An overview is given on materials developments of ferritic and Fe3Al-based iron aluminium alloys with strengthening precipitate phases for high-temperature applications currently underway at the Max-Planck-Institut für Eisenforschung GmbH (MPIE). The development of high-temperature alloys for structural applications is to be focussed on optimisation of strength, creep and corrosion resistance at high temperatures and sufficient ductility at lower temperatures. This is discussed with respect to recent studies on Fe-Al-based alloys with strengthening precipitates, such as κ-phase Fe3AlCx, MC-carbides, Laves phase, and the B2-ordered intermetallic phase NiAl. The following alloy systems have been investigated: Fe-Al-X (X=C, Ti, Ta, Mo, Zr), Fe-Al-Ti-Nb, Fe-Al-Ni-Cr, and Fe-Al-M-C (M=Ti, V, Nb, Ta).

The investigations have been focussed on the microstructure, constitution, mechanical properties, and high-temperature corrosion behaviour of Fe-Al-based alloys with Al contents ranging from 10 to 30 at. %.

In Al-rich TiAl crystals, several long-period superstructures may appear depending on Al composition, annealing temperature and annealing time. Amongst these, Al5Ti3 and h-Al2Ti contain pure Al (002) layers, as in the L10 structure of the matrix, alternating with Ti (002) layers that exhibit an ordered arrangement of the Al atoms in excess. In single crystals with compositions ranging from Ti-54.7at.%Al to Ti-62.5at.%Al annealed at 1200°C, the Al5Ti3 long-period superstructure embedded in the L10 matrix develops with increasing Al concentration to finally transform fully into h-Al2Ti for Ti-62.5at.%Al. On the other hand, Al5Ti3 precipitates grow with annealing time at 500°C in Ti-58.0at.%Al.

The effects of the Al5Ti3 and h-Al2Ti superstructures on slip properties of 1/2<110] ordinary dislocations are examined both at a macroscopic and a microscopic level. The CRSS for 1/2<110] ordinary slip increases with Al5Ti3 ordering depending on Al composition, or of annealing time in the case of Ti-58.0at.%Al. Dislocations with 1/2<110] Burgers vector group into fourfold configurations to avoid the trailing of extended APBs in Al5Ti3. The CRSS for slip in the <110] direction further increases with the formation of h-Al2Ti particles within the L10 matrix in Ti-62.5at.%Al. By contrast, Ti-62.5at.%Al fully transformed into Al5Ti3 exhibits a CRSS significantly lower than that of the two-phase alloy.

Structural properties and magnetic behavior in pseudobinary alloys CoFe1−x Alx are studied. The B2-phase exists stably over wide ranges of composition and temperature and the Heusler-phase appears around the stoichiometric composition (0.4< x< 0.6 ) below about 973K. The mean magnetic moment in the B2-phase region decreases nearly linearly with increase in x in x > 0.2. Further, it decreases with increase in quenching temperature in x < 0.4, while it remains almost constant in x > 0.4. The mean magnetic moment of Heusler alloy Co2FeAl (x = 0.5) has the value of 1.30μB/atom, nearly the same as in the B2-phase, and shows no remarkable quenching temperature dependence.

The creep properties at 750°C of TiAl alloys produced by cast and powder metallurgy processes are investigated by creep tests and subsequent TEM investigations. It is demonstrated that the same dislocation mechanism is controlling the primary and secondary creep stages in these two alloys. This mechanism is the mixed climb of ordinary dislocations. Explanations to the creep behaviour during primary and secondary stages are then proposed.

The high temperature capability of TiAl alloys can be significantly improved by the implementation of precipitation reactions. From the engineering viewpoint the challenge is to establish the hardening mechanism without compromising desirable low temperature properties such as ductility and toughness. With this perspective in mind, twin structures were investigated in a TiAl alloy containing Ti3AlC carbides, in order to identify mitigation strategies for overcoming precipitation induced embrittlement. The study is focussed on the nucleation and immobilization of twins at precipitates. The results are discussed in the context of the plastic anisotropy and fracture behaviour of TiAl alloys.

Microstructure evolution during cold rolling of binary stoichiometric Ni3Al single crystals was examined by the optical (OM) and transmission electron microscopy (TEM). In the case of the <001> initial RD, the banded structure is formed. Inside each matrix band, the localized shear deformations occur alternately on two {111} planes. In addition, huge amounts of widely extended superlattice intrinsic stacking faults (SISFs) are observed from relatively early stage of cold rolling. The occurrence of the localized shear deformation is considered to be controlled by the SISFs since they must be strong obstacles for the dislocation motion on the other glide plane. The extensive formation of the SISFs is therefore considered to be one of the most important microstructural features which control the cold rolling behavior of Ni3Al.