Surface Integrity And Mechanical Properties Of Gradient Nanostructured Commercial Titanium

With the development of mechanical industry, the mechanical properties of metal materials became more and more important. Failure of materials usually occurred at the surface or initiated from the surface. In order to improve the service lives of components, surface modification technologies have been extensively used. Coarse grains can be refined into nano-scale through severe plastic deformation, and the gradient structure of which the grain size increased from nanometers to micrometers can be obtained. This gradient structure could enhance the mechanical properties greatly.Ultrasonic Shot Peening produces severe plastic deformation with high strain rate to obtain excellent surface states. This study investigates the effect of USSP on surface integrity and mechanical properties of commercial titanium. The conclusions are as follow:(1)The metallographic experiment, transmission electrical microscope experiment was done to investigate the effect of USSP input parameters on the microstructure of commercial titanium. It was found that three regions were formed after treatment:severe plastic deformation layer, the transition layer and the matrix. The thickness of severe plastic deformation layer was increased with the increasing static pressure and processing times. Through TEM experiment in a different position, we found that the grain refinement mechanism was the combined action of mechanical twinning and dislocation slide.(2)Surface roughness was efficiently improved processed by USSP. This process could enhance for the induced nanocrystalline and strain hardening.The residual compressive stress field with a certain thickness was formed near the surface. Annealing at 400? for 1h, grains in the subsurface began to grow firstly.(3)Through the nanoindentation and uniaxial tension to obtain the micro and macro mechanical properties of treated specimen. After processing, nanohardness and yield strength increased significantly. The uniaxial tensile experiment shown that yield strength enhanced after processing by the ultrasonic shot peening, but toughness declined.(4) The tension-compression fatigue experiment shown that the fatigue crack initiated from the subsurface, and the fatigue limit was enhanced after the USSP treatment. The fatigue life of annealed specimen was further enhanced.