| Titanium alloy has low density,high specific strength,and excellent corrosion resistance,so it is widely used in aerospace,petrochemical and other fields and has the reputation of "modern metal" or "space metal".Ti-6Al-4V(TC4)alloy accounts for75%-85% of the total titanium alloy usage due to its excellent comprehensive properties.The fatigue failure of TC4 alloy mostly starts on the surface of the material,so surface nanocrystallization has become one of the most effective wa ys to improve its fatigue resistance.However,the high-cycle fatigue properties of metal materials has high microstructure sensitivity,so it is very important to relate the initial microstructure to the surface nanocrystallization closely.For the above purposes,surface gradient nanostructures were prepared on the surface of TC4 alloys with equiaxed and lamellar structures using high-energy shot peening(HESP)technology,and their nanocrystallization mechanism and fatigue properties were studied.The main research contents and results are as follows:The metallographic microscopy,D8 Advance A25 X-ray diffractometer,JEM-200 CX transmission electron microscopy,LEXT OLS4000 laser confocal microscope and 401 MVD vickers hardness tester were used to characterize the surface gradient nanostructures of the TC4 alloys in different microstructures treated by HESP.The results showed that: After the surface HESP treatment,the nanocrystalline size of equiaxed and lamellar structure on topmost surface reached 18.9nm and 17.1nm,respectively,and the surface roughness(Ra)was 4.34μm and 2.44μm,respectively;The maximum residual compressive stress of the TC4 alloy in equiaxed and lamellar structure after HESP treatment were both located in the subsurface layer,which were-906 MPa and-859 MPa,respectively;The microhardness of equiaxed structure increased from 320 HV to 394 HV,which increased by 23.1%,and the microhardness of lamellar structure increased from 340 HV to 407 HV,which increased by 19.7%.Transmission electron microscopy(TEM)was used to analyze the nanocrystalization mechanism of TC4 alloy in two kinds of structures.The results showed that: Twinning played a n important role in the nanocrystalization process of theα phase in equiaxed-structure TC4 alloy.Twins and dislocation cells and other structures divided the original coarse equiaxed grains into small-sized sub-crystals,and finally formed the randomly oriented equiaxed nanocrystalline;In the lamellar-structure TC4 alloy,the α phase was preferentially deformed compared to the β phase.The grain refinement of the lamellar α phase was carried out under the synergy of dislocation motion and twinning.The β phase grain refinement is dominated by the dislocation motion.At the large strain,the lamellar structure disappeared and the subcrystals rotated,eventually forming the equiaxed nanocrystals.The effects of surface nanocrystallization on the high-cycle fatigue properties of TC4 alloys with two microstructures were investigated by tensile-tensile fatigue tests.The results showed that: After HESP treatment,the fatigue limit of equiaxed TC4 alloy increased from 489 MPa to 555 MPa,increased by 13.5%;the fatigue limit of lamellar structure increased from 381 MPa to 506 MPa,increased by 32.8%;Fatigue fracture analysis showed that the HESP treatment could hinder and delay the initiation and propagation of fatigue cracks.The source of fatigue cracks in both equiaxed and lamellar microstructures treated by HESP were transferred to the subsurface layer,and the fatigue bands were denser than the untreated samples;The local fatigue strength of the surface layer after HESP treatment was greater than the applied load,so that the weak area of the material was transferred to the subsurface layer;The HESP treatment made the surface grains of the equiaxed TC4 more uniform,thereby reducing the stress and strain concentration.The coarse β grains of the lamellar TC4 were refined,thus reducing the effective slip distance of dislocations and delaying the initiation of cracks. |
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