A new method for evaluating the size of plate-like precipitates by small-angle scattering

Improving the reliability of small- and wide-angle X-ray scattering measurements of anisotropic precipitates in metallic alloys using sample rotation

Nanometric precipitates in metallic alloys often have highly anisotropic shapes. Given the large grain size and non-random texture typical of these alloys, performing small- and wide-angle X-ray scattering (SAXS/WAXS) measurements on such samples for determining their characteristics (typically size and volume fraction) results in highly anisotropic and irreproducible data. Rotations of flat samples during SAXS/WAXS acquisitions are presented here as a solution to these anisotropy issues. Two aluminium alloys containing anisotropic precipitates are used as examples to validate the approach with a -45°/45° angular range. Clear improvements can be seen on the SAXS I(q) fitting and the consistency between the different SAXS/WAXS measurements. This method-ology results in more reliable measurements of the precipitate's characteristics, and thus allows for time- and space-resolved measurements with higher accuracy.

Characterization of Zr-Containing Dispersoids in Al-Zn-Mg-Cu Alloys by Small-Angle Scattering

The characterization of Zr-containing dispersoids in aluminum alloys is challenging due to their broad size distribution, low volume fraction, and heterogeneous distribution within the grains. In this work, small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) were compared to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) regarding their capability to characterize Zr-containing dispersoids in aluminum alloys. It was demonstrated that both scattering techniques are suitable tools to characterize dispersoids in a multi-phase industrial 7xxx series aluminum alloy. While SAXS is more sensitive than SANS due to the high electron density of Zr-containing dispersoids, SANS has the advantage of being able to probe a much larger sample volume. The combination of both scattering techniques allows for the verification that the contribution from dispersoids can be separated from that of other precipitate phases such as the S-phase or GP-zones. The size distributions obtained from SAXS, SANS and TEM showed good agreement. The SEM-derived size distributions were, however, found to significantly deviate from those of the other techniques, which can be explained by considering the resolution-limited restrictions of the different techniques.

A new perspective on the precipitation sequence in a high-purity Al-1.74 at.% Cu alloy by employing positron annihilation spectroscopy: experiment and theory

We study Al-1.74 at.% Cu as a typical binary model alloy to obtain new information on the precipitation sequence. The alloy has been solution heat treated, rapidly quenched, and then isochronally annealed up to 540 °C. We reveal new effects on the evolution of the precipitation sequence by combining positron annihilation lifetime and coincidence Doppler broadening spectroscopies. Positron parameters do very sensitively respond to changes in the type of precipitates present. We find that in the as-quenched state after exposing the alloy just a few minutes to room temperature GP-I zones containing vacancies inside their copper platelet dominate. They represent about 80% of all detected GP-I zones. At 200 °C GP-II (Θ″) is the dominating type of precipitate. However, the GP-II zones have obviously expelled all structural vacancies on growing from larger GP-I zones. Also the transition from GP-II zones dissolving while precipitates of the Θ' phase do form at about 250 °C can be clearly observed. The signals support that Θ' precipitates contain vacancies on their Cu sublattice, since they have to grow in a copper-poor environment. Finally, our signals indicate coarsening of Θ'-precipitates forT> 400 °C and their dissolution until 450 °C. All our experimental results agree well withab initiotheoretical calculations of positron parameters. However, the formation of the equilibrium Θ-phase cannot be observed since these have a too low number density due to their large size.

Structural evolution and mechanism of strain glass transition in Ti48.7Ni51.3 shape memory alloy studied by anomalous small-angle X-ray scattering

The in-situ anomalous small-angle X-ray scattering (ASAXS) technique was used to investigate the strain glass transition (SGT) in as-quenched Ti_48.7Ni_51.3 shape memory alloy during a thermal cycle of 30 °C to the SGT temperature T_g (−50 °C) and then to 30 °C again. The Ni atoms play a critical role as point defects in the SGT mechanism and are very difficult to characterize using conventional tools. ASAXS identified the distribution of Ni atoms in nanodomains, which have a disk-like core–shell configuration with a Ni-rich shell and a highly Ni-rich core. Moreover, the morphological evolution, growth and shrinkage of the highly Ni-rich core domains during the thermal cycle through T_g are demonstrated. The enhancement and reversible behavior of the local lamellar ordering arrangement of nanodomains during the SGT process at T_g are revealed. The structural evolution and local ordering arrangement of nanodomains can play a role in hindering martensitic transformation. The ASAXS results provide new knowledge about the SGT beyond that from current simulation works. However, this corresponding structure of the nanodomains was destroyed when the specimen was heated to 250 °C.

Characterization of anisotropic pores and spatially oriented precipitates in sintered Mo-base alloys using small-angle neutron scattering

Small-angle neutron scattering (SANS) is a powerful method for the characterization of materials in the mesoscopic size range. For example, the method can be used to investigate the precipitation mechanisms in powder metallurgically processed materials. As a result of the processing route, the alloy matrix is usually heavily textured. If precipitates have an orientation relationship to the alloy matrix, they can produce an anisotropic scattering pattern showing streaks. The scattering is superimposed by a background with ellipsoidal shape, originating from deformed large-scale structures. The evaluation of such data quickly becomes elaborate and a quantitative analysis of precipitation is difficult. The present work reports a method for treating the anisotropic scattering from such samples. A systematic study of the ellipsoidal background reveals that it originates from uniaxially deformed sinter pores. Irrespective of the degree of deformation during the processing route, SANS shows that sinter pores remain present in the matrix, and their morphology and relative volume fractions are determined. Consequently, their scattering signal can be subtracted to reveal the scattering from aligned precipitates. The method is demonstrated on powder metallurgically produced pure Mo and an Mo–Hf–C alloy.

A Small-Angle Neutron Scattering Environment for In-Situ Observation of Chemical Processes

A new sample environment for the observation of ongoing chemical reactions is introduced for small-angle neutron scattering (SANS) experiments which enables structural changes to be followed continuously across a wide Q-range in response to changes in the chemical environment. The approach is demonstrated and validated by performing single and multiple potentiometric titrations on an aqueous anionic surfactant solution (oligo-oxyethylene alkylether carboxylic acid in D₂O) with addition times varying from 1 s to 2 h. It is shown that the continuous flow set-up offers considerable advantages over classical ‘static’ measurements with regards to sample throughput, compositional precision and the ability to observe fast structural transitions. Finally, the capabilities and ongoing optimisation of the sample environment are discussed with reference to potential applications in the fields of biology, colloidal systems and complex soft matter.

Direct view on the phase evolution in individual LiFePO4 nanoparticles during Li-ion battery cycling

Phase transitions in Li-ion electrode materials during (dis)charge are decisive for battery performance, limiting high-rate capabilities and playing a crucial role in the cycle life of Li-ion batteries. However, the difficulty to probe the phase nucleation and growth in individual grains is hindering fundamental understanding and progress. Here we use synchrotron microbeam diffraction to disclose the cycling rate-dependent phase transition mechanism within individual particles of LiFePO₄, a key Li-ion electrode material. At low (dis)charge rates well-defined nanometer thin plate-shaped domains co-exist and transform much slower and concurrent as compared with the commonly assumed mosaic transformation mechanism. As the (dis)charge rate increases phase boundaries become diffuse speeding up the transformation rates of individual grains. Direct observation of the transformation of individual grains reveals that local current densities significantly differ from what has previously been assumed, giving new insights in the working of Li-ion battery electrodes and their potential improvements.

Understanding phase transitions in electrodes under realistic conditions is important for future battery design. Here, the authors use synchrotron micro-beam diffraction to reveal the phase transition mechanism within individual particles of LiFePO₄, revealing a cycling rate transformation mechanism.

In situcharacterization of β′′ precipitation in an Al–Mg–Si alloy by anisotropic small-angle neutron scattering on a single crystal

A single crystal of an Al–Mg–Si alloy (Mg: 0.43 wt%, Si: 0.47 wt%) was aged at 453 K while small-angle neutron scattering experiments were carried outin situ. The scattering data recorded on a two-dimensional detector show the symmetry typical for needle-shaped β′′ precipitates oriented in the three [100] directions of the aluminium lattice and allow the calculation of the length, diameter and number density of the precipitates assuming cylindrical scattering objects of equal size and composition. The repetition time of the experiments was ∼12 min. The values obtained for the three quantities agree well with values measured by transmission electron microscopy (TEM) on a similar alloy. The impact of one week of natural ageing before artificial ageing on the evolution of the size and number density of precipitates is found to be pronounced, as expected from published TEM data.

Complementarity of Atom Probe, Small Angle Scattering and Differential Scanning Calorimetry for the Study of Precipitation in Aluminium Alloys

Two examples of precipitation studies (in Al-Li-Cu and Al-Li-Mg alloys) are shown to demonstrate the complementarity of atom probe tomography, small-angle-scattering and differential scanning calorimetry for precipitation studies. It will be used to unravel an unexpected two-step precipitation behavior of T1in Al-Li-Cu and to ascertain precipitates size in Al-Li-Mg. through a model free comparison between atom probe and SAXS.