Ni-Ti-Nb shape memory alloys have attracted considerable attention due to wide transformation hysteresis and excellent shape memory effect, which have been successfully applied as couplings and fasteners. However, the yield stress of present Ni-Ti-Nb alloys already can’t satisfy the practical requirements for the high pressure tube. As an important twin-type damping alloys, Ni-Ti alloy with high damping capacity is usually in the martensitic state, while the mechanical properties, especially the yield strength, are not ideal for further development requirements as structural material. Therefore, recent research focuses more eyes on improving the mechanical properties of the Ni-Ti-Nb alloys.
In this thesis, the work on improving the performance was taken by adjusting the Ni/Ti ratio and controlling the Nb and Mo content in Ni-Ti-Nb alloys. The effect of Mo and Nb on the performance of Ni-Ti-Nb alloys was investigated, such as the microstructure, phase transformation behavior, mechanical behavior, shape memory effect, transformation hysteresis and damping capacity, and two kind novel Ni-Ti-Nb-Mo quarternary shape memory alloys with high yield strength and high damping were obtained.
Based on the characteristics of the alloys with high yield strength, suitable Mo and Nb were added into the Ni-Ti-Nb alloy with 4.5at.％Nb to improve the mechanical properties. It is found that the uniform distribution of Mo depresses the appearance of coarse β-Nb particles on the grain boundaries and short stripped texture consisting of abundant fine disperse Nb-rich particles appears around the grain boundaries. The yield strength of the alloys was enhanced from 450MPa to 600MPa due to the solution strengthening of Nb and Mo and the elongation keeps in a high level with 28%. The yield strength promotion of the matrix increases the critical stress for slip, which is responsible for the improvement of the shape memory effect. The maximum recoverable strain of the alloy with 0.5at.%Mo is near 8% and has reached the high level of Ni-Ti binary alloys. Mo addition further enlarges the transformation hysteresis. When the Ni-Ti-Nb-Mo alloy pre-deformes 20%, the transformation hysteresis is close to 200℃ which is higher than 150℃ in traditional Ni-Ti-Nb alloys. This novel high-strength alloy is promising to be used for high pressure tube and the macro-scale coupling with higher-qulity requirements.
As for the alloys with high yield strength and high damping capacity, the Ni-Ti-Nb alloy with 9.0at.%Nb is employed as the based alloy for the addition of the Mo element. It is found that the addition of Mo increases the solubility of Nb and Mo in the NiTi matrix so that the degree of supersaturation increases at the ambient temperature to induce large quantities of fine Nb-rich particles precipitating from the NiTi matrix. Because of the precipitation strengthening of the Nb-rich particles and the solution strengthening of Nb atom and Mo atom, the yield strength of the Ni-Ti-Nb-Mo alloy in the martensitic state is enhanced from 150MPa to 302MPa, and the elongation reaches 48%. Mo addition in the alloy with 9.0at.%Nb induces R transformation and three step tansformations occur due to the stress field around the Nb-rich particles. The enlargement of NiTi matrix zone and the precipitation of Nb-rich particles increase the intrinsic damping. Hence the damping capacity of Ni-Ti-Nb alloy can be effectively improved by Mo addition, which can reach 0.015 in the martensitic state. In addition, the Ni-Ti-Nb-Mo alloy has good shape memory effect and wide transformation hysteresis. This kind of alloy owns both high yield strength and high damping capacity, which has widely potentiality for engineering application, such as intelligent damping components or energy dissipation devices.
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