Co-evolution of point defects and Cr-rich nano-phase in binary Fe-20 at.% Cr alloy: A comprehensive investigation using positron annihilation spectroscopy and atom probe tomography

The role of point defects in temporal evolution of Cr-rich alpha '-phase separation in Fe-20 at.% Cr alloy is elucidated by intercepting the long term (upto 1000 h at 773 K) aging at regular intervals and probing by a combination of atom probe tomography (APT) and positron annihilation spectroscopy (PAS). Since the Cr concentration of nanoscale alpha '-phase in Fe-20 at.% Cr alloy increases continually on aging, the point defects play active role throughout the aging duration. The near-atomic resolution of APT and self-seeking ability of positrons towards point defects make this analysis possible. The difference of positron affinities between Fe and Cr enables identification of the Cr-rich nano-phases that are specifically associated with defects; they would otherwise be indiscernible in the absence of defect. Thus, the temporal evolution of Cr-rich nano-phase along with the associated point defects can be fully characterized at each stage of aging using APT and PAS, respectively. The combined APT-PAS study shows that Cr-rich alpha '-precipitation is preceded by vacancy-Cr complex that act as nucleation site for Cr-clusters, which in turn, cause an early rise in micro-hardness due to cluster hardening. Interestingly, this is accompanied by redistribution of the quenched-in vacancies and their trapping in Cr-clusters. Our results depict that the vacancies present in the core of the alpha '-precipitates migrate towards the precipitates' interface during the course of aging and eventually get recovered resulting in virtually defect-free precipitates. This study chronicles the way the point defects shape the process of alpha '-phase separation throughout the entire length of aging.