no indication of special orientation relationships with their surrounding grains. Measurements indicated that the island grains initially had a size advantage over other matrix grains. The fraction of pinning phase was much lower at abnormal grain boundaries than at boundaries in the fine-grained matrix into which they were growing. A variety of simulations were made, including attempts to model that difference in pinning phase distribution, but none of these were successful in predicting abnormal grain growth.

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May be this can be a case study for the course?

Influence of interface mobility on the evolution of austenite–martensite grain assemblies during annealing

M J Santofimia et al

The quenching and partitioning (Q&P) process is a new heat treatment for the creation of advanced high-strength steels. This treatment consists of an initial partial or full austenitization, followed by a quench to form a controlled amount of martensite and an annealing step to partition carbon atoms from the martensite to the austenite. In this work, the microstructural evolution during annealing of martensite–austenite grain assemblies has been analyzed by means of a modeling approach that considers the influence of martensite–austenite interface migration on the kinetics of carbon partitioning. Carbide precipitation is precluded in the model, and three different assumptions about interface mobility are considered, ranging from a completely immobile interface to the relatively high mobility of an incoherent ferrite–austenite interface. Simulations indicate that different interface mobilities lead to profound differences in the evolution of microstructure that is predicted during annealing.

[1] Relationship between the parting limit for de-alloying and a particular geometric high-density site percolation threshold

Artymowicz et al

The parting limit or de-alloying threshold for electrolytic dissolution of the more reactive component from a homogeneous fcc binary alloy is usually between 50 and 60 at%. The system that has been most studied, dissolution of Ag from Ag-Au, shows a parting limit close to 55 at% Ag. Here, Kinetic Monte Carlo (KMC) simulations of ‘Ag-Au’ alloys and geometric percolation modeling are used to study the relationship between this parting limit and the high-density site percolation thresholds pc(m) for an fcc lattice, subject to the rule that atoms with coordination greater than nine are prevented from dissolution. The value of pc(9) is calculated from geometric considerations to be 59.97 ± 0.03%. In comparison, using KMC simulations with no surface diffusion and no dissolution allowed for ‘Ag’ atoms with more than nine total neighbors, the parting limit is found to be slightly lower (58.4 ± 0.1%). This slight discrepancy is explained by consideration of the local atomic configurations of ‘Ag’ atoms – a few of these configurations satisfy the percolation requirement but do not sustain de-alloying, while a larger number show the converse behavior. There is still, however, an underlying relationship between the parting limit and the percolation threshold, because being at pc(9) guarantees a percolation path in which successive ‘Ag’ atoms share at least one other ‘Ag’ neighbor. With realistic kinetics of surface diffusion for ‘Au’, the parting limit drops to 54.7 ± 0.3% because a few otherwise inaccessible dissolution paths are opened up by surface diffusion of ‘Au’.

[2] Non-equilibrium melting of colloidal crystals in confinement

E Villanova-vidal et al

A novel and flexible experiment is reported for investigation of the non-equilibrium melting behaviour of model crystals made from charged colloidal spheres. In a slit geometry, polycrystalline material formed in a low salt region is driven by hydrostatic pressure up an evolving gradient in salt concentration and melts at large salt concentration. Depending on particle and initial salt concentration, driving velocity and the local salt concentration, complex morphologic evolution is observed. Crystal-melt interface positions and the melting velocity are obtained quantitatively from time-resolved Bragg and polarisation microscopic measurements. A simple theoretical model predicts the interface to first advance, then for balanced drift and melting velocities to become stationary at a salt concentration larger than the equilibrium melting concentration. It also describes the relaxation of the interface to its equilibrium position in a stationary gradient after stopping the drive in different manners. The influence of the gradient strength on the resulting interface morphology and a shear-induced morphologic transition from polycrystalline to oriented single crystalline material before melting are discussed.

[3] Shear thinning in deeply supercooled melts

V Lubchenko

We compute, on a molecular basis, the viscosity of a deeply supercooled liquid at high shear rates. The viscosity is shown to decrease at growing shear rates, owing to an increase in the structural relaxation rate as caused by the shear. The onset of this non-Newtonian behavior is predicted to occur universally at a shear rate significantly lower than the typical structural relaxation rate, by approximately two orders of magnitude. This results from a large size—up to several hundred atoms—of the cooperative rearrangements responsible for mass transport in supercooled liquids and the smallness of individual molecular displacements during the cooperative rearrangements. We predict that the liquid will break down at shear rates such that the viscosity drops by approximately a factor of 30 below its Newtonian value. These phenomena are predicted to be independent of the liquid’s fragility. In contrast, the degree of nonexponentiality and violation of the Stokes–Einstein law, which are more prominent in fragile substances, will be suppressed by shear. The present results are in agreement with existing measurements of shear thinning in silicate melts.

[4] X-ray cross correlation analysis uncovers hidden local symmetries in disordered matter

P Wochner et al

We explore the different local symmetries in colloidal glasses beyond the standard pair correlation analysis. Using our newly developed X-ray cross correlation analysis (XCCA) concept together with brilliant coherent X-ray sources, we have been able to access and classify the otherwise hidden local order within disorder. The emerging local symmetries are coupled to distinct momentum transfer (Q) values, which do not coincide with the maxima of the amorphous structure factor. Four-, 6-, 10- and, most prevalently, 5-fold symmetries are observed. The observation of dynamical evolution of these symmetries forms a connection to dynamical heterogeneities in glasses, which is far beyond conventional diffraction analysis. The XCCA concept opens up a fascinating view into the world of disorder and will definitely allow, with the advent of free electron X-ray lasers, an accurate and systematic experimental characterization of the structure of the liquid and glass states.

Here is the summary section of a pre-print in arXiv by Leo Kadanoff on phase transitions, which pays rich tributes to Gibbs and condensed matter theorists:

We have focused upon the nature of phase transitions and the mean field theory which describes them. The description of the phase transition “leaped in full
armor” from the inspiration of Gibbs, and his insight that a phase transition is an abrupt change which occurs only in a infinite system. Many years of investigation were given theoretical clothing in a mean eld theory set forward originally by van der Waals. This theory provided a generally satisfactory, but numerically inaccurate, description of the main events which occur in the phase diagrams of typical materials. It explains the various ordering of these materials, and some of the similarities and differences among the materials. It explains the universality of the phase diagrams, so that different materials can have rather similar phase diagrams. It fails to explain why the diagrams are inaccurate near the critical point, exactly the region in which the general formulation of Landau would have them be most accurate. But, Landau’s variational method would be the language for the advances to come.

Tuning of the martensitic transformation temperature in Cu-Zn thin films by control of zinc vapor pressure during annealing

N Haberkorn et al

We report shape memory effect in Cu-Zn thin films grown by electrodeposition on pyrolytic graphite from pyrophosphate-based electrolytes. Cu-Zn films showing martensitic transformation could only be obtained after the optimization of thermal annealing parameters like annealing temperature, Zn vapour pressure and fast quenching. By means of the control of the Zn vapour pressure during annealing using a bulk reference Cu-Zn alloys the chemical composition of the films could be adjusted and martensitic transformation temperatures of the films could be tuned.

[1] Phase-Field Simulations of the Dendrite Morphologies and Selected Evolution of Primary Alpha-Mg Phases during the Solidification of Mg-rich Mg-Al Based Alloys

M Wang et al

A formulation of solid-liquid interfacial thermodynamic and kinetic anisotropic characteristics for hexagonal close-packed metals is proposed. The two- and three-dimensional dendritic growth of primary Mg in undercooled Mg-Al alloy melts are modeled using the phase-field method, with consideration of the integration of crystallographic lattice symmetry and experimental observations. The morphologies of three-dimensional dendrites are obtained and the calculated results have shown intricately hierarchical branched structures. The excess free energy of solution system is based on the Redlich-Kister model.

[2] Phase transformations in QE22 Mg alloy

G Barucca et al

The precipitation sequence in a QE22 Mg alloy is followed by differential scanning calorimetry, microhardness, electrical resistivity, positron annihilation spectroscopy and transmission electron microscopy (TEM) observations, after different thermal treatments. The decomposition of the supersaturated solid solution occurs via the formation of nanosized coherent structures (GP zones) followed by the co-precipitation of two metastable phases responsible for the peak ageing condition. The stable phase (Mg, Ag)12Nd appears at the highest annealing times, leading to over-ageing and hardness reduction. TEM observations provide information on the crystallographic structure of the forming phases, allowing some inconsistencies present in the literature to be clarified. Activation energies are derived from both calorimetric and resistometric measurements at different scanning rates.