| Metallic multilayered composites(MMCs)belong to a new kind of composite material,which is processed by the composite technology of same or different metals to achieve solid metallurgical bond on the interface.This kind of material combines the characteristics of each component metal,and has both high strength and excellent thermal stability,which is a major direction of the development of metal materials today.Compared with other severe plastic deformation technologies,the accumulative roll bonding(ARB)process is considered to be a simple and economically feasible process,which can continuously produce largescale ultra-fine grained multilayered composite materials in large-scale industrialization.In this paper,the preparation of Ti/Nb multilayered composite with theoretical single layer thickness of 300 nm was successfully achieved by ARB process using both pure Ti and Nb metals with an initial thickness of 1 mm.The macroscopic lamellar structures evolution and line scan composition analysis of RD-ND rolling surfaces from first cycle to eighth cycles were investigated systematically by using Field emission scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS)and X-ray diffractometry(XRD);The macroscopic mechanical properties of the composites were evaluated by the electronic universal tensile testing machine and the Vickers hardness tester;The microstructure evolution of Ti/Nb multilayered composites with different cycles was obtained by transmission electron microscopy(TEM).The lamellar structures evolution of RD-ND rolling surface of Ti/Nb multilayered composite displays that during the accumulative roll bonding,the component metals Ti and Nb are deformed harmoniously and Ti layers and Nb layers are well bonded.With the increasing number of ARB cycles,Ti layers preferentially starts to neck,fracture and even separation.In the fifth cycles,the shear bands initiate due to the accumulation of shear stress.In the eighth cycles,the Ti/Nb multilayered composite material having high density heterogeneous interfaces with shear bands spread over several layers is formed.It is characterized by EDS and XRD that in the Ti/Nb multilayered composites with different cycles,only the interdiffusion of Ti and Nb atoms occurs at the heterogeneous interface,that is,each metal atom exists as a solid solution in the adjacent metal matrix,and intermetallic compound is not formed.The microstructure evolution of RD-TD rolling surface of Ti/Nb multilayered with different cycles is studied by TEM.There are two typical characteristics in the Ti layers and the Nb layers:approximately equiaxed grians and layered grains.With the increasing number of ARB cycles,the layered grains gradually changed into equiaxed grains.In the eighth cycles,the grain sizes of the Ti layers and the Nb layers reach the nanometer scale,which are respectively 70nm and 200 nm.The Ti/Nb multilayered composite processed by ARB has excellent mechanical properties:high strength and limited plastic deformation ability.With the increasing number of ARB cycles,the yield strength and ultimate tensile strength of the eighth cycles composite are 518 MPa and 808 Mpa respectively,and the elongation is only 8%.There are many strengthening mechanisms in the mechanical properties of composites,which are mainly strain hardening caused by dislocation and fine grain strengthening,followed by interfacial coupling effects caused by the interaction of adjacent interfaces with heterogeneous interfaces of different mechanical properties.Besides,The Ti/Nb multilayered composites have different degrees of necking during the tensile process,which is characterized by ductile fracture.The microhardness indicates that the higher the cycles number is,the more uniform the internal stress distribution is.The microhardness distribution exhibits significant size effect.The thermal stability of the Ti/Nb multilayered composites with different cycles shows that with the annealing temperature increasing,the heterogeneous interface becomes rough and fuzzy,and the interdiffusion of Ti and Nb atoms is intensified.After annealing at 500℃for one hour,the microhardness decreased slightly,especially for the eighth cycles,the hardness hardly changed,indicating that the Ti/Nb multilayered composites possess outstanding thermal stability.When the annealing temperature reached 800℃,the microhardness of each cycle decreased significantly,indicating that the annealing temperature exceeded the critical temperature,and the microhardness and lamellar structures showed obvious instability.Above all,The high-density heterogeneous interface allows the Ti/Nb multilayered composite possess the characteristics of high strength and excellent thermal stability,breaking the contradiction between high strength and thermal stability of traditional metallic multilayered compositesfabricated by severe plastic deformation process. |
