The Systems Tantalum (Niobium)-Cobalt-Boron

Abstract

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 microanalysis, DTA and Pirani melting point measurements. Ternary phase equilibria were determined 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 crystallization from as-cast samples determined by SEM and EPMA measurements. The crystal structures of novel ternary compounds 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 temperature modification (ZrAlNi-type, a = 0.5953 nm, c = 0.3248 nm; a = 0.5926 nm, c = 0.3247 nm for Nb and Ta respectively), which was only present in as-cast alloys, but found to be stabilized by the addition of Fe to annealing temperatures 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 compounds {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; β = 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 homologue compound in the Ni-system exists. The crystallographic relations among the structure types of Re3B and pseudo-orthorhombic as well as monoclinic {Nb,Ta}Co2B were defined in terms of a Barnighausen scheme. DFT calculations 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, specific heat and electrical resistivity measurements on the compounds TaCo2B and Ta2Co21B6 reveal paramagnetic and ferromagnetic metallic ground states, respectively.