Positron annihilation study on deformation-induced Au precipitation in Fe–Au and Fe–Au–B–N alloys

Abstract

We have investigated the potential self-healing of deformation-induced defects by Au precipitation during isothermal aging at 550 °C in Fe–Au and Fe–Au–B–N alloys using positron annihilation spectroscopy. Two different samples with 0 and 24 % pre-strain were used to study the influence of dislocations on the Au precipitation. Dislocations introduced prior to the aging process play an essential role in the formation of Au precipitates. The Coincidence Doppler broadening (CDB) technique shows that Au precipitation in the matrix occurs in the pre-strained samples only. TEM observations confirm the heterogeneous nature of the Au precipitation which occurs exclusively on dislocations and grain boundaries. The evolution of S and W parameters derived from the CDB indicates a three-stage precipitation process for the pre-strained samples. Both the hardness tests and the positron annihilation spectroscopy indicate that the addition of boron and nitrogen to the Fe–Au alloy causes a deceleration of the Au precipitation in the pre-strained samples, but does not alter the defect-induced mechanism of the Au precipitation. The defect-induced Au precipitation provides a promising site-specific autonomous repair mechanism to extend the lifetime of Fe-based alloys for high temperature structural applications.