[1] Formation of Stray Grains during Directional Solidification of a Nickel-Based Superalloy

Y Zhou

Superalloy CMSX-4 is directionally solidified and initiated by bi-crystal seeds. It has been found that diverging boundaries are the most favorable location for stray grain formation. The phenomenon cannot be attributed to nucleation of crystals. Reasonable mechanism is bending or detachment of side arms during extension of secondary arms and development of tertiary branches at the diverging boundaries. Solute interaction of the neighboring dendrites promotes likelihood of bending or detachment and then leads to an enhanced frequency of stray grains.

► Stray grains are formed particularly around re-entrant features such as the platforms or shroud regions of the turbine blade airfoils and lead to rejection of single crystal superalloy components since the boundSuperalloy CMSX-4 is directionally solidified and initiated by bi-crystal seeds. It has been found that diverging boundaries are the most favorable location for stray grain formation. The phenomenon cannot be attributed to nucleation of crystals. Reasonable mechanism is bending or detachment of side arms during extension of secondary arms and development of tertiary branches at the diverging boundaries. Solute interaction of the neighboring dendrites promotes likelihood of bending or detachment and then leads to an enhanced frequency of stray grains.

[2] Sapphire surface pits as sources of threading dislocations in hetero-epitaxial GaN layers

F Y Meng et al

Sapphire substrates showed nanosized surface pits after the growth of GaN layers using a two-step process by hydride vapor phase epitaxy. Threading dislocations with Burgers vectors of c and c+a were found to originate from the pits. Mechanism of their generation is developed from cross-sectional transmission electron microscopy observations.

[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] 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.

In Nature this week

August 10, 2007