Formation of Hcp and Bcc phases in auslenitic iron alloys

Abstract

The formation of the hcp (c) and bcc (a) structures in pure iron under high pressure conditions , as well as the morphological and crystallographic aspects of martensitic transformation to these structures at atmospheric pressure in iron alloys, are reviewed. It is concluded that the unique features of a-or lath martensite formation are not dependent upon the presence of the ~ phase. Application of the phenomenological theory of martensitic transformation has not successfully rationalized the crystallography of lath-martensite. The criterion for c-and a-phase formation is established using the regular solution approximation and appropriate lattice stability parameters. In particular, the c phase can be formed in Fe-Ni-Cr compositions through stress-induced transformation attending a-martensite formation. Further consideration suggests that the c ~ a transformation is not expected at atmospheric pressure at temperatures below approximately 500°K in the alloys considered. Thus, two mar-tensitic transformations, y ~ c and y ~ a, can occur jointly in certain alloys. MANY austenitic (~) steels are known to transform partially to hcp (c) and bcc* (a) structures during *The present discussion willbe restricted tovery lowcarbon levels where a-martensite is cubic. This simplifying restriction can be removed later in order to broaden the range of alloys considered. quenching or deformation. While these crystallographic forms are observed in pure iron and certain iron alloys under high pressure, considerable controversy exists over the transformation sequencebetweenthe parent), and the daughter phases at atmospheric pressure. The alternatives are direct formation of c and a from aus-tenite or sequential martensitic transformation from through c to produce a as the end product where c may serve as a nucleation site for a in both cases. The present work seeks to establish the criterion for c-and a-phase formation in order to clarify the relationship between these structures in iron alloys. Before reviewing the transformation products found in binary and ternary iron alloys and attempting to characterize their stability, consideration is first given to the influence of pressure on transformations in iron and its alloys. These observations which offer clear examples of many reaction sequences, can be coupled with other results, to estimate the differences in free energy between the ~,, c, and a phases. Structural transformation at fixed compositions in iron-basealloys normally take place when the daughter phase has a lower free energy than the parent phase. The magnitude of the free energy difference is called the driving force. ~'~ The driving force for fcc(~)-~ bcc(a), fcc(y) hcp(c) and hcp(c) ~-bcc(a) reactions in iron or iron base alloys is alteredby changing the temperature or the pressure.3's The magnitude of the driving force at J. F. BREEDIS, formerly AssociateProfessor,Department ofMetal-lurgy and Materials Science, M.I.T., Cambridge, Mass., is now with AMF, Inc., Stamford, Conn. L. KAUFMAN is Director of Research, ManLabs, Inc., Cambridge, Mass. This paper is based on an invited talk presentedat a symposium on Formation of Martensitein Iron Alloys sponsored by the 1MD Ferrous Metallurgy Committee and held at the Spring Meeting ofThe Metal-lurgical Society ofAIME, May 1970,in Las Vegas, Nev. transformation is consideredto reflect the size of the barrier required for nucleation of the parent-daughter phase reaction.2 Thus, the transformation temperature at which martensite (a) begins to form from austenite on cooling (M s) can be characterized by a drivingforce which is usually of the order of 200 to 400 cal per g-atom in iron base alloys, hz'~'g Similar values hold at A s where martensite reverts to austenite (~). To discuss the formation of the hcp (e) phase, the temperature at whichE forms from ~ on cooling requires specification. Defining this temperature as E s would appear adequate , but specifying the reversion of E to y on heating6'7 and the temperature/pressure conditions for e ~ a reactions requires a further expansion of the above noted definitions. Accordingly, we define MS~/ as the temperature-pressure conditions under which marten-site forms from austenite on cooling in the classical sense. However,MS~/ wilt also cover the conditions for the formation of the a phase from ~ by release of pressure, Fig. 1. By analogy MSe denotes the condi-tionsunder which the bcc phase forms from the phase on cooling1° or release of pressure.~'~ Thus, ESa for iron at 480°K is approximately 120 kbar, Fig. 7 of Ref. 5. Under these conditions, the parent a phase is converted to the daughter hcp (~) phase as the pressure increases. When all of the a is converted to e (by raising the pressure) and the process is reversed by pressure release, the conversion of e to a begins at Mse. Location of the precise Mse pressure at 480°K is difficult because of the problem of pressure measurement in solid pressure transmitting media during unloading. Bearing this difficulty in mind, Fig. 6 of Ref. 5 indicates that the reversalbegins near 100 kbar and is completed near 60 kbar. Although these estimates of pressure are crude, they are in keeping with the fact that Mao, etal., Table I of Ref. 8, report lattice parameter data for the e phase retained duringdecom-pression of iron at 80 kbar and 300°K. By analogy, ES~ specifies the conditions under which the e daughterforms from the parent ~. Thus, cooling an alloy containing 84.6 pct Fe and 15.4 pct Ru from 800°C to room temperature at 40 kbar results in a y ~ e reaction6 which begins at EsV ~ 850°K. When the METALLURGICAL TRANSACTIONS VOLUME 2,SEPTEMBER 1971-2359