Fabrication And Strengthening-toughening Mechanism Of Titanium With Multiscale Structure

Titanium alloy have the characteristics of low density,high specific strength and modulus and good corrosion resistance.It has broad application prospects in the fields of aerospace and high performance automobile.With the rapid development of aerospace vehicles and high-performance cars,the service performance requirements of titanium alloy parts are constantly increasing.Not only the strength of titanium alloy needs to be greatly improved,but also the toughness(plasticity)must be maintained.However,the plasticity of titanium alloy prepared by traditional methods will decrease dramatically with the increase of strength.There is a strength-plasticity inversion problem.In order to solve this bottleneck problem,titanium plates with different grain sizes and titanium foils were laminated stacking.Then titanium with multi-scale grains were prepared by vacuum hot pressing sintering combined with rolling technology.The microstructural characteristics of multi-scale titanium were characterized by electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM).The effects of stacking mode and CNTs on the mechanical properties of multi-scale titanium were clarified.The crack initiation and propagation of multiscale titanium were studied by in situ tensile technique combined with SEM and three-dimensional X-ray microscopy(XRM),and the strengthening mechanism and fracture mechanism were revealed.The gradient structure titanium alloy sheets with decreasing grain size from core to surface and the layered structure titanium alloy sheets with alternating coarse and fine grains were prepared by different stacking methods.The gradient structure titanium alloy sheet can be rolled by lamination again,which can prepares laminated rolling titanium alloy sheet.In addition,the CNTs gradient structure titanium alloy sheet was successfully prepared through introducing CNTs between fine grained layers on the surface of gradient structure.The microstructures of multi-scale titanium were characterized by EBSD technology.The grain size of gradient-structured titanium varies gradiently from core to surface,which decreases gradually form core to surface.The average grain sizes are 1.1um,500 nm and 300 nm,respectively.The texture of gradient structure also varies with the grain size from core to surface.The texture changes from basal texture to bimodal texture,and the deviation angle of bimodal texture becomes larger.The gradient structure titanium alloy is rolled again to obtain the laminated rolling titanium alloy.The grain size of the laminated titanium alloy will be further refined,which is about 200 nm.The texture change of the laminated rolling titanium alloy is similar to that of thegradient structure titanium alloy.The results of mechanical properties test show that the yield and tensile strength of gradient structure titanium alloy sheet are 525 MPa and 619 MPa,respectively.Compared with the original titanium alloy sheet(yield and tensile strength are 271 MPa and 357 MPa respectively),the yield and tensile strength of gradient structure titanium alloy sheet increased by 93.7% and 73.3%,while the fracture elongation is still as high as 30.8%.The yield and tensile strength of the layered structure titanium alloy sheets are 573 MPa and 683 MPa,respectively.Compared with original titanium alloy sheets,the yield and tensile strength of laminated titanium alloy sheets are increased by 111.4%and 91.3%,while the fracture elongation is still 24%.The yield and tensile strength of laminated rolling titanium alloy sheets are as high as 726 MPa and 819 MPa,respectively,which are 167.9% and 129.4% higher than that of original titanium alloy sheets,and still maintain sufficient fracture elongation(about 17.6%).The yield strength and tensile strength of CNTs gradient structure titanium alloy are 650 MPa and 761 MPa,respectively,which are 139.9% and 113.2% higher than that of original titanium alloy plate,and the elongation at break is 24.3%.It can be seen that the strength of titanium alloy sheet with fine grain layer is obviously increased and its elongation at break is not significantly reduced.Therefore,the combination of multi-scale structure design and rolling technology can provide a feasible method to solve the problem of Strength-plasticity inversion of titanium alloy.The reason why multiscale titanium exhibit excellent strength and toughness is that coarse grains provide enough plasticity and fine grains provide high strength.In-situ tension combined with three-dimensional X-ray tomography study shows that for laminated rolling titanium,the crack propagation can be divided into three stages: Crack initiation and initial propagation: corresponding to the uniform deformation stage of the specimen until the macro-strain reaches 6.7%;The rapid crack growth stage: corresponding to the macro-strain range of 6.7%-12.7%;Crack instability propagation stage: when the macroscopic strain range is 12.7% to fracture.