Effect of Supersonic Fine Particle Bombardment on Microstructure and Mechanical Properties of Cold-rolled Fe-28Mn-8Al-1C Low-density Steel
Fe-Mn-Al-C low-density steel has become a preferred material for energy conservation and emission reduction in the automotive industry owing to its low density,high strength,excellent toughness,and significant potential for weight reduction.However,failures often originate from the material surfaces.Moreover,the surface integrity and microstructure directly determine the service life and safety of the material.Efforts have been made to optimize the properties and surface integrity of low-density steel to achieve a good match between its strength and ductility,along with better surface integrity,in addition to extending its service life in operating environments,thereby enhancing the reliability and safety of vehicle operation.Nanocrystallization can be achieved through plastic deformation as a novel surface strengthening technology that utilizes supersonic airflow to carry hard particles and bombard the surface of materials with extremely high kinetic energy,after SFPB,which has the advantages of a high strengthening efficiency,solid particle reuse,and simple and convenient operation,can be used for metal components with complex shapes and large sizes.In this study,to prevent the premature surface failure of low-density steel during service and improve its service life,the surface of cold-rolled Fe-28Mn-8Al-lC low-density steel was nano-treated using supersonic particle bombardment technology.Scanning electron microscope,transmission electron microscope,X-ray diffractometer,micro-hardness tester,universal material testing machine,and other testing methods were used to systematically study the effects of the SFPB impact time and gas pressure on the surface morphology,microstructure,and mechanical properties of cold-rolled Fe-28Mn-8Al-1C low-density steel.The results are as follows:after SFPB treatment,under the impact of high-energy and high-speed Al2O3 particles,gradient nanostructures consisting of the severe plastic deformation layer,micro-plastic deformation layer,and core matrix formed on the surface of cold-rolled Fe-28Mn-8Al-lC low-density steel.The grain size of the surface layer was refined to the nanometer level by"dislocation segmentation."With the increase in the impact time and gas pressure,the grain size of the surface layer decreased gradually and was refined to 8.68 nm at 1.0 MPa for 150 s.When the gas pressure was 1.0 MPa and the impact time was 120 s,the corresponding ultimate tensile strength and yield strength reached 1 679 MPa and 1 543 MPa,with increases of 15.7％and 26.4％,respectively.As the impact time and gas pressure increased,the surface micro-hardness and plastic deformation layer depth gradually increased,whereas the surface grain size gradually decreased.When the gas pressure was 1.0 MPa and the impact time was 150 s,the surface grain size was 8.68 nm,and the corresponding surface micro-hardness and plastic deformation layer depth were 569 HV and 16 μm,respectively.However,if the impact time was too long or gas pressure too high,stress concentration occurred on the surface of the cold-rolled Fe-28Mn-8Al-1C low-density steel,leading to the initiation and expansion of cracks and resulting in a decrease in its strength.The elongation of the cold-rolled Fe-28Mn-8Al-1C low-density steel under different SFPB process parameters did not change significantly,ranging between 4％and 5％,and the tensile fracture morphology exhibited a mixed mode of ductile and brittle fractures.SFPB technology could achieve controllable preparation of material microstructures by constructing gradient nanostructures on the surface of materials.Simultaneously,the mechanisms of back-stress strengthening and residual compressive stress inhibiting crack initiation and propagation could effectively improve the comprehensive mechanical properties of low-density steel.A novel surface-strengthening technology for the study of Fe-Mn-Al-C low-density steel with high strength and toughness is presented,offering a reference for its production and application in the automotive field in the future.
cold-rolled Fe-28Mn-8Al-1C low-density steelsupersonic fine particle bombardment(SFPB)surface morphologymicrostructuremechanical properties
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