Numerical modelling of mass resolution in a scanning atom probe.

We have recently suggested a novel method for improving mass resolution in the scanning atom probe (SAP), based on a post-deceleration scheme. A two-conductor counter electrode is used, and the high voltage pulse is applied to the front conductor, with the rear conductor being held at ground. For a separation between the two conductors of 100 microm or less, ions travel between the two while the pulse is essentially constant, so that the ion leaves the counter electrode with an energy equivalent to the applied d.c. potential. In this paper, we have used a numerical model for the electric fields in the SAP to verify the results of the simpler analytical approach used earlier. In particular, the ion acceleration in the vicinity of the tip, previously assumed to be instantaneous, was modelled using a hyperboloidal field approximation. The numerical model was used to calculate the flight time for ions having a range of masses and evaporating over a range of times at the peak of a high voltage pulse. Modelled mass resolutions, calculated in this way, were then compared with analytical expressions, and were found to agree very well. This shows that the earlier assumption of an instantaneous acceleration did not seriously affect the validity of the approach

A procedure for quantification of precipitate microstructures from three-dimensional atom probe data.

New analysis software for selecting and quantifying particles in three-dimensional atom maps has been designed. The selection of solute-rich regions is performed by connecting solute atoms which lie within a fixed distance (d), and taking clusters above a certain minimum number of solute atoms (N(min)). Other atoms within some distance L greater than d are taken to belong to the cluster. However, this results in the inclusion of a shell of matrix atoms, which must be removed through an erosion step, to define the final cluster. Data filtered in this way can be used for subsequent quantification of parameters such as size, shape, composition, number density and volume fraction with better accuracy than by manual selection. The choice of d, N(min) and L values is discussed and some methods of evaluation of these parameters are proposed. Examples are presented on the application of this new software to the analysis of early stage clustering in an Al-Mg-Si-Cu alloy and a copper-containing steel

An atom probe study of fine scale structure in AlMgSi(Cu) alloys

The 6xxx series (AlMgSi(Cu)) aluminium alloys are currently being used for automotive body sheet applications in which the in-service strength of the body panel is achieved through age hardening during the automotive paint bake cycle (typically in the range 170-200 degreesC for times of up to 30 minutes). It has been shown that preageing treatments in the range 50-120 degreesC may improve the subsequent ageing response, causing the material to reach peak hardness more rapidly during paint-baking.In the present work, the precipitate microstructure evolution in AlMgSi(Cu) alloys during preageing has been studied using 3-dimensional atom probe (3DAP) microanalysis. The effect of quenching and preageing followed by natural ageing on the microstructure evolution in Cu-containing and Cu-free 6xxx alloys have been studied by 3DAP analysis. Dedicated software has been developed to automatically detect clusters of solute atoms within the volume of 3DAP microanalysis. This allows better quantification of precipitate parameters such as shape, composition, number density, and volume fraction. Hardness measurements have also been performed to link the mechanical properties to the microstructural information

A study on the early-stage decomposition in the Al-Mg-Si-Cu alloy AA6111 by electrical resistivity and three-dimensional atom probe

Electrical resistivity measurements and three-dimensional atom probe (3DAP) analysis were employed to investigate early-stage decomposition of the Al alloy AA6111 in the temperature range 60-180C where electrical resistivity initially increased with ageing time. 3DAP measurements provided information on the shape, number density and solute content of the precipitates, as well as the solute concentration of the matrix, for the ageing conditions corresponding to the resistivity maxima. Using the 3DAP results, the precipitate size distributions for these ageing conditions were determined in terms of the measured number of solute atoms per precipitate. The number density and the Cu content of the precipitates decreased with increasing temperature, whereas the Mg/Si ratio increased. The size distribution of precipitates at the higher ageing temperatures showed the addition of larger size precipitates to the precipitate population. A modification to Matthiessen's law was employed to describe the anomalous resistivity increase by considering the effect of solutes and precipitates on the resistivity evolution. Using the 3DAP results in analysing the resistivity anomaly, it was found that the decrease in the resistivity maxima with increasing temperature was associated with the decrease in the number density of precipitates and not the scattering power of precipitates. The 3DAP results were further used to provide information on the mechanisms of early-stage decomposition and the temperature dependence of the nucleation rate. From this, the nucleation rate appeared to be controlled by the migration of solute atoms, which was assisted by quenched-in vacancies

Effect of unloading brace treatment on pain and function in patients with symptomatic knee osteoarthritis: the ROTOR randomized clinical trial

Evidence is still inconclusive for the benefits of bracing in patients with knee osteoarthritis. To assess the effect of REBEL RELIEVER unloading knee brace in conservative treatment of knee osteoarthritis, a randomized controlled trial was conducted in 67 patients with symptomatic medial knee osteoarthritis, who randomly received 6-week treatment with either REBEL RELIVER unloading knee brace + usual care (Brace group, N = 32) or usual care alone (Control group, N = 35). Primary outcome was the global last 24h-pain relief (100-mm visual analogic scale [VAS]) at 6 weeks. Secondary endpoints included pain on motion (100-mm VAS), function (Lequesne index), safety and observance. At 6 weeks, mean [SD] last 24h-pain decreased significantly more in Brace group versus Control group (-41.35 [3.37] vs -15.37 [3.23], difference - 25.98, 95% CI - 41.64 to - 10.33, P < 0.0001). Higher mean [SD] pain on motion decrease (-51.91 [3.49] vs - 19.91 [3.34], difference - 32.01, 95% CI - 48.21 to - 15.80, P < 0.0001) and better improvement of Lequesne index score (-5.8 [0.5] vs - 2.3 [0.5], difference - 3.5, 95% CI-5.0 to - 2.0, P < 0.0001) were observed in Brace group. Safety and observance to the brace were excellent. The additive clinical benefit of wearing REBEL RELIEVER unloading knee brace was demonstrated in knee osteoarthritis patients

Atom probe tomography of solute distributions in mg-based alloys

Atom probe tomography (APT) has been carried out on three magnesium-based alloys: M1 (Mg-1 wt pct Mn), AZ31(Mg-3 pct Al-1 pct Zn), and ME10 (Mg-1 pct Mn- 0.4 pct misch metal). The aims of this experiment were to measure the composition of the matrix and to investigate solute clustering in the matrix of the three different alloys. For AZ31, the matrix composition was variable but close to the bulk composition. For ME10 and M1, the matrix was depleted in alloying additions, with the remainder residing in precipitates. Most alloying additions were found to exhibit clustering to some extent, with misch metal having the strongest partitioning behavior to clusters. Solute clusters did not appear to affect mechanical twinning. It has been proposed that the clustering behavior of misch metal contributes to its ability to modify the recrystallization texture.<br /

A model for predicting the yield stress of AA6111 after multi-step heat treatments

A model has been developed to predict the yield stress of the aluminum alloy AA6111 after multi-step heat treatments which involve combinations of ambient temperature ageing and high temperature artificial ageing. The model framework follows the internal state variable framework where the two principal state variables are i) the volume fraction of clusters which form at ambient temperature and ii) the volume fraction of metastable phases which form during high temperature ageing. The evolution of the these state variables has modeled using a set of coupled differential equations. The mechanical response (the yield stress) is then formulated in terms of the state variables through an appropriate flow stress addition law. To test the model predictions a series of experiments were conducted which examined two scenarios for multi-step heat treatments. In general, good agreement was observed between the model predictions and the experimental results. However, for the case where a short thermal excursion at 250oC was applied immediately after the solution treatment, the results were not satisfactory. This can be understood in terms of the importance of the temperature dependence for the nucleation density of metastable precipitates.Materials Engineering, Department ofApplied Science, Faculty ofReviewedFacultyResearche

On the use of density-based algorithms for the analysis of solute clustering in atom probe tomography data

Because atom probe tomography (APT) provides three-dimensional reconstructions of small volumes by resolving atomic chemical identities and positions, it is uniquely suited to analyze solute clustering phenomena in materials. A number of approaches have been developed to extract clustering information from the 3D reconstructed dataset, and numerous reports can be found applying these methods to a wide variety of materials questions. However, results from clustering analyses can differ significantly from one report to another, even when performed on similar microstructures, raising questions about the reliability of APT to quantitatively describe solute clustering. In addition, analysis details are often not provided, preventing independent confirmation of the results. With the number of APT research groups growing quickly, the APT community recognizes the need for educating new users about common methods and artefacts, and for developing analysis and data reporting protocols that address issues of reproducibility, errors, and variability. To this end, a round robin experiment was organized among ten different international institutions. The goal is to provide a consistent framework for the analysis of irradiated stainless steels using APT. Through the development of more reliable and reproducible data analysis and through communication, this project also aims to advance the understanding between irradiated microstructure and materials performance by providing more complete quantitative microstructural input for modeling. The results, methods, and findings of this round robin will also apply to other clustering phenomena studied using APT, beyond the theme of radiation damage