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Direct Observations of Sigma Phase Formation in Duplex Stainless Steels Using In-Situ Synchrotron X-Ray Diffraction

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The formation and growth of sigma (σ) phase in 2205 duplex stainless steel (DSS) was observed and measured in real time using synchrotron radiation during 10 hour isothermal heat treatments at temperatures between 700 °C and 850 °C. Sigma formed in near-equilibrium quantities during the isothermal holds, starting from a microstructure which contained a balanced mixture of metastable ferrite and austenite. In-situ synchrotron diffraction continuously monitored the transformation, and these results were compared to those predicted by thermodynamic calculations. The data were further analyzed using a modified Johnson–Mehl–Avrami–Kolmogrov (JMAK) approach to determine kinetic parameters for sigma formation over this temperature range. The initial JMAK exponent, n, at low fractions of sigma was found to be approximately 7.0; however, toward the end of the transformation, n decreased to values of approximately 0.75. The change in the JMAK exponent was attributed to a change in the transformation mechanism from discontinuous precipitation with increasing nucleation rate, to growth of the existing sigma phase after nucleation site saturation occurred. Because of this change in mechanism, it was not possible to determine reliable values for the activation energy and pre-exponential terms for the JMAK equation. While cooling back to room temperature, the partial transformation of austenite resulted in a substantial increase in the ferrite content, but sigma retained its high-temperature value to room temperature.

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ACKNOWLEDGMENTS

The LLNL portion of this work was performed under the auspices of the United States Department of Energy (DOE), Lawrence Livermore National Laboratory, under Contract No. W-7405-ENG-48. The ORNL portion of this work was sponsored by the United States DOE, Division of Materials Sciences and Engineering, under Contract No. DE-AC05-00OR22725 with UT–Battelle, LLC. The UNICAT facility at the APS is supported by the United States DOE under Award No. DEFG02-91ER45439, through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana–Champaign, the Oak Ridge National Laboratory (U.S. DOE Contract No. DE-AC05-00OR22725 with UT–Battelle LLC), the National Institute of Standards and Technology (U.S. Department of Commerce), and UOP LLC. The APS is supported by the U.S. DOE, Basic Energy Sciences Office of Science, under Contract No. W-31-109-ENG-38. The authors thank Professor T. DebRoy, Pennsylvania State University, for reviewing this article and adding helpful suggestions and also thank Mr. Jackson Go, LLNL, for performing the optical metallography.

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Authors and Affiliations

  1. Lawrence Livermore National Laboratory, Livermore, CA, 94551, USA

    J. W. Elmer (Program Element Leader for Stockplie Metallurgy and Joining) & T. A. Palmer (Metallurgist)

  2. Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA

    E. D. Specht (Scientist)

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  1. J. W. Elmer
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  2. T. A. Palmer
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  3. E. D. Specht
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Correspondence to J. W. Elmer.

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Manuscript submitted July 24, 2006.

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Elmer, J.W., Palmer, T.A. & Specht, E.D. Direct Observations of Sigma Phase Formation in Duplex Stainless Steels Using In-Situ Synchrotron X-Ray Diffraction. Metall Mater Trans A 38, 464–475 (2007). https://doi.org/10.1007/s11661-006-9076-3

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