About the contrast of [delta]' precipitates in bulk Al-Cu-Li alloys in reflection mode with a field-emission scanning electron microscope at low accelerating voltage

Summary Characterising the impact of lithium additions in the precipitation sequence in Al-Li-Cu alloys is important to control the strengthening of the final material. Since now, transmission electron microscopy (TEM) at high beam voltage has been the technique of choice to monitor the size and spatial distribution of [delta]' precipitates (Al3Li). Here we report on the imaging of the [delta]' phase in such alloys using backscattered electrons (BSE) and low accelerating voltage in a high-resolution field-emission scanning electron microscope. By applying low-energy Ar+ ion milling to the surface after mechanical polishing (MP), the MP-induced corroded layers were efficiently removed and permitted the [delta]'s to be visible with a limited impact on the observed microstructure. The resulting BSE contrast between the [delta]'s and the Al matrix was compared with that obtained using Monte Carlo modelling. The artefacts possibly resulting from the sample preparation procedure were reviewed and discussed and permitted to confirm that these precipitates were effectively the metastable [delta]'s. The method described in this report necessitates less intensive sample preparation than that required for TEM and provides a much larger field of view and an easily interpretable contrast compared to the transmission techniques. Lay description Since several decades, Al-Li alloys were developed to answer the difficult question: 'Can a light weighted alloy provide high strength and failure resistance?' Through time, other alloying elements such as copper, magnesium and zirconium were added to control and optimise the mechanical and chemical performances of these materials. These controlled additions then allowed these alloys to be used mostly in aerospace applications where they are now widely part of aircraft's fuselages and wings, and spacecraft's tanks. Due to these highly critical applications, materials science engineers need to understand the relation between the mechanical properties and the microstructure of these alloys to match the strict requirements necessary to ensure maximum security of the travellers. In that regard, the behaviour of lithium needs to be known accurately, specially the formation of coherent Al3Li ([delta]') precipitates, which is critical to obtain the desired mechanical behaviour. Historically, scanning (SEM) and transmission (TEM) electron microscopy techniques have been used to investigate these microstructures but only TEM has been used