Microstructural Modification of Laser-Deposited High-Entropy CrFeCoNiMoWC Alloy by Friction Stir Processing: Nanograin F
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INTRODUCTION
IN most conditions, metal failures occur on the surface in the forms of abrasion, fatigue fretting, crack, and corrosion, which seriously decrease the reliability of metal during its service in harsh circumstances, such as the surface wearout of the steam turbine blade under strong alternating stress and corrosion conditions. It is well known that the surface failures rely highly on the strength and microstructure of the surface. For many metal materials, low surface strength and poor wear properties are significantly hindering their engineering application. Consequently, microstructural modification on the surface is an ideal means to enhance the service lifetime and safe reliability of metals. It is common knowledge that the ultrafine grain or nanograin favors the mechanical properties from the Hall–Petch formulation. Metals could be refined to nanograin by severe plastic deformation (SPD) on the surface with gradient variation from Nano to micro.[1,2] Optimization of surface strengths by surface
RUIDI LI, MINBO WANG, and TIECHUI YUAN are with the State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, P.R. China. Contact email: liruidi@csu. edu.cn, [email protected] BO SONG and YUSHENG SHI are with the State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, 430074, P.R. China. Manuscript submitted May 30, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
nanocrystallization can widen the application foreground. At present, nanograin metals, such as Al, Ni, Cu, Ti, Co, and Fe base alloy, have been made by SPD.[3] Nevertheless, the traditional SPD methods, such as high-pressure torsion (HPT)[4] and accumulative rolling bonding,[5] could not enable the surface nanocrystallization and the deformation of highstrength metals. Surface mechanical attrition treatment (SMAT)[6] is a developed technique that can enable surface nanocrystallization via random and repetitive impact of milling balls onto the surface. However, SMAT suffers from low processing efficiencies, limited nanostructure, and microstructural inhomogeneity due to its insufficient material flowability, which have hindered its widespread application.[7,8] Different from other SPD process, friction stir processing (FSP) can create surface microstructure containing ultrafine grains or nanograins with large grain boundary (GB) misorientations, features that are important for enhanced properties.[9,10] FSP is based on the fundamental concepts of friction stir welding by which a rotary tool is inserted into the workpiece, produces friction heat, and then plasticizes the material.[11,12] Thus, FSP has been used as a unique thermomechanical process by which the large plastic strain and temperature lead to smaller grains and breakup of the constituent second phase.[12–14] Recently, increasing attention has been paid to the research and development of high-entropy alloys (HEAs), in which five or more elements are mixed in
near-equimolar rations, such that the
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