In Situ Local Measurement of Austenite Mechanical Stability and Transformation Behavior in Third-Generation Advanced Hig
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THIRD-GENERATION Advanced High-Strength Steels (3GAHSS) are typically transformation induced plasticity (TRIP) steels with microstructures that contain different volume fractions of ferrite (a), retained
FADI ABU-FARHA is with Clemson University, Greenville, SC, 29607. XIAOHUA HU is with the Pacific Northwest National Laboratory, Richland, WA, 99354. XIN SUN is with the Oak Ridge National Laboratory, Oak Ridge, TN, 37930. Contact e-mail: [email protected] YANG REN is with the Argonne National Laboratory, Argonne, IL, 60439. LOUIS G. HECTOR Jr. and TYSON W. BROWN are with the GM Global R&D Center, Warren, MI, 48092-2031. GRANT THOMAS is with AK Steel Research, Middletown, OH, 45005. Manuscript submitted April 25, 2017. The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. US Department of Energy (DOE) will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energ y.gov/downloads/doe-public-access-plan).
METALLURGICAL AND MATERIALS TRANSACTIONS A
austenite (c or RA), bainite and martensite (a¢) produced from various alloying and heat-treatment schemes. In addition to dislocation-mediated plasticity, the diffusionless phase transformation of retained austenite into martensite with plastic straining, or the so-called TRIP-effect[1,2] denoted by c fi a¢, is also very important for 3GAHSS. This transformation delays necking and fracture and leads to exceptional ductility and strength and hence mechanical properties that are attractive for many applications. In some cases, austenite transformation to a¢-martensite may also involve an intermediate step through e-martensite, which is the hexagonal close-packed (hcp) form of martensite that can nucleate with straining because of the low difference in Gibbs free energy between the c and e phases. The subsequent transformation of e-martensite to a¢-martensite has been experimentally confirmed elsewhere.[3–5] The 3GAHSS are generally distinct from other TRIP steels by virtue of a higher (initial) retained austenite volume fraction (RAVF) and often higher manganese (Mn) content.[6] There are two significant challenges for the wider applications of 3GAHSS: (1) the desired ductility and work hardening are sensitive to the initial volume
fraction and the stability of the metastable retained austenite during plastic straining[7]; (2) the constitutive models used in finite element-based component forming and performance simulations are dependent upon accurate measurement and description of the retained austenite transformation behavior, i.e., RAVF vs strain.[8] Regarding (1), ductility is adversely affected if the austenite transformation proceeds too rapidly. Alternatively, strength is adversely affected if the austenite transformation proceeds too slowly. The transf
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