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[en] We report a stress-induced phase transfonnation in stoichiometric UO2 from fluorite to the α-PbO2 structure using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. MD simulations, performed on nanocrystalline microstructure under constant-stress tensile loading conditions, reveal a heterogeneous nucleation of the α-PbO2 phase at the grain boundaries followed by the growth of this phase towards the interior of the grain. The DFT calculations confinn the existence of the α-PbO2 structure, showing that it is energetically favored under tensile loading conditions.
[en] Here, in situ ion irradiation and rate theory calculations were employed to directly compare the radiation resistance of an oxide dispersion strengthened alloy with that of a conventional ferritic/martensitic alloy. Compared to the rapid buildup of dislocation loops, loop growth, and formation of network dislocations in the conventional ferritic/martensitic alloy, the superior radiation resistance of the oxide dispersion strengthened alloy is manifested by its stable dislocation structure under the same irradiation conditions. Thus, the results are consistent with rate theory calculations, which show that high-density nanoparticles can significantly reduce freely migrating defects and suppress the buildup of clustered defects.
[en] Excellent mechanical properties of oxide-dispersion-strengthened (ODS) alloys arise from a high density of Y-Ti nano-oxides finely dispersed in the matrix. Characteristics of this precipitation can strongly be influenced by oxygen contamination during milling. We studied an as-received and annealed (1 h 1150 C) oxygen-enriched ferritic ODS alloy by High Resolution Transmission Electron Microscopy. The as-received sample has unknown b.c.c nano-oxides, while the annealed one has orthorhombic nanoparticles. We propose a phase relaxation during the annealing, confirmed by the observation of a particle where both cubic and orthorhombic structures coexist. The influence of oxygen on the particle structure is also discussed. (authors)
[en] Using first-principles calculations based on density-functional theory, the energetics of different vacancy-type defects, including voids, stacking fault tetrahedra (SFT) and vacancy loops, in Ni are investigated. It is found that voids are more stable than SFT at 0 K, which is also the case after taking into account the volumetric strains. By carrying out ab initio molecular dynamics simulations at temperatures up to 1000 K, direct transformations from vacancy loops and voids into SFT are observed. Our results suggest the importance of temperature effects in determining thermodynamic stability of vacancy clusters in face-centered cubic metals.
[en] Nanocrystalline 3Y-ZrO2 with densities ranging from 90.4% to 98.7% of theoretical and grain sizes from 55 nm to 160 nm were tested by Vickers indentation. The hardness of nanocrystalline and submicron (Tosoh) 3Y-ZrO2 was a strong function of density but independent of grain size and could be expressed as Hardness (GPa) = 0.549*Density (in %) + 40.449 with a correlation coefficient, R, of 0.919. The fracture toughness of both submicron and nanocrystalline 3Y-ZrO2 was low, about 2.5--4.5 MPa·m1/2, in contrast with the 8.4 MPa·m1/2 for a micron-grained 3Y-ZrO2 sample. In addition, the fracture toughness was relatively constant over all density and grain size ranges studied (55--400 nm; 90--100% density). The low fracture toughness, combined with no perceptible grain size dependence, implies that there is not a significant room temperature ductility mechanism peculiar to nanocrystalline materials. Minor effects may exist, but they are not sufficient to noticeably increase the fracture toughness. Because the data fit with existing trends of decreasing transformation toughening with decreasing grain size, it is presumed the poor fracture toughness of the nanocrystalline samples is due to a diminished capacity to transformation toughen via the tetragonal-to-monoclinic transformation
[en] In any composite development effort it is important to demonstrate that modifications in constituents or in the fabrication process do not adversely affect the other constituents or the resulting composite properties. Previous work has demonstrated that the extracted fiber strength distribution in as-consolidated composites is different than typical virgin fiber strength distributions and that composite heat-treatments in an inert environment may further degrade fiber strength. The degree to which a composite can withstand elevated temperature exposure depends on the kinetics of the fiber-matrix interaction which is related to the matrix chemistry and microstructure. Smith et al. have characterized the reaction kinetics of several Ti-Al-Nb alloys with SiC fiber, but the effect of the reaction on fiber strength had not been measured. The goal of this work was to establish the effect of composite consolidation in a Ti-22Al-23 Nb (atomic percent) matrix on the fiber strength distribution. The consequences of post-consolidation heat treatments, as well as prolonged high temperature exposure, on fiber strength were also evaluated
[en] The present work was undertaken to understand the effect of contamination by the milling atmosphere on the crystallization reaction during milling. Milling was performed with an already amorphous Fe36Ni36B28 alloy which has a relatively high crystallization point (approximately 520 C). Milling conditions were selected so as to allow for contamination while measures were taken to make sure that milling was a mechanical rather than a thermal treatment. Structural changes were monitored and compared with the thermal crystallization behavior of as-cast ribbons
[en] The newly developed Al-Fe-V-Si aluminum alloy, produced by melt spinning into ribbons, comminution of ribbon to particles, and then consolidation of particles by extrusion and forging, is being considered for high temperature applications due to the material's characteristics of high elevated temperature strength, low density, good toughness and thermal stability. In order to extend the near-net shaping capability of the material, the authors have proposed a new process that Al-Fe-V-Si aluminum alloy particles can be consolidated by casting, in which the liquid aluminum alloy was infiltrated around the Al-Fe-V-Si particles to form a FVS1212/A201 composite material. Preliminary study of the Al-Fe-V-Si particle reinforced A201 aluminum alloy composite demonstrated that the compression strength at 300 C can be twice as high as A201 aluminum alloy. This work constitutes a continuation of the previous efforts to understand the microstructural evolution sequences, particularly the precipitation events during infiltration of the liquid aluminum into Al-Fe-V-Si preform
[en] The (Fe,Cr)2(Mo,W) Laves phase found in the studied 10.5% chromium steel, which had been creep tested for nearly 1,500 h at 600 C, contained 0.8 at.% carbon. Thus, the solubility of carbon in this phase cannot be considered to be negligible. When compared with matrix analyses of the same material in the as-tempered state, the matrix surrounding the Laves phase in the creep tested material showed a slight enrichment in carbon. Besides carbon, Laves phase was enriched in silicon (3.7 at.%) and phosphorus (0.7 at.%). Further studies of the role of these elements on the nucleation and stability of Laves phase in 9%--12% chromium steels should be performed
[en] All materials exhibit some viscoelastic response, which can manifest itself as creep, relaxation, or, if the load is sinusoidal in time, a phase angle δ between stress and strain. Recently, a study of pure elements with low melting points, Cd, In, Pb, and Sn disclosed that cadmium exhibited a substantial loss tangent of 0.03 to 0.04 over much of the audio range of frequencies, combined with a moderate stiffness G = 20.7 GPa. Lead, by contrast, exhibited tan δ of 0.005 to 0.016 in the audio range. Indium exhibited a high loss tangent exceeding 0.1 at very low frequency. A eutectic alloy of indium and tin was found to exhibit substantial damping exceeding 0.1 below 0.1 Hz, and this alloy was used to make a composite exhibiting high stiffness and high damping. It is the purpose of this communication to present viscoelastic properties of two additional low melting point alloys, SnCd and SnSb. Both InSn and SnSb are used as solders. Although the melting point of Sb is 630.74 C, TH > 0.55 at ambient temperature for the alloy of SnSb (95 wt% Sn/5 wt% Sb) which melts near 240 C. Eutectic SnCd melts at 177 C so TH ∼ 0.65 at room temperature