In Situ Analysis of Plastic Deformation of Lath Martensite During Tensile Process
SHI Zengmin1(), LIANG Jingyu1, LI Jian2(), WANG Maoqiu3, FANG Zifan1
1.Hubei Key Laboratory of Hydroelectric Machinery Design & Maintenance, China Three Gorges University, Yichang 443002, China 2.School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China 3.Special Steel Institute, Central Iron and Steel Research Institute, Beijing 100081, China
Cite this article:
SHI Zengmin, LIANG Jingyu, LI Jian, WANG Maoqiu, FANG Zifan. In Situ Analysis of Plastic Deformation of Lath Martensite During Tensile Process. Acta Metall Sin, 2021, 57(5): 595-604.
Lath martensitic steels are widely used in high strength structural materials. Coherency strains in quenched lath martensite induce huge dislocation densities, which are the sources of the alloys' strength, whereas the way its microstructure functions is still unclear. The plastic deformation behavior of lath martensite in ultrahigh strength steel was investigated using in situ neutron diffraction technology. Diffraction data were analyzed using the Z-Rietveld and convolutional multiple whole profile (CMWP) fitting procedures. Transformation dislocations in the as-quenched martensite were mixed with edge and screw components and showed characteristics of random distribution. Significant work hardening of lath martensite can be better understood by considering the increase in dislocation density along with changes in dislocation arrangement. With increased tensile strain, the total dislocation density increased with the increasing amount of edge-type components and the decreasing amount of screw-type components. The hard orientation packets showed characteristics of work hardening with an increased dislocation density, whereas the soft orientation packets showed characteristics of work softening with a decreased dislocation density. The partitioning of the applied load was carried out within two types of packets, which further promoted the formation of long-range internal stresses after deformation.
Fund: National Basic Research Program of China(2010CB630802);National Natural Science Foundation of China(51201093);Hubei Technology Innovation Project(2017AAA113)
About author: LI Jian, professor, Tel: 15071161133, E-mail: lijian@hust.edu.cn SHI Zengmin, professor, Tel: 18872517438, E-mail: shzm@ctgu.edu.cn
Fig.1 Schematic to explain in situ neutron diffraction during tensile deformation (AD—axial direction, TD—transverse direction)
Fig.2 SEM (a) and EBSD (b) images of the 22SiMn2TiB steel
Fig.3 STEM-BF (a, c) and ADF (b, d) images with the incident beam being parallel to the <111> (a, b) and <001> (c, d) orientations of the 22SiMn2TiB steel (STEM—scanning transmission electron microscopy, BF—bright field, ADF—annular dark-field. Insets show SAED patterns. The region framed with circle in Fig.3a is for obtaining ADF image)
Fig.4 The true stress-true strain curves for the 22Si-Mn2TiB steel (Dot lines show the plastic deformation regime with strains being increased stepwise to arbitrary strains followed by unloading)
Fig.5 Neutron diffraction profiles of the 22SiMn2TiB steel in the axial direction obtained during tensile loading (a) and changes in the relative integral intensities of martensite (α) for different diffraction peaks vs the applied force (b)
Fig.6 Changes in full width at half maxima (FWHM) of martensite diffraction profiles obtained from Z-Rietveld fitting in the axial direction and the (200) peak profile before and after the tensile deformation (inset) (a), and dislocation densities obtained from convolutional multiple whole profile (CMWP) ?tting in the axial direction and the CMWP-calculated pattern (inset, measured profiles are presented in open circles and calculated profiles are presented in lines) (b) (γ—retained austenite, d—interplanar spacing, ε—true strain)
Fig.7 Volume fractions of retained austenite measured after plastic tensile deformation in the unloaded states
Fig.8 Lattice strains measured during tensile defor-mation (a) and the residual lattice strains measured in unloaded states after plastic tension (b) (Arrowed dot lines in Fig.8a indicate the unloading process)
Fig.9 Schematic of the soft orientation (SO) and hard orientation (HO) packets with the active Burgers vectors (b1 and b2) in- or out-of-lath-plane relative to the applied stress (σ), respectively (a); the measured data of martensite (200) diffraction profile, the CMWP-calculated total profile in the axial direction and the CMWP-calculated sub-peaks corresponding to the SO and HO packets (b)
Fig.10 Dislocation density (a), the parameter q (b) depending on the dislocation character and the stress distribution in the HO packet or SO packet (c) obtained from CMWP fitting in the axial direction
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