Work Hardening Behavior in Steel with Multiple TRIP Mechanisms
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TRANSFORMATION-INDUCED plasticity (TRIP) steels are expected to take a leading role in new vehicle designs that meet the 54.5 mpg corporate fuel economy average in 2025.[1] The target properties of these new steels would be combinations of ultimate tensile strengths and elongation to failures of 1000 MPa and 30 pct or 1500 MPa and 20 pct, respectively.[2] Modified TRIP steels have been reported to achieve 1000 MPa ultimate tensile strength and 30 pct elongation in a Fe-0.4C-1.2Mn-1.2Si steel consisting of ferrite, bainite, and retained austenite that transformed to a-martensite during deformation. Multiple martensitic transformation mechanisms have been reported where e-martensite is sometimes reported as an intermediate phase as austenite transforms to a-martensite.[3,4] Saturation of e-martensite in the microstructure has led to early fracture in TRIP alloys that do not subsequently transform to a-martensite.[5] It has been shown that the e-martensite plates act as stress concentrators and limit the strength and ductility of the alloy.[6, 7] Sun et al.[8] modeled the ductile failure mechanism in dual-phase steels and showed that the overall ductility was primarily influenced by the mechanical strength disparity between the two phases when the martensite volume fraction was greater than 0.15. A large M.C. MCGRATH, former Graduate Student, and D.C. VAN AKEN, Curator’s Professor, are with the Materials Science and Engineering Department, Missouri University of Science and Technology, Rolla, MO. Contact e-mail: [email protected] N.I. MEDVEDEVA, Principal Research Scientist, is with the Institute of Solid State Chemistry, Yekaterinburg, Russia, J.E. MEDVEDEVA, Associate Professor, is with the Department of Physics, Missouri University of Science and Technology, Rolla, MO. Manuscript submitted August 3, 2012. Article published online June 6, 2013 4634—VOLUME 44A, OCTOBER 2013
discrepancy between ferrite and martensite leads to void nucleation at lower strains, which reduces the uniform elongation.[8] The focus of this paper was to demonstrate improved mechanical properties in a TRIP steel when ferrite was avoided and TRIP behavior to e-martensite and a-martensite was promoted by manipulation of the generalized stacking fault energy curve. Formation of e-martensite is dependent upon the intrinsic stacking fault energy (cISFE). Thermodynamic methods for calculating cISFE are commonly expressed as cISFE ¼ 2qDGce þ 2rc=e ; where DGce is the difference between Gibbs free energies of c-austenite and e-martensite, q is the planar atomic density of the {111}, and rc=e is the interfacial energy between c and e.[9,10] The Gibbs free energies of the phases are most often estimated from regular solution models for multicomponent systems.[11, 12] Alloys with cISFE less than 20 mJ/m2 have been shown to exhibit TRIP behavior, where the austenite transforms to e-martensite during deformation,[11] while alloys with cISFE greater than 20 mJ/m2 will not transform.[11] Alloys with cISFE between 12 and 35 mJ/m2 exhibit twinning-induced
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