Lithium Detection in Aluminium Lithium Alloys Using PEELS

Abstract

Aluminium lithium alloys are important alloys for the aerospace industry on account of their good strength to weight properties. However, these alloys suffer from poor short-transverse fracture toughness. Segregation of Li onto the grain boundaries is one mechanism proposed to account for this. Different AlLi alloys have been studied in a 200keV TEM equipped with a Parallel Electron Energy Loss Spectrometer.Typical Li concentrations in these alloys range from 10 to 14 atomic percent (2 to 3 weight percent). This, combined with the proximity of both the Al plasmon and Al L edge and the strong deviation of the Li K edge from the ideal sharp Fermi step makes the detection of Li difficult. Negative “jump ratios”, as shown in fig. 1, make the edge very difficult to see in an unprocessed spectrum and a cursory inspection of such a spectrum might miss the Li the edge. The presence of the Li edge, and also the edge threshold energy, is more visible if the second difference of the spectrum (obtained by convolution with a top-hat filter) is displayed as shown in fig. 2.