New insights into microstructure of neutron-irradiated tungsten

The development of appropriate materials for fusion reactors that can sustain high neutron fluence at elevated temperatures remains a great challenge. Tungsten is one of the promising candidate materials for plasma-facing components of future fusion reactors, due to several favorable properties as for example a high melting point, a high sputtering resistivity, and a low coefficient of thermal expansion. The microstructural details of a tungsten sample with a 1.25 dpa (displacements per atom) damage dose after neutron irradiation at 800 °C were examined by transmission electron microscopy. Three types of radiation-induced defects were observed, analyzed and characterized: (1) voids with sizes ranging from 10 to 65 nm, (2) dislocation loops with a size of up to 10 nm and (3) W–Re–Os containing σ- and χ-type precipitates. The distribution of voids as well as the nature of the occurring dislocation loops were studied in detail. In addition, nano-chemical analyses revealed that the σ- and χ-type precipitates, which are sometimes attached to voids, are surrounded by a solid solution cloud enriched with Re. For the first time the crystallographic orientation relationship of the σ- and χ-phases to the W-matrix was specified. Furthermore, electron energy-loss spectroscopy could not unambiguously verify the presence of He within individual voids.

New microstructural insights into neutron-irradiated tungsten

The development of appropriate materials for fusion reactors that can sustain high neutron fluencies at elevated temperatures remains a great challenge. Tungsten is one of the promising candidate materials for plasma-facing components of future fusion reactors, due to several favorable properties as for example a high melting point, a high sputtering resistivity, and a low coefficient of thermal expansion. The microstructural details of a tungsten sample with a 1.25 dpa (displacements per atom) damage dose after irradiation at 800°C were examined by transmission electron microscopy (TEM). Three types of radiation-induced defects were observed, analyzed and characterized: (i) voids with sizes ranging from 10 nm to 65 nm, (ii) dislocation loops with a size of up to 10 nm and (iii) W-Re-Os containing σ- and χ-type precipitates. The distribution of voids as well as the nature of the occurring dislocation loops were studied in detail. In addition, nano-chemical analyses revealed that the σ- and χ-type precipitates, which are sometimes attached to voids, are surrounded by a solid solution cloud enriched with Re. For the first time the crystallographic orientation relationship of the σ- and χ-phases to the W-matrix was specified. Furthermore, electron energy-loss spectroscopy could not unambiguously verify the presence of He within individual cavities.

Preparation of MgAl2O4-Coated Al2O3np and Migration of Ceramic Nanoparticles during Ultrasonic Processing of Aluminum Matrix Composites

Composites reinforced by nano-ceramic particles typically result in the formation of clustering and a weak interface. The spatial distribution of particles and the wetting behavior remarkably affect the targeted properties. Here, a surface modification combined spatial control solution was demonstrated to prepare nanocomposites with homogeneous micro-structures. Poly-crystalline nano-MgAl2O4 particles that possess a good crystallographic orientation relationship with Al were coated on the surface of ceramic particles, and they were macro- and then microscopically dispersed in the melt by ultrasonic vibration with variable frequency. The reason this is that the acoustic pressure distributed in the Al melt can induce the acoustic streaming and cavitation. A model for calculating equilibrium particle migration velocity was proposed, based on which the distribution of particles could be controlled by adjusting the solidification rate and the size of particle clustering. The experimental results were validated by the prediction of the model. In addition, it was found that the relationship of the maximum radius angle with the contact angle was ω0=180°−θ, and ultrasonic vibration could provide enough energy for the later stage entering of particles to overcome the energy barrier.

The origin of clinopyroxene - titanomagnetite clustering during crystallisation of synthetic trachybasalt

<p>Crystal clustering impacts rheology and differentiation in magmatic systems, and also offers insights into nucleation processes. Electron backscatter diffraction (EBSD) is ideal for studying interactions between crystals at interfaces. Clinopyroxene (Cpx) – titanomagnetite (Timt) clusters formed in time series experiments on synthetic trachybasaltic melt were studied using EBSD to understand the cause of clustering. Experiments were performed at 400 MPa and the NNO +2 buffer, at both anhydrous and hydrous (2 wt.% H2O) conditions, by cooling from 1300 °C (superliquidus) to 1100 °C with a rate of 80°C/min and holding at the target temperature for 4 – 8 hours before isobaric quenching.</p><p>All experiments crystallize dendritic Cpx (Lmax = 50 – 60 µm) and isometric euhedral to hopper-shaped Timt (Lmax = 5 – 6 µm). Infrequent (~ 10 mm<sup>-2</sup>) unmelted Cr-oxide crystals are surrounded by polycrystalline Cr-bearing Timt rims (Lmax Cr-oxide + rim = 20 µm). Cpx dendrite “rosettes” radiate from the polycrystalline rims, but many dendrites do not belong to rosettes, at least in 2D. Individual Timt crystals (Cr-free) are strongly associated with the sides and tips of Cpx dendrites. About 75% of Timt grains are in contact with Cpx in 2D. Cpx-Timt interfaces are irregular, and Timt is often attached only by thin necks. Timt grain centers are weakly clustered (R = 0.87 – 0.95, 1 = random).</p><p>Timt shows a strong crystallographic orientation relationship (COR) with Cpx, with 75 – 89% of Timt grains in contact with Cpx lying within 6° of a single fixed (“specific”) COR, OR1 = Cpx [010] // Timt <110>; Cpx (100) // Timt <111>; Cpx [001] // Timt <112>. The axes Cpx [010] // Timt <110> show the least dispersion (< 3°) from the ideal alignment. Relative to Cpx, individual Timt may be rotated up to 6° away from OR1, around an axis close to Cpx [010]. There are two peaks in this continuous distribution, corresponding to OR1 (above) and OR2 = Cpx [010] // Timt <110>; Cpx (-101) // Timt <111>; Cpx [101] // Timt <112>. The misorientation between OR1 and OR2 is 5.3°. OR1 and OR2 together represent 68 – 77% of Timt grains in contact with Cpx (tolerance angle 2.6°).</p><p>Cpx dendrite branches bend around Cpx [010]. The anhydrous sample with dwell time 4 hours shows continuous bending of up to ~15°, whereas the hydrous sample with dwell time 8 hours shows bending of up to only ~7° and subgrain boundaries (1 - 2°) separating undistorted domains, suggesting recovery of bent crystals during annealing. Initial Cpx nucleation likely occurred heterogeneously as rosettes on Cr-bearing Timt rims around Cr-oxide crystals. Multiple Timt grains touching different branches of the same bent Cpx crystal all maintain a close COR with the Cpx orientation immediately adjacent to the Cpx-Timt interface, indicating that Timt nucleated on (or attached to) dendrite branches during or after their growth.</p><p>In conclusion, EBSD is a powerful method for understanding crystallization and cluster formation. Future work will study the effect of annealing time, water content, and undercooling on Cpx – Timt cluster development.</p>

Atomic resolution structural and analytical characterization of layered precipitates in special alloys for high-temperature applications

Ni-base superalloys (e.g. Alloy 718) are largely used nowadays for manufacturing metallic components like gas turbine engine disc components for land-based power generation and aircraft propulsion to be exploited at high temperatures, due to their excellent mechanical properties and resistance to corrosive or oxidizing environments. The VDM 780 Premium alloy has been recently developed [1] in order to push even further the temperature limits of the widely used Alloy 718. This newly developed Ni-base superalloy is based on the  matrix (Ni-based cubic Fm-3m structure) and ’ hardening precipitates (Ni3Al-based cubic Pm-3m structure). The amount of the various crystal phases, the size and morphology of the different precipitates depend on the particularities of the applied heat treatments. The crystal phases obtained during the thermal treatment at high-temperature determine the grain size evolution and the mechanical properties of the alloy operating at high temperature. This HRTEM study brings a decisive contribution in understanding the structure of the high-temperature precipitates formed in the newly developed alloy VDM 780 Premium. For this purpose, we combined various techniques of analytical electron microscopy, including electron diffraction, atomic-resolution imaging, chemical characterization by EDS and EELS, elemental mapping by STEM-EDS ans STEM-EELS as well as structural modelling. Our results corroborated from SAED patterns, HRTEM imaging and local FFT diagrams have revealed that the high-temperature precipitates in the alloy VDM 780 Premium exhibit a layered structure consisting of alternating  and  phases in the crystallographic orientation relationship [010] || [100], (001) || (010), (100) || (001).

Effect of Rare-Earth Element Y on the Microstructure and Mechanical Properties of the Mg-6.0wt.%Zn-0.5wt.%Zr Alloy

The extruded Mg-6.0wt.%Zn-0.5wt.%Zr (ZK60) alloys with 0.5wt.% and 1.0wt.% Y additions were prepared. The microstructures and tensile properties of the alloys aged in different conditions were investigated. The results show that the specimen with 1.0wt.% Y additions and aged at 453K for 15 h had the best tensile strength. The ultimate tensile strength and yield strength reached 365MPa and 352MPa, respectively. The grain refinement, the formation of the fine dispersed Mg-Zn particles and the W-Mg3Zn3Y2 ternary phase may be the main reasons for the improvement. The average grain size reached to ~ 3μm. The size of the fine and dispersed particles was only several nanometers and the particle number density (Nv) was very high. The crystallographic characteristics of the ternary W-Mg3Zn3Y2 phase were studied by the transmission electron microscope and selected area electron diffraction techniques. The crystallographic orientation relationship between the W phase and the α-Mg matrix is identified as: [2 0]α//[ ]w, (0002)α plane near parallel to the (220)w plane with a 3.5 degree angle difference. This orientation relationship has rarely been studied and reported.

On the forbidden and the optimum crystallographic variant of rutile in garnet

In many inclusion–host systems with similar oxygen packing schemes, the optimum crystallographic orientation relationship (COR) between the inclusion and the host is mostly determined by matching the similar oxygen sublattices of the two structures. In contrast, the prediction of the optimum COR or even just the rationalization of the observed COR(s) between an inclusion and host with incompatible oxygen sublattices, like rutile–garnet, is not straightforward. The related documentation for such cases is therefore limited. Given the abundant crystallographic data for the rutile–garnet system acquired by transmission electron microscopy and electron backscatter diffraction methods recently, this problem can now be examined in detail for the critical structural factors dictating the selection of optimum COR in such a structurally complicated system. On the basis of the unconstrained three-dimensional lattice point match and structural polyhedron match calculated for the observed CORs, it becomes clear that the prerequisite of optimum COR for rutile (rt) in garnet (grt) is to have most of their octahedra similarly oriented/inclined in space by aligning 〈103〉rtand 〈111〉grtfor needle extension growth. Further rotation along the 〈103〉rt//〈111〉grtdirection then leads to the energetically most favorable COR-2 variant with a good lattice point match defined by the coincidence site lattice (CSL) and a good topotaxial match of the constituent polyhedra at the CSL points, leaving unfavorable COR-1′ in the forbidden zones. This understanding sheds light not only on hierarchical energetics for the selection of inclusion variants in a complicated inclusion–host system, but also on yet-to-be-explored [UVW]-specific CORs and hetero-tilt boundaries for composite materials in general.