Martensite Enables the Formation of Complex Nanotwins in a Medium Mn Steel
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Strong and ductile metallic materials are desirable to develop lightweight structural components for different engineering applications.[1,2] However, improving strength often results in a reduced ductility, which is known as strength-ductility trade-off in metals and alloys.[3,4] To alleviate this trade-off, coherent nanotwin boundaries have been employed in varied metallic materials.[5–17] The nanotwin boundaries can be not only effective barriers but also slip planes for dislocation glide.[18–20] It is proposed that the hierarchical nanotwins can enhance the interaction between coherent nanotwin boundaries and dislocations, achieving a better strength and ductility combination compared with monolithic nanotwins.[21,22] Such hierarchical nanotwins can be generated in single-phase metallic
B.B. HE is with the Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China and with the Shenzhen Institute of Research and Innovation, The University of Hong Kong, Shenzhen 518000, China and also with the Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen, 518055, China. M.X. HUANG is with the Department of Mechanical Engineering, The University of Hong Kong and also with the Shenzhen Institute of Research and Innovation, The University of Hong Kong. Contact e-mail: [email protected] Manuscript submitted on August 02, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
materials by using various techniques.[3,23–26] For example, the hierarchical nanotwins in steels with a single austenitic phase can be developed by using severe plastic deformation, such as pre-torsion and uni-axial tension,[3] surface mechanical grinding treatment (SMGT)[24] and surface mechanical attrition treatment (SMAT).[26] In this article, we revealed a pathway to generate a nanotwinned structure with a complex morphology in a dual-phase medium Mn steel. Such complex nanotwins have a very high twin density. By detailed microstructural observation, we found that the quenched martensite, which results from the strong first-order solid-state phase transformation,[27] can facilitate the development of these complex nanotwins. A medium Mn steel with a chemical composition of Fe-10 pct Mn-0.45 pct C-1 pct Al (in wt pct) is employed for the present investigation. It is prepared by casting and forging, followed by hot rolling to strips with a final thickness of 4 mm. The ASTM E-8 sub-sized tensile samples are wire cut along the rolling direction. The tensile samples are annealed at 1000 deg for 1 hour, followed by water quenching (WQ) and immersing in liquid nitrogen (LN) for 10 minutes to generate lath martensite. The martensite start (Ms) temperature of the present steel is predicted to be 75 deg according to the empirical equations.[28,29] The tensile samples are tempered at 620 deg for 5 hours to allow C partitioning.[30] The tensile samples are plastically deformed to varied engineering strains at room temperature under a quasistatic strain rate of 5 9 10 4 s 1. The microstructure is char
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