Tensile deformation behavior of mechanically stabilized Fe-Mn austenite
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I.
INTRODUCTION
T H E influence of austenite (3/) predeformation on the austenite-to-martensite transformation during subsequent cooling has been well studied for the case of bcc ( a ' ) martensitic steels.[l'2] The mechanical stabilization of 3/is a v e ~ important phenomenon utilized in both ausformed steels tl and in steels processed for transformation-induced plasticity (TRIP) t4J to prevent extensive a ' martensite transformation during cooling. Because hcp e martensite often forms as a precursor to ct' martensite during deformation, it is anticipated that ausforming can also be utilized to stabilize 3/ against e formation. Tamura et al. reported that the Ms temperature for the y--~ e transformation decreases monotonically with increasing prior deformation of y in an Fe-Cr-Ni alloy containing both a ' and e martensites, lSj The effect of thermo-mechanical treatment on the transformation characteristics and mechanical properties of Fe-Mn-C-(X) alloys was discussed by Drobnjak and P a r r 16t and Sipos et al.t7~ and the effect on an Fe-Cr-Ni alloy was studied by Tomota et al. I81However, a ' martensite was induced along with e in these alloys. More recently, Sipos et al. demonstrated the mechanical stabilization of 3/ against the deformation-induced y --~ e transformation in Fe-26Mn-0.2C, I91 in which only e martensite is induced by deformation. The role of thermomechanical treatments such as ausforming or controlled rolling is important for the development of high-manganese steels since the y ~ e transformation behavior is quite sensitive to the austenite substructure. In this study the influence of austenite stabilization through ausforming on the resultant mechanical properties is contrasted for Fe-17Mn and Fe-25Mn binary alloys. Only the y ~ e transformation occurs during cooling or deformation of 25Mn steel, while a ' is easily induced by deforma-
Y. TOMOTA is Associate Professor, Departmentof Metallurgical Engineering, Ibaraki University, Hitachi, Japan. M. STRUM, formerly Candidate for Ph.D. at the University of California at Berkeley, is with LawrenceLivermoreNational Laboratoryand ResearchAssistant, Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94720. J.W. MORRIS, Jr. is Professor of Metallurgy, Department of Materials Science and Mineral Engineering, and Program Leader, Structural Materials, Center for Advanced Materials, LawrenceBerkeley Laborntnrv University of California. Berkeley. CA 94720.
tion in 17Mn steel, t~~ Significant differences in the tensile behavior of ausformed 17 and 25Mn alloys accompany the martensitic transformations during subsequent deformation, and several new observations on the influence of ausforming are presented. The differences in mechanical behavior between the two alloys are shown to be associated with the formation of both e and or' during tensile deformation in 17Mn while the y --> e martensite transformation occurs exclusively in Fe-25Mn.
II.
EXPERIMENTAL P R O C E D U R E
Binary Fe-17Mn and Fe-25Mn steels were inductionmelted in an
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