The Systems Tantalum (Niobium)-Cobalt-Boron
- Autor(en)
- J. Wind, O. Romaniv, G. Schoellhammer, J. Bursik, H. Michor, G. Giester, P. Rogl
- Abstrakt
Constitution of the ternary systems Nb-?Co-?B and Ta-?Co-?B was studied, employing optical and electron microscopy, x-?ray powder, single crystal diffraction, electron probe microanal., DTA and Pirani m.p. measurements. Ternary phase equil. were detd. within an isothermal section at 1100 °C. For the Co-?rich part (=50 at.?% Co) of the system, a liquidus surface projection and a corresponding Schulz-?Scheil reaction scheme were constructed in combination with data for primary crystn. from as-?cast samples detd. by SEM and EPMA measurements. The crystal structures of novel ternary compds. have been elucidated by x-?ray powder and single crystal diffraction and were supported by TEM. {Nb,?Ta}?CoB with NbCoB-?type exhibits a high temp. modification (ZrAlNi-?type, a = 0.5953 nm, c = 0.3248 nm; a = 0.5926 nm, c = 0.3247 nm for Nb and Ta resp.)?, which was only present in as-?cast alloys, but found to be stabilized by the addn. of Fe to annealing temps. of 1400 °C. Ta3Co4B7 (a = 0.3189 nm, b = 1.8333 nm, c = 0.8881 nm) was proven to be isotypic with Nb3Co4B7. The novel orthorhombic compds. {Nb,?Ta}?Co2B3 (TaCo2B3-?type with space group Pnma; a = 0.53628 nm, b = 0.32408 nm, c = 1.24121 nm for TaCo2B3; a = 0.53713 nm, b = 0.32442 nm, c = 1.2415 nm for NbCo2B3) adopt unique structure types with branched boron zig-?zag chains. {Nb,?Ta}?Co2B were found to crystallize in a unique monoclinic structure type (space group P21/c; a = 0.9190 nm, b = 0.64263 nm, c = 0.63144 nm; ß = 109.954°, for Nb) very close to an orthorhombic setting (Cmce, a = 0.63162 nm, b = 1.72810 nm, c = 0.64270 nm, for Nb)?. Substitution of Co by Ni stabilizes a smaller orthorhombic lattice with Re3B-?type structure (Cmcm) although no homolog compd. in the Ni-?system exists. The crystallog. relations among the structure types of Re3B and pseudo-?orthorhombic as well as monoclinic {Nb,?Ta}?Co2B were defined in terms of a Baernighausen scheme. DFT calcns. revealed very close stabilities for the three competing structure types for {Nb,?Ta}?Co2B. Detailed transmission electron microscopy (TEM) for Nb(Co,?Fe)?B, {Nb,?Ta}?Co2B, {Nb,?Ta}?(Co,?Ni)?2B, and Ta3Co4B7 confirmed lattice geometries and crystal symmetry. Vickers hardness was measured for {Nb,?Ta}?Co2B, {Nb,?Ta}?(Co,?Ni)?2B, {Nb,?Ta}?2-?xCo21+xB6 and {Nb,?Ta}?Co2B3 exhibiting the highest value of hardness of HV = 22.4 ± 1.1 GPa for TaCo2B3. Magnetic, sp. heat and elec. resistivity measurements on the compds. TaCo2B and Ta2Co21B6 reveal paramagnetic and ferromagnetic metallic ground states, resp.
- Organisation(en)
- Institut für Physikalische Chemie, Institut für Mineralogie und Kristallographie
- Externe Organisation(en)
- Czech Academy of Sciences, Technische Universität Wien, Universität Wien
- Journal
- Journal of Phase Equilibria and Diffusion
- Band
- 35
- Seiten
- 43-85
- Anzahl der Seiten
- 43
- ISSN
- 1547-7037
- DOI
- https://doi.org/10.1007/s11669-013-0275-5
- Publikationsdatum
- 02-2014
- Peer-reviewed
- Ja
- ÖFOS 2012
- 104017 Physikalische Chemie, 105113 Kristallographie
- Schlagwörter
- ASJC Scopus Sachgebiete
- Condensed Matter Physics, Metals and Alloys, Materials Chemistry
- Link zum Portal
- https://ucrisportal.univie.ac.at/de/publications/276f1c37-0c4f-4493-bd0f-967dee39aa04