Criteria for Phase Selection

Candidate phases for materials developments are selected in view of specific applications, i.e. they must show certain properties which make them promising for a particular application. For permanent magnets the energy product B H is a figure of merit and should reach high values (e.g. Stadelmaier et al., 1991), whereas for other functional materials other physical parameters are decisive. 

Selection criteria with respect to structural applications at high temperatures have already been discussed by Sauthoff (1989). First, such phases must have sufficient strength at the service temperature which also means sufficient creep resistance. The creep resistance scales with the diffusion coefficient and with the shear modulus (e.g. Jung et al., 1987), and both parameters scale with the melting temperature (Frost and Ashby, 1982). Thus the 

The main problem with strong intermetallics is their brittleness, which makes processing and application difficult or impossible. However, the brittleness of intermetallics should be less severe than that of ceramics because the atomic bonding of intermetallics is, at least partially, still metallic, whereas it is primarily covalent or 

Plastic deformation is more difficult in intermetallics than in metals and conventional metals alloys because of the stronger atomic bonding and the resulting ordered atomic distribution, which gives rise to more complex crystal structures (Paufler, 1985). Experience indeed shows that the brittleness of intermetallics increases with decreasing lattice symmetry and increasing 

Finally, it must already be possible to prepare an intermetallic of sufficient quality in the laboratory, and from a practical point of view this is the basic requirement for any materials development. The processing of high-strength intermetallics is difficult because of their brittleness, and development of the necessary processing techniques is a very demanding task. It is, however, also a very important task, because a poor quality increases the apparent brittleness and reduces the strength which may then preclude any applications of the tested material. 

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