| Pure titanium has been widely used in aerospace,biomedical,automotive and shipbuilding fields because of its low density,high specific strength,strong corrosion resistance and good biocompatibility.However,with the rapid development of modern industry,people have put forward higher requirements for the comprehensive properties of materials,that is,to maintain good plasticity while having high strength.Inspired by biological materials with high strength and high toughness in nature,this project designed a pure titanium plate with micro-nano gradient mixed crystal structure by stacking pure titanium plate with different grain size,combining with SPS rapid sintering and temperature control clad rolling.Metallographic microscope(OM),scanning electron microscope(SEM),electron backscattering diffraction(EBSD)and transmission electron microscope(TEM)were used to reveal the evolution process of micro-nano gradient mixed grain structure during the rolling process,and the mechanical properties were evaluated.The effects of different annealing temperatures and holding time on microstructure were revealed by EBSD technique,and the relationship between annealing temperature and holding time,microstructure and mechanical properties was established.The local strain evolution of micro-nano gradient mixed crystal pure Ti was characterized by in-situ tensile combined with digital image correlation technique(OM-DIC)of metallographic microscope.The fracture behavior of three-dimensional crack initiation,propagation and crack density distribution evolution in micro-nano gradient mixed crystal pure Ti was studied by in-situ tensile combined with three-dimensional X-ray microscopy(XRM).The mechanism of strengthening and toughening of gradient structure is preliminarily revealed.Firstly,the grain sizes of pure titanium plates with different initial thicknesses were controlled by heat treatment,and then stacked according to the decreasing grain sizes from the center to the surface.A gradient structure preform with the core grain size of90 ?m and the edge grain size of 5.48 ?m was constructed by SPS rapid sintering(700?,10mins).Then,combined with temperatures-controlled clad rolling(600?,90% total deformation),the micro-nano gradient mixed crystal pure Ti plate was constructed.The microstructure evolution of different deformation in rolling process was studied by using EBSD technology.Results show that after a 90% of rolling deformation fine-grain ratio decreased gradually from the edge to center,coarse grain percentage increase gradually,present a gradient structure,and even in the same layer,the grain size of apparent micron grade coarse grain and the mixture of nanoscale fine-grain distribution,presents the micro-nano mixed crystal structure,so to achieve the micro-nano gradient mixed crystal structure of the building.Moreover,after further secondary rolling,the grain size of the whole material is further refined and still maintains the micro-nano gradient mixed grain structure.The microhardness results show that the microhardness of both primary and secondary rolled materials shows a gradual gradient change from the center to the edge.The results of tensile properties show that the yield strength of the micro-nano gradient mixed titanium plate is increased by 118%-197% to 682 MPa,and the tensile strength is increased by 91%-108% to 739 MPa,while the elongation remains at 24%.After the second rolling,the yield strength is 701 MPa,which is slightly higher than that of the first rolling,but the tensile strength is increased by 26% to 930 MPa,and the fracture elongation remains at 12.3%.The fracture morphology shows that the fracture of primary and secondary rolled materials decreases gradually from the center to the edge of the dimple,and has a trend of transition from plastic fracture to brittle fracture.In order to further control the microstructure and properties of materials,respectively,on a rolling and the second rolling plate heat treatment,found a titanium plate in 400 ? main recovery process of rolling,450 ? recrystallization is obvious,with the increase of temperature,holding time recrystallization area increase gradually,gradually transforms the low Angle grain boundary large Angle grain boundaries,intracrystalline local orientation difference decreased;The second rolled titanium plate was completely recrystallized at 475? for 5min.At the same time,the tensile properties show that the strength decreases and the plasticity increases with the increase of annealing temperature and holding time.The tensile strength is 823 MPa and the elongation is up to 22.4% after being held at 425? for 5min.The plasticity of the secondary rolled titanium plate is improved greatly while the strength is basically maintained.The study on the local strain evolution of OM-DIC pure titanium plate with micro-nano gradient structure shows that the plastic deformation tends to occur in the soft central layer at the low strain stage,resulting in the strain zoning behavior.When the macroscopic strain reaches 2.2%,the local strain distribution at the center and the edge of the material tends to be the same,and the coarse-grained grain at the center and the fine-grained grain at the edge deform harmonically.The fracture behavior of pure titanium plate with micro-nano gradient structure was studied by in-situ tensile combined with 3D-XRM.It was found that the crack first appeared at the edge of the plate during the uniform deformation stage and the early stage of the inhomogeneous deformation stage,but its initiation and propagation were very slow.In the late stage of non-uniform deformation,the material has obvious necking phenomenon,and the central part of the crack initiation and rapid expansion along the RD direction,followed by rapid instability fracture.In addition,the CT imaging of the fracture shows that the large volume cracks are almost all concentrated in the center.The results show that the gradient structure can effectively restrain the crack initiation and propagation in the process of hard and brittle surface deformation,and the coarse-grained zone in the center bears the main strain in the plastic deformation,which coordinates the deformation of the material. |
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