Characterizing the nano-structure and defect structure of nano-scaled non-ferrous structural alloys

Abstract

The presence and interaction of nanotwins, geometrically necessary dislocations, and grain boundaries play a key role in the mechanical properties of nanostructured crystalline materials.Therefore, it is vital to determine the orientation, width and distance of nanotwins, the angle and axis of grain boundary misorientations as well as the type and the distributions of dislocations in an automatic and statistically meaningful fashion in a relatively large area. In this paper, such details are provided using a transmission electron microscope-based orientation microscopy technique called ASTAR™/precession electron diffraction. The remarkable spatial resolution of this technique (~2 nm) enables highly detailed characterization of nanotwins, grain boundaries and the configuration of dislocations. This orientation microscopy technique provides the raw © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/data required for the determination of these parameters. The procedures to post-process the ASTAR™/PED datasets in order to obtain the important (and currently largely hidden) details of nanotwins as well as quantifications of dislocation density distributions are described in this study. KEYWORDSnanoscale orientation microscopy, nanotwin characterization, ASTAR™, precession electron diffraction, spatial distribution of geometrically necessary dislocations, severely deformed metallic materials

Characterization of the near-surface nanocrystalline microstructure of ultrasonically treated Ti-6Al-4V using ASTAR™/precession electron diffraction technique

“…The surface-treated area can be studied more quantitatively by determining the dislocation density distributions of geometrically necessary dislocations [39,40] and disclinations [59], developing phenomenological equations to correlate microstructure and composition with the mechanical response (e.g., yield strength) [60,61] as well as investigating the microstructure and texture evolutions as a function of surface deformation (e.g., indentation) [62].…”

“…Precessing the direct beam results in a considerable improvement in the quality 1 /reliability 2 of the recorded diffraction patterns due to the removal (or reduction) of any dynamical diffraction effects [36] and formation of diffraction patterns with quasi-kinematical diffraction condition [37,38]. ASTAR/PED technique has been used to study dislocation density evolutions in severely deformed metallic materials [39], characterization of nanotwins [40], determination of grain boundary character distributions of nanocrystalline copper [41], investigation of the early stages of Zircaloy-4 oxidation behavior [42], etc.…”

Ti-6Al-4V microstructural orientation at different length scales as a function of scanning strategies in Electron Beam Melting in additive manufacturing

“…TEM samples were prepared with a FEI Helios DualBeam TM (focused ion beam/SEM) from the bottom, middle, and top of the samples in the XZ plane. PED scans were carried out to obtain crystallographic information at nanoscale and quantify the dislocation density (density of dislocations) affected by the three different scan strategies using a MatLab code developed previously [13]. Scans for PED [14] with 2.5 x 4 µm area were run on the TEM foils using ASTAR on FEI Tecnai G2-F20 STEM with 10 nm step size, 0.917 precession angle, 10 precessions per frame and 0.65 gamma.…”

Application of ASTAR/precession electron diffraction technique to quantitatively study defects in nanocrystalline metallic materials

“…The diffraction pattern of a crystal oriented close to 2110 zone axis when the electron beam is not precessed ("PED off" condition) and (b) the same situation as "a" with precessing the direct beam for 1.3° are shown. (c) The diffraction pattern of a crystal oriented far from 0001 zone axis in a PED off condition and (d) the same situation as "c" with PED on condition are depicted [68].…”

“…In this chapter, the characterization capability of the ASTAR  /PED technique is presented by providing two challenging examples. The first example is about the characterization of a UNSM surface treated Ti-6Al-4V sample [182] and the second example is about the characterization of nanotwins in a severely deformed Inconel 718 sample which is a nickel-based superalloy [68]. These two examples represent the general capabilities of this technique and highlight the advantages of the ASTAR  /PED technique with respect to the EBSD technique.…”