Papers by Keyword: TEM

Abstract: Morphology of dispersoids in an annealed Al-Mg alloy were investigated using TEM. Five kinds of dispersoids with different structures and morphologies were observed in an annealed Al-Mg alloy. The 1st, or spherical-like one is monoclinic structured θ-Al₄₅(Mn,Cr)₇ phase with twin and orientation domain. The 2nd or plate-shaped one is η-Al₅(Mn,Cr) phase with monoclinic or pesuo-tetragonal structure. The 3rd or prismatic-like one is a new hexagonal structured Al_6.4Mn phase with a unit cell of a=1.72nm, c=1.27nm, and γ=120°, and the 4th or big rod-shaped one is orthorhombic structured Al₆(Mn,Fe) phase which is often reported. The 5th one is E-Al₁₈Mg₃(Mn–Cr)₂ phase with twin or triple twin observed occasionally in Al-Mg annealed alloy. The first two of dispersoids are in majority, followed by the middle two and a small number of the fifth. Formation mechanisms of these particles in Al-Mg alloy are discussed according to phase diagram and possible formation of the twins in the particles are described based on minimum energy.

Abstract: The effects of helium concentration and displacement damage on microstructural evolution at low dpa and low helium concentration were mainly investigated in specimens of austenitic stainless steel 316FR or SUS304 and a high chromium martensitic steel (HCM12A). The 316FR and HCM12A specimens were implanted uniformly with helium at 823 K up to 30 appm-He or 50 appm-He by 50 MeV cyclotron accelerator using energy degraders. After the helium implantation, the microstructures were examined by a transmission electron microscopy and positron annihilation lifetime measurements. Irradiation hardening behaviors were analyzed using SUS304 and HCM12A steels at 823 K implanted with He ion up to 100 appm with different He/dpa ratios in the HIT ion irradiation experiments and the hardening behaviors were examined by nano indentation method. In the irradiation and annealing specimens, these mechanical properties and microstructures were examined to understand the effects of helium production, displacement damage and annealing on microstructural development, and kinetic Monte Carlo (kMC) simulations were also performed to understand the microstructural development, and the results were compared with the results of TEM observation and positron annihilation lifetime measurements. Important some differences in the microstructural developments such as cavity formation and growth between austenitic stainless steel and martensitic steel were observed in low dpa and low helium concentration conditions.

Abstract: Embedding smart materials in the composite to enhance mechanical strength have become a research hotspot owing to their unique properties. The present research also focus on novel way to fabricate composite by embedding Shape Memory Alloy (SMA) wire and montmorillonite (MMT) nanoclay by varying clay concentration (0-7 wt.%). The extent of dispersion of nanoclay in epoxy resin was studied using Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD). Fabricated samples were examined for tensile, flexural and impact characteristics. Scanning Electron Microscopy (SEM) was used to study the adhesion, delamination and damage occurred within the composite due to tensile loading. Results shows that the tensile strength, flexural strength and impact energy of SMA/MMT/glass/epoxy composite was improved by 23%, 21% and 57% respectively, when it was compared with composite with glass/epoxy composite.

Abstract: Quenching and partitioning produces advanced high-strength steels that utilise transformation-induced plasticity for improved strength and deformability. Microstructures of these steels consist mainly of tempered martensite and carbon-enriched retained austenite. A novel processing route of direct-quenching and partitioning (DQP) facilitates carbon partitioning from supersaturated martensite to untransformed austenite directly from the quench-stop temperature in a decelerated cooling that simulates slow cooling of a coiled strip. A major advantage of DQP steels is that they keep both the costs and emissions down by inexpensive alloying and energy-efficient processing. In this study, we investigate the microstructures of 0.2C and 0.4C laboratory hot-rolled DQP steels with comparison to a direct-quenched variant with high-resolution transmission electron microscopy as the main research technique. We show that the structures of DQP steels have frequent nanotwinned regions and can contain three different crystal structures with characteristic length scales ranging from few nm to ~200 nm. This is in remarkable contrast to the traditional lath-martensitic microstructure of the as-quenched material. Density functional theory calculations provide further insight into these findings with the calculated results of energetics, and show that carbon helps in stabilising the newly found omega phase. These results give further insight to the aspects that must be considered when assessing their effect on essential mechanical properties like strain hardening and toughness.

Abstract: Chemical vapour deposited HfN can be utilised as a component of multilayer systems in protective coatings on cutting tools. In this study, related AlHfN coatings were synthesized through a reaction of metallic hafnium and aluminium with HCl gas forming gaseous HfCl₄ and AlCl₃, which were subsequently transported into a heated coating reactor. Via high temperatures and separately introduced NH₃ and N₂ as reaction gases, AlHfN coatings were deposited on hardmetal inserts. By varying the ratio between AlCl₃ and HfCl₄, compositionally different AlHfN coatings were examined. Additionally, surface morphology, composition as well as crystalline phases of the obtained coatings were analysed by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Finally, the microstructure of the cross section of a coating was investigated via transmission electron microscopy. The observations revealed a great impact of the gas composition on the morphology and crystal structures of the coatings. Within the layer, the growth of columnar microstructures was detected. Additionally, the formation of an amorphous HfN intermediate layer between the substrate and the AlHfN with a thickness of approximately 2 nm was found.

Abstract: This work describes the development of a new post-implant crystal recovery technique in 4H-SiC using XeCl (l=308 nm) multiple laser pulses in the ns regime. Characterization was carried out through micro-Raman spectroscopy, Photoluminescence (PL), Transmission Electron Microscopy (TEM) and outcomes were than compared with 1h thermally annealed at 1650-1770-1750 °C P implanted samples (source implant) and P and Al implanted samples for 30 minutes at 1650 °C (source and body implants). Experimental results demonstrate that laser annealing enables crystal recovery in the energy density range between 0.50 and 0.60 J/cm². Unlike the results obtained with thermal annealing where stress up to 172 Mpa and high carbon vacancies (V_c) concentration is recorded, laser annealing provides almost stress free samples and much less defective crystal avoiding intra-bandgap carrier recombination. Implant was almost preserved except for step-bouncing and surface oxidation phenomena leading to surface roughening. However, the results of this work gives way to laser annealing process practicability for lattice damage recovery and dopant activation.

Abstract: We have studied the microstructure of the growth surface of the 4H-SiC grown by the m-face solution growth. Atomic Force Microscopy (AFM) revealed the micro-striped morphology with the asperity of several nm in the band-like morphology region. The cross-sectional Transmission Electron Microscopy (XTEM) showed that the growth surface consisted of a bunch of nanofacets and vicinal surface. This peculiar morphology is totally different from that of conventional spiral growth on c-face, which can be closely related with the growth mechanism of the m-face solution growth.

Abstract: Two types of carrot defects with and without a shallow pit were found by mirror projection electron microscopy (MPJ) inspection in 4H-SiC epi wafer. Surface morphology and cross-sectional structure of prismatic stacking faults (PSFs) were investigated using MPJ and atomic force microscopy (AFM), transmission electron microscopy (TEM) and high-resolution scanning transmission electron microscopy (STEM). The depths of the surface grooves due to the PSFs, the stacking sequences around the PSFs and the structure of the Frank-type stacking faults which were connected to the PSFs were different. We discuss the difference between the two types of carrot defects.

Abstract: Stacking faults (SFs) in 4H-SiC epitaxial wafers were inspected by using a mirror projection electron microscope (MPJ) [1, 2]. Dark and bright line contrasts of SFs in MPJ images represent surface morphology and crystal defects located in the epitaxial layer. Inspected SFs were classified into three types of SFs on the basis of the MPJ images. After classification, a cross section of each type of SF was observed by transmission electron microscopy (TEM) to verify the classification result. Complex SFs classified by MPJ images consisted of prismatic plane and basal plane SFs.

Abstract: Ga₂O₃ is a hopeful wide-band-gap semiconductor material for a next-generation power semiconductor. We performed crystal structure analysis on Ga₂O₃ film on sapphire substrate using cross-sectional transmission electron microscope (TEM) and atomic resolution plan-view scanning transmission electron microscopy (STEM). The TEM analysis suggested that the main Ga₂O₃ film is composed of κ-Ga₂O₃ or mixed crystal of κ-Ga₂O₃ and ε-Ga₂O₃. But, it is difficult to distinguish these two possibilities only by cross-sectional TEM. Contrast modulation of Ga atomic columns in the atomic resolution HAADF-STEM image showed that the main part of the Ga₂O₃ film was κ-Ga₂O₃ monolayer grown along the c-axis direction, and twins are formed.