Self healing in Fe-based systems: From model alloys to designed steels

When high-temperature steels are loaded under under industrially relevant conditions not only creep (i.e. a time dependent strain increase even under nominally constant loading conditions) occurs, but also local damage is formed. At relatively short exposure times quasi-spherical micron-sized cavities form preferentially at the grain boundaries oriented perpendicular to the principal loading direction. These cavities subsequently grow and coalesce into micro and macro cracks, which ultimately lead to failure of the structure. The concept of self healing, in which such damage is healed in-situ and under the applied loading conditions rather than is being prevented by a special microstructure, provides a new principle to extend the creep lifetime. Well-selected supersaturated solute atoms can selectively segregate at the free internal surface of the grain boundary cavities and fill them, thereby preventing the coalescence of cavities. This reduction in coalescence rate leads to an extended lifetime. The potential of the concept has been demonstrated in previous studies for binary Fe-based model alloys in which only one healing reaction can take place. The current work aims to take the validation one step further and to demonstrate it for a Fe-3Au-4W (in weight percent) model system in which two healing reactions can take place simultaneously, but without any intention to achieve decent mechanical properties. This work also aims to apply the selfhealing concept for two multi-component steels designed to have both decent mechanical properties and to demonstrate self-healing behaviour when exposed to the right conditions.