Study On Functionally Graded NiTi Shape Memory Alloys

Near equiatomic NiTi shape memory alloy undergoes a reversible thermoelastic martensitic transformation between a B2 structured austenite and a B19' structured martensite phase,leading to the unique shape memory effect and superelasticity,which enable NiTi alloy recovery from large deformation of up to 10%.Besides,NiTi alloys also show high damping properties,excellent mechanical performance and good biocompatibility,which make NiTi alloys widely used in medical and non-medical fields.Functionally graded materials(FGMs)contain a spatial variation of composition and/or microstructure for the specific purpose of controlling the mechanical and/or functional properties along certain directions.The design and fabrication of functionally graded NiTi shape memory alloys through tailoring the microstructure,chemical composition and/or macroscopic geometry,could lead to the appearance of novel properties,which could not be obtained in conventional NiTi alloys.In this work,NiTi alloys with graded microstructure and chemical composition were fabricated by ultrasonic shot penning and laser additive manufacturing,respectively.Moreover,the geometrically graded NiTi alloys were fabricated by modifying the shape of samples.The microstructure,phase transformation behavior as well as the mechanical/functional properties were studied,and the main conclusions could be summarized as follows:(1)Ultrasonic shot penning(USP)generates nano-crystallization at surface,and thus leads to the formation of graded microstructure along the thickness of the sample,which composes nanocrystalline at surface and coarse grains in the interior.Moreover,USP generates high density of dislocations due to sever plastic deformation at surface.The density of dislocations decreases from the surface to the interior of the sample.The nanosized grains at the surface and dislocation networks in the interior could act as the heterogeneous nucleation sites for nanosized Ni4Ti3 precipitates during the subsequent low-temperature(250?)aging treatment,leading to the homogenous distribution of Ni4Ti3 nanoprecipitates.The homogeneous aging microstructure could improve largely the strength of NiTi matrix,and thus suppressing the plastic deformation associated with the martensitic transformation.As a result,the functional stability of NiTi alloys is improved.(2)The geometrically graded NiTi alloys were fabricated by modifying the shape of tensile samples.The stepwise NiTi samples with the width of 3-4 mm and length of 5-15 mm were fabricated,which show multiple stress plateau or linear superelasticity during tensile.The conventional NiTi alloy undergoes stress-induced martensite transformation upon loading,giving rises to a stress plateau.However,it is hardly to develop certain relation between stress and strain within the range of transformation plateau.The geometrically graded NiTi alloy show multiple plateau stress or linear superelasticity,and thus the relation between stress and strain could be developed.Moreover,the method combining thermal cycling and low-temperature aging treatment were applied to optimize the aging microstructure,and thus to improve the functional stability of the geometrically graded NiTi alloys.(3)The layer-structured NiTi alloys with graded composition were fabricated by selective laser melting(SLM)techniques.Since Ni has a lower boiling point as compared with Ti,a loss of Ni will occur during SLM process.It is well known that the transformation behavior of NiTi alloys is very sensitive to the Ni concentration in the matrix.As a result,the transformation behavior of SLM-fabricated NiTi alloy varies under different SLM process conditions.Inspired by this concept,the microstructure with graded composition,which shows austenite/martensite layered structure at room temperature,was fabricated by modifying the scanning speed during SLM process.The unique microstructure could expand largely the transformation interval(i.e.the temperature range between transformation start and finish temperature)of martensitic transformation in NiTi alloys,reaching a maximum of 64?.Within the transformation interval,the austenite phase transforms gradually into martensite phase upon cooling,which improves largely the damping properties of NiTi alloys.