Microstructure and property evolution of Fe-N ferrite undergoing early-stages of precipitation

The precipitation-hardening phenomenon is well-known in nitrogen-supersaturated ferrite and its precipitation sequence has received extensive attention. Thus far the α″-Fe16N2 phase has been known as the main hardening precipitates in the alloy upon ageing at low temperatures. Here we report that distinct precursors (pre-α″) of the α″-phase exist and they also play a crucial role in the precipitation-hardened alloy. Using (in-situ) high-resolution transmission electron microscopy, property characterization and first-principle energy calculations in association with varying thermal processes, it is shown that lying on the {001}α planes, the pre-α″ precipitates have a plate-like morphology and are the major hardening precipitates in natural aged Fe-N ferrite. They are stable without much change even after tempering at 60 °C for 2 h. Furthermore, the plate-like pre-α″ precipitates typically consist of needle-like domains, due to energy minimization. The in-situ observations demonstrate that the precipitation sequence upon ageing is as follows: N-supersaturated α-Fe→N-rich clusters→pre-α″ (GP zones)→α″, where GP zones stands for Guinier-Prestone zones. The pre-α″ precipitates can have even more significant effects on properties of the alloy, as compared with the well-known α″ precipitates.