APT based reliable quantification of Cr-rich phase separation and its relationship with mechanical properties in thermally aged Fe-20 at.% Cr alloy

Abstract

In this work, thermal aging (at 773 K from 1 h to 1000 h) induced change in micro-hardness is directly correlated with underlying microstructural evolution at nano-scale for high-Cr binary ferritic Fe-20 at.% Cr alloy. The reason for hardening of the alloy is attributed to the decomposition of ferrite phase into the ultrafine Cr-rich α/ phase in the Fe-rich matrix. Extremely small spatial dimension (diameter < 5 nm), identical bcc crystal structure and similar lattice parameter of α/ with matrix pose extreme challenge in their characterization and reliable quantification. A combinatorial methodology utilizing atom probe tomography (APT), transmission electron microscopy (TEM) and small angle neutron scattering (SANS) is followed for reliable quantification of α/ precipitates (diameter > 2 nm) with reduced uncertainty for samples aged for 50 h onwards. In addition, Cr-clusters (diameter < 2 nm) are quantified for samples below 50 h of aging using APT and SANS. Such reliable quantification is utilized to estimate the change in micro-hardness of the alloy through various theoretical dispersion strengthening models. The current work demonstrates that selection of suitable hardening model (Friedel–Kroupa–Hirsch model (with fixed barrier strength of 0.2), Orowan model (with variable barrier strength value between 0.047 and 0.146)) along with reliable quantification of microstructural evolution (α/ here), can successfully estimate the experimentally observed mechanical response of the alloy.