| As a shape memory alloy with superior property, NiTiNb alloy was widely usedin aviation, watercraft and automobile industry. As well known, the shape memoryeffect is mainly controlled by martensitic phase transformation and reversetransformation. Therefore, the research on the mechanism of martensitic phasetransformation and reverse transformation is critical for improving the property ofNiTiNb alloy. However, not only the research on the related field, but also the effectof the hot or cold treatment on martensitic phase transformation in NiTiNb alloy isscarce currently. In this thesis, the item about the reliability of NiTiNb alloy used tolink with Ti alloy tube and stainless steel tube was researched carefully, which issupported by national important project<The application research on memory alloytube joint>. Based on DSC, SEM and EDS method, the effect of heat treatment onphase transformation temperature of NiTiNb alloy was researched. Then, in order toquantize the martensitic phase transformation and reverse transformationtemperature and develop the theoretic basis of NiTiNb alloy, the mechanism leadingthe evolution of martensitic phase transformation and reverse transformationtemperature was analyzed.In the process of exploring the effect of heat treatment technology on phasetransformation temperature of NiTiNb memory alloy, the influence of someparameters in heat treatment (such as: aging temperature, aging time, cooling speedand cool cycling treatment way) on various main physical performance (such as:phase transformation temperature, transformation hysteresis, the average inothermicenthalpy and the average endothermic enthalpy) was researched. The mechanisms ofaging and cold treatment were also explored. The results show that the parameterssuch as aging temperature, cooling speed and frequency of cold cycling treatmentobviously affect the martensitic phase transformation and reverse transformation,while other parameters such as: aging time and cryogenic treatment nearly can notobviously affect the basic property of NiTiNb alloy.In the process of exploring the effect of aging temperature on martensitic phasetransformation and reverse transformation, an sensitive temperature T could betestified existing between300~400℃aging temperature range. When the aging occur in this sensitive temperature, the phase transformation temperature Ms appearobvious change. Nevertheless, when the aging temperature is higher or lower thanthe sensitive temperature, the phase composition of NiTiNb alloy has nearly nochange. From the microstructure analysis, for one side, it can be concluded that withthe addition of element Nb, both the yield strength of matrix in the temperature ofMs and the average inothermic enthalpy of martensitic phase decrease. For anotherside, due to the diffusion of element Nb, the position of Ni was replaced and theNi/Ti atomic proportion occur slight evolution. As a result, the martensitic phasetransformation temperature finally occur obviously change.The cooling speeds in the same aging temperature also greatly affects themartensitic phase transformation and reverse transformation, especially formartensitic phase transformation temperature Ms. Compared with original sample,the influence of cooling way on Ms could follow with this sequence, water coolingwhich could obviously improve Ms, air cooling and furnace cooling which slightlyimprove Ms. Therefore, it can be concluded that with the increase of cooling speed,the evolution range of Ms also increase. This can be credited to the slight evolutionof Ni/Ti atomic proportion, the evolution of release stress existed in the sample andthe dislocation. However, the cooling speed can not obviously affect thetransformation hysteresis, the average inothermic enthalpy.Cold cycling treatment parameters could affect NiTiNb alloy by two means.Firstly, when the treated temperature is higher than Ms or in the range of Ms-Mf,cold cycling treatment can not seriously influence the transformation hysteresis, theaverage inothermic enthalpy. Secondly, when the temperature is below Mf, coldcycling treatment could seriously influence the transformation hysteresis. Ms, Mf, Asand Afincrease with the decrease of cycling number, especially for Mswhose changerange could attain30℃. The transformation hysteresis of NiTiNb memory alloyincrease with cycling number. When the cycling number is15, the transformationhysteresis could reach73.9℃. This can be credited to the slight evolution of Ni/Tiatomic proportion, the evolution of release stress existed in the sample and thedislocation. |
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