Study On The Effect Of Surface Nanocrystallization On Fatigue Property Of TC4 Alloy

Surface nanocrystallization(SNC) is a technique which is from the defects produced by the severe plasticity deformation, causing dislocation annihilation, reorganization, then form crystal in submicrometer or nanometer, and with the temperature increasing, the surface organization with high deformation energy storage will be recrystallized and form nanocrystals. The method can improve material surface structure to improve the overall performance of the material. This study uses the ultrasonic shot peening technology(USSP)of TC4 nano surface, and the surface structure and mechanism of nano, testing the bending fatigue performance and the influence factors of stability of the residual, the compressive stress distribution and stress will also be analyzed and discussed.The surface organization and grain size were analyzed of TC4 samples by ultrasonic shot peening(USSP) with different durations by X-ray diffraction(XRD), Optical microscope(OM),scanning electron microscope(SEM) and transmission electron microscopy(TEM). And the macroscopic residual stress were evaluated by X stress tester, the fatigue limit of TC4 samples by USSP with different durations was determined by Four-point bending fatigue experiment.And uses SEM to observe the fatigue fracture characteristic. In addition, the surface microhardness and the surface roughness were determined of TC4 samples.The results indicate that the crystallite size of the treated surface layer can be decreased to 26 nm from 55 nm as the process duration increased from 15 min to 60 min, and the corresponding microstrain increases from 0.173% to 0.263%. With the strain increases, the dislocation density of the surface layer increases, and developed the dislocation wall,dislocation cell can be developed to accommodate the strains, then the formation of subgrainis resulted. With further increasing strains, grains are further refined, misorientations increase,and the refined grains become random, finally the TC4 samples with gradient nanostructured surface layer are obtained. The USSP treated fatigue limit increased by 10.64 %; the source of crack initiation is in the sample surface, fracture extension area transform from untreated river to the wavy after processing; if the smaller the roughness after processing, the smaller gap of stress concentration, the better the performance of the corresponding material fatigue. The residual compressive stress value exceeds the yield strength of TC4 material after surface nanocrystallization;when load is larger, the residual stress in rapid release period; when loads close to or smaller than the fatigue limit, with increase of cyclic time load size is no longer to significantly alter the size of the residual stress value, its to reach a stable value; dislocation and dislocation cell has take place configuration and number of changes in the process of fatigue, reduce dislocation density led to the relaxation of compressive residual stress.