Scripta METALLURGICA Vol. 26, pp. 523-528, 1992 Pergamon Press plc et MATERIALIA Printed in the U.S.A. All rights reserved ATOM PROBE STUDIES OF EARLY STAGES OF PRECIPITATION REACTIONS IN MARAGING STEELS H. Ti-FREE MODEL ALLOY AND Co-FREE T-300 STEEL W. Sha, A. Cerezo and G.D.W. Smith Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom. (Received October 29, 1991) Introduction Our work has already established that in a Fe-Ni-Co-Mo model maraging steel the equilibrium precipitate is the Fe7Mo6 ~t-phase (1) while in a Co-free T-300 steel (2) the age-hardening was produced by two intermetaUic phases namely Ni3Ti and Fe7Mo6 (3). Comparing the results of Co and Ti-containing C-300 steel, Ti-free model alloy and Co- free T-300 steel, the role of different alloying elements in the complex maraging steel systems has been investigated. Fe-Ni-Co-Mo model alloys have been widely investigated. There are two purposes of the current study of the model alloy:. 1) To understand a system with fewer components and to serve as a comparison with the more complex commercial C- 300 steel which was discussed in the accompanying paper (4). 2) To compare the atom probe field-ion microscopy (APFIM) study result with other studies using more conventional techniques, especially the systematic work by Servant et al on very similar model alloys (5,6,7,8). To summarise, Servant et al's conclusions for Fe-Ni-Co-Mo alloys concerning early stage reactions include: a) There are Mo-dch clusters (radius approximately 0.5-0.6 nm) in as-quenched alloys. However, the quenching was conducted at a cooling rate of 600"C.h -1, which is about 2 orders of magnitude lower than the normal water quenching procedure. b) At lower/shorter ageing temperatures/times (for example 414"C/15 h 50 min), an to-phase is present. The to- phase has a hexagonal structure (a=0.4058 nm, c=0.2485 nm). It was initially proposed as Mo-rich zones after experiments using scattering techniques, but in their later work when direct observation using transmission electron microscopy (TEM) was involved this was understood as spheroidal t0-phase precipitates rather than simple zones. The structure of the to-phase was characterised using electron microdiffraction and the composition obtained by the small angle x-ray scattering technique. We have already shown that an important role of Co in maraging steels is to lower the solubility of Mo in martensite, thus producing denser Mo-containing precipitates to increase the strength (3). We now examine the influence of Co on the eady stages of Mo precipitation, by comparing the microchemistry of Co-containing C-300 and Fe-Ni-Co- Mo model alloy with Co-free T-300 steel. Experimental The compositions of the alloys studied are shown in table 1. The Fe-Ni-Co-Mo alloy was made at Sheffield University Materials Advisory Centre, UK. The 750 g vacuum cast melt was hot rolled to about 14 mm diameter. The composition of the model alloy was chosen to match the composition of an alloy which Servant et al investigated (5-8) and of the commet~'ial Co-containing C-300 steel studied. The T-300 steel samples were originally made by Vanadium Alloy Steel Company, USA, with a target strength level 2 GPa/300 ksi (UTS). As-quenched specimeus were produced by water quenching samples annealed at 816"C for 2 h. Fe-Ni-Co-Mo and T-300 specimens were aged at 414"C for 15 h 50 rain and at 450"C for 5 min, respectively, after being annealed at 816"C for 2 h. FIM images were obtained with neon image gas. Atom probe analyses were usually conducted with a specimen temperature 65 K for conventional AP or 45 K for the position-sensitive atom probe (POSAP) experiments and a pulse fraction of 20%. The composition obtained from the conventional atom probe from the as-quenched Fe-Ni-Co-Mo material was in excellent agreement with the nominal composition (table 1). Examples of atom probe mass spectra from T-300 maraging steel obtained in the two types of instruments are given in reference 3. For details about APFIM techniques and data analysis, see reference 9. 523 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press plc