Phase decomposition and ordering in Ni-11.3 at.% Ti studied with atom probe tomography.

The decomposition behavior of Ni-rich Ni-Ti was reassessed using Tomographic Atom Probe (TAP) and Laser Assisted Wide Angle Tomographic Atom Probe. Single crystalline specimens of Ni-11.3 at.% Ti were investigated, the states selected from the decomposition path were the metastable γ″ and γ' states introduced on the basis of small-angle neutron scattering (SANS) and the two-phase model for evaluation. The composition values of the precipitates in these states could not be confirmed by APT data as the interface of the ordered precipitates may not be neglected. The present results rather suggest to apply a three-phase model for the interpretation of SANS measurements, in which the width of the interface remains nearly unchanged and the L12 structure close to 3:1 stoichiometry is maintained in the core of the precipitates from the γ″ to the γ' state.

TEM and HREM study of Al3Zr precipitates in an Al-Mg-Si-Zr alloy.

The structure of Al(3)Zr precipitates in Al-1.0Mg-0.6Si-0.5Zr (in wt.%) alloy was investigated using conventional transmission electron microscopy (TEM) and high-resolution TEM (HREM). After annealing of the alloy in the temperature range 450-540 degrees C, spherical precipitates of metastable L1(2)-Al(3)Zr phase appeared nearly homogeneously within the matrix, and elongated particles were found at grain boundaries. L1(2)-structured Al(3)Zr were about 20-30 nm in diameter and coherent with the matrix. Inside some of them, planar faults parallel to {100} planes were revealed by use of HREM. Most probably, these faults are an indication of the transition stage of transformation to the stable D0(23)-type Al(3)Zr phase. The elongated precipitates (about 100 nm) were identified as D0(22)-type Al(3)Zr. Energy-dispersive X-ray analysis showed that they contain, apart from Al, mainly Zr with small amounts of Si. The substitution of Al by Si increased the stability of the D0(22)-Al(3)Zr as compared with D0(23)-Al(3)Zr.

Ni(3)Si(Al)/a-SiO(x) core-shell nanoparticles: characterization, shell formation, and stability.

We have used an electrochemical selective phase dissolution method to extract nanoprecipitates of the Ni(3)Si-type intermetallic phase from two-phase Ni-Si and Ni-Si-Al alloys by dissolving the matrix phase. The extracted nanoparticles are characterized by transmission electron microscopy, energy-dispersive x-ray spectrometry, x-ray powder diffraction, and electron powder diffraction. It is found that the Ni(3)Si-type nanoparticles have a core-shell structure. The core maintains the size, the shape, and the crystal structure of the precipitates that existed in the bulk alloys, while the shell is an amorphous phase, containing only Si and O (SiO(x)). The shell forms around the precipitates during the extraction process. After annealing the nanoparticles in nitrogen at 700 °C, the tridymite phase recrystallizes within the shell, which remains partially amorphous. In contrast, on annealing in air at 1000 °C, no changes in the composition or the structure of the nanoparticles occur. It is suggested that the shell forms after dealloying of the matrix phase, where Si atoms, the main constituents of the shell, migrate to the surface of the precipitates.

Nano-structured materials produced from simple metallic alloys by phase separation.

A method which is able to produce different types of nano-structured materials, namely nano-particles, nano-structured surfaces and nano-porous membranes, from two-phase metallic alloys is reviewed. The new process first establishes nano-structures in the bulk alloy and then separates them by selective phase dissolution. Variation in processing makes it possible to produce different types of nano-structure even from the same alloy. The process can be applied to many different alloy systems. An overview is presented emphasizing the versatility of the process with examples of different nano-structure types that can be produced. Further, the new method is discussed in relation to similar processes (specifically, electrochemical processes) which have been used for nano-structure synthesis.

Bulk interfaces in a Ni-rich Ni-Au alloy investigated by high-resolution Z-contrast imaging.

Interfaces between Au-rich precipitates and the Ni-rich matrix in a decomposed Ni-10 at.% Au alloy were investigated by low-magnification and high-resolution Z-contrast imaging. During aging at 923 K, the originally single crystalline sample decomposed and recrystallized resulting in a microstructure consisting of subgrains separated by small-angle grain boundaries. These small-angle grain boundaries are decorated by Au-rich precipitates. The interfaces between the Au-rich precipitates and the Ni-rich matrix were characterized with respect to the orientation relationship between precipitates and matrix, misfit dislocations and concentration gradients. Two transformation modes were identified that are involved in the decomposition of bulk Ni-rich Ni-Au alloys. While in the first mode the interface is semi-coherent, in the second mode the interface corresponds to an incoherent twin boundary. It is further shown that strain fields around misfit dislocations can result in systematic errors in the determination of the concentration gradients across interfaces between precipitates and matrix.

Self-assembled semiconductor quantum dots with nearly uniform sizes.

Self-assembled PbSe quantum dots on PbTe quasisubstrates, where the quasisubstrate layer is grown on Si(111), show a size distribution as low as 2%, below any other reported size distribution. The result is explained by nonoverlapping diffusion radii for most of the dots and their nucleation occurring mainly at defects (glide steps of the quasisubstrate), conditions which are not met by self-assembled quantum-dot structures in other material systems.

Quantitative characterisation of chemical inhomogeneities in Al-Ag using high-resolution Z-contrast STEM.

The incoherent imaging model is applied to interpret high-resolution Z-contrast micrographs. A simple method for a column-by-column resolved characterisation of Ag-rich precipitates in Al-Ag is developed. No information on the detailed imaging process is required. Evaluating the high-angle scattering intensities of Al and Ag by image analysis, the number of Ag atoms contained in individual atomic columns can be determined accurately and moreover, the thickness of the thin foil can be calculated. Multislice simulations confirm the broad validity of the incoherent imaging model for Z-contrast STEM and are used to check the method presented. Finally, the image analysis is applied to experimental Z-contrast images of Guinier-Preston zones in Al-3 at% Ag. The Ag content of the individual atomic columns can be determined with an accuracy better than +/-10%.

The asymptotic leading term of anisotropic small-angle scattering intensities. II. Non-convex particles.

For anisotropic particulate samples with scattering contrast (delta n)(2), the leading asymptotic term of the scattering intensity, along a direction q (q/q) of reciprocal space, is [4 pi(2)(delta n)(2)/q(4)]sigmaj [1/kappaG.j(+/-q)]. Here, kappaG.j(+/-q)denotes the Gaussian curvature value at the points (labelled by j) of the interphase surface where the normal is either parallel or antiparallel to q. If the Gaussian curvature vanishes at, say, the jth of these points, the corresponding contribution takes the form Cj/qalpha with 2< or = alphaj<4, Cj and alphaj being determined by the local behaviour of the surface. However, the intensity detected by a counter pixel, with opening solid angle deltaomega(q0) along (mean) direction q0, asymptotically still behaves as 4pi(2) (delta n)(2)(deltaomega(q0))/q(4), where S(deltaomega(q0)) is the area of that part of the interface that has its normals inside deltaomega(q0).

Bloch waves and weak-beam imaging of crystals

The influence of the number of diffracted beams on weak-beam contrast simulations of thickness contour lines and dislocation images is investigated. For large deviation parameters s(g)-->thickness contour lines from two-beam simulations are similar to those from many-beam simulations. In many-beam simulations of wedge-shaped bent samples extra thickness contour lines appear at locations with ((g)-->, 3(g)-->). These extra lines occur between the imaging condition ((g)-->, -(g)-->) and ((g)-->, 3(g)-->). Therefore, in the case of a more symmetrical imaging condition many-beam simulations are mandatory. In bent samples the contributions of different Bloch waves to weak-beam images change as a function of the imaging conditions ((g)-->, x(g)-->). Near ((g)-->, 3.5(g)-->) two Bloch waves dominate. In the case of x <3 two other Bloch waves with different wavelengths are most important for the image contrast. The 'classical' ((g)-->, 3(g)-->) weak-beam condition is not suitable to determine signs and magnitudes of Burgers vectors from terminating thickness contour lines. Higher deviation parameters s(g)--> are necessary, especially for dense dislocation arrangements.