Mo-9Si-8B alloys with additons of Zr – microstructure and creep properties

Three phase Mo-9Si-8B (at.%) alloys are a prominent example for a potential new high temperature structural material. Due to their high melting point and excellent creep resistance. In this study the effect of Zr addition (0…4 at.%) on the microstructure and creep properties of Mo-9Si-8B (at.%) alloys is investigated. Two powder metallurgical processes, hot isostatic pressing (HIP) and spark plasma sintering (SPS), are used to prepare specimens. The resulting microstructures are examined using SEM and TEM analysis. SPS alloys exhibit smaller grain sizes and fewer oxides compared to the HIP alloys, because of the oxygen availability during HIP. The more Zr is present in the alloys, the more and finer the observed particles are. With addition of Zr the formation of SiO2 on the grain boundaries can be prevented completely, due to the formation of ZrO2. High temperature tensile creep tests are carried out under vacuum to determine the influence of the microstructure on creep properties. The creep rates are one order of magnitude lower for the Zr containing alloys. However with a level of 4 at.% Zr the minimum creep rates increase again. Please click Additional Files below to see the full abstract

Microstructural analysis and high temperature creep of Mo-9Si-8B alloys with Al and Ge additions

Refractory metals and their alloys show potential for high temperature applications, due to the increased melting point and creep resistance. Spark plasma sintering technology as well as argon arc melting is used to prepare quaternary and quinternary Mo-9Si-8B-xAl-yGe (x is 0 or 2; y is 0 or 2) samples. Compositions are stated in at.%. All the compositions consist of a Mo solid solution (α-Mo) and two intermetallic phases: Mo3Si (A15) and Mo5SiB2 (T2). On the one hand, no zirconium is added to the alloys to avoid evaporation of MoO3 due to the phase transformation from a monoclinic to a tetragonal crystallographic structure of ZrO2 at 1150°C. On the other hand, fractions of Al and Ge are alloyed to reduce the melting point of the intermetallic phases. The specimens are homogenized and coarsened by a subsequent heat treatment in a vacuum radiation furnace at 1850°C for 24 h. Both the reduction of the melting point and the heat treatment at a temperature of 1850°C result in an increase in diffusion rate. This procedure is expected to generate an α-Mo interpenetrating network. The resulting microstructures are investigated using SEM, EDX and XRD analyses. A creep testing device for a very short specimen heated in a radiation furnance up to 1400°C usable in air or vacuum is presented. Creep tests are performed at elevated temperatures in vacuum to investigate the influence of different fabrication techniques. Please click Additional Files below to see the full abstract

Frictionless motion of marginally resolved diffuse interfaces in phase-field modeling

We investigate the influence of artificial grid friction in different phase-field models by considering the stationary motion of an interface between two phases at different bulk free energy levels. Following the striking idea of a Sharp Phase-Field Model (SPFM) from Finel et al., we proof that restoring translational invariance indeed eliminates artificial grid friction effects during stationary interface propagation. Over a largely extended range of possible driving forces the theoretic interface velocities are reproduced by orders of magnitude more accurately, even if the diffuse interface profile is only marginally resolved by just one grid point. We propose a new variant of the SPFM, which restores translational invariance locally in the direction of interface motion. It is shown that, even for marginally resolved interface-profiles, the new SPFM variant provides frictionless motion for arbitrarily oriented planar interfaces. Finally, by considering thermal diffusion limited solidification, we demonstrate the capability of the SPFM-approach to also deal with inhomogeneous driving forces using a one-gird point interface resolution.Comment: 7 pages, 8 figures, 3 supplemental animation

Influence of the core hole on Kß emission following photoionization or orbital electron capture: a comparison using MnO and 55Fe2O3

The Mn K ß fluorescence emission in MnO after photoionization and in "Fe 2 O 3 after radioactive electron capture decay from the K shell have been measured using a crystal array spectrometer with an instrumental energy bandwidth of 0.7 eV (full width at half maximum). Both compounds have a 3d 5 valence electron configuration in the ionic approximation. It is found that the spectral features after K capture in 55 Fe 2 O 3 are shifted in emission energy and are sharper, compared to the spectra following photoionization in MnO, i.e., the spectra exhibit a dependence on the mode of excitation. Crystal-field multiplet calculations including ligand-to- metal charge transfer have been carried out for the 1s intermediate states as well as for the 3p to 1s (K ß) radiative transition. The populated 1s intermediate states after photoionization are found to be spread over several eV. In comparison, only the lowest-lying 1s intermediate states split by the weak (1s,3d) exchange interaction are populated after K capture. It is proposed that the differences in population of the 1s intermediate states together with a term-dependent final-state lifetime broadening can account for the changes in the spectral shapes due to the different modes of excitation

High Resolution K Capture X-ray Fluorescence Spectroscopy: A New Tool for Chemical Characterization

The ability to probe specific chemical sites in complex systems would make X-ray spectroscopy a far more versatile spectroscopic tool. In vibrational and magnetic resonance spectroscopies, isotopic substitution is commonly employed to allow characterization of particular species. Except in a few special cases, such as gas-phase spectra of light elements, isotope effects are too small to be observed in X-ray absorption spectra. An alternative approach is to examine the X-ray emission that results after electron capture by a radioactive isotope.^1,2 Controlled introduction of electron-capture isotopes could result in specific labeling of chemically distinct sites. In this paper, we show that highresolution electron capture fluorescence spectra can be obtained on a reasonable time scale. Chemical shifts in these spectra can be used to identify elemental spin states, oxidation states, and even the types of neighboring atoms. In the electron-capture process an inner shell electron reacts with a nuclear proton to yield a neutron and a neutrino