In-Situ Measurements of Load Partitioning in a Metastable Austenitic Stainless Steel: Neutron and Magnetomechanical Meas
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NTRODUCTION
THE mechanical response of metastable austenitic stainless steels is dominated by the complex evolution of microstructure attending plastic deformation. The transformation from a fully austenitic microstructure to a mixture of austenite and martensite (a0 or -martensite) gives these steels their impressive combination of strength and ductility,[1–3] but also makes these properties challenging to predict.[4] The ‘‘dynamic composite’’ nature of these steels means that the work-hardening rate is determined by the combination of the rate of phase transformation and the intrinsic mechanical properties of the parent and product phases. While the macroscopic rate of phase transformation can be readily measured, the bulk mechanical response of the individual phases cannot, since they cannot be formed as unstrained, monolithic materials. Models for the work-hardening behavior of these steels have therefore often relied on strong assumptions regarding the mechanical response of the austenitic and martensitic phases (e.g.,[5]). Extraction DAVID MARE´CHAL, Graduate Student, and CHAD W. SINCLAIR, Professor, are with the Department of Materials Engineering, The University of British Columbia (UBC), Vancouver, BC V6T 1Z4, Canada. Contact e-mail: [email protected] PHILIPPE DUFOUR, Graduate Student, and PASCAL J. JACQUES, Professor, are with the Institute of Mechanics, Materials and Civil Engineering, IMAP, Universite´ Catholique de Louvain, 1348 Louvain-la-Neuve, Belgium. JEAN-DENIS MITHIEUX, Research Engineer, is with the Research Center, Aperam, 62330 Isbergues, France. Manuscript submitted November 2, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
of the in-situ mechanical response of co-deforming phases is possible by means of diffraction measurements, where elastic strains carried by individual phases can be measured based on the shifting of Bragg peaks.[6] Neutron[7,8] and X-ray[9–11] diffraction measurements have been performed in-situ and ex-situ on metastable austenitic stainless steels and used to estimate the relative contributions of the two phases to the overall workhardening rate. In the experiments mentioned previously, the maximum volume fraction of -martensite is below 5 pct, and thus the results of these experiments have been considered in relation to the parent austenite and the a¢-martensite only. These experiments have revealed that the martensitic phase can be considered as neither perfectly plastic nor perfectly elastic, contrary to previous assumptions. In the work of Spencer et al.,[8] the complexities arising from the concomitant phase transformation and co-deformation were reduced by first predeforming the material at subambient temperatures, where the transformation proceeded rapidly with plastic strain, and then continuing the testing during in-situ neutron diffraction experiments at ambient temperature where the rate of transformation was negligible. In this way, the elastic strains carried by a fixed volume fraction of straininduced a¢-martensite were followed during tensile t
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