ADVANCED MATERIALS & PROCESSES | JANUARY/FEBRUARY 2025 26 sample emits. An electron microprobe works by bombarding a solid material to an accelerated, focused electron beam, which is strong enough to trigger the sample to emit both matter and energy[4]. Muniandy et al. effectively used EPMA to reveal elemental segregation in a nominally equiatomic CrMnFeCoNi HEA. The authors established through EPMA that this HEA exhibits compositional heterogeneities that are largely dependent on processing methods (Fig. 3). The average compositions in percentage of 21.3 Cr, 17.8 Mn, 20.2 Fe, 21 Co, and 19.7 Ni (at.%) as shown in the advent of sophisticated analytical instruments is helping scientists to develop new HEAs with the most desirable properties. This article explores some of these advanced tools. ELECTRON ENERGY LOSS SPECTROSCOPY Electron energy loss spectro- scopy (EELS) is a spectroscopic tool that examines the energy distribution of electrons that emerge from a thin specimen following inelastic interactions[2]. EELS may be used as an additional instrument attached to a standard TEM (Fig. 2a). The EELS spectrum offers valuable insights into the specimen’s atomic structure, including details on its electronic structure, bonding states, nearest-neighbor arrangements, coordination numbers, dielectric characteristics, and band gaps in addition to its chemical elemental compositions. Depending on the quantity of energy lost, these features can be seen in various energy areas. As an example in Fig. 2b, an annealed CoCrCuFeNiAl0.5 HEA was analyzed using EELS for its elemental composition and precipitation. High spatial resolution of EELS element mapping helped researchers confirm the relationship between chemical segregations and L12 crystal structure ordering. The EELS mapping technique revealed that the L12 structure is primarily rich in FeCoCr (Ni), and in the fcc matrix, Cu is enriched. ELECTRON PROBE MICROANALYSIS Electron probe microanalysis (EPMA) examines the local chemical composition of materials using x-ray spectroscopy. This nondestructive tech- nique provides in situ chemical analysis together with sample imaging. The method is based on assessing the strength of distinctive x-rays that a Fig. 2 — (a) Schematic view of EELS; and (b) EELS results for a CoCrCuFeNiAl0.5 sample showing the elemental mapping of its L12 crystal structure[3]. (a) (b) Fig. 3 — EPMA maps indicating a predominantly uniform distribution of all five elements: (a) with only slight local variations including a marginally decreased Mn content and a slightly increased Cr content; and (b) material with another processing strategy where heterogenous elemental segregation is revealed[5]. (a) (b)
RkJQdWJsaXNoZXIy MTYyMzk3NQ==