Early subnanometre cluster formation during quenching of a high-strength AA7449 aluminium alloy was investigated using in situ small angle X-ray scattering. Fast quench cooling was obtained by using a laser-based heating system. The size and number density of homogeneous nucleated clusters were found to be strongly dependent on the cooling rate, while the volume fraction of cluster formation is independent of the cooling rate. Heterogeneous larger precipitation starts at higher temperatures in volume fractions that depend on the cooling rate. V C 2014 AIP Publishing LLC. The fabrication of aluminium alloys involves a number of thermomechanical steps such as solute-heat treatment followed by fast cooling, i.e., quenching. It is well known that the cooling rate influences the homogeneous and heterogeneous formation of precipitates and that the effect of these differences on the mechanical properties cannot be removed by additional aging. The typical precipitation sequence of the equilibrium g phase (Mg(Zn,Cu,Al) 2 ) in the Al-Zn-Mg(-Cu) system is: GPZ ! g 0 ! g. 1 Early studies by Lendvai and L€ offler on Al-Zn-Mg alloys report that vacancy-rich clusters (VRC), acting as nucleation sites for Guinier Preston zones (GPZ), can form during the cooling from the solutionizing temperature. 2,3 The number of VRCs strongly depends on the quench parameters influencing the excess vacancies in the structure. Clusters formed by only a few atoms 4 have been identified using 3D atom probe. Despite their small size, the clusters are effective in increasing the yield strength of the material. 5 In addition to the VRCs, heterogeneous precipitation of the equilibrium g phase occurs at higher temperatures. 1 It is therefore not surprising that when producing thick Al alloy plates, the resultant precipitation size, density, and volume fraction are expected to differ across the plate because of the difference in cooling rates. The latter creates residual stresses at as quenched temper that are reduced by stress relief. The remaining residual stresses at final temper may lead to machining distortions. To investigate the influence of precipitation on residual stresses, thermomechanical models linking solid-state transformations to the final stress distribution have to be developed. Such simulation schemes need input on size, density, and volume fraction of precipitation as function of cooling rates that can only be provided by experimentation.Small angle X-ray scattering (SAXS) has proven to be a useful tool for the investigation of precipitation phenomena. 6 SAXS is a well established technique for investigating clusters with high contrast in atomic number with respect to the matrix. SAXS has been used extensively to study precipitation phenomena in Al alloys ex situ and also in situ during aging. [6][7][8] Providing information on the size and volume fraction of the precipitates, SAXS validated thermodynamic-based precipitation model predictions 7,9 of the effect of aging on precipitation. There is however no information to be gathered...
In the presence of phosphate anions, spindle‐shaped mesoscale hematite particles can be prepared via controlled precipitation of iron(III) chloride. The aspect ratio of the particles is determined by the concentration of phosphate anions selectively covering specific crystal surfaces and thus enabling anisotropic growth of the particles. The scattering function for suspensions of polydisperse spindles is derived and used to analyse the small‐angle scattering resulting from these particles. In the presence of an external magnetic field, the particles align perpendicular to the field direction as a result of the negative anisotropy of their magnetic susceptibility . Hereby, an isotropic–nematic phase transition can be induced in external magnetic fields.
The dynamic behavior of charge-stabilized colloidal particles in suspension was studied by photon correlation spectroscopy with coherent X-rays (XPCS). The short-time diffusion coefficient, D(Q) , was measured for volume concentrations phi < or = 0.18 and compared to the free particle diffusion constant D(0) and the static structure factor S(Q) . The data show that indirect, hydrodynamic interactions are relevant for the system and hydrodynamic functions were derived. The results are in striking contrast to the predictions of the PA (pairwise-additive approximation) model, but show features typical for a hard-sphere system. The observed mobility is however considerably smaller than the one of a respective hard-sphere system. The hydrodynamic functions can be modelled quantitatively if one allows for an increased effective viscosity relative to the hard-sphere case.
The scattering function for Schulz±Flory distributed spherical core±shell particles is derived analytically. A constant ratio of core to shell radii is assumed. The analytical expression, which does not require any numerical integration, provides a fast way to model experimental data by nonlinear leastsquares ®tting. The asymptotic behavior for large momentum transfers coincides with the different power laws expected for homogenous spheres and thin spherical shells. In the second part, the derived expression is applied to describe experimental small-angle X-ray scattering data from core±shell particles with different particle sizes, polydispersity and ratio of core to shell radii. For large particles, a resolution correction by numerical convolution with a Gaussian resolution function is applied.research papers
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