Intermetallic alloys
Intermetallic alloys are formed because of special bonding properties between at least two types of atoms in a certain stoichiometric ratio; in the case of binary phases, the general designation is thus AmBn. In the structure of pure metals and solid solutions, on the one hand, and that of ceramics, on the other hand, intermetallic alloys are distinguished by the fact that their bonding character is neither purely metallic nor completely covalent or ionic. A certain share of metallic bonding is always present, however.
The limit for the application of conventional Fe-, Co-, and Ni-based materials is around 1100 °C; for very low mechanical stress, the limit is slightly higher. The disadvantages of ceramic materials are their low ductility and fault tolerance. In view of these aspects, materials with an intermetallic matrix have been developed. High-strength Ni alloys consist predominantly of the intermetallic phase γ’-Ni3Al, of course. However, the creep strain at low stress occurs in the softer coherent γ’ structure of the solid solution. With a basic intermetallic mass, however, the strength and strain within this phase are decisive for the mechanical properties. The purposes of intermetallic alloys are to raise the temperature limit for applications beyond that of conventional superalloys and to ensure sufficient corrosion resistance at the same time. The ductility and fault tolerance should be sufficient for manufacturing components with reasonable effort and expense, and for ensuring reliability in operation. These alloys are intended for closing the gap between classical high-temperature alloys and ceramics.
Intermetallic phases are characterised by strong bonding between the unlike types of atoms. In the ideal case, the preferential A-B bonding in the superlattice phases results in the maximal possible number of unlike neighbours, which is designated as structural disorder. As dictated by the phase type, covalent as well as ionic bonding components can occur; nevertheless, a certain measure of metallic bonding character always persists. These structural features give rise to high values of Young’s modulus and high Peierls stresses; consequently, the strength values are very high. Because of the remaining metallic bonding component, the expected brittleness is at least lower than that of ceramics.
For employing intermetallic phases, they must be solid-solution-hardened with the addition of foreign elements and particle-hardened by second phases. The strain and ductility characteristics are usually similar to those of ceramics, at least at low temperatures. The brittle fracture range can extend to about 0.5 Ts.
The commercially available intermetallic alloys are designated as Triballoys. As indicated by the manufacturers, these alloys are characterised by “excellent resistance to abrasive and adhesive wear as well as corrosion, even at high temperatures”, /Delo99/. They consist of a hard intermetallic (Laves) phase which is dispersed in a softer matrix. The hard Laves phase, which imparts strength to the Triballoys, is stable up to 900 °C. These Laves phases have the following properties:
- Stoichiometric ratio AB2
- Very high packing density of the atoms; maximal volume filling of 71 per cent at an atomic radius ratio rA : rB = √3/2 = 1.225; actual fluctuation width: 1.05 to 1.68; Laves phases classified among the TCP phases (topologically close-packed)
- Cubic or hexagonal lattice structure
- Predominantly metallic bonding
The intermetallic Co-based alloy Triballoy 400 combines excellent wear resistance with high corrosion resistance. The intermetallic Ni-based alloy Triballoy 700 has a higher Cr content than Triballoy 400 for improved oxidation and corrosion resistance. The intermetallic Co-based alloy Triballoy 800 likewise has a higher Cr content than T 400; it is harder and has a higher wear resistance than T 400 and T 700.