Deformation-induced phase transformation and strain hardening in type 304 austenitic stainless steel
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E plastic deformation of type 304 austenitic stainless steel results in the transformation of parent austenite (g) to martensite of hexagonal close-packed (e) and/or body-centered cubic (a#) crystal structures. The extent of transformation is dependent on the deformation temperature, strain, and grain size.[1,2,3] Earlier research on type 304 stainless steel postulated that e-martensite is an intermediate or transient phase during g ! a# transformation with an orientation relationship with parent austenite as (111)g//(00.1)e, and that the plates of bcc martensite having {225}g habit plane nucleate from the hcp structures.[2,4,5] These observations are rationalized considering the low stacking fault energy of type 304 steels. More recent observations with high-resolution electron microscopy have demonstrated that both forms of martensite can be produced independently during deformation—i.e., the presence of e-martensite is not a prerequisite to a#-martensite formation.[6,7] While e-martensite forms from overlapping stacking faults on every other {111}g plane by the passage 2.g Shockley partial dislocations, a#-martensite of a6 ,11" is reported to form at intersections between two shear AMAR K. DE, formerly with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO, is now Senior Research Engineer with Mittal Steel, USA Research and Development Center, 3001 E. Columbus Drive, East Chicago, IN 46312. JOHN G. SPEER, Professor, is with the Advanced Steel Processing and Products Research Center, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. DAVID K. MATLOCK, Armco Foundation Fogarty Professor and Director, is with Advanced Steel Processing and Products Research Center, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] DAVID C. MURDOCK, Metallurgical Engineer, is with Micromotion, Division of Emerson, 7070 Winchester Circle, Boulder, CO 80301. MARTIN C. MATAYA, Technical Staff Member, is with the Los Alamos National Laboratory, Los Alamos, NM 87545. ROBERT J. COMSTOCK, Jr., Research Engineer, Carbon Steel Product Research, is with AK Steel Corporation, Middletown, OH 45043. Manuscript submitted August 10, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
bands.[6–10] The shear bands can be e-martensite, mechanical twins, dense stacking faults, or twin boundaries. However, during plastic deformation, the e-martensite eventually transforms to a#-martensite at higher strains, so that at larger strains only a#-martensite is observed in 304 stainless steels. A possible mechanism proposed for this transformation is the shifting of faulted planes in the hcp phase by a second [8,11] a " 12 ,112.g shear resulting in a bcc martensite structure. The transformation sequence and the mechanism significantly influence the stress-strain behavior of type 304 austenitic steel, and distinct stages are observed in flow behavior corresponding to inherent microstructural
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