Aluminum Σ3 grain boundary sliding enhanced by vacancy diffusion

N Du et al

Grain boundary sliding is an important deformation mechanism for elevated temperature forming processes. Molecular dynamics simulations are used to investigate the effect of vacancies in the grain boundary vicinity on the sliding of Al bi-crystals at 750 K. The threshold stress for grain boundary sliding was computed for a variety of grain boundaries with different structures and energies. These structures included one symmetrical tilt grain boundary and five asymmetrical tilt grain boundaries. Without vacancies, low energy Σ3 grain boundaries exhibited significantly less sliding than other high energy grain boundaries. The addition of vacancies to Σ3 grain boundaries decreased the threshold stress for grain boundary sliding by increasing the grain boundary diffusivity. A higher concentration of vacancies enhanced this effect. The influence of vacancies on grain boundary diffusivity and grain boundary sliding was negligible for high energy grain boundaries, due to the already high atom mobility in these boundaries.

Critical grain size for dislocation storage and consequences for strain-hardening of nanocrystalline materials

O Bouaziz et al

We consider strain-hardening of nanostructured materials and propose a physically based interpretation of their low strain-hardening capability in terms of a reduced storage rate of dislocations. The model suggested provides a modification of the Kocks-Mecking-Estrin (KME) evolution law for dislocation storage for nanostructured materials and predicts a critical grain size below which the strain-hardening rate drops off.

[1] On grain growth in the presence of mobile particles

V.Yu. Novikov

The ability of second phase particles to migrate along with grain boundaries is shown to be determined not only by the particle mobility but also by the migration rate of the grain boundary where they locate. This leads to a duality in the mobile particle behaviour: they behave as either movable or immovable depending on the boundary migration rate. In the first case, they reduce the boundary mobility; in the second one they decrease the driving force for boundary migration. It is demonstrated by numerical modeling that mobile particles with low mobility can suppress grain growth even in nanocrystalline material, the limiting grains size being several times smaller than in the case of randomly distributed immobile particles. It is also shown that the Zener solution to the problem of the grain growth retardation by disperse particles is a specific case of the proposed approach.

[2] Neutron Larmor diffraction measurements for materials science

J. Repper et al

Neutron Larmor diffraction (LD) is a high-resolution diffraction technique based on the Larmor precession of polarized neutrons. In contrast to conventional diffraction, LD does not depend on the accurate measurement of Bragg angles, and thus the resolution is independent of the beam collimation and monochromaticity. At present, a relative resolution for the determination of the crystal lattice spacing d of Δd/dnot, vert, similar10-6 is achieved, i.e. at least one order of magnitude superior to conventional neutron or X-ray techniques. This work is a first step to explore the application of LD to high-resolution problems in the analysis of residual stresses, where both the accurate measurement of absolute d values and the possibility of measuring type II and III stresses may provide additional information beyond those accessible by conventional diffraction techniques. Data obtained from Inconel 718 samples are presented.

Preface to the view point set Jean-Marc Chaix et al

Thermodynamics and Kinetics of Grain Boundary Triple Junctions in Metals – Recent Developments

G Gottstein et al

We assess the contribution of grain boundary triple junctions to the driving force for grain growth and the “energetic” effect of boundary junctions on grain growth in nanocrystalline materials. The first measurement of grain boundary line tension allows to estimate quantitatively the fraction of the driving force due to boundary triple junctions. For polycrystals with a grain size in the range of not, vert, similar50 nm, it is comparable with the driving force from grain boundaries.

On the triple line in infiltration of liquid metals into porous preforms

J M Molina et al

We address here two questions of current interest: i) are contact angles measured by means of the sessile drop technique of any help to understand liquid metal infiltration into solid porous preforms?, and, ii) to what extent are contact angles derived from either the capillary law or drainage curves valid?. These questions have neither a simple nor a unique answer, as infiltration may occur under very different scenarios, namely, non-reactive and reactive infiltration, both spontaneous and forced. On the other hand, while the use of the simplest version of the capillary law relies upon the questionable slug-flow hypothesis, analyzing drainage curves by means of Brooks and Corey model seems only justified for particular distributions of pore sizes such as a power law. Anyhow, experimental studies indicate that threshold pressures, and therefore contact angles, derived from those two methods are not so different.

Triple lines

March 7, 2010

Did you also notice the explosion of papers dealing with triple lines in Scripta? Here are some more.

[1] The role of triple line in solid state microstructural evolutions: interplay with interfaces and consequences on instabilities and pattern selection

YvesS Brechet

The present contribution illustrates the role of the triple line in solid state microstructural evolution. In single phase materials, the role of the triple line in grain growth phenomena is discussed. In surface alloying, it is shown that the contact angle at the interface plays a key role on the kinetics of the process. In discontinuous precipitation, it is shown that the conditions at the triple line removes the degeneracy of the problem of spacing selection.

[2] Early Stages of Dissolutive Spreading

E Saiz et al

The initial instants of spreading in high-temperature dissolutive systems (Cu and Au on Ni) are recorded using high-speed video and a drop transfer set-up. The results suggest the existence of an initial regime where the advance of the triple junction is too fast compared with dissolution and spreading occurs on a flat unreacted surface. The analysis indicates that spreading is then controlled by the triple line friction. The calculated friction coefficients are similar to those for non-reactive high-temperature systems.

[3] Approaches to atomistic triple-line-properties from first principles

A Hashibon and C Elsaesser

Different approaches to study wetting and adhesion by applying density-functional theory (DFT) methods are highlighted. The ab-initio thermodynamics method is used to demonstrate the link between the calculated work of separation and the work of adhesion and wetting angles from sessile-drop measurements. An approach to extend DFT calculations to the case of large scale interfaces relevant for wetting systems is also discussed.

Triple lines and triple lines

February 13, 2010

A couple of more triple lines papers:

[1] Triple lines in materials science and engineering

A H King

We assess the impact of triple lines in materials preparation and use by considering several examples of materials behavior in which they have identifiable effects. The microstructural roles of triple lines are also considered and some persistent scientific questions are raised.

[2] Grain boundary ridges and triple lines

B B Straumal et al

The classification of various grain boundary (GB) and surface triple lines is given. The observations of “degenerated” GB triple lines like first-order and second-order facet-to-facet, rough-to-facet and rough-to-rough ridges, as well as there influences on the GB mobility are discussed. Some interesting phenomena remain undisclosed in experiments and should be studied in the future.

Triple lines and nucleation

February 5, 2010

A letter from Greer in Scripta.

Some recent papers from scripta:

[1] Kinetics and size effect of grain rotations in nanocrystals with rounded triple junctions

F Yang and W Yang

A kinetic model is developed to quantify the rate of grain rotations driven by either grain boundary energy or stress. The critical roles of triple junctions and grain shape are emphasized. The size effects for the rotation rate are analyzed. As the grain size decreases, the model predicts shifts in the dominating driving forces and dissipation mechanisms.

[2] Direct non-destructive observation of bulk nucleation in 30% deformed aluminum

S S West et al

A 30% deformed aluminum sample was mapped non-destructively using Three-Dimensional X-ray Diffraction (3DXRD) before and after annealing to nucleation of recrystallization. Nuclei appeared in the bulk of the sample. Their positions and volumes were determined, and the crystallographic orientations were compared with the orientations of the deformed grains. It was found that nuclei with new orientations can form and their orientations have been related to the dislocation structure in the deformed grains.

[3] Dynamic abnormal grain growth: A new method to produce single crystals

J Ciulik and E M Taleff

Dynamic abnormal grain growth (DAGG) is a newly discovered phenomenon which can be used to produce large single crystals from polycrystalline material in the solid state at temperatures above approximately half the melting temperature. The unique aspect of DAGG, compared to previously understood abnormal grain growth phenomena, is the requirement of plastic straining for initiation and propagation of abnormal grain growth. Our findings demonstrate that DAGG can be used to produce large single crystals of molybdenum in the solid state.

[4] Evaluation of the liquid-solid interfacial energy from crystallization kinetic data

J Torrens-Serra et al

The kinetic data obtained from the analysis of experimental measurements of nanocrystallization in Fe65Nb10B25 metallic glass are used to successfully estimate the molten alloy viscosity, Fe23B6 crystallization driving force and solid-liquid interface energy in the framework of the classical theory of nucleation and growth. We use a Vogel-Fulcher-Tamman law for the viscosity and linear temperature dependence for the crystallization driving force and interfacial energy. A negative temperature coefficient for the crystal-melt interfacial energy is obtained. Both the thermal stability and the glass forming ability of this alloy are discussed.

[5] Experimental study of the miscibility gap and calculation of the spinodal curves of the Au–Pt system

X N Xu et al

The miscibility gap (MG) of the Au–Pt binary system in the temperature range 600–1050 °C has been experimentally determined by the diffusion couple technique. The results show that the determined MG deviates from the currently accepted one, which shifts to the Au-rich side of the Au–Pt system. Based on the present experimental data, the Au–Pt system has been thermodynamically reassessed, with the result that the critical point of the miscibility gap is not, vert, similar1200 °C at 56 at.% Pt, in contrast to the currently accepted 1260 °C at 61 at.% Pt. The chemical and coherent spinodals of the Au–Pt system have been thus calculated.

[6] Estimation of dislocation density in bainitic microstructures using high-resolution dilatometry

C Garcio-Mateo et al

It is possible by means of high-resolution dilatometry, together with a model based on isotropic dilatation and atomic volumes, to estimate the dislocation density introduced in the microstructure as a consequence of the isothermal decomposition of austenite into bainitic ferrite. The relatively high dislocation density associated with this microstructure is attributed to the fact that the shape deformation accompanying this displacive transformation is accommodated by plastic relaxation.

[7] Magnetic phase transition and magneto-optical properties in epitaxial FeRh0.95Pt0.05 (0 0 1) single-crystal thin film

W Lu et al

This paper reports an investigation of the structure, magnetic phase transition and magneto-optical properties of FeRh0.95Pt0.05 thin film. A first-order magnetic phase transition occurs at a temperature around 180 °C, accompanied by a lattice expansion in the c-axis. The effect of substitution on the phase transition in ordered FeRh-based alloy systems is discussed. The nucleation and growth mechanism of the phase transition is quite similar to that of the crystallization of solids. In addition, the Kerr rotation spectrum was also studied.