Microstructure Origin for Thermal Fatigue of TiNi Films
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Microstructure Origin for Thermal Fatigue of TiNi Films Shulin Wen , Jibao He HREM Analytical Center, College of Mater. Sci. & Eng., Shandong University 73 Jingshi Rd., Jinan, Shandong, 250061 , CHINA ABSTRACT In order to improve the performance and prolong the life of shape memory alloy material (SMA), it is very important to trace and study the microstructure change on the fatigue of SMA. The microstructure features between the samples before and after thermal fatigue (about 100,000 thermal cycles) of the NiTi-SMA films were examined and compared with each other by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). It was found that there is no difference of microstructures between the two kinds of samples except some precipitates appeared in the fatigue specimen. These precipitates which may be identified as TiNi3 phase with a grain size of 10-20 nm may impede transformation from martensite to austenite which works as mechanism of SMA during thermal fatigue. Therefore, these precipitates result in the serious decay on SMA performance and further fracture of the SMA material. INTRODUCTION Fatigue is a progressive and localized permanent structural change. It occurs in such a material which subjected to repeated and fluctuating strain. However, the maximum value of this strain must be less than the tensile strength of the materials. After much fluctuating strain cycling, fatigue may cause cracks or fractures in the materials. The simultaneous cyclic stress, tensile stress and plastic strain only cause the fatigue. If any one of these three is not present, then the fatigue will not initiate and propagate. Fatigue occurs in TiNi shape memory alloy (SMA) due to repeated and fluctuating strains during its application in great deal of low-high temperature cycling. The TiNi-SMA is a kind of materials which have an ability to return some previously defined shape and size when subjected to the appropriate thermal procedure. These materials can be plastically deformed at a relatively low temperature, and up exposure to a higher temperature will return to their shapes prior to the deformation [1-5]. Therefore, the SMA undergoes the repeated simultaneous cyclic stress, tensile stress and plastic strain and the fatigue then occur during long-term application of the material. One of the goals of TiNi-SMA application is to make miniature valve as smart microvalve which is applied to combustion engine fuel delivery system or closed-loop control drug delivery system. The material used for this purpose is NiTi film with phase transformation temperature around 60 C and
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temperature hystereses as low as 10 degrees. The valve needs to work for millions of low-high temperature cycles at least. Fatigue then occurs and causes the decay of the performance of SMA first and then further fracture produced. Therefore, it is very important to trace and investigate the microstructural reason on the fatigue of SMA in order to retain SMA performance and prolong SMA life. EXPERIMENTAL The most reasonable
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