Invited Lecture
How oxygen can get a precipitate forming alloying element?

Interaction of dislocations with dispersion of precipitates is the most effective strengthening mechanism in creep resistant structural materials. A spontaneous precipitation is conditioned by a sufficient supersaturation of the matrix by precipitate-forming elements like carbon. In steels, this is achieved by dissolution of a sufficient amount of carbon in austenite and by its transformation to supersaturated ferrite by cooling. To provoke precipitation of oxides, being chemically much more stable than carbides, this method is useless since oxygen exhibits a very low solubility in austenite. However, a high amount of oxygen can be captured (“dissolved”) in a heavily deformed matrix with a very high density of defects acting as traps for oxygen. This is just the case of mechanically alloyed powders, where trapping of oxygen even stabilizes defects against recovery and, thus, an immense amount of oxygen of the order of several atomic percent can be captured. After severe reduction in density of defects due to dynamic recrystallization induced by a hot consolidation of the powder, the matrix gets strongly supersaturated by oxygen, which provides an enormous chemical driving force for precipitation of very fine oxides. This new idea (recrystallization induced precipitation) promotes oxygen to a precipitate-forming alloying element.

The new family of creep resistant structural materials is called oxide precipitation hardened alloys (OPHAs). A pilot environment friendly creep and oxidation resistant Fe–11Al–1O based OPHA with 4vol.% of 10-20 nm sized Al₂O₃ having creep properties comparable to top creep resistant materials (polycrystalline nickel-based superalloys or ODS alloys) has been developed. The processing and properties of the pilot OPHA are presented in detail. Some attempts to increase the stability of oxides by micro-alloying are promising.