Phase chemistry and precipitation reactions in maraging steels: Part III. Model alloys
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I.
INTRODUCTION
THE general background to our atom-probe (AP) studies o f maraging steels was set out in Part I o f this series, m Here we review briefly the literature on precipitation in certain m o d e l alloys and in high silicon-pluschromium o r manganese-containing steels. W e also identify areas in w h i c h field-ion microscopy ( F I M ) and A P microanalysis can contribute new insight into the nature o f the reaction sequence. A . Fe-Ni(-Co)-Mo Alloys The aging behavior o f Fe-Ni(-Co)-Mo m o d e l alloys has been widely investigated. F o r example, Servant and co-workers have carried out systematic X-ray, neutron scattering, and transmission electron microscopy (TEM) studies o f a range o f alloys. 12-7] Their conclusions include the following: (1) There are Mo-rich clusters (radius approximately 0.5 to 0.6 nm) in as-quenched alloys. However, the quenching carried out by Servant et a l . [41 w a s conducted at a cooling rate o f 600 °C" h -~, which is 2 to 3 orders o f magnitude l o w e r than the normal water-quenching procedure. (2) At l o w e r aging temperatures (for example, 414 °C) o r after short aging at higher temperatures, an w phase is present. The o9 phase is a nonequilibrium, submicroscopic phase which forms as a nucleation growth product and is often thought to be a transition phase. It occurs in metastable alloys, typically during aging at l o w temperature o r athermally upon quenching.I8,9] The o9 phase has a hexagonal structure (a = 0.4058 n m and c = W . SHA, formerly Graduate Student with the Department of Materials, Oxford University, is with the Department of Materials Science and Metallurgy, Cambridge University, Cambridge CB2 3QZ, United Kingdom. A. CEREZO, Royal Society Research Fellow, and G.D.W. SMITH, University Lecturer, are with the Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom. Manuscript submitted July 3 1 , 1992. METALLURGICAL TRANSACTIONS A
0.2485 nm). The aging product was initially proposed to consist o f Mo-rich zones, on the basis o f experiments using scattering techniques, but in theirl a t e r work, when direct observation using TEM was involved, spheroidal o9 phase precipitates were reported, rather than simple zones. The structure o f the o9 phase was characterized using electron microdiffraction and the composition obtained by small-angle X-ray scattering technique. (3) At l a t e r stages o f aging, the more stable F e - M o (Fe2Mo) phase forms. It can form simultaneously with the o9phase o r separately if the aging temperature is sufficiently high o r the aging time sufficiently long. In the present work, we have studied two m o d e l alloys, Fe-18.2 pct Ni-2.9 pct Mo and Fe-18.2 pct Ni-8.8 pct Co-2.9 pct Mo. These alloys were chosen because their base composition was the same as that o f the commercial C-300 steel studied in Part I o f this series. Ill Also, they correspond closely to two o f the m o d e l alloys studied previously by Servant et al. W e have examined the aging process at two temperatures, 510 °C (identical to
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