[1] Dislocation dynamics simulations of dislocation interactions and stresses in thin films

R S Fertig III and S P Baker

The dislocation interactions that stop threading dislocations (threads) during relaxation at increasing applied strains in single-crystal thin films are investigated using large-scale three-dimensional dislocation dynamics simulations. Threads were observed to stop via interactions with both threads and misfit dislocations (misfits). Both types of interactions were shown to depend on stress inhomogeneity. Low-stress regions enabled threads to stop in weak thread–misfit interactions even at high average film stresses. Threads were also concentrated in low-stress regions, which facilitated their interaction with other threads. Threads accumulated in thread–thread interactions, and stopped only temporarily in thread–misfit interactions. The mean free path for dislocation motion is shown to be accurately predicted from details of the inhomogeneous stress state arising from the applied strain and the misfit structure. These behaviors are analyzed to present a more complete picture of film strength, strain hardening and relaxation.

[2] Comparing grain boundary energies in face centered cubic metals: Al, Au, Cu and Ni

E A Holm et al

The energy of 388 grain boundaries in Al, Au, Cu and Ni were calculated using atomistic simulations. Grain boundary energies in different elements are strongly correlated. Consistent with a dislocation model for grain boundary structure, the boundary energy scales with the shear modulus. Boundaries with substantial stacking fault character scale with the stacking fault energy. There is more scatter in the data for Al, which has a high stacking fault energy, than for the low stacking fault energy elements.

In situ TEM observation of stress-induced martensitic transformations and twinning processes in CuAlNi single crystals

N Zarubova et al

Stress-induced martensitic transformations and twinning processes were studied in thin foils of CuAlNi single crystals strained in situ in a transmission electron microscope. The nucleation and growth of the martensite plates were monitored for three transformation processes known from bulk experiments: (i) the transformation of austenite into 2H martensite at low-stress levels; (ii) the twinning/detwinning processes in 2H martensite; and (iii) the transformation between austenite and 18R martensite at higher stress levels. The morphology of the austenite/martensite habit planes was examined, and the existence of planar interfaces between a single variant of 2H martensite and austenite on the microscopic level was proven.

The role of self-shadowing on growth and scaling laws of faceted polycrystalline thin films

C Ophus et al

We investigate, via both experiment and simulation, the effects of self-shadowing on the growth of faceted polycrystalline thin films. Faceted aluminum thin films were sputtered and the anomalous scaling behaviour of their surfaces was characterized. To understand the causes of this anomalous behavior, growth of faceted thin films was simulated by coupling a level set construction to a ballistic deposition model. The angular distribution function of deposition flux was varied to control the degree of self-shadowing. We show how differing degrees of self-shadowing strongly modify film surface morphologies and compare these results with experimental findings.

[1] Enhanced multiferroic properties and domain structure of La doped BiFeO3 thin films

F Yan et al

BiFeO3 (BFO) and La-doped BFO (BLFO) thin films are grown on Pt/TiO2/SiO2/Si substrate using pulsed laser deposition. The domain structure of BFO and BLFO are investigated via piezoresponse force microscopy. Highly enhanced ferroelectric properties with great remanent polarization (Pr) of 102 μC/cm2 and decreased leakage current density are obtained via La-doping. Magnetic property is also increased by La doping ascribed to spatial homogenization of spin arrangement. The mechanisms for the enhancement of ferroelectric and ferromagnetic characteristics are discussed.

[2] Experimental method for true in situ measurements of shear-coupled grain boundary migration

T Gorkaya et al

A novel set-up developed to continuously observe and measure stress-driven grain boundary migration is presented. A commercial tensile/compression scanning electron microscope hot stage was utilized for in situ observations of mechanically loaded samples at elevated temperatures up to 850 °C by recording orientation contrast images of bicrystal surfaces. Two sample holders were designed and fabricated for applying a shear stress to the boundary in bicrystals of various geometries. The results of the first measurements in Al bicrystals are presented.

Some recent papers!

May 6, 2010

[1] Structural and compositional homogeneity of InAlN epitaxial layers nearly lattice-matched to GaN

J M Manuel et al

A group of InAlN films was fabricated by molecular beam epitaxy and investigated by X-ray diffraction, transmission electron microscopy and element nano-analyses. All top InxAl1−xN layers have compositions around lateral lattice-matching to GaN (x ≈ 0.18) and are pseudomorphic. For a growth rate of 350 nm h−1, each InAlN film separated into two sublayers with different In/Al-ratios. Micrographs reveal sharp transitions both at the InAlN/GaN and at the InAlN/InAlN interfaces. In contrast to these separated layers, an optimized epitaxy using an AlN interlayer and a lower growth rate, 100 nm h−1, enabled the fabrication of a single-phase InxAl1−xN layer on GaN, homogeneous on a nanoscopic scale.

[2] Microstructural stability in multi-alloy systems: Nanostructured two-phase, dual alloy multilayers

X Pan et al

Interdiffusion and microstructural stability in multilayers consisting of two nanostructured two-phase alloys were investigated using phase field simulations. A prototype ternary system containing a miscibility gap was used as the model system. Alloys with various compositions within the miscibility gap were chosen to form 20 μm bilayer repeating units in the multilayers. The initial microstructures in the alloys were produced by spinodal decomposition, which yielded a uniform distribution of precipitates having an average diameter of about 50 nm. Two types of multilayers were investigated; one in which the alloys in the repeating unit had the same matrix phase and the other in which the alloys had a different matrix phase. In general the microstructural instability measured as the size of the reaction zone increased with the increase in composition difference between the two initial alloys and in atomic mobility difference between the diffusing species. In particular, when the matrix phase was the same a precipitate-free zone (i.e. a single-phase layer) and a coarse interconnected precipitate zone formed at the interface between the two alloys, while when the matrix phase differed two precipitate-free zones formed at the interface. The microstructural instabilities were analyzed in terms of variations in the effective diffusivity with composition, which produced a singularity in the diffusion path at the initial alloy interface. The instability caused the diffusion path to exit the two phase region of the phase diagram and enter the single phase region.

[3] The Shape of Bubble in He Implanted Cu and Au

Q Wei et al

Bubble evolution under thermal annealing has been studied in He implanted Cu and Au by in situ transmission electron microscopy (TEM). We show that, under the minimum energy requirement of system, the bubble developed into an octahedron ( or truncated octahedron) shape consisting of {111} planes in the manner predicted by the Wulff construction. Nonspherical shape of bubbles and sessile dislocations along the edges of octahedron provide a barrier to Ostwald ripening and migration of bubbles, leading to the low mobility of bubble under thermal annealing.

[1] A new ultrahigh-strength stainless steel strengthened by various coexisting nanoprecipitates

W Xu et al

A general computational alloy design approach based on thermodynamic and physical metallurgical principles and coupled with a genetic optimization scheme is presented. The model is applied to develop a new ultrahigh-strength maraging stainless steel. The alloy composition and heat treatment parameters are integrally optimized so as to achieve microstructures of fully lath martensite matrix strengthened by multiple precipitates of MC carbides, Cu particles and Ni3Ti intermetallics. The combined mechanical properties, corrosion resistance and identification of actual strengthening precipitates in the experimental prototype produced on the basic of the model predictions provide a strong justification for the alloy design approach.

[2] An investigation of the effect of structural order on magnetostriction and magnetic behavior of Fe–Ga alloy thin films

A Javed et al

This paper reports results from a comprehensive study of Fe–Ga films fabricated over a wide range of growth conditions. Polycrystalline Fe100−xGax films (14 less-than-or-equals, slant x less-than-or-equals, slant 32) were deposited (using three different combinations of growth parameters) on Si(1 0 0) using a co-sputtering and evaporation technique. The growth parameters (sputter power, Ga evaporation rate and chamber pressure) were used primarily to control the Fe:Ga ratio in the films. X-ray diffraction showed that all films had left angle bracket1 1 0right-pointing angle bracket crystallographic texture normal to the film plane. The lattice parameter increased with % Ga up to x = 22 and was independent of growth parameters. Conversion electron Mössbauer spectroscopy studies showed a predominance of the disordered A2 phase in all films. It appears that the use of vacuum deposition with appropriate parameters can effectively suppress the D03 ordered phase. Systematic studies of the effective magnetostriction constant as a function of composition support this conclusion. It was found that films of high effective saturation magnetostriction constant and low stress could be fabricated using low Ar pressure, irrespective of sputter power or evaporation rate, giving properties useful for application in microelectromechanical systems.

[3] Coarsening of a multimodal nickel-base superalloy

K Coakley et al

The coarsening of γ′-Ni3Al precipitates in the nickel superalloy Ni115 has been examined and compared to the results of a numerical model based on LSW coarsening theory. Ni115 has a γ′ fraction of around 60%, and at the coarsening temperatures of interest the γ′ distribution is bimodal, with two populations not, vert, similar5 nm and not, vert, similar90 nm in radius. It is found that during the initial transient (around 2000 h at 800 °C), the fine γ′ dissolve, leading to a rapid increase in the mean radius followed by a plateau. At long times, the expected steady-state unimodal t1/3 coarsening is observed. The model reproduces these features in form and approximately in magnitude, a first for LSW model-experiment comparisons in nickel superalloys.

[4] Growth morphologies in peritectic solidification of Fe–C: A phase-field study

A Choudhury et al

We use a thermodynamically consistent multi-phase, multi-component phase-field model, where the evolution equations for the different fields are derived from an entropy functional, for simulating peritectic growth structures in two and three dimensions. Different solidification morphologies are obtained in the computations and the characteristic properties of the growth forms are discussed. The phase-field method allows for a prediction of the surface energies in the three-phase system δ-ferrite, γ-austenite and liquid based on comparison between experimentally observed and simulated structures. Additionally an investigation of possible nucleation sites in evolving domains is presented and its dependence on the solid–solid surface energy is examined.

[5] Topological characteristics of plane sections of polycrystals

G S Rohrer and H M Miller

Homology metrics have been used to assess the connectivity of grain boundary networks in plane sections of polycrystals. The analysis is based on orientation maps, and four characteristic microstructure types were examined: SrTiO3 microstructures with normal and bimodal grain size distributions and two Ni microstructures with different concentrations of Σ3 grain boundaries. The inverse connectivity, defined as the ratio of the number of independent pieces of the network to the number of closed loops, is proposed as a metric for the extent to which certain types of grain boundaries are connected. The variation in inverse connectivity with disorientation threshold, below which boundaries are excluded from the network, produces distinct signatures for the different microstructures.

[6] Controlling Ag whisker growth by using very-thin metallic films

H Tohmyoh et al

The selective growth of Ag nano-whiskers on polycrystalline films has been realized by introducing an additional artificial layer onto the films. Ag nano-whiskers with diameters of about 200 nm and lengths of around 3 μm have been successfully generated from Ag films covered with a 1 nm-thick SiO2 layer. On the other hand, the formation of Ag whiskers/hillocks on the top surface of the film could be suppressed by using thick SiO2 layers or ductile Au layers.

[1] The effects of grain grooves on grain boundary migration in nanofilms

A Novick-Cohen et al

Using numerical computations and asymptotic analysis, we study the effects of grain grooves on grain boundary migration in nanofilms, focusing for simplicity on axisymmetric bicrystals containing an embedded cylindrical grain located at the origin. We find there is a critical initial grain radius, R*, such that if RR*, groove growth during grain shrinkage leads to film break-up. The central cross-section of the grain boundary profile is seen to be parabolic, and an ordinary differential equation which depends on the tilt angle and the groove depth is seen to govern the location of the groove root. Near the annihilation–pinch-off transition, temporary stagnation occurs; thereafter, the shrinking grain accelerates rapidly, then disappears.

[2] Misfit strain–film thickness phase diagrams and related electromechanical properties of epitaxial ultra-thin lead zirconate titanate films

Q Y Qiu et al

The phase stability of ultra-thin (0 0 1) oriented ferroelectric PbZr1–xTixO3 (PZT) epitaxial thin films as a function of the film composition, film thickness, and the misfit strain is analyzed using a non-linear Landau–Ginzburg–Devonshire thermodynamic model taking into account the electrical and mechanical boundary conditions. The theoretical formalism incorporates the role of the depolarization field as well as the possibility of the relaxation of in-plane strains via the formation of microstructural features such as misfit dislocations at the growth temperature and ferroelastic polydomain patterns below the paraelectric–ferroelectric phase transformation temperature. Film thickness–misfit strain phase diagrams are developed for PZT films with four different compositions (x = 1, 0.9, 0.8 and 0.7) as a function of the film thickness. The results show that the so-called rotational r-phase appears in a very narrow range of misfit strain and thickness of the film. Furthermore, the in-plane and out-of-plane dielectric permittivities ε11 and ε33, as well as the out-of-plane piezoelectric coefficients d33 for the PZT thin films, are computed as a function of misfit strain, taking into account substrate-induced clamping. The model reveals that previously predicted ultrahigh piezoelectric coefficients due to misfit-strain-induced phase transitions are practically achievable only in an extremely narrow range of film thickness, composition and misfit strain parameter space. We also show that the dielectric and piezoelectric properties of epitaxial ferroelectric films can be tailored through strain engineering and microstructural optimization.

[3] A more accurate two-dimensional grain growth algorithm

E A Lazar et al

We describe a method for evolving two-dimensional polycrystalline microstructures via mean curvature flow that satisfies the von Neumann–Mullins relation with an absolute error O(Δt2). This is a significant improvement over a different method currently used that has an absolute error O(Δt). We describe the implementation of this method and show that while both approaches lead to indistinguishable evolution when the spatial discretization is very fine, the differences can be substantial when the discretization is left unrefined. We demonstrate that this new front-tracking approach can be pushed to the limit in which the only mesh nodes are those coincident with triple junctions. This reduces the method to a vertex model that is consistent with the exact kinetic law for grain growth. We briefly discuss an extension of the method to higher spatial dimensions.

[4] Point defects in multicomponent ordered alloys: Methodological issues and working equations

R Besson

The aim of this work is to give the independent-point-defect thermodynamics of ordered compounds a sufficiently general flavour, adapted to and working for multicomponent alloys. Generalizing previous approaches, we first show that an appropriate description for a crystal with point defects allows treatment of the practically important pressure and defect volume parameters in the grand canonical framework, the equivalence of which is explicited with the closer to experiments isothermal–isobaric conditions. Since industrial applications often involve multialloyed compounds, we then derive an operational tool for atomic-scale investigations of long-range order alloys with complex crystallographies and multiple additions.

[5] Misorientation texture development during grain growth. Part II: Theory

J Gruber et al

A critical event model for the evolution of number- and area-weighted misorientation distribution functions (MDFs) during grain growth is proposed. Predictions from the model are compared to number- and area-weighted MDFs measured in Monte Carlo simulations with anisotropic interfacial properties and several initial orientation distributions, as well as a dense polycrystalline magnesia sample. The steady-state equation of our model appears to be a good fit to all data. The relation between the grain boundary energy and the normalized average boundary area is discussed in the context of triple junction dynamics.

[6] Spatial correlations in symmetric and asymmetric bicontinuous structures

A L Genau and P W Voorhees

Spatial correlations of interfacial curvature are compared for symmetric and asymmetric two-phase mixtures produced following spinodal decomposition as given by a numerical solution to the Cahn–Hilliard equation in three dimensions. By calculating radial distribution functions of the density of interfacial area as a function of the mean interfacial curvature of these bicontinuous microstructures, it is found that long-range diffusive interactions, in combination with the morphology of the system, yield a variety of correlations and anticorrelations over a range of length scales. The asymmetric mixtures show some similarities to the symmetric mixtures, as well as other unique features.

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.