Splitting phenomenon in the precipitation evolution in an Fe–Ni–Al–Ti–Cr stainless steel

The evolution of precipitation and mechanical properties of an Fe–Ni–Al–Ti–Cr stainless steel was studied during ageing at 525 °C. Atom probe tomography and transmission electron microscopy were applied to follow the microstructural evolution. An initial hardening reaction, which is remarkable in terms of extent, is reported to be caused by the formation of complex multi-component clusters. They are composed mainly of Ni, Al and Ti. After ageing to peak hardness (3 h), splitting of these clusters into spherical and elongated particles was observed. Based on the chemical composition, the spherical precipitates were identified to be of type NiAl B2, and the elongated particles were associated with the η-phase (Ni3(Ti, Al)). Βoth types of precipitates contribute to the strength of the material.