Effect Of Residual Stress And Microstructure Morphology On Fatigue Property Of Surface Nanocrystallized TC4

Titanium alloys are usually used to manufacture aeroplane compressor blades and structural parts because of their high specific strength and adaptability to higher temperature conditions.Titanium alloys often suffer from fatigue failure in long-term service under harsh conditions,resulting in huge losses.Therefore,the study of the fatigue failure mechanism of titanium alloys and the improvement of the fatigue limit of titanium alloys have become a hot topic in recent years.In order to improve the fatigue limit of materials,there are many strengthening methods.Ultrasound shot peening is one of the commonly used methods for surface strengthening of materials.It can realize nano-scale surface of materials.In order to improve the fatigue limit by ultrasonic shot peening,two aspects can be analyzed: firstly,the material treated by ultrasonic shot peening can obtain a certain depth of stress field,which effectively counteracts part of the external tension stress and delays the time when the material failure;secondly,nanocrystals on the surface of the material can effectively prevent the crack initiation,thereby improving the fatigue limit of the material.At the same time,the coarse grains of the matrix can keep the plasticity of the material,and achieve the effect of both strong plasticity and strengthening.In this paper,TC4 alloy was treated by ultrasonic peening to achieve surface nanocrystallization.Some residual stresses were released by low temperature heat treatment at 350℃,and the surface grains remained at nanometer level.It was found that the fatigue failure mode of ultrasonic shot peening for 30 min was single crack initiation crack,and that of ultrasonic shot peening for 30 min + 350 ℃ for 60 min was multi-source initiation,but all of them originated from sub-surface layer,while the original crack originated from surface layer.Aluminum enrichment was found in the fracture surfaces of samples shot-peening for 30 minutes,shot-peening for 30 minutes and heat-treated at 350 ℃ for different time.The relationship between fatigue limit and different stress fields is obtained by fitting Goodman equation,and the average stress influence factor m is 0.3386.In addition,the equiaxed grains were obtained from the long axis grains of the original TC4 alloy after heat treatment at 970 ℃.The effects of morphology on the tensile properties and four point bending fatigue properties were studied.It is found that the tensile limit and elongation of long-axis grains are increased by 8.2% and 4% respectively after ultrasonic peening,and the tensile strength and elongation of equiaxed grains are increased by 6.6% and 8% respectively.The fatigue limit of equiaxed grains is 433 MPa and that of long grains is 416 MPa.The fatigue limit of equiaxed grains is better than that of long grains.