Characterization of nano-precipitates in custom 455 stainless steel by transmission electron microscopy and atom probe tomography

The nano-scale precipitate phases in Custom 455 stainless steel quenched at 850 °C and aged at 480 °C for 4 h have been examined in detail by using transmission electron microscopy and atom probe tomography. The steel contains a large number of small, spherical Cu-rich precipitates densely distributed in the martensite matrix and some rod-shaped Ni3Ti precipitates, adjacent to the Cu-rich precipitates, together with spherical Ni-rich G-phase particles with a composition close to the stoichiometry of Ni16Si7Ti6 phase. The average size of the G phase is about 4.0 ± 0.8 nm, and the number density is high (9.8 × 1023 m−3), similar to that of the Cu-rich precipitates (9.6 × 1023 m−3). The length of Ni3Ti phase particle is about 15.0 ± 3.0 nm, and the number density is low (5.2 × 1022 m−3); The orientation relationships between the Cu-rich precipitates, the Ni3Ti phase and the martensitic matrix are: [−1−12]Cu//[−1100]Ni3Ti//[21−1]M, (111)Cu//(0001)Ni3Ti//(011)M. For the G-phase precipitates, the following orientation relationships between the G-phase, the martensitic matrix and the Ni3Ti phase exist: [100]G//[100]M//[−12−10]Ni3Ti, (022)G//(011)M//(0001)Ni3Ti.