Effect Of Ultrasonic Shot Peening On The Microstructure And Mechanical Properties Of FeCoNiCrMn High-entropy Alloy

The thermodynamic high-entropy effect,lattice distortion effect,kinetic slow diffusion effect,and "cocktail effect" of high-entropy alloys make them one of the most promising new materials in recent years.In this paper,the FeCoNiCrMn high-entropy alloy with equal atomic ratio is focused on.Numerical simulations and experimental researches were carried out to explore the microstructure evolution and mechanical properties of the alloy under different ultrasonic shot peening time.On this basis,the influence mechanism of ultrasonic shot peening on the strength and toughness of the alloy was analyzed.The main findings of the thesis are as follows.(1)A single shot peening single point superimposed impact model and a full-coverage impact model were established by numerical simulations using Abaqus software.It was concluded that the residual compressive stress field generated by single shot peening superimposed impact became deeper and the surface roughness was increased.A residual compressive stress layer of certain thickness was formed on the surface of the target material after full-coverage impact,and the surface roughness was decreased due to the mutual extrusion of multiple shot peen.(2)Comparing the microstructure of the original FeCoNiCrMn high-entropy alloy sample with the one after ultrasonic shot peening for 3 min,6 min and 9 min,it was found that a certain thickness of residual compressive stress strengthening layer was formed after ultrasonic shot peening.The depths reached 234 μm,303 μm,and397 μm,respectively.The crystal structure of the alloy changed from the original single-phase FCC structure to the mixed structure of FCC+BCC,and the intensity of the diffraction peak corresponding to the BCC structure became stronger.With the prolongation of the ultrasonic shot peening time,the original surface grain structure became flocculent,the grain boundary was compressed,the entanglement and increment of dislocations was intensified,and the surface roughness was increased.(3)The mechanical properties test results showed that the hardness of FeCoNiCrMn high-entropy alloy after ultrasonic shot peening was greatly improved.The surface hardness of the alloy after ultrasonic shot peening for 3 min,6 min and 9min was increased by 145.35%,151.32% and 162.25% respectively compared with the original one.With the extension of the sonic peening time,the strength of the alloy increased and the elongation after break remained at around 30%,with the 6min specimen performing best in terms of strength and elongation after break,with its tensile strength increasing by 1.53 times,yield strength increasing by 2.53 times and elongation after break remaining at 29.9%.(4)The tensile fracture of the original specimen showed a typical ductile fracture with a uniform distribution of tough nests.The specimens after 3 min and 6 min ultrasonic shot peening exhibited a mixture of tough nest and quasi-dissociation fracture,and the specimen after 9 min ultrasonic shot peening showed a mixture of tough nest and dissociation fracture.The crack initiation location was shifted from the surface to the sub-surface of the original specimen,and the crack expansion direction shifted to the internal tissue after encountering the surface reinforcement layer,which delayed the fracture of the material effectively.