| The effects of adding Cr and heat treatment on the microstructure, transformation, low temperature deformation and room temperature cyclic deformation characteristics of Ti-Ni shape memory alloys (SMA) were investigated by optical microscope, thermo-gravimetric, X-ray diffraction, differential scanning calorimetry and tensile test using Ti-50.8Ni, Ti-50.8Ni-0.3Cr and Ti-51.1Ni SMAs as the samples in this paper. The results are as follows:The composition phases of Ti-50.8Ni, Ti-50.8Ni-0.3Cr and Ti-51.1Ni alloys are parent phase (B2) (CsCl structure), and the low and intermediate temperature annealed Ti-50.8Ni, Ti-50.8Ni-0.3Cr and Ti-51.1Ni alloys'deforming microstructures are fibrous. With increasing the annealing temperature (θa), the fibrous microstructure evolves gradually to equiaxed grain. The oxidation of this there alloys speeds up whenθa is over 600℃.With increasingθa, the transformation types of Ti-50.8Ni alloy change from Aâ†'Râ†'M / Mâ†'Râ†'A to Aâ†'Râ†'M / Mâ†'A to Aâ†'M / Mâ†'A (A-parent phase, R-R phase, M-martensite), and one of Ti-50.8Ni-0.3Cr alloy change from Aâ†'R / Râ†'A to Aâ†'Râ†'M / Mâ†'Râ†'A to Aâ†'Râ†'M / Mâ†'A to Aâ†'M / Mâ†'A upon cooling / heating, and the types of Ti-51.1Ni alloy change from Aâ†'Râ†'M / Mâ†'Râ†'A to Aâ†'Râ†'M / Mâ†'A upon cooling / heating. With increasingθa, the R transformation temperatures and M temperature hysteresises decrease, the M transformation temperatures increase, and R temperature hysteresises are nearly not change as about 4℃for Ti-50.8Ni, Ti-50.8Ni-0.3Cr and Ti-51.1Ni alloys. With increasingθa, the platform-stress (σm) in the stress-strain curves decreases firstly and then increases, tensile strength decreases, ductility and energy loss per cycle (WD) increase for Ti-50.8Ni, Ti-50.8Ni-0.3Cr and Ti-51.1Ni alloys. When deforms at low temperature (10℃), the 400-500℃annealed Ti-50.8Ni and Ti-51.1Ni alloys show shape memory effect (SME), the 550-650℃annealed ones show SME + superelasticity (SE), while the 400-650℃annealed Ti-50.8Ni-0.3Cr alloy shows SE.When cyclic deformation at room temperature and low temperature (10℃), the characteristics of Ti-50.8Ni and Ti-51.1Ni alloys evolved from SME to linear SE, and the stress inducing martensitic critical stress and the accumulation residual strain (εr*) increased rapidly with increasing cyclic deformation number n, while the characteristics of Ti-50.8Ni-0.3Cr alloy evolved from incomplete superelasticity to complete superelasticity, and the shape of stress-strain curve is stable. With increasing n, the superelastic residual strain (εr) decreased and the superelastic strain recovery ratio (ηs) increased in Ti-50.8Ni and Ti-51.1Ni alloys; while theεr andηs in Ti-50.8Ni-0.3Cr alloy kept at lower and higher values, respectively. The superelasticity of Ti-50.8-0.3Cr alloy was stable.With increasing n, the equivalent stiffness (keq) of Ti-50.8Ni and Ti-51.1Ni alloy increase firstly and then tend to constant, the WD increases firstly and then decreases, and the equivalent damping (ξeq) decreases. While the keq and WD of Ti-50.8-0.3Cr alloy turn to stable,ξeq decrease firstly and then turn to stable.With increasingθa, the characteristics of Ti-50.8Ni and Ti-51.1Ni alloy springs evolved from SME to SE, Ti-50.8Ni-0.3Cr alloy spring shows SE all through, and theσm of three alloy springs decrease firstly and then increase.A structure design method of a temperature sensitive switch device based on shape memory alloy spring is put forward, and a new idea for the exploiture and design of new product is provided. |
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