Study of the microstructure resulting from brazed aluminium materials used in heat exchangers

Re-solidification of AA4343 cladding after brazing as well as the related precipitation in the modified AA3003 core material have been investigated. Analysis of the re-solidified material showed that partial dissolution of the core alloy occurs in both the brazing joints and away of them. Far from the brazing joints, the dissolution is, however, limited and diffusion of silicon from the liquid into the core material leads to solid-state precipitation in the so-called “band of dense precipitates” (BDP). On the contrary, the dissolution is enhanced in the brazing joint to such an extent that no BDP could be observed. The intermetallic phases present in the resolidified areas as well as in the core material have been analyzed and found to be mainly cubic alpha-Al(Mn,Fe)Si. These results were then compared to predictions made with available phase diagram information

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

Aspects of the observation of clusters in the 3-dimensional atom probe

The maximum separation method is a useful technique for the identification of solute clusters within 3-dimensional atom probe (3DAP) data. However, the method requires the selection of appropriate parameters that will correctly identify solute clusters without incorrectly identifying random variation in solute atom separations as being due to cluster formation. A simple analytical approach has been used to estimate the apparent number of clusters that would be expected using the maximum separation method in a random solute distribution of a given overall composition. Results of the analytical model have been found to give a good match with tests where the maximum separation method has been applied to randomised experimental 3DAP data. The model allows calculation of suitable parameters for accurate identification of clusters under a range of different conditions. A Poisson probability distribution has been used to formalise the errors involved in the measurement of the number densities of clusters or precipitates within 3DAP data. This approach also allows an upper limit for the precipitate density to be defined in cases where no precipitates are observed within a volume of analysis. Copyright (C) 2007 John Wiley and Sons, Ltd

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