Stress and Strain Partitioning of Ferrite and Martensite during Deformation
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IN the last decade, the manufacture of high-quality steels for reducing fuel consumption has become an important concern for the automatic industry.[1] While sufficient plasticity remains, the dual-phase (DP) steels show an advantage in the high-strength over traditional productions such as the interstitial-free steels and low carbon steels. Compared to the high-strength low-alloy (HSLA) steels, the DP steels exhibit a higher ultimate strength, higher work hardening rate, and elimination of yield point elongation with a considerable increase in ductility and formability.[1] In fact, the mechanical behavior of DP steels has been widely studied in the past years,[2–4] including the influence of the volume fraction (Vm) of the harder phase (martensite) on the yield and ultimate strengths. It was already established that during deformation the strain may be transferred into the martensite islands, after the ferrite matrix is excessively strained, and the shearing of the interface between the martensite and ferrite occurred in DP steels with a high value of Vm. Less strain in martensite and a higher strain in the ferrite during deformation of the DP steels lead to delayed necking and retarded strain to fracture.[5–8] Previous attempts have been made to Z.H. CONG, Graduate Student, N. JIA, Lecturer, and Y.D. WANG, Professor, are with the Key Laboratory for Anisotropy and Texture of Materials (MOE), Northeastern University, Shenyang 110004, P.R. China. Contact e-mail: [email protected] X. SUN, Chief Scientist, is with the Computational Science and Mathematics Division, Pacific Northwest National Laboratory, Richland, WA 99352. Y. REN and J. ALMER, Physicists, are with the X-ray Science Division, Argonne National Laboratory, Argonne, IL 60439. Manuscript submitted October 3, 2008. Article published online April 8, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
quantitatively model the stress and strain partitioning of ferrite and martensite of duplex steel with various numerical or analysis methods.[4,9,10] While those micromechanical models were used to understand the local mechanics of deformation and mechanisms governing the macroscopic elastic-plastic deformation of heterogeneous solids, the real material parameters used for describing the constitutive laws of the representative volume element and matrix are difficult to obtain from experiments. In principle, it is possible to measure the yield stress and hardening behavior of different phases by the X-ray or neutron diffraction technique if the two phases exhibit different crystallographic structures. However, the direct measurement of both strain and stress state in each phase is still difficult as the traditional diffraction methods cannot distinguish the ferrite and martensite that exhibit the same crystal structure with similar lattice parameters. In this article, we study the micromechanical behavior of the DP980 steel by the in-situ high-energy X-ray diffraction (HEXRD) technique with a good resolution in the reciprocal space. The (200) lattice strains for each
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