Phase decomposition in an Fe-40 at.% Cr alloy after isothermal aging and its effect on hardening

The phase decomposition process of an Fe-40 at.% Cr alloy was studied after isothermal aging at 475 and 500 °C using a high-resolution transmission electron microscope, as well as hardness measurements. High-resolution transmission electron microscope observations showed that the hardening behavior is associated with the formation of the nanometric coherent decomposed Cr-rich and Fe-rich phases with irregular shape and interconnected as expected for a spinodally-decomposed alloy. As the aging progressed, coherent rounded Cr-rich phase precipitates were observed in the Fe-rich phase matrix. The coarsening process of the Cr-rich phase was observed for aging times up to 750 h. Nevertheless, no decrease in hardness with time was observed because of the nanometric size of the Cr-rich phase, less than 10 nm. Aging hardening was higher at 500 °C because of the higher decomposition kinetics.

Research Highlights
► Spinodally-decomposed phases showed an interconnected and irregular shape in aged Fe–Cr alloy.
► Further aging promoted the formation of nanometric coherent rounded Cr-rich precipitates.
► Nanometric Cr-rich phases are responsible for the age hardening.
► Coarsening process of these nanometric Cr-rich precipitates caused no decrease in hardness.