Design And Fabrication Of NiTi Two-way Shape Memory Alloy With A Small Transformation Temperature Range

This thesis work aims at developing NiTi two-way shape memory alloy with a smalltransformation temperature range. In this study, nickel and titanium powers were used tofabricate dense NiTi alloys by using non-consumable vacuum arc smelting process; and theNiTi two-way shape memory alloy (TWSMA) with a small transformation temperature rangewas successfully obtained through the constraint aging treatment. Phase analysis (XRD),microanalysis (SEM) and thermal analysis (DSC) were employed to characterize theconstituents, microstructure and phase transformation behavior of the alloys systematically, inparticular, the two-way shape memory effect (TWSME) and the effect of constraint agingtreatment on the response temperature range of TWSME were analyzed.The results show that the magnitude of electric current for smelting and lumen shape ofthe suction mould are crucial to the quality of suction cast NiTi alloy samples. Smooth lumenof the suction mould and an appropriate current is helpful for making quality NiTi alloys.The temperature and dwelling time in constraint aging treatments have significantinfluence on the phase transformation behavior and response temperature range of TWSMEof the NiTi alloy. XRD analysis results show that the major phases in the suction cast NiTialloy are B2and NiTi2, but after pre-aging and the subsequent constraint aging treatment,Ni4Ti3and Ni3Ti form and the martenstitic transformation behavior changes from one-step totwo-step. With increasing constraint aging temperature and dwelling time, the transformationtemperature range between the R phase and its reverse transformation becomes narrow.The NiTi alloy shows better TWSME after the constraint aging treatment with hightemperature and prolonged dwelling time. Under the same constraint aging treatment, theshape recovery rate of TWSME of the Ni-rich NiTi alloy decreases with nickel content.Finally, the electrothermal driving characteristics of the NiTi alloy were studied. Theresults show that the response time and the response temperature range vary after differentconstraint aging treatments, and the alloy undergoing constraint aging at450°C for100hshows the shortest response time and the smallest response temperature range.