Study On Design And Preparation Of W/NiTi Shape Memory Alloy Composites

Freestanding nanowires have ultrahigh elastic strain limits and yield strengths,but it has proven difficult to exploit their intrinsic mechanical properties in bulk composites.Based on the principle of strain matching between the ultrahigh elastic strain of nanowires and the transformation strain of phase-transforming matrix,we have demonstrated that the Nb nanowires embedded in NiTi phase-transforming matrix can exhibit ultrahigh elastic strain,which yields excellent mechanical properties of the composites.Inspired by the principle of strain matching,it is expected to obtain more excellent mechanical properties by combining tungsten(W),which has the highest elastic modulus among metals,with NiTi phase-transforming matrix.Besides,based on the concept of transformation-induced plasticity,the W/NiTi shape memory alloy composite could overcome the brittleness of W at room temperature and exhibit excellent plasticity.The W/NiTi(Nb)composites were prepared W/NiTi shape memory alloy composites by powder metallurgy,accumulative roll-bonding and infiltration method.The microstructure,transformation behavior and mechanical properties of the composite are investigated by SEM,TEM,XRD,DSC and material testing machine.The in-situ high energy X-ray diffraction is used to investigate the deformation mechanism of the composites.To prepare W/NiTi shape memory alloy composites by powder metallurgy method,TiNiNb shape memory alloy powders are efficiently prepared by the hydrogen-dehydrogen process of the cold-drawn TiNiNb wire composed of amorphous and nanocrystalline,which significantly decreased hydriding temperature and shortened hydriding time in this study.The obtained TiNiNb powers with the particle size in range of 20-50 ?m,exhibit almost the same reversible phase transformation behavior as that of the original TiNiNb SMA ingot.Then the W/NiTi(Nb)shape memory alloy composites were prepared using the hydrogenated TiNiNb powders.The oxidation of TiNiNb powders and mutual diffusion between TiNiNb powders and W resulted in the chemical composition change of the TiNiNb.A novel W/NiTi micro-laminated composite has been fabricated by vacuum hot processing and forging.In situ synchrotron diffraction experiments during compression test provided clear evidence on the martensitic transformation of NiTi,which reduces stress concentration and prevents crack propagation.The composite has exhibited an excellent combination of strength and ductility despite the brittle nature of W.The results suggest a new guideline for the alloy design of high-performance tungsten-based composites.Novel W/NiTi shape memory alloy composites with above 70% tungsten in volume fraction were successfully fabricated by infiltration and hot pressing process.A W fiber/NiTi shape memory alloy composite was subsequently prepared by further forging and rolling of the sintered W/NiTi composite.The results suggest that NiTi matrix in the sintered W/NiTi shape memory alloy composites deforming by martensite transformation effectively hindered the propagation of micro-cracks in tungsten particles during loading,which resulted in a comparable tensile strength and ductility with conventional tungsten heavy alloys.Meanwhile we also found that the martensitic transformation of the NiTi matrix in the W fiber/NiTi shape memory alloy composite during tensile test.The W fibers exhibit a large elastic strain limit up to ~0.83%,which is far larger than that of monolithic tungsten.The W fibers exhibit high load carry capacity during deformation as they were capable of carrying ~90% of the applied load.The W fiber/NiTi composite exhibited a fracture tensile strength of 2300 MPa.