Study Of Bulk W/NiTi Shape Memory Alloy Composite

High strength and large elastic strain of materials have always been the goal of human pursuit.Nanowires have large elastic strain and ultra-high strength due to lack of dislocations.Researchers have predicted that nano-phase reinforced composites have excellent mechanical properties.However,a large number of studies have proven that the mechanical properties of nanowires were difficult to reflect in the composites.Recently,based on the concept of strain matching between the transformation strain of phase-transforming matrix and the elastic strain of nanowires,our research group designed a kind of Nb/NiTi SMA composite based on the Nb-NiTi eutectic phase change,found that the elastic strain of Nb nanowires in the phase-transforming matrix reached 6.5%.However,the volume percentage of Nb in Nb-NiTi eutectic structure is low(less than 25%),and the elastic modulus of Nb is low(95 GPa).We are based on the elastic modulus of W(400 GPa)higher than Nb,and the use of W and TiNi can get W/TiNi composites with high volume percentage of W.By this,W with the highest elastic modulus has been selected instead of Nb.W and NiTi are combined by infiltration and vacuum hot pressing.High strength W-fiber/NiTi SMA composites were prepared by rolling.The effects of NiTi and NiTiNb pre-alloy components on the microstructures,phase transformation behaviors,phase compositions and mechanical properties of W/NiTi composites were investigated by SEM,DSC,XRD and tensile test.The deformation behavior of high strength W-fiber/NiTi SMA composites during tensile deformation was studied by in-situ synchrotron X-ray diffraction.The results show that brittle phase Ni3 Ti and Ti2 Ni were formed in the W/NiTi SMA composites prepared by binary NiTi alloy due to the change of NiTi composition caused by inter diffusion between W and NiTi during the infiltration process at high temperature.The addition of a small amount of Nb to the pre-alloy could inhibits the formation of the brittle phase.We successfully fabricated composites composed of W and NiTi by Ni42Ti53Nb5 alloy.High strength W-fiber/NiTi SMA composites were prepared by rolling method.The result of in-situ synchrotron X-ray diffraction shows that,stress induced martensite transformation of NiTi matrix occurred during tensile test,which ensures that the lattice strain of W fibers reaches 0.83%.The maximum stress of W fiber bore reached 3280 MPa,much higher than the strength of the individual W previously reported.The W fiber with a volume fraction of 70% exhibit excellent load carry capacity,as they were capable of carrying 90% of the applied load.This enables the fracture strength of the composite up to 2300 MPa.