Precipitation in 9Ni-12Cr-2Cu maraging steels

Two maraging steels with the compositions 9Ni-12Cr-2Cu-4Mo (wt%) and 9Ni-12 Cr-2Cu and with small additions of Al and Ti were investigated using atom p robe field ion microscopy. Tomographic atom probe investigations were perfo rmed to clarify the spatial distribution of elements in and close to the pr ecipitates. Materials heat treated at 475 degrees C for 5, 25 min, 1, 2, 4 and 400 h were analyzed. Precipitates in the Mo-rich material were observed already after 5 min of aging, while in the material without Mo, precipitat ion started later. In both materials precipitation begins with the formatio n of Cu-rich particles which work as nucleation sites for a Ni-rich phase o f type Ni-3(Ti,Al). A Mo-rich phase was detected in the Mo-rich steel after 2 h of aging. The distribution of alloying elements in the precipitates, t heir role in the precipitation process, and the mechanism of hardening in t he two materials are discussed. (C) 1998 Acta Metallurgica Inc. Published b y Elsevier Science Ltd. All rights reserved.