Study On The Microstructure And Mechanical Properties Of NiTi/Nano Metal Film Composites

Nanostructured metallic thin films are important for the high-performance micro element devices,micro-electromechanical systems and interconnection structures.Its service reliability in complex stress environment and mechanical specificity in extreme spatial scale are hot issues.However,the ductility of the free-standing nano metal film is very poor,and it is usually brittle fracture at the end of elastic deformation.In the film substrate system,the nano metal film deposited on the substrate usually breaks when only a few percent（<5%）of external strain is applied.The limited mechanical properties of nanometer metal thin films have become a major challenge for their development and application.Recently,it has been found that the uniformly deformed substrate or component can effectively alleviate the stress and strain concentration at the interface,thus stabilizing the deformation process of nano component.NiTi shape memory alloy is a typical lattice shear alloy,which can output uniform lattice shear through martensitic transformation under external force.The stress or strain concentration caused by NiTi shape memory alloy is far less than the atomic scale stress or strain concentration caused by dislocations in the plastic deformation process;If the nano-metal film is composite with NiTi shape memory alloy,it is expected that the uniform lattice shear deformation of NiTi component can alleviate the stress or strain concentration in the film and avoid the plastic deformation and instability of the film.In addition,the martensitic NiTi alloy after phase transformation has a high work hardening index,which is expected to harden locally（such as at the crack tip）and hinder crack propagation in the thin film.Therefore,NiTi based film matrix composites or multilayer composites are expected to achieve high ductility or high strength（hardness）.In this paper,NiTi（phase transformation substrate）/X（film）,Steel（ferritic stainless steel,dislocation slip substrate）/X（film）,Kapton（polymer substrate）/X（film）three membrane matrix composites were prepared by magnetron sputtering,among them X=Nb,W.Ni Ni Nb Fe is the dislocation slip component,and NiTi is the lattice shear component.Scanning electron microscope,transmission electron microscope,X-ray diffractometer,atomic force microscope,resistance meter and other equipment were used to characterize the microstructure and morphology,and uniaxial tensile,in-situ synchrotron radiation tensile,nano indentation and other methods were used to characterize the mechanical and phase transformation behavior.The results show that the crack surface density（total crack length per unit area）of the nanocrystalline Cu film on NiTi memory alloy substrate is only 1×10^-3μm/μm²at24%applied strain.In contrast,the cracking of nanocrystalline Cu films on the Steel and Kapton substrates is earlier（the external strain is respectively With 6%and 15%）,and when the external strain is 24%,the crack surface density of Cu films on their surfaces(8μm/μm²,5×10^-2μm/μm²)are 3 and 1 orders of magnitude higher than that on NiTi.In particular,we found that the Cu nanocrystals on NiTi substrate grew with the increase of external strain.The results of in situ grazing incident X-ray study show that the fracture of nanocrystalline Cu film on NiTi shape memory alloy substrate is synchronous with the end of substrate lattice shear deformation.The lattice strain of Cu film increases slowly during lattice shear deformation of substrate（i.e.,stress-induced martensitic transformation）.When the lattice deformation of substrate ends（i.e.,the plastic deformation of oriented martensite begins）,the lattice strain of Cu film begins to decrease,that is,the initiation or propagation of cracks.This indicates that lattice shear deformation can effectively alleviate stress or strain concentration.This mechanism is also applicable to other lattice types of nanocrystalline films--both bcc lattice type nanocrystalline Nb films and hcp lattice type nanocrystalline Zn films exhibit excellent ductility on NiTi substrates.In addition,when the nanocrystalline thin film is NiTi thin film coating to form the“sandwich”configuration（the substrate is still NiTi）,the effect of relieve stress and strain concentration is better,the maximum lattice strain of nanocrystalline Nb films in this configuration is about 2.5%,which is higher than the maximum lattice strain（about 2%）of monolayer nanocrystalline Nb films located directly on the NiTi substrate.The results of multilayer composites show that the effects of NiTiNbFe and NiTi components on the mechanical behavior of the composites are significantly different.The nano indentation hardness of single layer NiTi film is only 1/2 of that of single layer NiTiNbFe film,but the contribution of NiTi component to the hardness of multilayer is obviously higher than that of NiTi Ni Fe component.When the modulation period is 5 nm（the modulation ratio is 1:1）,the hardness values of NiTi/W multilayer and NiTiNbFe/W multilayer are close to each other.When the modulation ratio is0.57（the modulation cycle is 7 0 nm）,the hardness of NiTi/W nano-multilayers is higher than that of NiTiNbFe/W nano-multilayers.The above law obviously deviates from the mixing law for the strength of composite materials.When the modulation period is 100nm to 2 nm,the hardness of NiTi/Nb multilayers is significantly higher than that of X/Nb multilayers（X=Cu,Ti,Al,Mg,etc.）.When the modulation period is 20 nm to 5nm,the hardness of NiTi/W multilayers and the slope of the relationship between hardness and modulation period are higher than that of X/W multilayers（X=Cu,Fe,etc.）,indicating that lattice shear components can effectively inhibit stress or strain concentration and improve the strength（hardness）of multilayer films.In addition,it is also found that the strain rate sensitivity index（m）of NiTi/Nb is closely related to the shear of NiTi element lattice,m increases monotonically with the decrease of the modulation period and begins to decrease when the modulation period is reduced to 10nm（phase transformation is inhibited）.The m of NiTi/W and Ni Ni Nb Fe/W increased first and then decreased with the decrease of the modulation period,but the increase slope of the m value of NiTi/W was higher than that of NiNiNbFe/W.