Abstract
A high resolution scanning ion microprobe has been used, in conjunction with transmission electron microscopy, to study the distribution of the metastable δ' (Al 3Li) phase in Al-Li alloys. Various secondary ions were used for chemical imaging (lateral resolution ∼ 80 nm) and their contrast aspects and formation mechanisms are discussed. δ' particles appeared dark in the 27Al +, 7Li + and 34AlLi + distribution maps despite their higher Li content than the α matrix (aluminum-lithium solid-solution). Maps generated from two molecular ions, 14Li + 2 and 41Li 2Al +, exhibited true chemical contrast in the images: the δ' precipitates appeared bright in accordance with the known Li distribution. Using retrospective image analysis, measurements were made of the secondary ion intensities displayed by the δ' particles and the α matrix. Efforts to calibrate the secondary ion intensities for quantitative analysis are also described. The unusual ion formation mechanism from δ' precipitates illustrates the coupled channel model of secondary particle emission: the observed favored emission of Li 2 causes Li emission to be depressed.
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