Phase chemistry and precipitation reactions in maraging steels: Part IV. Discussion and conclusions
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I.
INTRODUCTION
THE work described in the current series o f articles ti,z,3~ represents an atomic-scale microscopical and microchemical approach to the understanding o f phase transformations in various types o f maraging steels. Maraging steels are characterized by their high supersaturation o f alloying elements. The effects o f the combination o f these elements are difficult to characterize and understand, due to the complexity o f the systems and particularly the ultrafine scale o f the microstructure. Fortunately, atom-probe field-ion microscopy (APFIM) techniques have the advantage o f atomic-scale resolution, and the direct observation and analysis o f atoms involve no averaging process. The microchemistry information provides us with an opportunity to understand the role o f different chemical species. Comparing the results o f commercial C-300 and T-300 steels,tt,zl m o d e l Fe-Ni(-Co)-Mo alloys, and less-common Cr- and Mncontaining systems,pl the role o f different phases and alloying elements in the complex maraging steel systems can be understood. II. B E H A V I O R OF D I F F E R E N T PHASES I N M A R A G I N G STEELS
A. Ni3Ti Many previous investigations o f early stages o f decomposition have been made on Fe-Ni(-Co)-Mo m o d e l 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
alloys, and it has often been reported that Mo clustering or segregation occurred. The formation o f the Ni3Ti phase at a very early stage was suggested over 20 years ago, t4J but no direct observation o f such an effect has been m a d e . The present study shows definitely that T i is much more heavily redistributed than Mo in the very early stage o f aging, when both T i and Mo are present in material. The existence o f the metastable Ni3Ti can be explained by its l o w e r lattice misfit with the matrix, tSj and, therefore, presumably, a l o w e r barrierto nucleation than for the more stable Mo-rich phases. The current study suggests that the Ni3Ti precipitate forms at the very early stage o f aging and is Mo-enriched in the C o containing C-300 steel, mzl Although Mo cosegregates with Ti-rich particles, there is still a sufficient amount o f Mo in the martensite matrix f o r the subsequent formation o f Fe-Mo phases. As aging proceeds to longer t i m e s , FeTMO 6/x-phase precipitates begin to form, which results in a drop o f Mo content in the Ni3Ti-type precipitate. After a g i n g for 8 hours, this type o f precipitate becomes nearly pure Ni3Ti, as f a r as stoichiometry is concerned. It is also found that, in the C-300 steel, the M o - r i c h regions o f some Ni3Ti precipitates do not quite overlap with the Ti-rich part
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