Frank Kasper phases

File:Nb3Sn-unit-cell-from-xtal-1991-3D-SF-B.png

Unit cell of the A15 phases of Nb₃Sn

File:C14-MgZn2.jpg

Unit cell of Laves phase with MgZn₂ structure (Mg atoms are green).

Frank-Kasper (FK) phases or topologically close-packed phases are one of the largest groups of intermetallic compounds, known for their complex crystallographic structure and physical properties. Owing to their combination of periodic and aperiodic structure, FK phases belong to the class of quasicrystals. Applications of FK phases as high-temperature structural and superconducting materials have been highlighted; however, they have not yet been sufficiently investigated for details of their physical properties. Also, their complex and often non-stoichiometric structure makes them good subjects for theoretical calculations.

History

In 1958, Frank and Kasper, in their original work investigating many complex alloy structures, showed that non-icosahedral environments form an open-end network which they called the major skeleton, and is now identified as the declination locus. They came up with the methodology to pack asymmetric icosahedra into crystals using other polyhedra with larger coordination number and atoms. These coordination polyhedra were constructed to maintain topological close packing (TCP).

Unit-cell geometries classification

Based on the tetrahedral units, FK crystallographic structures are classified into low and high polyhedral groups denoted by their coordination numbers (CN) referring to the number of atom centering the polyhedron. Some atoms have an icosahedral structure with low coordination, labeled CN12. Some others have higher coordination numbers of 14, 15 and 16, labeled CN14, CN15 and CN16, respectively. These atoms with higher coordination numbers form uninterrupted networks connected along the directions where the five-fold icosahedral symmetry is replaced by six-fold local symmetry.

Classic FK phases

The most common members of a FK-phases family are: A15, Laves phases, σ, μ, M, P, and R. A15 phases are intermetallic alloys with an average coordination number (ACN) of 13.5 and eight A3B stoichiometry atoms per unit cell. Where two B atoms are surrounded by CN12 polyhedral (icosahedra), and six A atoms are surrounded by CN14 polyhedral. Nb₃Ge is a superconductor with A15 structure. Laves phases are intermetallic compounds composed of CN12 and CN16 polyhedrons with AB₂ stoichiometry, commonly seen in binary metal systems like MgZn₂. Due to the small Solubility of AB₂ structures, Laves phases are almost line compounds.

The sigma (σ) phase is an AB stoichiometric intermetallic compound formed at the electron/atom ratio range of 6.2 to 7. CrFe is a typical alloy crystallizing in the σ phase at the equiatomic composition. With physical properties adjustable based on its structural components, or its chemical composition provided a given structure. The μ phase has an ideal A₆B₇ stoichiometry, with its prototype W₆Fe₇, containing rhombohedra cell with 13 atoms. While many other Frank Kasper alloy types have been identified, more continue to be found. The alloy Nb₁₀Ni₉Al₃ is the prototype for the M phase. It has orthorhombic space group with 52 atoms per unit cell. The alloy Cr₉Mo₂₁Ni₂₀ is the prototype for the P-phase. It has a primitive orthorhombic cell with 56 atoms. The alloy Co₅Cr₂Mo₃ is the prototype for the R-phase which belongs to the rhombohedral space group with 53 atoms per cell.

Applications

FK phase materials have been pointed out for their high-temperature structure and as superconducting materials. Their complex and often non-stoichiometric structure makes them good subjects for theoretical calculations. A15, Laves and σ are the most applicable FK structures with interesting fundamental properties.The A15 compounds are forming important intermetallic superconductor with major applications in materials used in wires for superconducting such as: Nb₃Sn, Nb₃Zr and Nb₃Ti. A majority of superconducting magnets are constructed out of Nb₃Ti alloy. Small extents of σ phase considerably decreases the flexibility and impairment in erosion resistance. While addition of refractory elements like W, Mo or Re to FK phases helps to enhance the thermal properties in such alloys as steels or nickel-based superalloys, it increases the risk of unwanted precipitation in intermetallic compounds.