The Phase Transition Point And Surface Modification Of Ti-Ni Shape Memory Alloy

Abstract:In this paper, the effect of heat treatment on phase transformation and micro structure were studied. The designed alloy was sufficient for the actual requirement. PVA/SiO2multilayer coatings were fabricated on TiNi alloy by sol-gel method. The main results are as follows:1)B2phase, M phase, R-phase and Ti2Ni phases were found in Ti-49.98at%Ni shape memory alloys after heat treatment, and these phases except R-phase were also found in Ti-53.60at%Ni alloy;.2)Heat treatment can change the content of precipitates, leading to the changes of phase transformation behavior of the Ti-Ni shape memory alloy. In both Ti-53.60at%Ni and Ti-49.98at%Ni alloys, the proportion of Ti2Ni phase increased with the increase of aging temperature, accompanied by the decreased of Ti content in matrix, leading to the decrease of Af and increase of Ms. The results coincided with those of measured phase transformation temperatures by DSC;3)Ti-53.60at%Ni shape memory alloy had moderate austenite transformation finishing temperature (Af) to meet the need of the sterilization bolt device. The optimal heat treatment process was solid-solution treated at800℃, followed by450℃aging for60mins;4)The optimal processing parameters to produce PVA/SiO2coating were as follows:the mass ratio of polyvinyl alcohol (PVA) to ethyl orthosilicate (TEOS) was1:4, the pulling rate was2mm/min, and the sintering temperature was550℃;5)White spherical SiO2-rich phase were uniformly distributed on the surface of TiNi alloy after optimal treatment. PVA and SiO2were chemically bonded, and formed coral shaped particles with size smaller than100nm. High density was obtained between the layers. SEM analysis indicated that the content of Ni ions linearly decreased from the substrate to the surface;6)The good combination between coating and the substrate were showed from the nano indentation test results. Increasing the sintering temperature could increase the hardness and elastic modulus of the coating. Electrochemical test results showed that the polarization resistance of coated TiNi shape memory alloy sample was significantly higher than uncoated one, suggesting that the coated alloy had better corrosion resistance.