STRUCTURE OF THE COBALT/NICKEL-CONTAINING ALUMINIDES WITH FILLED PENTAGONAL CLUSTERS

M. Ellner¹, U. Burkhardt¹, K. Gotzmann¹ and Yu. Grin²

¹Max-Planck-Institut für Metallforschung, Seestr. 92, 70174 Stuttgart, Germany ²Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany ellner@vaxww1.mpi-stuttgart.mpg.de

The binary system Co-Al [1] shows in the concentration range 0.5 < x_Al < 1.0 the phase Co₂Al₅ (P6₃/mmc, hP28) formed by peritectic reaction at 1455 K, a phase bundle (in the concentration range 0.745 < x_Al < 0.760) and the aluminium-richest phase Co₂Al₉ (P2₁/a, mP22) formed by peritectic reaction at 1247 K. The decagonal phase d-CoAl₃(m) is a metastable phase in the binary system Co-Al. The multi-phase bundle consists of CoAl₃ formed by peritectic reaction at 1426 K, the high-temperature phase Co₄Al₁₃(h) formed by peritectic reaction at 1400 K and undergoing an eutectoidal decomposition at 1356 K, the orthorhombic o-Co₄Al₁₃ (Pmn2₁, oP102) formed by peritectic reaction at 1365 K and the monoclinic m-Co₄Al₁₃ (C2/m, mC102-7) formed by peritectoid reaction from CoAl₃ and Co₄Al₁₃(h) at 1363 K.

For the aluminium-rich alloys of the ternary system Co-Ni-Al, phase equilibria were investigated at the sections Co_25-xNi_xAl₇₅ (0 < x < 25) and Co_22-yNi_yAl₇₈ (0 < y < 22) [2]. Nickel substitutes cobalt in Co₂Al₉ up to x_Ni = 0.047. The pseudobinary high-temperature phase (Co,Ni)₄Al₁₃(h) (C2/m, mC34-1.8, homeotype with Os₄Al₁₃ [3]) is stabilized by nickel-addition up to x_Ni = 0.07. The ternary phase Co₂NiAl₉ (= "Y₂") is formed by ternary peritectoid reaction from (Co,Ni)₄Al₁₃(h), the nickel-containing Co₂Al₉ and the ternary decagonal phase at 1153 K. It shows only a limited homogeneity range between Co₁₇Ni_8.2Al_74.8 and Co_16.2Ni₉Al_74.8. In contrast to the binary system Co-Al, the ternary decagonal phase is a stable phase in the system Co-Ni-Al.

The Co₂Al₅ structure shows infinitive pentagonal channels built up by intergrowth of coordination polyhedrons formed by aluminium and cobalt atoms. The periodicity of a = 7.67 Å corresponds to a sequence of two pentagonal antiprisms and one pentagonal prism [4]. The structures o-Co₄Al₁₃ [5] and m-Co₄Al₁₃ are formed by two flat (AA´) and two puckered (BB´) layers of atoms in the sequence ABA´B´. These structures can be described in terms of pentagonal and rhombic clusters with dimensions d = 4.7 Å and h = 8.1 Å. The edges of pentagonal clusters are formed by transition metal (TM) atoms. Two TM atoms are situated in the middle of the prisms at y = 0.25 and y = 0.75. Each of them is surrounded by five aluminium atoms in form of puckered pentagons. The basal faces of pentagonal prisms are alternate occupied by one aluminium atom or five aluminium atoms forming distorted pentagon.

The structure of the high-temperature phase (Co,Ni)₄Al₁₃(h) (homeotypical with Os₄Al₁₃) is formed by one flat and one puckered layer (AB´); they form infinitive pentagonal channels with pentagonal prismatic clusters of h = 4.1 Å. Although a metrical relationship between the neighbouring phases (Co,Ni)₄Al₁₃(h) and Co₂NiAl₉ exists, both structures differ in the environment and packing of some common fragments remarkably: in the structure of Co₂NiAl₉ the common fragments are separated by slabs of tetragonal antiprisms and cuboctahedrons, in the structure of (Co,Ni)₄Al₁₃ the common fragments are shifted to (0 1/2 0) and connected without any intermediate parts.

[1] T. Gödecke and M. Ellner, Z. Metallkd. 87(1996)854-864.
[2] T. Gödecke and M. Ellner, Z. Metallkd. 88(1997)382-389.
[3] B. Zhang, V. Gramlich and W. Steurer, Z. Kristallographie 210(1995)498-503.
[4] Yu. Grin, U. Burkhardt, M. Ellner and K. Peters, Z. Kristallographie 209(1994)479-487.
[5] Yu. Grin, U. Burkhardt, M. Ellner and K. Peters, J. Alloys and Comp. 206(1994)243-247.