NanoSIMS Analysis of Trace Element Segregation during the Al-Si Eutectic Reaction
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NanoSIMS Analysis of Trace Element Segregation during the Al-Si Eutectic Reaction CHRISTIAN J. SIMENSEN, ØYVIND NIELSEN, FRANC¸OIS HILLION, and JORUNN VOJE The purpose of the present article was to test, for the first time, the potential of the NanoSIMS for quantifying the microscale spatial distribution of trace elements in aluminum alloys. As a suitable test case, two variants of an as-cast industrial A356 alloy have been chosen and analyzed quantitatively. The results show that Sr segregates preferentially to the a-Si particles rather than to the Al matrix. The Na segregates preferentially to a-Si, but in contrast to Sr, Na is inhomogeneously distributed in the particles and tends to accumulate at parts of their surfaces. The Ca segregates preferentially to the Al matrix and was not found in a-Si. This suggests that impurity-induced twinning is probably not the mechanism responsible for the flake-to-fibrous transition observed in Ca-modified Al-Si-alloys. The P was difficult to analyze as the 31P-peak was close to the peak of the molecule fragment 31Si 1 1H. The complication can probably be resolved in future studies. The present work demonstrated that NanoSIMS is a tool with great potential for providing new insight into microscale trace element segregation in Al alloys. I. INTRODUCTION
WHEN a solid sample is sputtered by primary ions, a fraction of the particles emitted from the target is ionized. Secondary ion mass spectrometry (SIMS) consists of analyzing these secondary ions with a mass spectrometer. The SIMS provides a unique combination of extremely high sensitivity for all elements (detection limit down to parts per billion level for many elements), high lateral resolution imaging (down to 40 nm), and a very low background that allows high dynamic range (more than 5 decades). The technique is destructive by its nature (sputtering of material), which can be an advantage because it ensures that the measured trace element distribution is an actual feature of the sample and not artifacts induced by the sample preparation. The SIMS has previously been used for analyzing aluminum alloys in a few studies.[1,2] During recent years, a new instrument, NanoSIMS50, has been developed. The advantage of the new instrument is the high lateral resolution in combination with a high sensitivity. The purpose of the present article is to test, for the first time, the potential of the NanoSIMS for quantifying the microscale spatial distribution of trace elements in Al alloys. An Al-7 wt pct Si alloy is chosen as a test case, due to the importance of trace element effects in the microstructure evolution of the Al-Si binary eutectic, as will be outlined subsequently. The Al-Si alloys constitute the majority of Al foundry alloys due to their excellent combination of castability and mechanical properties. An important reason for this is the fact that a range of trace elements has been proven to cause a flake-to-fibrous transition or so-called ‘‘modification’’ in CHRISTIAN J. SIMENSEN, Senior Research Scientist, and ØYVIND NIELSEN, Resear
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