Morphology and crystallographic orientation relationship in isothermally transformed Fe–N austenite

The 225 °C isothermal transformation of a high-nitrogen austenite with Fe–2.71 wt.% N was investigated by means of electron microscopy. It was found that the transformation products were composed of ultrafine α-Fe and γ′-Fe4N plus retained austenite γ, which were in two types of morphologies, namely, (i) with the retained austenite patches dispersed among the (α-Fe + γ′-Fe4N) packets and (ii) with the ultrafine α-Fe and γ/γ′-Fe4N laths interwoven with each other within a single bainitic packet. A cube–cube orientation relationship between the γ (austenite) and γ′-Fe4N, and a near Greninger–Troiano (G–T) one between the γ (austenite) and the bainitic α-ferrite were detected. The morphology, orientation relationship and high hardness (> 1000 HV) of the transformation products indicated that the isothermal transformation of the high nitrogen austenite was analogous to a bainitic one.