3DAP measurements of Al content in different types of precipitates in aluminium alloys

Abstract

Three-dimensional atom probe and field ion microscopy have been used to determine the composition of precipitates formed in several types of aluminium-based alloys. Special attention has been paid to local magnification artefacts, which are often suspected to strongly disrupt the precipitates' apparent composition. The charge state of Al ions has been used to identify the nature of precipitates (lower or higher evaporation field than the matrix) and also to determine the origin of Al ions observed in 3D reconstructions. The use of a cluster identification method, which allows erosion concentration profiles to be drawn, has revealed the importance of trajectory overlaps in the measurement of the precipitates' composition. By applying this method to low-evaporation-field precipitates in a 7XXX alloy, good agreement was found between atom probe measurements and observations in transmission electron microscopy and small angles X-ray scattering.

Influence of alloy composition and heat treatment on precipitate composition in Al–Zn–Mg–Cu alloys

“…Several algorithms can be used to find the precipitates in the recorded volumes and their interfaces with the matrix, such as a cluster identification algorithm based on concentration thresholds [3], or a maximum separation distance algorithm [5]. Since the heat treatments investigated in the present work lead to the formation of well-defined precipitates, these algorithms lead to the identification of the same objects, even though some subtle differences in the location of the interface can remain.…”

“…The precipitate compositions have been measured using a protocol described in Ref. [3], in order to suppress as much as possible the influence of the diffuse interface between the precipitates and the matrix, caused by the difference in evap- oration fields of the two-phases. However, due to the very small size of the precipitates, this protocol was not sufficient to completely suppress this effect and we had to make some assumptions in order to know how much Al had to be removed from the measured composition.…”

“…Evaluating the composition of nanoscale precipitates in multi-constituent alloys, and in particular Al alloys, has been the subject of much research effort in the past decade [1][2][3][4][5][6][7][8][9][10]. In fact, the question of the composition of nanoscale precipitates in multi-constituent alloys raises many difficult questions.…”

Cluster formation at the Si/liquid interface in Sr and Na modified Al–Si alloys

“…Since the clusters are located in the Si phase, their Si content could not be determined and instead the focus was put on the content of Al and modifier. The local composition of the clusters measured by APT can be influenced by ion trajectory overlaps due to local magnification effects [20]. When considering 2 -5 nanometre sized segregations present in these alloys, this artefact may lead to a convolution of the matrix with the precipitate resulting in an overestimation of the matrix element [20][21][22][23][24].…”

Comparison of segregations formed in unmodified and Sr-modified Al–Si alloys studied by atom probe tomography and transmission electron microscopy

“…This local magnification artefact results in a deformation of the segregation in the x-y direction [27]. Lefevbre et al [28] reported a 1.5 to 2 nm region affected by trajectory overlaps at the interface of low-evaporation-field precipitates. Considering this biased area, we expect the significant Al depletion detected at the centre of the large segregations (13-18 nm diameter) not to be an artefact from trajectory aberrations.…”