| Research Abstract |
Establishment of the structural control method for the most suitable functional properties and the material inspection technique of Ti-Ni shape memory alloys are the main object of this project.
Transformation temperatures, observation of microstructures and mechanical properties of variously heat-treated Ti-Ni-Co and Ti-Ni-Cu alloys were estimated. Mechanical properties of the alloys, which were at T < AィイD2fィエD2 (martensite phase), T = AィイD2fィエD2 (austenite + martensite phase) and T > AィイD2fィエD2 (austenite phase) ( T : test temperature, AィイD2fィエD2 : reverse transformation finish temperature ), were investigated by using a servo-hydraulic type material testing machine with an environmental chamber. Then fracture surfaces were observed by a high resolution scanning electron microscopy and X-ray diffractometer, and microstructure of near fracture region was observed by a transmission electron microscopy. The tensile deformation behaviors and fatigue properties are strongly affected by deformation speed. All the values of inflection stresses on the stress - strain curves increased with increasing tensile speed. The fatigue life obtained at 20 Hz was about 2 times longer than that at 0.5 Hz, referring to the maximum stress versus the number of cycles to failure curve. Striation patterns are clearly observed on the fatigue fracture surface, and the crack growth rates, dl/dN, were measured from these striation patterns. The deformation process model of Ti-Ni shape memory alloys was proposed on the basis of the above experimental results.
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