Formation And Microstructure And Properties Of Small-Diameter And Thin-Wall NiTi Alloys Tubes

Forming technology of the tube NiTi alloys was studied in this thesis, and succeeded in preparing the NiTi alloys tube with small-diameter and thin-wall. Microstructure, phase transformation, mechanical properties, shape memory effect and superelasticity of the tube NiTi alloys were investigated by TEM, DSC, tensile test, and bend test .The results of the tube forming test indicate that the surface defects of the raw materials seriously influenced on the forming properties for the tubes and pipes. The size of the raw materials is directly related to the aim dimension. Empirical equation about how to choose the dimension of the raw materials is set up by experimental data analysis. And the optimal processing parameter for forming tube is determined to be: annealing temperature 750℃, the pass radial shrink 0.1mm, drawing velocity 90m/h, non-core drawing velocity 40m/h.TEM results show that the characteristic of the martensite structure pattern of 12%, 29% cold-forming tube is self-accommodation following variant coarsen and deforming micro-twins and hunch martensite intercross respectively, as well as 8%, 22% cold-forming filament. Therefore, it is proved that the stress of the tube is less than the filament during cold-draw processing.DSC results indicate that the phase transformation temperatures decrease and hysteresis temperature of the transformation widen with increasing the deformation strain for the samples with the same annealing temperature. Whereas the phase transformation temperatures increase and the transformation hysteresis decrease with the annealing temperature increasing for the samples with the same deformation strain.Tensile test results show that the elastic modulus, yield strength, tensile strength of the test sample increased and elongation decrease when the deformation strain increases. The yielding plateau in the stress-strain curve becomes obvious gradually with the increasing of the annealing temperature, while the plateau stress in unloading curve decreases.Bending test results show that, for the same deformation temperature, the...