The effect of annealing time on shape memory and superelastic behavior of NiTi bi-layer composite

NiTi alloys (SMAs) are unique alloys, which have two attractive properties, shape memory effect and super-elasticity. Each property strongly depends on the composition, temperature and structure. To have shape memory and superelastic behavior at the same time, a bi-layer composite, austenitic (A)/martensitic (M) NiTi alloy was designed and made to investigate the properties. The superior properties of superelasticity and shape memory behavior in NiTi are result of a thermoelastic martensite transformation. It is expected that incorporation of these two behaviors simultaneously in the bi-layer composite specimen, introduce unique properties in this material. Layers with 2:1 ratio (M:A) were bonded under diffusion bonding process in vacuum tube furnace at 1000 ⁰C, for 3 and 8 hours under 20 MPa compressive stress. The interface microstructure was investigated by optical microscopy, and chemical composition gradient in the interface zone was analyzed using line scan analysis with energy dispersive X-ray spectroscopy. Mechanical properties of the interface were studied using micro-hardness measurements. The shape memory and superelastic behavior of bi-layer were investigated using loading-unloading test and in-situ thermal heating by applying electrical current. The results depicted that annealing time had significant effect on the width of interface, and thus a microstructural gradient has been developed, within the thickness of the specimen. The average of the stress plateau is found to increase with increasing annealing time. It was found that the bi-layer specimens could act as a functionally graded material due to their chemical composition gradients that is desirable for better controllability in actuation applications.

Keywords:

NiTi alloy , Shape memory , Superelasticty , Bi-layer composite , Diffusion bonding