Structure And Properties Of Surface Nanometer Layer In TB6 Alloy

Titanium alloy TB6 is widely used because of their the high specific strength,perfect forgeability, good fracture toughness, small anisotropy and high corrosion resistance. However, its high notch sensitivity, low hardness and poor abrasion resistance limit its further applications in aerospace. In this paper, the surface nanocrystallization(SNC) on TB6 alloy was investigated by using the high-energy shot peening(HESP). The effects of the shot peening time and shots size on the surface microstructure, roughness and microhardness of the TB6 alloy were discussed.Furthermore, the mechanism of surface nanocrystallization of the TB6 alloy was analyzed.The results of the TB6 alloy treated at different durations and shots sizes show that the thickness of deformation layer is increasing with the increasing durations, and then remains constant. When the shots are in the diameter of 1 mm, 2 mm and 4 mm,the maximum thickness of deformation layer are 600 μm, 800 μm and 1600 μm,respectively. However, the thickness of deformation layer increases with the increasing shots size. When treated for 3minutes, the thickness of deformation layer increases from 509 to 1507 μm as the the diameter of shots increases from 1 to 4 mm.The results of surface roughness and microhardness of the TB6 alloy after HESP treatment show that the hardness of the TB6 alloy surface layer is enhanced significantly, which can be increased from 280 to about 380 HV. The value of surface roughness is also increased. After shot peening for 45 min, the value of surface roughness of the treated sample is increased from 0.102 to 1.184 μm.XRD results show that the TB6 alloy consists of single β phase before HESP,while β â†' α"martensite transformation occurs in the surface of the TB6 alloy after HESP. The amount of α" martensite increases with the increasing treatment durations,and then remains unchanged. Under the same treatment durations, the smaller shots is helpful to promote the βâ†'α"martensite transformation.After treated for 45 min with shots in the diameter of 4 mm, the TEM images of the nanometer layer of the TB6 alloy show that coarse grain changes to exquiaxed nanocrystalline with the grain size of 100~150 nm. The formation of intersection of lamellar twins and high density of dislocations are found around the lamellar twins. It indicates that the twining deformation occurs and deformation twin forms during the HESP. The initial mechanism of the strengthing of the TB6 surface layer is the grainrefinement and the occurance of gradient structure of the multi-directional twins,single-directional twins to the strain-free matrix during the HESP.