Microstructures and Compositions of Nano-Precipitates and Reverted Austenite in Custom 455 Stainless Steel in Over-Aged Condition Studied by Transmission Electron Microscopy and Atom Probe Tomography

Abstract

The microstructures and nano-scale precipitate phases of Custom 455 stainless steel quenched at 850 °C and over-aged at 480 °C for 256 h have been studied in a quantitative way by using transmission electron microscopy and atom probe tomography. The results show that plate reverted austenite is formed between the martensite laths. In the reverted austenite, a rod-shaped Ni3Ti phase particle of about 40 nm long is precipitated at the interphase interface between martensite and reverted austenite, with an adjacent Cu-rich precipitate with a size of ~20 nm. In the martensite, spherical G-phase particles with a diameter of about 8 nm are precipitated adjacent to the Cu-rich precipitates with a size of about 10 nm. These Cu-rich precipitates in the martensite are much smaller than those in the reverted austenite, but have a ten times higher number density than that in the latter. The orientation relationships between the martensitic matrix, reverted austenite, Ni3Ti phase, G-phase and Cu-rich precipitates are: (01-1)M//(1-11)γ//(0004)η//(02-2)G//(111)Cu,
[111] M //[011]γ//[2-1-10]η//[111]G//[11-2]Cu.

Keywords: reverted austenite, Ni3Ti phase, G-phase, Cu-rich precipitates, Atom probe tomography