Investigation Of Mechanical Properties And Superelasticity Of Cold-Rolled Ni_50.7Ti_49.3Shape Memory Alloy Sheet

The NiTi shape memory alloy has been developed into a new functional materials and widely used in many fields due to a series of advantages,such as good shape memory effect and superelasticity,as well as good corrosion-and wear-resistance and biocompatibility,and so on.The mechanical properties and superelasticity of near-equal atomic ratio NiTi shape memory alloy are intimately related to heat treatment process,pre-deformation temperature and amount,and cyclic loading number,therefore,the investigation of the influences of these factors on the mechanical properties and superelasticity has also important practical significance.In the thesis,the influences of heat treatment process,pre-deformation temperature and amount on the mechanical properties and superelasticity of cold-rolled Ni50.7Ti49.3 shape memory sheet,as well as cyclic loading number effect on the superelasticity under the optimum process conditions,are systematically investigated using electronic all-purpose tester,X-ray diffraction tester,transmission electron microscopy(TEM)and differential scanning calorimeter(DSC).The aim is to provide reference data for the optimization of processing technology of the cold-rolled Niso.7Ti49.3 shape memory sheet.The following results are obtained.The dislocation density in the as-cold-rolled Ni50.7Ti49.3 sheet is higher and they are in the tangled or cellar shape,meanwhile,a number of lamellar structure are observed.After the alloy sheet is heat treated at 400 ℃ the dislocation density notably decreases and the grain size increases,meanwhile,the lamellar structure in many regions becomes unclear and a lot of uniformly distributed and grained particles with a diameter of 20nm are precipitated;With increasing heat treatment temperature,the hardness of Ni50.7Ti49.3 alloy sheet first slowly increases and reaches the maximum at 400℃ and then rapid decreases and remains constant above 600℃;The yield strength for sheets treated at different temperatures is not dependent on the orientation of sheet,and is less than that of the as-cold-rolled,but with increasing heat treatment temperature,it rapid increases and then slowly drops.The tensile strength along rolling direction is more than that along transverse direction for the as-cold-rolled sheet and sheets treated at 300℃ and 400℃,and the tensile strength along two directions first increases and then decreases,and the maximum is obtained at 400℃.The notable martensitic reorientation features appear on the engineering stress and strain curves for the sheets treated at 300℃ and 400℃ whether being loaded along rolling or transverse directions,and the maximum tensile strength is obtained along rolling direction for sheet treated at 400℃.For the sheet held for 2h at 400℃ and then cooled in water,as it is pre-strained to 6%at 60℃,the better superelasticity is obtained.