Results 1 - 10 of 186
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[en] During the last decade, aluminum alloys discontinuously reinforced with ceramic particulates have received increasing attention for various high-performance applications in automotive, aerospace, and transportation industries due to their high strength-to-weight ratio and superior wear resistance. Particulate-reinforced MMCs can be produced by ingot metallurgy (IM) and powder metallurgy (PM) processing routes. The advantages of PM over IM processing in the fabrication of MMCs include near-net-shape fabrication, lower processing temperatures, and improved microstructural homogeneity. In this study, the authors attempt to fabricate hybrid composites composed of Al-4 wt pct Cu matrix, boron nitride (BN), and SiC particulates. The BN particulates are selected because of their low density (2.3 g/cm3) and self-lubricant property. It is expected that BN additions can further substantially improve the wear performance of SiC/Al-4 pct Cu composites
[en] The vaporization of alloys of the Ni-Hf system was investigated in the temperature range between 1,200 and 1,650 K by Knudsen effusion mass spectrometry. The different compositions of the 16 alloy samples investigated covered the complete homogeneity range of the Ni-Hf system. The partial pressure of Ni was determined over all samples. The thermodynamic activities of Ni and Hf in the alloys were evaluated from these pressures and by Gibbs-Duhem integration. In addition, Gibbs energies of formation, enthalpies of formation, and entropies of formation resulted for the nine intermetallic phases of the Ni-Hf system. Beside similar thermodynamic data for the evaporation reactions were studied. The data obtained are discussed and a method for distinguishing the congruent melting compounds from the peritectic ones by defining stability factors calculated from the Gibbs energies of formation is suggested
[en] Arc cast, extruded, and heat-treated in situ composites of niobium silicide (Nb5Si3) intermetallic with niobium phases (primary--Nbp and secondary--Nbs) exhibited high fracture resistance in comparison to monolithic Nb5Si3. In toughness tests conducted at 298 K and slow applied loading rates, the fracture process proceeded by the microcracking of the Nb5Si3 and plastic deformation of the Nbp and Nbs phases, producing resistance-curve behavior and toughnesses of 28 MPa√m with damage zone lengths less than 500 microm. The effects of changes in the Nbp yield strength and fracture behavior on the measured toughnesses were investigated by varying the loading rates during fracture tests at both 77 and 298 K. Quantitative fractography was utilized to completely characterize each fracture surface created at 298 K in order to determine the type of fracture mode (i.e., dimpled, cleavage) exhibited by the Nbp. Specimens tested at either higher loading rates or lower test temperatures consistently exhibited a greater amount of cleavage fracture in the Nbp, while the Nbs always remained ductile. However, the fracture toughness values determined from experiments spanning six orders of magnitude in loading rate at 298 and 77 K exhibited little variation, even under conditions when the majority of Nbp phases failed by cleavage at 77 K. The changes in fracture mode with increasing loading rate and/or decreasing test temperature and their effects on fracture toughness are rationalized by comparison to existing theoretical models
[en] The formation of phase bands in in situ diffusion couples of the V-N system was studied by the reaction of vanadium sheet with pure nitrogen within the temperature range 1,100 C to 1,700 C and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase bands of β-V2N and δ-VN1-x develop. The morphology of the β-V2N/α-V(N) interface depends on the saturation state of the α-V(N) core. If the nitrogen content in α-V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the α-V(N) phase, the interface is planar. Electron probe microanalysis (EPMA) was used to measure the diffusion profiles within the couples. The homogeneity regions of the nitride phases were established and the phase diagram accordingly corrected. From the growth rates of the phase bands, the mean composition-independent nitrogen diffusivities in β-V2N and δ-VN1-x were derived. These diffusivities follow an Arrhenius equation with activation energies of 2.92 (β-V2N) and 2.93 eV (δ-VN1-x). By using δ-VN1-x as a starting material and a low nitrogen pressure during annealing, it could be shown that the direction of nitrogen diffusion can be reversed, i.e., β-V2N is formed on the surface of the couple as a result of out-diffusion of nitrogen
[en] Microstructure evolution during annealing of a wrought near-gamma titanium aluminide alloy, Ti-45.5Al-2Nb-2Cr (at. pct), in the temperature range 1,200 C to 1,320 C was investigated. The mean grain size of the alpha phase as well as the volume fraction and size of the gamma particles were evaluated as a function of annealing temperature and time. Isothermal annealing at temperatures above the alpha transus, Tα = 1,300 C, led to rapid grain growth of the alpha phase, the kinetics of which could be described by a simple power-law type expression with a grain growth exponent p = 2.3. Alpha grain growth was significantly retarded during annealing at subtransus temperatures (1,200 C ≤ T ≤ 1,300 C) by the pinning influence of gamma-phase particles. Limiting grain size values predicted by computer simulation models applicable for high-volume fractions of precipitates/particles were in good agreement with experimental findings. The kinetics of alpha grain growth in the presence of gamma particles were analyzed, and the results showed that a grain growth exponent of p ∼ 2.6 could satisfactorily account for the experimental results
[en] Bulk amorphous metals (BAMs) are an interesting class of new materials possessing unique properties that offer exciting possibilities for applications to a broad range of technologies. In contrast to the previous generation of amorphous metals, BAMs can be produced in bulk form at cooling rates as low as ∼1 K/s. The understanding of the structure, properties, and required cooling rates for BAM formation is hindered by the large number of constituents in typical alloys. In this article, the authors present the results of first principles local density approximation studies of the electronic structure and energetics of model Ni-Pd-P, Zr-Ni-Cu, and Zr-Ni-Al amorphous alloys that relate to two of the simplest BAMs, namely, Ni0.4Pd0.4P0.2 and Zr0.6Al0.15Ni0.25. The calculations are based on large unit cell (∼300-atom) structural models for which the electronic structure is calculated using the first principles order-N locally self-consistent multiple scattering method
[en] Crack growth and fatigue life predictions made with the MMCLIFE 3.0 code are compared to test data for unidirectional, continuously reinforced SCS-6/Ti-14Al-21Nb (wt pct) composite laminates. The MMCLIFE 3.0 analysis package is a design tool capable of predicting strength and fatigue performance in metal matrix composite (MMC) laminates. The code uses a combination of micromechanic lamina and macromechanic laminate analyses to predict stresses and uses linear elastic fracture mechanics to predict crack growth. The crack growth analysis includes a fiber bridging model to predict the growth of matrix flaws in 0 degree laminates and is capable of predicting the effects of interfacial shear stress and thermal residual stresses. The code has also been modified to include edge-notch flaws in addition to center-notch flaws. The model was correlated with constant amplitude, isothermal data from crack growth tests conducted on 0- and 90 degree SCS-6/Ti-14-21 laminates. Spectrum fatigue tests were conducted, which included dwell times and frequency effects. Strengths and areas for improvement for the analysis are discussed
[en] Microstructural evolution and variation in phase composition of W-8 pct Mo-7 pct Ni-3 pct Fe alloy were investigated with respect to various isothermal holding times, ranging from 5 to 240 minutes, at a sintering temperature of 1480 C. Mass transfer was found to proceed actively in both the liquid matrix phase and the tungsten-based solid phase during the initial stage of the isothermal hold. Formation of large jagged grains was found to be a result of excessive interdiffusion between molybdenum and tungsten. The jagged grains gradually developed into spheroidal grains with the reprecipitation of supersaturated tungsten atoms in the liquid matrix phase, which also resulted in crystallization of the matrix phase with less lattice dilation during cooling. Based on model fitting, reprecipitation of tungsten atoms from the liquid matrix phase to grains is proposed to be controlled by a first-order interfacial reaction
[en] The structure of hyper-eutectic ZrxPt100-x (73 ≤ x ≤ 77) metallic glasses produce by melt spinning was examined with high-energy synchrotron X-ray diffraction (HEXRD) and fluctuation electron microscopy. In addition, details of the amorphous structure were studied by combining ab initio molecular dynamics and reverse Monte Carlo simulations. Crystallization pathways in these glasses have been reported to vary dramatically with small changes in compositions; however, in the current study, the structures of the different glasses were also observed to vary with composition, particularly the prepeak in the total structure factor that occurs at a Q value of around 17 nm-1. Results from simulations and fluctuation electron microscopy suggest that the medium-range order of the amorphous structure is characterized by extended groups of Pt-centered clusters that increase in frequency, structural order, or spatial organization at higher Pt contents. These clusters may be related to the Zr5Pt3 structure, which contains Pt-centered clusters coordinated by 9Zr and 2Pt atoms.
[en] A multiscale methodology was developed to predict the evolution of thermal conductivity of polycrystalline fuel under irradiation. In the mesoscale level, phase field model was used to predict the evolution of gas bubble microstructure. Generation of gas atoms and vacancies were taken into consideration. In the macroscopic scale, a statistical continuum mechanics model was applied to predict the anisotropic thermal conductivity evolution during irradiation. Microstructure predicted by phase field model was fed into statistical continuum mechanics model to predict properties and behavior. Influence of irradiation intensity, exposition time and morphology were investigated. This approach provides a deep understanding on microstructure evolution and property prediction from a basic scientific viewpoint.