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

The microstructures and nano-scale precipitate phases of Custom 455 stainless steel quenched from 850 degrees C and over-aged at 480 degrees 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 similar to 20 nm. In the martensite, spherical G-phase particles with a diameter of about 8.0 +/- 1.6 nm are precipitated adjacent to the Cu-rich precipitates with a size of about 10.0 +/- 2.0 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)(gamma)//(0004)(eta)//(02-2)(G)//(111)(Cu), [111] (M) //[011](gamma)//[2-1-10](eta)//[111](G)//[11-2](Cu).