The use of combined three-dimensional electron backscatter diffraction and energy dispersive X-ray analysis to assess the characteristics of the gamma/gamma-prime microstructure in alloy 720Li™

Abstract

Multiple three-dimensional reconstructions of a γ/γ' phase structure in Alloy 720Li have been carried out by employing a serial milling technique with simultaneous electron backscatter diffraction (EBSD) and energy dispersive x-ray (EDX) analysis data collection. Combining EBSD data with EDX is critical in obtaining maps to distinguish between the chemically differing, but crystallographically similar γ and γ' phases present in the alloy studied. EDX is shown to allow the differentiation of γ and γ' phases, with EBSD providing increased grain shape accuracy. The combination of data sources also allowed identification of coherent γ/γ' phase interfaces that would not be identified using solely EBSD or EDX. The study identifies a region of grain banding within the alloy, which provides the basis for a three-dimensional comparison and discussion of γ' phase size between coarse and fine grain regions, whilst also identifying coherent γ' phase interfaces, possible only using both EDX and EBSD systems simultaneously. The majority of the γ' phase lies in the range of 1-10 μm in non-banded regions, with a detectable particle size limit of 500 nm being established. The validity of the reconstruction has been demonstrated using an electron interaction volumes model, and an assessment of the validity of EBSD and EDX data sources is discussed showing γ' phase connectivity in all dimensions.