A High Entropy (Hf-Ta-Zr-Nb)C Ultra-High Temperature Ceramic (UHTC) was fabricated by ball milling and Spark Plasma Sintering (SPS) with a density of 99%. The microstructure characteristics were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) in combination with electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). Atomic structure and local chemical disorder was determined by means of scanning transmission electron microscopy (STEM) in conjunction with energy dispersive X-ray spectroscopy (EDS). According to the results, high purity, dense and homogeneous high entropy carbide with Fm-3m crystal structure was successfully produced. The grain size ranged from approximately 5 µm to 25 µm with average grain size of 12 µm. Chemical analyses proved that all grains had the same chemical composition at the micro as well as on the nano/atomic level without any detectable segregation. The approximately 1.5 nm thin amorphous grain boundary phase contained impurities that came from the starting powders and the ball milling process.

Stankov, S.; Yue, Yuanzheng; Miglierini, M.; Sepiol, B.; Sergueev, I.; Chumakov, A. I.; Hu, L.; Svec, P.; Rüffer, R.

Recently we determined the iron-partial density of vibrational states ͑DOS͒ of nanocrystalline Fe 90 Zr 7 B 3 ͑Nanoperm͒, synthesized by crystallization of an amorphous precursor, for various stages of nanocrystallization separating the DOS of the nanograins from that of the interfaces ͓S. Stankov, Y. Z. Yue, M. Miglierini, B. Sepiol, I. Sergueev, A. I. Chumakov, L. Hu, P. Svec, and R. Rüffer, Phys. Rev. Lett. 100, 235503 ͑2008͔͒. Here we present quantitative analysis of the evolution of various thermoelastic properties calculated from DOS such as mean-force constant, mean atomic displacement, vibrational entropy, and lattice specific heat as the material transforms from amorphous, through nanocrystalline, to fully crystallized state. The reported results shed new light on the previously observed anomalies in the vibrational thermodynamics of nanocrystalline materials.

No abstract

The formation of a nanocrystalline structure and its influence on the magnetic and mechanical properties in a ternary Fe 80.5 Nb 7 B 12.5 alloy has been investigated using a variety of complementary methods. The crystallization studies performed by DSC calorimetry, magnetization and electrical resistivity measurements have confirmed a two-stage nature of the primary crystallization process. The microstructure in the series of heat-treated amorphous and nanocrystalline specimens with different volume fractions of crystalline phase was examined by transmission electron microscopy, x-ray diffraction and 57 Fe Mössbauer spectrometry. The results obtained by a combination of static magnetic measurements and Mössbauer spectrometry have indicated a higher degree of structural and magnetic inhomogeneity of the residual amorphous phase after nanocrystallization. Striking differences in the magnetic hardening regime at elevated temperatures have been observed for the samples with different volume fractions of nanocrystalline particles. The strongest magnetic hardening effects are visible for the samples exhibiting a medium degree of crystallinity, while the best soft-magnetic properties are obtained for the samples where the primary crystallization process is nearly finished. The ductility tests have revealed that the transition from ductile to brittle behaviour develops predominantly in an amorphous phase just before crystallization and the subsequent crystallization causes only slight changes in the embrittlement level. On the other hand, the hardness is rather insensitive to the structural relaxation processes before crystallization and its value increases proportionally to the volume fraction of precipitated nanocrystalline grains.

A FePt-based hard-magnetic nanocomposite of exchange spring type was prepared by isothermal annealing of melt-spun Fe52Pt28Nb2B18 (atomic percent) ribbons. The relationship between microstructure and magnetic properties was investigated by qualitative and quantitative structural analysis based on the x-ray diffraction, transmission electron microscopy, and F57e Mössbauer spectrometry on one hand and the superconducting quantum interference device magnetometry on the other hand. The microstructure consists of L10-FePt hard-magnetic grains (15–45 nm in diameter) dispersed in a soft magnetic medium composed by A1 FePt, Fe2B, and boron-rich (FeB)PtNb remainder phase. The ribbons annealed at 700 °C for 1 h exhibit promising hard-magnetic properties at room temperature: Mr/Ms=0.69; Hc=820 kA/m and (BH)max=70 kJ/m3. Strong exchange coupling between hard and soft magnetic phases was demonstrated by a smooth demagnetizing curve and positive δM-peak in the Henkel plot. The magnetic properties measured from 5 to 750 K reveals that the hard characteristics remains rather stable up to 550 K, indicating a good prospect for the use of these permanent magnets in a wide temperature range.

No abstract

No abstract