Autonomous repair of creep damage can be achieved during high-temperature creep in iron based alloys by the addition of gold as a solute healing agent. The creep lifetime at a temperature of 550 °C is found to be extended significantly by the introduction of about 1 at.% of solute Au in the ferritic Fe matrix, when the alloy has been homogenized at elevated temperature and subsequently quenched. Combined electron microscopy techniques demonstrate that the improved creep properties result from the selective Au precipitation at the early-stage creep cavities, preferentially formed on grain boundaries oriented perpendicular to the applied stress. The selective precipitation of gold atoms at the free surface of a creep cavity results in pore filling, and, thereby, self healing of the creep damage. The large difference in atomic size between the Au and Fe strongly reduces the nucleation of precipitates in the matrix. As a result, the matrix acts as a reservoir for supersaturated solute until damage occurs. Grain boundaries and dislocations act as fast routes for solute gold transport from the matrix to the creep damage. The efficiency to heal creep damage is found to depend strongly on the applied stress. For lower stress levels filling fractions of up to 80% have been observed for the open-volume creep damage.
This chapter summarises the main findings of the work already published in the open literature.
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