Using transmission Kikuchi diffraction in a scanning electron microscope to quantify geometrically necessary dislocation density at the nanoscale

High Temperature Mechanical Properties and Microstructure Evolution of Ti-6Al-4V Alloy Linear Friction Welding Joints

The combination of linear friction welding (LFW) and hot forming processes is limited due to a lack of research on the high-temperature flow behavior and microstructure evolution of welded joints. In this paper, an electric-assisted high-temperature uniaxial tensile test platform based on digital image correlation (DIC) is built, and uniaxial tensile tests of Ti-6Al-4V alloy with LFW joints are performed at different temperatures (923–1023 K) and different strain rates (0.001 s−1–0.01 s−1). Then the microstructure of the LFW joints under different hot deformation conditions have been analyzed by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results indicate that the high-temperature flow behavior of LFW joints shows an obvious correlation between temperature and strain rate: the yield stress decreases from 203 MPa at 923 K to 105 MPa at 1023 K, and increases from 85 MPa to 130 MPa when the strain rate increases from 0.001 s−1 to 0.01 s−1 at 973 K. The hot deformation mechanisms with deformation conditions have been characterized, which changes from the mechanism of dislocation creep to the mechanism of self-diffusion as the deformation temperature increases from 923 to 1023 K. Especially, the fraction of high angle boundaries (HABs) rapidly rise from 49.2% to 64.1% with the increasing temperatures, the discontinuous dynamic recrystallization (DDRX) become the primary mechanism of nucleation during high-temperature deformation of LFW joints.

Recent Advances in EBSD Characterization of Metals

Electron backscatter diffraction (EBSD) has been attracting enormous interest in the microstructural characterization of metals in recent years. This characterization technique has several advantages over conventional ones, since it allows obtaining a wide range of characterization possibilities in a single method, which is not possible in others. The grain size, crystallographic orientation, texture, and grain boundary character distribution can be obtained by EBSD analysis. Despite the limited resolution of this technique (20–50 nm), EBSD is powerful, even for nanostructured materials. Through this technique, the microstructure can be characterized at different scales and levels with a high number of microstructural characteristics. It is known that the mechanical properties are strongly related to several microstructural aspects such as the size, shape, and distribution of grains, the presence of texture, grain boundaries character, and also the grain boundary plane distribution. In this context, this work aims to describe and discuss the possibilities of microstructural characterization, recent advances, the challenges in sample preparation, and the application of the EBSD in the characterization of metals.

Surface treatment and electron backscatter diffraction (EBSD) analysis of the body-centered cubic phase in Mg-Li based alloys processed by severe plastic deformation

Although electron backscatter diffraction (EBSD) analysis for Mg-Li based alloys is difficult to conduct, this technique is necessary to understand the properties of these alloys. Numerous studies have addressed the use of electron backscatter diffraction (EBSD) analysis as an effective technique for quantifying Mg-Li based alloys. However, a crystallographic orientation determination of β-Mg by EBSD has not been achieved thus far. In this study, to conduct EBSD analysis of the body-centered cubic (BCC) phase in Mg-Li based alloys with various applied equivalent strains prepared by severe plastic deformation, surface treatment techniques were established. By the use of both Ar⁺ ion milling and Ar⁺ ion etching under ultra-high vacuum conditions, the Kikuchi patterns of Mg-Li based alloys were detected. According to the results of the EBSD analysis, it was concluded that the structural evolution of BCC Mg-Li based alloys can be understood with this approach. From the results of the EBSD analysis, it is clarified that the high strength of this alloy is attributed to the combination of Hall-Petch relationship and Bailey-Hirsch relationship.

Correlating results from high resolution EBSD with TEM- and ECCI-based dislocation microscopy: Approaching single dislocation sensitivity via noise reduction

High resolution electron backscatter diffraction (HREBSD), an SEM-based diffraction technique, may be used to measure the lattice distortion of a crystalline material and to infer the geometrically necessary dislocation content. Uncertainty in the image correlation process used to compare diffraction patterns leads to an uneven distribution of measurement noise in terms of the lattice distortion, which results in erroneous identification of dislocation type and density. This work presents a method of reducing noise in HREBSD dislocation measurements by removing the effect of the most problematic components of the measured distortion. The method is then validated by comparing with TEM analysis of dislocation pile-ups near a twin boundary in austenitic stainless steel and with ECCI analysis near a nano-indentation on a tantalum oligocrystal. The HREBSD dislocation microscopy technique is able to resolve individual dislocations visible in TEM and ECCI and correctly identify their Burgers vectors.

Extraction of plutonium-containing microcrystals from Hanford soil using a focused ion beam for single-crystal X-ray diffraction analysis

Herein, the successful use of a focused ion beam/scanning electron microscope to prepare microsamples of radioactive single crystals for X-ray diffraction analysis is reported. This technique was used to extract and analyze crystalline Pu-containing particles as small as 28 µm3 from Hanford soil taken from the 216-Z-9 waste crib, which were then crystallographically characterized using single-crystal X-ray diffraction to confirm the cubic structure of PuO2. As a systematic proof of concept, the technique was first tested using UO2 crystals milled into cubic shapes with approximate volumes of 4620, 1331, 125, 8 and 1 µm3, in order to empirically determine the crystal size limits for characterization by a laboratory-based diffractometer with a sealed tube Mo or Ag anode X-ray source and a charge-coupled device detector.