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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Modeling of dislocation channel width evolution in irradiated metals

Defect-free dislocation channel formation has been reported to promote plastic instability during tensile testing via localized plastic flow, leading to a distinct loss of ductility and strain hardening in many low-temperature irradiated materials. In order to study the underlying mechanisms governing dislocation channel width and formation, the channel formation process is modeled via a simple stochastic dislocation-jog process dependent upon grain size, defect cluster density, and defect size. Dislocations traverse a field of defect clusters and jog stochastically upon defect interaction, forming channels of low defect-density. And based upon prior molecular dynamics (MD) simulations and in-situ experimental transmission electron microscopy (TEM) observations, each dislocation encounter with a dislocation loop or stacking fault tetrahedron (SFT) is assumed to cause complete absorption of the defect cluster, prompting the dislocation to jog up or down by a distance equal to half the defect cluster diameter. Channels are predicted to form rapidly and are comparable to reported TEM measurements for many materials. Predicted channel widths are found to be most strongly dependent on mean defect size and correlated well with a power law dependence on defect diameter and density, and distance from the dislocation source. Due to the dependence of modeled channel width on defect diameter and density, maximum channel width is predicted to slowly increase as accumulated dose increases. The relatively weak predicted dependence of channel formation width with distance, in accordance with a diffusion analogy, implies that after only a few microns from the source, most channels observed via TEM analyses may not appear to vary with distance because of limitations in the field-of-view to a few microns. Furthermore, examinations of the effect of the so-called “source-broadening” mechanism of channel formation showed that its effect is simply to add a minimum thickness to the channel without affecting channel dependence on the given parameters.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ion irradiation induced nucleation and growth of nanoparticles in amorphous silicon carbide at elevated temperatures

Ion irradiation induced crystallization in as-deposited amorphous SiC films is investigated using grazing-angle incidence x-ray diffraction (GIXRD), transmission electron microscopy (TEM) and Raman spectroscopy. Irradiation with 5 MeV Xe to fluence of 115 Xe/nm2 at 700 K results in a homogenous distribution of 3C-SiC grains with an average crystallite size of ~5.7 nm over the entire film thickness (~1 µm). The nucleation and growth processes exhibit a weak dependence on dose in displacements per atom (dpa) in the dose range from 6 to 20 dpa. A transformation of homonuclear C-C bonds from sp3 to sp2 hybridization is observed in the irradiated films, which may be partly responsible for the observed grain size saturation. The results from this study may have a significant impact on synthesis of nanograins in amorphous SiC and other similar materials with effective control of grain size and density by ion irradiation.

phase transformation, SiC, ion irradiation↗
DOE OSTI2018

Microstructural evolution of 3C-SiC exposed to simultaneous neutron irradiation and helium implantation

The synergism of transmutant helium and irradiation defects is critically important to understand the performance of SiC when in service in nuclear environments. In this study, we exposed 3C-SiC in contact with a 2 μm Ni foil to neutron irradiation at 500 °C to 29 dpa in the high flux isotope reactor to simulate the simultaneous introduction of helium and irradiation defects into SiC. The combination of TEM observations and thermal desorption measurements helps to elucidate the impact of helium on the microstructural evolution in 3C-SiC. Thermal helium desorption spectra from ion-implanted and neutron-irradiated SiC showed a completely different desorption behavior in terms of the peak position and helium desorption flux. The identification of possible helium trapping sites was attempted by applying the first order dissociation model to the measured thermal helium desorption spectra. TEM observations of the neutron-irradiated samples showed that the presence of helium facilitated the stabilization of the defect clusters and promoted the formation of visible helium bubbles in SiC subject to simultaneous neutron irradiation and in-situ He implantation. Following the subsequent heat treatment during the thermal desorption measurements, large faceted helium bubbles were found along grain boundaries in the neutron-irradiated SiC with in-situ He implantation, while the helium bubbles in the grain interior were relatively smaller than those found in the neutron-irradiated sample without in-situ He implantation due to the reduced mobility of He-defect clusters. In conclusion, the He contents contained in the neutron-irradiated samples were quantified based on the experimental data from TEM and thermal desorption measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and fission products in the UCO kernel of an AGR-1 TRISO fuel particle after post irradiation safety testing

Tristructural isotropic (TRISO) coated particle fuel as a key fuel concept for the High Temperature Gas-cooled Reactors (HTGRs) and a candidate accident tolerant fuel (ATF) has been currently investigated under the US-DOE Advanced Gas Reactor (AGR) fuel development and qualification program. Over the past decades, extensive studies have been conducted to evaluate the fission product release, diffusion on Ag, Pd and Cd in the SiC layer and the TRISO coating system, and post-safety-test performance. However, to date, there are limited reported results on the fuel kernels’ response to irradiation with or without post irradiation high temperature annealing. To incrementally fulfill this knowledge gap extensive studies using transmission electron microscopes (TEM) and atom probe tomography (APT) were conducted on a TRISO fuel particle kernel with a 19.74% 235U enrichment, irradiated to 18.63% FIMA and subsequently subjected to post-safety testing at 1600ºC for 300 hours.. Microstructure characterizations, elemental analysis, and phase identification were conducted using conventional TEM and scanning TEM (STEM) imaging, energy-dispersive X-ray spectroscopy (EDS), selected-area electron diffraction (SAED) and atom probe tomography (APT). The following findings were made: (1) significant reconstructions and phase evolutions occurred in the irradiated and post-safety annealed fuel kernel, and its microstructure consists of two primary phases, namely a “high-Z (atomic mass)” UC phase and a “low-Z” UO phase, (2) no fission gas bubbles are identified within the fuel kernel, that can be attributed to the high temperature post safety annealing, (3) fission products Zr, Nb, Mo, Ru, Tc and Rh were found to preferentially segregate into UC phase or to form metallic precipitates, while the lanthanide fission products tend to stay in the solution of UO phase, (4) Pd was detected in the rod-shaped precipitates, (5) APT data reveals segregation of Te, Pd, and Ag at the UO and UC phase boundary, and (6) Pd is detected in both the UO and UC phases but with an enrichment in the former one.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Post-irradiation characterization of a high burnup mixed oxide fuel rod with minor actinides

The Advanced Fuels Campaign performed a series of irradiation tests of minor actinide-bearing mixed oxide fuel (MA-MOX), the so-called AFC-2C&D experiments, to investigate the transmutation of long-lived transuranic actinide isotopes contained in spent nuclear fuel via fast reactor technology at burnups exceeding 10 % fission of initial metallic atoms. This manuscript reports the test results derived from one of the five MA-MOX rodlets taken to higher burnup in the AFC-2D irradiation. This includes both non-destructive investigations, such as gamma and neutron spectrometry, and destructive investigations, such as fission gas release, ceramography, and chemical burnup analysis. In addition, the microstructure of the fuel was investigated using advanced electron microscopy techniques including electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Here, it was observed with EBSD that the pellet had subdivision of the grains and the TEM observed migration of cladding material into the 5 metal precipitates in the fuel which could have been from the higher than desired oxygen/metal ratio. The TEM also showed an enrichment of Cr in fuel clad chemical interaction (FCCI) layer.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

Inhomogeneous structural evolution of silver-containing Alpha-MnO2 nanorods in sodium-ion batteries investigated by comparative transmission electron microscopy approach

Alpha-MnO 2 is an attractive cathode candidate for sodium-ion batteries attributed to its unique one-dimensional 2×2 tunnels for facile sodium-ion diffusion, in addition to its incomparable cost advantage. In particular, α-MnO2 shows superior rate capability with silver stabilizing ions at the center of tunnels that improve electrical conductivity. In this work, we directly compare structural transformation of silver-containing α-MnO 2 nanorods (Ag 1.22 Mn 8 O 16-x or L-Ag-HOL and Ag 1.66 Mn 8 O 16-y or H–Ag-HOL), containing higher and lower concentrations of oxygen vacancies respectively, by transmission electron microscopy (TEM). The elaborate comparative and statistical TEM studies eliminate concerns regarding generalization errors and facilitate rational structural development of nanorods with improved functionality. It is found that sodium ions favorably diffuse through the area where oxygen vacancies are concentrated, and the samples with more silver ions and fewer oxygen vacancies (H–Ag-HOL) show more significant structural deformation with more inhomogeneous sodiation. The difference in functional electrochemistry coupled with the observed difference in inter- and intra-nanorod inhomogeneous structural evolution emphasizes the significance of the uniform electrical conductivity of the electrode. This work helps to improve the α-MnO 2 electrode material for sodium-ion batteries as well as suggesting the importance of delicate statistical approaches for TEM investigations.

25 ENERGY STORAGE↗
DOE OSTI2013

Direct synthesis of Al-SBA-15 containing aluminosilicate species plugs in an acid-free medium and structural adjustment by hydrothermal post-treatment

A series of Al-SBA-15 with controllable aluminosilicate plug structures inside straight mesopores has been hydrothermally synthesized in a one-step synthesis in an environmentally friendly acid-free medium, using triblock copolymer Pluronic P123 as a structure-directing agent, water as solvent, tetraethyl orthosilicate (TEOS) and aluminum nitrate (Al(NO){sub 3}·9H{sub 2}O) as silica and aluminum sources, respectively. The effects of the P123/Si molar ratio in the initial solution and aging temperature on the structural properties of the resulting materials were investigated by powder X-ray diffraction (XRD), nitrogen adsorption–desorption at 77 K, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric (TG), FT-IR spectra and inductively coupled plasma (ICP) analyses. The nature of the Al species and the acidity of the resultant samples were studied by solid state {sup 27}Al MAS NMR and pyridine adsorption measurements. The specific surface area (935–755 m{sup 2}g{sup −1}), pore volume (1.03–0.56 cm{sup 3}g{sup −1}) and especially the concentration and distribution of open type mesopores (0–68% to the total pores) of the synthesized Al-SBA-15 can be controlled by a simple adjustment of the P123/Si molar ratio in the initial solution. Moreover, increasing the aging temperature higher than 363 K can remarkably decrease the formation of plug structures to obtain “open” form mesopores. The observation by TEM of alternate defined gray and white areas inside the mesopores gives the strong evidence of isolated microporous aluminosilicate plugs inside the channels. In addition, a moderate hydrothermal post-treatment can finely modify the mesostructures through the partial or complete dissolution of the aluminosilicate plugs. - Graphical abstract: The plugs-containing structures can be interpreted as the distribution of individual isolated plugs along the mesoporous channel. - Highlights: • Al-SBA-15 with controllable plug structures was synthesized in an acid-free medium. • Its structural properties can be finely modified through moderate hydrothermal post-treatment. • TEM analysis gave the direct evidence of isolated aluminosilicate plugs inside the channels.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

The structure of nano-palladium deposited on carbon-based supports

Nano-palladium catalysts, prepared using the same procedure with the same metal content (3 wt%) and two different supports, activated carbon (Pd/C) and activated carbon—multiwalled carbon nanotubes (Pd/C/CNT), are discussed. The simple technique of deposition reduction was applied in the preparation of these two types of Pd catalysts. TEM, XRD analysis, EXAFS signal analysis, and XANES were used for sample characterization. In both samples, transmission electron microscopy identified nanosized Pd particles with nearly spherical morphology but different sizes. The mean diameters of the particles on Pd/C and Pd/C/CNT were estimated to be 5.4 nm and 7.8 nm, respectively. The EXAFS signal analysis showed that Pd atoms on the particle surfaces were coordinated by 4 oxygens to form a PdO monolayer covering a metallic core. The XANES signal analysis indicated a smaller particle size for Pd/C (∅ 5 nm) than for Pd/C/CNT (∅ 10 nm), in good agreement with the TEM observations. - Graphical abstract: Visualization of metallic core (left), oxide monolayer (middle) and nanoparticle of diameter 5 nm (right). - Highlights: • Pd catalysts were prepared on two types of supports: carbon and carbon nanotubes. • BET, TEM, XRD characterization of prepared catalysts. • XAFS: Concentration of Pd in samples Pd/C and Pd/C/CNT. • EXAFS and XANES signal analysis of catalysts. • Visualisation of atoms arrangement at the Pd nanoparticle surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Origin of “memory glass” effect in pressure-amorphized rare-earth molybdate single crystals

The memory glass effect (MGE) describes the ability of some materials to recover the initial structure and crystallographic orientation after pressure-induced amorphization (PIA). In spite of numerous studies the nature and underlying mechanisms of this phenomenon are still not clear. Here we report investigations of MGE in β′-Eu{sub 2}(MoO{sub 4}){sub 3} single crystal samples subjected to high pressure amorphization. Using the XRD and TEM techniques we carried out detailed analysis of the structural state of high pressure treated single crystal samples as well as structural transformations due to subsequent annealing at atmospheric pressure. The structure of the sample has been found to be complex, mainly amorphous, however, the amorphous medium contains evenly distributed nanosize inclusions of a paracrystalline phase. The inclusions are highly correlated in orientation and act as “memory units” in the MGE. - Graphical abstract: Schematic representation of pressure-induced amorphization and “memory glass” effect in rare-earth molybdate single crystals. The XRD and TEM measurements have revealed the presence of the residual identically oriented paracrystalline nanodomains in the pressure-amorphized state. These domains preserve the information about initial structure and orientation of the sample. They act as memory units and crystalline seeds during transformation of the amorphous phase back to the starting single crystalline one. - Highlights: • Pressure-amorphized Eu{sub 2}(MoO4){sub 3} single crystals were studied ex-situ by XRD and TEM. • Tiny residual crystalline inclusions were found in amorphous matrix of sample. • The inclusions keep in memory the parent crystal structure and orientation. • The inclusions account for “memory glass” effect in rare-earth molibdates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Thermal annealing dynamics of carbon-coated LiFePO{sub 4} nanoparticles studied by in-situ analysis

The thermal behavior of core-shell carbon-coated lithium iron phosphate (LiFePO{sub 4}-C) nanoparticles made by flame spray pyrolysis (FSP) during annealing was investigated by in-situ transmission electron microscopy (TEM), in-situ X-ray powder diffraction (XRD) as well as ex-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Crystallization of the initially glassy LiFePO{sub 4}-C nanoparticles starts at quite low temperatures (T=400 °C), forming single crystals inside the confinement of the carbon shell. Upon increasing the temperature to T≥700 °C, LiFePO{sub 4} starts to diffuse through the carbon shell resulting in cavities inside the mostly intact carbon shell. By increasing the temperature further to T≥800 °C, the initial core-shell morphology converts into open carbon shells (flakes and cenospheres) and bulky LiFePO{sub 4} particles (diameter in the range 300–400 nm), in agreement with ex-situ experiments. - Graphical abstract: TEM images of a typical sample area recorded at room temperature and after heating in-situ heating reveal the growth of particles and the formation of empty carbon cages. - Highlights: • LiFePO{sub 4} coated by a carbon shell is produced by flame spray pyrolysis. • The amorphous LiFePO{sub 4} starts to crystallize at 400 °C as revealed by in-situ XRD. • Crystal growth was visualized by TEM heating experiments. • The formation of empty carbon cages starts at 700 °C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Gold nano-particle formation from crystalline AuCN: Comparison of thermal, plasma- and ion-beam activated decomposition

In this work, in addition to the conventional thermal process, two non-conventional ways, the plasma and ion beam activations are described for preparing gold nanoparticles from microcrystalline AuCN precursor. The phase formation at plasma and ion beam treatments was compared with that at thermal treatments and the products and transformations were characterized by thermogravimetry-mass-spectrometry (TG-MS), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). TG-MS measurements in Ar atmosphere revealed that AuCN decomposition starts at 400 °C and completes at ≈700 °C with evolution of gaseous (CN){sub 2}. XPS and TEM show that in heat treatment at 450 °C for 1 h in Ar, loss of nitrogen and carbon occurs and small, 5–30 nm gold particles forms. Heating at 450 °C for 10 h in sealed ampoule, much larger, 60–200 nm size and well faceted Au particles develop together with a fibrous (CN){sub n} polymer phase, and the Au crystallites are covered by a 3–5 nm thick polymer shell. Low pressure Ar plasma treatment at 300 eV energy results in 4–20 nm size Au particles and removes most of the nitrogen and part of carbon. During Ar{sup +} ion bombardment with 2500 eV energy, 5–30 nm size Au crystallites form already in 10 min, with preferential loss of nitrogen and with increased amount of carbon residue. The results suggest that plasma and ion beam activation, acting similarly to thermal treatment, may be used to prepare Au nanoparticles from AuCN on selected surface areas either by depositing AuCN precursors on selected regions or by focusing the applied ionized radiation. Thus they may offer alternative ways for preparing tailor-made catalysts, electronic devices and sensors for different applications. - Graphical abstract: Proposed scheme of the decomposition mechanism of AuCN samples: heat treatment in Ar flow (a) and in sealed ampoule (b); Ar{sup +} ion treatment at 300 eV (c) and at 2500 eV (d). Cross section sketches illustrate the Au phase formation and the corresponding TEM micrographs are shown as top views. The dotted lines represent the original boundaries of the AuCN crystals. - Highlights: • AuCN decomposes between 400 and 700 °C with evolution of gaseous (CN){sub 2}. • Epitaxial relationship is established between Au and parent AuCN crystals. • Ar plasma treatment results in similar phase transition than the thermal treatment. • Plasma and ion-beam activation offer alternative way for preparing Au nanocrystals. • Reaction mechanisms of Au particle formation at the three treatments are proposed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Characterization of twin boundaries in an Fe–17.5Mn–0.56C twinning induced plasticity steel

A twinning-induced plasticity steel of composition Fe–17.5 wt.% Mn–0.56 wt.% C–1.39 wt.% Al–0.24 wt.% Si was analyzed for the purpose of characterizing the relationship between tensile strain and deformation twinning. Tensile samples achieved a maximum of 0.46 true strain at failure, and a maximum ultimate tensile strength of 1599 MPa. Electron backscatter diffraction (EBSD) analysis showed that the grain orientation rotated heavily to < 111 > parallel to the tensile axis above 0.3 true strain. Sigma 3 misorientations, as identified by EBSD orientation measurements, and using the image quality maps were used to quantify the number of twins present in the scanned areas of the samples. The image quality method yielded a distinct positive correlation between the twin area density and deformation, but the orientation measurements were unreliable in quantifying twin density in these structures. Quantitative analysis of the twin fraction is limited from orientation information because of the poor spatial resolution of EBSD in relation to the twin thickness. The EBSD orientation maps created for a thin foil sample showed some improvement in the resolution of the twins, but not enough to be significant. Measurements of the twins in the transmission electron microscopy micrographs yielded an average thickness of 23 nm, which is near the resolution capabilities of EBSD on this material for the instrumentation used. Electron channeling contrast imaging performed on one bulk tensile specimen of 0.34 true strain, using a method of controlled diffraction, yielded several images of twinning, dislocation structures and strain fields. A twin thickness of 66 nm was measured by the same method used for the transmission electron microscopy measurement. It is apparent that the results obtain by electron channeling contrast imaging were better than those by EBSD but did not capture all information on the twin boundaries such as was observed by transmission electron microscopy. - Highlights: • Performed tensile tests to assess mechanical performance of TWIP alloy • Analyzed tensile specimens using EBSD, TEM, and ECCI • EBSD showed that most twinning occurred at or near the < 111 >//TA orientation. • EBSD, TEM and ECCI were used to measure average twin density. • Compared spatial resolution of EBSD, ECCI and TEM for the instrumentation used.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Interfacial characterization of alumina-to-alumina joints fabricated using silver–copper–titanium interlayers

Two Ag–Cu–Ti interlayers with different compositions (Ag–35.3Cu–1.75Ti and Ag–26.7Cu–4.5Ti) were used to join sintered polycrystalline Al{sub 2}O{sub 3} having different amounts of porosity to investigate the effect of titanium and porosity contents on evolution of interfacial chemistry and microstructures. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) were used to characterize the interfacial microstructure. Two reaction layers, Ti{sub 2}O and Cu{sub 3}Ti{sub 3}O, were found at the interface of Ag–Cu–Ti interlayers and Al{sub 2}O{sub 3} using a series of simulated and experimental selected area diffraction patterns (SADP) of TEM and EDS. The total thickness of Ti{sub 2}O and Cu{sub 3}Ti{sub 3}O reaction layers at the interfaces increases with increasing amounts of Ti in the Ag–Cu–Ti interlayers but is independent of the porosity content in the Al{sub 2}O{sub 3}. Two possible formation mechanisms of Ti{sub 2}O and Cu{sub 3}Ti{sub 3}O reaction layers at the interface of Ag–Cu–Ti interlayers and Al{sub 2}O{sub 3} have been proposed based on the interfacial characterization performed in the present study. - Highlights: • Effect of alumina porosity and % Ti in interlayer on interface structure delineated • SEM, EDS, XRD, TEM with SADP used to delineate phases, compositions, crystallography • Ti{sub 2}O and Cu{sub 3}Ti{sub 3}O reaction layers identified and their formation mechanisms proposed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Characterization of grain-boundary phases in Li{sub 7}La{sub 3}Zr{sub 2}O{sub 12} solid electrolytes

Two types of Li{sub 7}La{sub 3}Zr{sub 2}O{sub 12} (LLZ) pellets were prepared containing Al and Si impurities, and their grain boundaries characterized using scanning electron microscopy (SEM) with energy-dispersed X-ray (EDX) elemental mapping, transmission electron microscopy (TEM), selected-area electron diffraction (SAED), electron energy-loss spectroscopy (EELS), and high-resolution transmission electron microscopy (HRTEM). In the LLZ pellets containing a small amount of both Al and Si, the total conductivity and density were found to be greater than that of LLZ containing Al alone. Using EDX and TEM, the Al and Si were confirmed to accumulate predominantly at grain-boundary regions. EELS and SAED were used to determine the presence of crystallites of LiAl{sub 5}O{sub 8} and LiAlSiO{sub 4} in the Al-only and Si and Al containing specimens, respectively. HRTEM observations clarified that in the case of LiAlSiO{sub 4}, nano-sized particles exist within an amorphous phase matrix, with a microstructure similar to that in a glass-ceramic. The presence of grain-boundary phases is shown to greatly affect the grain boundary resistance, in the case of the LiAlSiO4 glass-ceramic-type phase resulting in a noticeable improvement of the total conductivity. - Highlights: • Total conductivity of LLZ improved by inclusion of Al and Si impurities • Presence of the impurities at grain boundaries detected using SEM and TEM • Structure of grain-boundary phases identified using SAED, EELS, and HRTEM • Improved conductivity correlated with formation of LiAlSiO{sub 4} in grain-boundary phase.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Microstructural characterization of Charpy-impact-tested nanostructured bainite

In this work, a possible cause of the extraordinary low impact toughness of nanostructured bainite has been investigated. The microstructure of nanostructured bainite consisted chiefly of carbide-free bainitic ferrite with retained austenite films. X-ray diffractometry (XRD) measurement indicated that no retained austenite existed in the fractured surface of the Charpy-impact-tested specimens. Fractographs showed that cracks propagated mainly along bainitic ferrite platelet boundaries. The change in microstructure after impact loading was verified by transmission electron microscopy (TEM) observations, confirming that retained austenite was completely transformed to strain-induced martensite during the Charpy impact test. However, the zone affected by strained-induced martensite was found to be extremely shallow, only to a depth of several micrometers from the fracture surface. It is appropriately concluded that upon impact, as the crack forms and propagates, strain-induced martensitic transformation immediately occurs ahead of the advancing crack tip. The successive martensitic transformation profoundly facilitates the crack propagation, resulting in the extremely low impact toughness of nanostructured bainite. Retained austenite, in contrast to its well-known beneficial role, has a deteriorating effect on toughness during the course of Charpy impact. - Highlights: • The microstructure of nanostructured bainite consisted of nano-sized bainitic ferrite subunits with retained austenite films. • Special sample preparations for SEM, XRD and TEM were made, and the strain-affected structures have been explored. • Retained austenite films were found to transform into martensite after impact loading, as evidenced by XRD and TEM results. • The zone of strain-induced martensite was found to extend to only several micrometers from the fracture surface. • The poor Charpy impact toughness is associated with the fracture of martensite at a high strain rate during impact loading.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Microstructural investigation of plastically deformed Ti{sub 20}Zr{sub 20}Hf{sub 20}Nb{sub 20}Ta{sub 20} high entropy alloy by X-ray diffraction and transmission electron microscopy

The microstructure evolution in body-centered cubic (bcc) Ti{sub 20}Zr{sub 20}Hf{sub 20}Nb{sub 20}Ta{sub 20} high entropy alloy during quasi-static compression test was studied by X-ray line profile analysis (XLPA) and transmission electron microscopy (TEM). The average lattice constant and other important parameters of the microstructure such as the mean crystallite size, the dislocation density and the edge/screw character of dislocations were determined by XLPA. The elastic anisotropy factor required for XLPA procedure was determined by nanoindentation. XLPA shows that the crystallite size decreased while the dislocation density increased with strain during compression, and their values reached about 39 nm and 15 × 10{sup 14} m{sup −2}, respectively, at a plastic strain of ~ 20%. It was revealed that with increasing strain the dislocation character became more screw. This can be explained by the reduced mobility of screw dislocations compared to edge dislocations in bcc structures. These observations are in line with TEM investigations. The development of dislocation density during compression was related to the yield strength evolution. - Highlights: • Ti{sub 20}Zr{sub 20}Hf{sub 20}Nb{sub 20}Ta{sub 20} high entropy alloy was processed by arc-melting. • The mechanical was evaluated by RT compression test. • The microstructure evolution was studied by XLPA and TEM. • With increasing strain the dislocation character became more screw. • The yield strength was related to the development of the dislocation density.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Transmission electron microscopy analysis of hydroxyapatite nanocrystals from cattle bones

In this present study, hydroxyapatite which was obtained from cattle bones has been heat treated at temperature 400 °C and 600 °C. The microstructure after the treatment has been studied in detail using Transmission electron microscopy (TEM) and X-ray diffraction techniques. The TEM results indicate that natural bone consists of collagen and hydroxyapatite nano-crystals which are needle shaped. The heat treatment influences the crystallinity and growth of these hydroxyapatite nano-crystals known as ‘crystal maturation’ or ‘crystal ageing’. - Highlights: • Hydroxyapatite is obtained from cattle bones. • Material has been characterised using XRD and TEM. • Crystal growth and orientation has been studied in detail.

36 MATERIALS SCIENCE↗