Design and Validation of Novel Potential High Entropy Alloys

The design approach and validation of single phase senary refractory high entropy alloys (HEAs) MoNbTaTiVW and HfNbTaTiVZr were presented in first part of this dissertation. The design approach was to combine phase diagram inspection of available binary and ternary systems and Calculation of Phase Diagrams (CALPHAD) prediction. Experiments using X-ray diffraction and scanning electron microscopy techniques verified single phase microstructure in body centered cubic lattice for both alloys. The observed elemental segregation agrees well with the solidification prediction using Scheil model. The lattice constant, density and microhardness were measured to be 0.3216 nm, 4.954 GPa and 11.70 g/cm3 for MoNbTaTiVW and 0.334 nm, 5.5 GPa and 9.36 g/cm3 for HfNbTaTiVZr. To elaborate the single-phase stability of HEAs, CrxMoNbTaVW was examined over a certain range of Cr content in the second part of this dissertation. The change in composition led to different BCC structures with different microstructures and physical properties. Microstructure characterizations were performed using X-ray diffraction and scanning electron microscopy. Chemical micro-segregation during solidification predicted using the Scheil model generally agrees with the experimental results. The lattice constant, density, and Vickers\u27 micro-hardness of the high-entropy alloy samples in the as-cast state are measured and discussed. For CrxMoNbTaVW, x=2.0 case appears exceeding the upper limit of maintaining a single BCC phase HEA, determined by the XRD patterns. The elemental dependence of the mixing thermodynamic properties (entropy, enthalpy and Gibbs energy) in BCC phase in the senary system is analyzed. The calculated entropy of mixing and enthalpy of mixing for CrMoNbTaVW are 14.7 J/K/mol and −662.5 J/mol respectively. Phase predictions and characterizations on as-solidified septenary refractory high-entropy alloy, CrMoNbReTaVW, are presented in the third part of the dissertation. The simulated solidification process predicts a single body-centered-cubic (BCC) crystal structure with the tendency of compositional segregation. X-ray diffraction results confirm the “single-phase-like” BCC structure, while further experimental characterizations reveal the existence of multiple grains with significantly different compositions yet the same crystal structure and similar lattice. For better understanding of corrosion properties of high entropy alloys, the CALPHAD method was further used to simulate the Pourbaix diagram and the corrosion layer evolutions under equilibrium conditions for CoCrFeNi based HEAs in the last part of the dissertation. The oxidation layer pitting and forming potential were calculated and compared favorably with published experimental results on CoCrFeNi, CoCrFeNiCu and CoCrFeNiAl0.5 HEAs

Multi-lingual Common Semantic Space Construction via Cluster-consistent Word Embedding

We construct a multilingual common semantic space based on distributional semantics, where words from multiple languages are projected into a shared space to enable knowledge and resource transfer across languages. Beyond word alignment, we introduce multiple cluster-level alignments and enforce the word clusters to be consistently distributed across multiple languages. We exploit three signals for clustering: (1) neighbor words in the monolingual word embedding space; (2) character-level information; and (3) linguistic properties (e.g., apposition, locative suffix) derived from linguistic structure knowledge bases available for thousands of languages. We introduce a new cluster-consistent correlational neural network to construct the common semantic space by aligning words as well as clusters. Intrinsic evaluation on monolingual and multilingual QVEC tasks shows our approach achieves significantly higher correlation with linguistic features than state-of-the-art multi-lingual embedding learning methods do. Using low-resource language name tagging as a case study for extrinsic evaluation, our approach achieves up to 24.5\% absolute F-score gain over the state of the art.Comment: 10 page

Project Overview of the Beijing-Arizona Sky Survey

The Beijing-Arizona Sky Survey (BASS) is a wide-field two-band photometric survey of the Northern Galactic Cap using the 90Prime imager on the 2.3 m Bok telescope at Kitt Peak. It is a four-year collaboration between the National Astronomical Observatory of China and Steward Observatory, the University of Arizona, serving as one of the three imaging surveys to provide photometric input catalogs for target selection of the Dark Energy Spectroscopic Instrument (DESI) project. BASS will take up to 240 dark/grey nights to cover an area of about 5400 deg$^2$ in the $g$ and $r$ bands. The 5$\sigma$ limiting AB magnitudes for point sources in the two bands, corrected for the Galactic extinction, are 24.0 and 23.4 mag, respectively. BASS, together with other DESI imaging surveys, will provide unique science opportunities that cover a wide range of topics in both Galactic and extragalactic astronomy.Comment: 10 pages, submitted to PAS