3D ChemiSTEM™ Tomography of Nano-scale Precipitates in High Entropy Alloys

J.M. Sosa, D.E. Huber, B. Welk, J.K. Jensen, R.E.A. Williams, S. Lambert, H.L. Fraser
2014 Microscopy and Microanalysis  
Electron tomography is a three-dimensional technique well suited for characterizing fine-scale microstructural features on the order of 10-100 nm. Typically performed in a transmission electron microscope (TEM), it relies on successive image acquisition at multiple sample tilts. Images may be formed through various techniques including conventional and scanning transmission electron microscopy (CTEM and STEM), energy-filtered TEM (EFTEM), and energy dispersive x-ray spectroscopy (EDS). The
more » ... opy (EDS). The variety of available image formation mechanisms has defined electron tomography as a robust fine-scale characterization tool. Conventional TEM tomography has relied on diffraction contrast as the primary imaging mode. However, this contrast may be insufficient for discriminating different phases in a complex microstructure. In these cases, more informative contrast may be obtained through compositional mapping, where characteristic x-ray emission is recorded over an area of interest. FEI's recently developed ChemiSTEM™ technology employs a detection configuration with four silicon drift detectors (SDD), providing a very large collection angle (0.8str), coupled with high incident beam current, resulting in the ability to collect x-ray maps on the order of electron micrographs [1]. Despite such efficiency, few tomographic reconstructions have employed ChemiSTEM™ mapping as the primary imaging mode. This paper will present novel microstructural characterization of the emerging material system of high-entropy alloys (HEAs) using ChemiSTEM™ tomography. HEAs offer an attractive balance of properties including high strength and corrosion resistance [2, 3]. These materials often involve microstructures that result from phase separation and also spinodal decomposition, such that the resulting microstructures are three-dimensionally interconnected. It is important that the true distribution of these phases be established so that accurate models of the deformation processes may be developed, and hence it is necessary to invoke direct 3D characterization. The size-scale of the microstructural features (<100 nm) is well suited for electron tomography. Rather than acquire a tilt-series from a thin foil, a needle-shaped specimen was excised using a DualBeam™FIB/SEM, which offered several advantages. Firstly, when tilted about its longitudinal axis, the needle's symmetry avoided the projected thickness variations that occur when tilting thin foils. Secondly, the gradation of the needle's thickness allowed the authors to explore the limits of various reconstruction algorithms and evaluate their efficacy as a function of sample thickness. For this work, a FEI Titan 60-300 ChemiSTEM™ equipped with a quad-detector SDD was used to collect a tomographic tilt series of EDS spectral images from the aforementioned needle of HEA microstructure. Incident beam dwell times were 20 μs/px, with a live time of 600 s. The tomographic tilt series was collected about an axis parallel to the needle from -62° to +62° in 1° increments. In order to collect adequate signal, beam currents of ~2 nA were used. Beam amperages of this magnitude can threaten a thin specimen's integrity, especially when repeated imaging of the same local area is required, as with tomography. Therefore, one objective of this work was to optimize the sampling frequency (i.e. tilt step size) so as to minimize specimen damage while still obtaining a high-fidelity reconstruction. 764
doi:10.1017/s1431927614005546 fatcat:duytchw7wfbojgwqyee463iljm