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

Abstract

Comparison of segregations formed in unmodified and Sr-modified Al-Si alloys studied by atom probe tomography and transmission electron microscopy, 2014, Journal of Alloys and Compounds, (611) AbstractThe mechanical properties of Al-7 wt.% Si can be enhanced by structural modifications of its eutectic phase. Addition of low concentrations of certain elements, in this case 150 wt-ppm Sr, is enough to cause a transition from a coarse plate-like Si structure to a finer coralline one. To fully understand the operating mechanism of this modification, the composition of the eutectic Si phase in unmodified and Sr-modified alloys was analysed and compared by Atom Probe Tomography and (Scanning) Transmission Electron Microscopy. The unmodified alloy showed nanometre sized Al-segregations decorating defects, while the Sr-modified sample presented three types of Al-Sr segregations: (1) rod-like segregations that promote smoothening of the Al-Si boundaries in the eutectic phase, (2) particle-like segregations comparable to the ones seen in the unmodified alloy, and (3) planar segregations favouring the formation of twin boundaries. Al and Sr solubilities in Si after solidification were determined to be 430 ± 160 at-ppm and 40 ± 10 at-ppm, respectively. Sr predominantly segregates to the Si phase confirming its importance in the modification of the eutectic growth.

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

“…Ahead of the growing Si crystal a diffusion profile is formed by segregation leading to constitutional undercooling. The solubilities of Al and Sr in Si are ~400 at-ppm and ~40 at-ppm, respectively [16], which is less than the expected concentration in the liquid. Therefore, both these elements are expected to be enriched ahead of the solidification front.…”

“…Recent investigations have revealed that Al forms segregations with Sr or Na in the Si eutectic phase [16][17][18]. The presence of Al in combination with the modifier was not considered in previous theories when explaining the origin of the modification [5,11].…”

“…These defects are necessary for the repeated change of Si growth direction and growth rate that give rise to the desired coralline-structure [16]. It was observed that the defects in the Si lattice are enriched by the modifier (Sr / Na) and Al by correlating TEM and APT data.…”

3D Atom Probe Tomography Study on Segregation of Yttrium in Modified Al-Si Alloys

“…The ability of these three elements to cosegregate was regarded as indicative that these promote formation of new twins. Barrirero et al 10 similarly identified rod-like nanoscale cosegregations (responsible for smoothening of Al-Si boundaries in eutectic phase) and particle-like and planar cosegregations (favoring formation of twin boundaries). The particle-like segregations were also observed in the unmodified alloy, though obviously containing exclusively Al.…”

“…Atom probe tomography (APT) has been used by several researchers to look into the segregation of the modifying element within Al-Si alloys. [7][8][9][10][11][12] Past studies have explored the application of Y as a modifier of Al-Si alloys. Researchers used varying amounts of Y, between 200 ppm and 6000 ppm, and observed that, in presence of Y, the Si eutectic forms a refined plate-like morphology rather than a fibrous one.…”