[1] Kinetic study of phase transformation in a highly concentrated Fe–Cr alloy: Monte Carlo simulation versus experiments

Pareige et al

An atomic scale analysis of phase separation in a thermally aged Fe–25 at.% Cr alloy at 500 °C using 3-D atom probe (3DAP) and atomistic kinetic Monte Carlo (AKMC) simulation is presented. Treatment of the simulation data with the Lifshitz–Slyozov–Wagner and Huse laws shows that, whereas diffusion along the interfaces and through the bulk both occur at early stages, diffusion through the matrix quickly controls the growth of domains whereas the structure is still interconnected. Comparison of AKMC results with experimental ones showed that AKMC simulation in the two-band model approximation on a rigid lattice is able to reproduce the behaviour of the concentration field and of the width of the domains observed with 3DAP. This comparison also strongly support that, in real Fe–Cr alloys, as well as in simulated systems, diffusion is predominantly through the bulk and controls the growth of domains, while the structure is still interconnected.

[2] Capillarity-driven migration of a thin Ge wedge in contact with a bicrystalline Au film

Radetic et al

We have investigated the retraction of a single-crystalline Ge wedge in epitaxial contact with a bicrystalline Au film using in situ electron microscopy. The rate of retraction was close to that predicted for capillarity-driven surface diffusion, following kinetics proportional to tn, with n = 0.22–0.35, but crystal anisotropy caused migration to be significantly faster along left angle bracket1 0 0right-pointing angle bracket directions than along left angle bracket1 1 0right-pointing angle bracket. The bicrystalline Au substrate was not inert, but underwent abnormal grain growth in the area swept by the receding Ge wedge. Cross-sections made from plan-view transmission electron microscopy samples revealed that this was related to ridge formation during the retraction process. In situ observations of the process in an inclined orientation showed direct evidence of substrate grain boundaries being dragged by the receding Ge wedge. The results can be understood in the framework of capillarity models for isotropic solid-state wedges and reactive wetting in high-temperature liquid–solid experiments.

[3] Near-zero thermal expansivity 2-D lattice structures: Performance in terms of mass and mechanical properties

Palumbo et al

The coefficient of thermal expansivity (CTE), α, of a 2-D dual-material lattice and the effects of varying the constituent materials and geometry were explored in a parametric study. The lattices had geometries similar to those found in lightweight structures in many transport applications including aerospace and spacecraft. The aim was to determine how to reduce the CTE of such structures to near zero, by using two constituent materials with contrasting CTEs, without incurring penalties in terms of other elastic and failure properties, mass and manufacturability. The results are scale independent and so generic to all such lattices. Lattice CTE was primarily driven by the geometry of the lattice and the mismatch in the constituent’s CTE and elastic moduli, with zero CTE attainable if (i) the relative lengths of internal members a and b were in the range of 1.4–1.6, and (ii) the contrast between αb and αa was at least 4. Large negative CTEs could be obtained easily if in addition the ratio of moduli Eb and Ea was more than 10. It was shown that pairings of commonly used materials, in lattices with commonly used geometries, can give near-zero and negative CTEs. It was shown that this dual-material mechanism effectively exchanges distortion for internal stress. With carefully chosen material pairings there were either small or no penalties for the reduced CTE in terms of other key mechanical performance indices, e.g. premature failure. Two lattices were manufactured, one monolithic and one dual-material (grade 2 titanium and aluminium 6082). Their thermal expansivity was measured and found to match closely the analytical model’s prediction.

[1] Influence of constraints and twinning stress on magnetic field-induced strain of magnetic shape-memory alloys

Chmielus et al

Magnetic-field-induced strain of hard 14M and soft 10M martensitic samples of Ni-Mn-Ga single crystals was measured and optically observed in a rotating magnetic field in one-sided and two-sided constraint conditions. The soft sample showed smooth, continuous deformation of 0.6% when two-sided, and 2.7% when one-sided constrained. The hard sample produced no strain when two-sided, and discontinuous deformation of 1.8% when one-sided constrained. Thus, the constraints reduce, even block, twin boundary motion. This finding must be considered in experiment and applications.

[2] Repulsive force vs. source number: Competing mechanisms in the yield of twinned gold nanowires of finite length

Guo and Xia

Nanoscale twin boundaries play a significant role in the yield behavior of nanowires. They serve as both a source of dislocation nucleation and a generator of the repulsive force which acts against the nucleation of dislocations. In the present paper molecular dynamics (MD) simulations are performed to investigate the tensile deformation of twinned gold nanowires (NWs) of finite length. Emphasis is placed on competing mechanisms in the initial yield of nanowires: dislocation source number vs. repulsive force, both of which are controlled by the twin boundary spacing. The simulation results reveal that with decreasing twin boundary spacing there is a transition from softening to strengthening due to a change in the dominant mechanism of plastic deformation. An analytical model based on kinetic rate theory is also established to provide an insight into the competing mechanisms indicated by the simulation results.

[3] Plastic behavior of fcc metals over a wide range of strain: Macroscopic and microscopic descriptions and their relationship

Csanadi et al

The room temperature macroscopic and microscopic plastic behavior of four face-centered cubic metals (Al, Au, Cu and Ni) is investigated experimentally over a wide strain range, and theoretical modeling is used to simulate the established major micromechanisms describing the evolution of mobile and forest dislocations during plastic flow. It is shown that forest dislocations develop primarily due to interaction between mobile dislocations, while the contribution from forest–mobile interactions is only minor. The trapping of mobile dislocations and the annihilation of forest dislocations are both controlled by the same thermally activated dislocation motion. These observations permit a simplification of the theoretical model that leads to an analytical relationship for the evolution of the total dislocation density as a function of strain. From this analysis, correlations are drawn between the macroscopic parameters describing the stress–strain relationship and the fundamental characteristics of the microscopic processes.

Some interesting papers

January 16, 2011

[1] Effect of aging on martensitic transformation, microstructures and mechanical properties in off-stoichiometric Mn–Ni–Ga alloys

W Cai et al

(Mn,Ni)4Ga-type γ precipitates were obtained in Mn53Ni25Ga22 alloys aged at 573–973 K. Aging greatly affects the transformation temperature by modifying the matrix composition and changing the unit cell volume. When the aging temperature is 573 K, needle-like γ precipitates 70–80 nm in length can be seen by transmission electron microscopy. Rotational twin martensite variants were discovered to be related with an angle rotation of 15° in the [1 1 1] direction. A crystalline orientation relationship between the tetragonal martensites and face-centered cubic γ phases was observed as [1 1 1]γ//[1 1 0]M and View the MathML sourceγ//View the MathML sourceM. A substructure of View the MathML source type-I twins for γ phase was found in the 973 K/3 h aged sample in which the coarsened precipitates reveal a plate-like morphology. Compression tests show that a compressive strength of 1850 MPa with a fracture strain up to 25.6% can be achieved in 773 K/10 min aged samples due to a substantial dispersion strengthening from small γ precipitates.

[2] Multiscale modelling of MgO plasticity

J Amodeo et al

We propose a multiscale model of plasticity of pure MgO single crystals. The core structure of the rate controlling ½left angle bracket1 1 0right-pointing angle bracket screw dislocations has been modelled by the Peierls–Nabarro–Galerkin method. This model relies on γ-surfaces calculated ab initio for the {1 1 0}, {1 0 0} and {1 1 1} planes. The ½ left angle bracket1 1 0right-pointing angle bracket screw dislocations spread mostly in the {1 1 0} planes. The Peierls friction values are 150 MPa and 1.6 GPa for glide on the {1 1 0} and {1 0 0} planes, respectively. The kink pair theory is applied to model thermal activation of dislocation glide over the Peierls barrier below the athermal temperature Ta and to build a velocity law for this regime. The critical resolved shear stresses are deduced below Ta from the Orowan law. Above Ta the athermal stress τμ is obtained from discrete dislocation dynamics simulations to account for dislocation–dislocation interactions. This model is found to satisfactorily reproduce the critical resolved shear stresses observed experimentally, provided the contribution of impurities (unavoidable in experiments) is subtracted.

[3] Evolution of defects in copper deformed by high-pressure torsion

J. Čížek et al

Lattice defects in Cu deformed by high-pressure torsion (HPT) were investigated by positron annihilation spectroscopy (PAS) combined with transmission electron microscopy, X-ray diffraction (XRD) and Vicker’s microhardness (HV) measurements. The evolution of the microstructure during HPT processing was studied on samples subjected to various numbers of HPT revolutions using pressures of 2 and 4 GPa. Since strain in torsion deformation increases with the radial distance from the center of rotation, one can expect a non-uniform microstructure across the sample diameter. To examine this, HV was measured at various distances from the center of the HPT-deformed sample and the microstructure at the center was compared with that at the periphery. It was found that HPT-deformed Cu contains a high density of dislocations and also small vacancy clusters formed by the agglomeration of deformation-induced vacancies. The center of the sample exhibits coarser grains, a slightly lower density of dislocations and smaller vacancy clusters compared to the periphery. The dislocation density and concentration of vacancy clusters were evaluated from the combination of the PAS and XRD results. The theoretically estimated concentration of deformation-induced vacancies is of an order of magnitude comparable to that determined in experiment.

[4] A phase field model for isothermal crystallization of oxide melts

J Heulens et al

We present a multicomponent multi-phase field model for isothermal crystallization of oxide melts. The bulk thermodynamic properties of the liquid as a function of composition are retrieved from the FACT thermodynamic database for oxide systems. For solid phases modeled as stoichiometric in the thermodynamic database, a paraboloid Gibbs energy is introduced with specific constraints to ensure correct phase equilibria and minimal solubility in the stoichiometric phase. The interfacial mobility can show strong anisotropy and the interfacial energy can have weak anisotropy, since both faceted and dendritic growth morphologies are important for crystallization in oxide systems. The possibilities of the model are illustrated with three case studies considering crystallizing and dissolving solid phases in a CaO–Al2O3–SiO2 melt. These case studies show the influence of the diffusion mobilities on the diffusion path, the tie-line selection in a ternary system and the effect of the surface energy on dendritic growth.

[5] Effect of pore architecture on magnetic-field-induced strain in polycrystalline Ni–Mn–Ga

X X Zhang et al

Monocrystalline Ni–Mn–Ga alloys show magnetic-field-induced strains (MFIS) of up to 10% as a result of reversible twinning; by contrast, polycrystalline Ni–Mn–Ga shows near-zero MFIS due to strain incompatibilities at grain boundaries inhibiting twinning. Recently, we showed that porous polycrystalline Ni–Mn–Ga exhibits a small, but non-zero, MFIS value of 0.12% due to reduction of these incompatibilities by the porosity. Here, we study the effect of pore architecture on MFIS for polycrystalline Ni–Mn–Ga foams. Foams with a combination of large (not, vert, similar550 μm) and small (not, vert, similar80 μm) pores are fabricated by the replication method and exhibit thinner nodes and struts compared to foam containing only large (not, vert, similar430 μm) pores. When magnetically cycled, both types of foams exhibit repeatable MFIS of 0.24–0.28% without bias stress. As the cycle number increases from a few tens to a few thousands, the MFIS drops due to damage accumulation. The rate of MFIS decrease is lower in the dual-pore foam, as expected from reduced constraints on the twin boundary motion, since twins span the whole width of the thinner nodes and struts.

[6] A quantitative study of solute diffusion field effects on heterogeneous nucleation and the grain size of alloys

Da Shu et al

The nucleation ability of inoculating particles inside the solute diffusion zone around growing grains during alloy solidification is studied using a spherical, equiaxed dendritic grain model coupled with a new modified free growth model to predict the final grain size of cast aluminium alloys with improved accuracy. We show that the nucleation potency of inoculating particles is reduced by the solute field that develops close to existing, growing equiaxed grains under near isothermal conditions. Solute suppressed nucleation leads to much lower nucleated grain densities, higher nucleation undercoolings and longer times to recalescence when further nucleation events are halted. Under solute suppressed conditions, nucleation events occur in two stages: an initial transient nucleation before significant solute build-up and then continuous nucleation. The significance of the transient nucleation regime depends upon the size of the transient solute diffusion zone, and has been explored using the model. Model predictions suggest that the grain refinement of alloys of high solute content is controlled primarily by solute suppressed nucleation conditions.

[1] Twinning Mechanism via Synchronized Activation of Partial Dislocations in Face-Centered-Cubic Materials

B Q Li et al

In situ straining in a high-resolution transmission electron microscope and molecular dynamics simulations reveal a new deformation twinning mechanism in the face-centered-cubic structure. A twin forms via the simultaneous and cooperative activation of different Shockley partial dislocations on three (111) layers. The synchronized slip produces a zero net Burgers vector; such twining relieves local stress concentration in a shear confined to adjacent atomic layers, but induces no macroscopic shape change of the surrounding crystal.

[2] Importance of secondary and ternary twinning in compressed Ti

W Tirry et al

Twin formation during uniaxial compression of high-purity α-Ti at room temperature is investigated for both quasi-static and dynamic conditions using electron backscatter diffraction techniques. The initial texture is favorable for View the MathML source twinning, yet it is observed that secondary and ternary twins occur for both strain rates, showing a higher propensity in the dynamic case. While secondary twins may explain the difference in texture change and strain hardening between the two loading conditions, the ternary twins mainly contribute to grain fractioning.

[3] Limit of steady-state lamellar eutectic growth

Wang and Trivedi

Eutectic microstructure under rapid solidification conditions becomes unstable beyond a certain velocity that places a limit on the finest spacing and the largest interface undercooling that can be achieved for a eutectic microstructure. Expressions are developed to characterize the physics that lead to the limit of eutectic growth. Activation energy for diffusion and eutectic temperature are shown to play key roles at the limit of eutectic growth, and this limit modifies the high velocity branch of the coupled growth regime.

Carbon Diffusivity in Multi-Component Austenite

S-J Lee et al

The diffusivity of carbon in multi-component austenite has been investigated using a thermodynamic-based analysis. The activity coefficient of carbon as a function of alloying elements and temperature was used to derive the carbon diffusivity as a function of composition and temperature. The strong influence of chromium (which decreases carbon diffusion rates) was incorporated in the diffusivity equation. The equation for carbon diffusivity was verified by comparing calculated results with measured diffusivity of carbon for various alloys.

Carbide grain growth in cemented carbides

K Mannesson et al

Abnormal grain growth is often observed in cemented carbides during sintering, but cannot be understood in terms of the classical LSW theory. In this work the grain growth behavior during sintering at 1430 °C is studied both experimentally and by means of computer simulations. A model based on several processes—2-D nucleation of growth ledges, mass transfer across the interface and long-range diffusion coupled in series—is formulated and the equations are solved numerically. Both computer simulations and experimental studies reveal that the grain growth behavior is strongly influenced by the initial size distribution.