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xiii ABSTRACT Ti50Ni25Cu25 ribbon is widely studied because of its one stage B2 (austenite) ↔B19 (martensite) transformation with small hysteresis and good shape memory property, which can be useful in microactuator appliances. Structure-property relationships are fundamental to optimize properties of shape memory alloys. In order to understand the effects of initial melt-spun structure and post-heat treatment on the shape memory properties (transformation behavior and shape recovery strain) of Ti50Ni25Cu25 ribbon, three different initial structures were selected: fully amorphous, fully crystalline and partially amorphous. In addition, the crystal structure and transformation behavior of bulk Ti-Ni-Cu alloys with different Cu contents were investigated in order to understand the mechanism of Cu addition on the transformation characteristics from a thermodynamic viewpoint. SEM, XRD, TEM, DSC and TMA techniques were used as characterization methods. The initially amorphous ribbon crystallized at 450°C. With an initial amorphous structure, it was possible via proper annealing to obtain nano-sized martensite without twinning or micro-sized martensite with only one pair of twin variants. Strong preferential orientation was observed to change with annealing from [211]B19 fiber to (111)B19[ 2 11]B19 sheet texture, B11 TiCu precipitates formed at annealing temperature above 550°C. The initially partial amorphous ribbon shared a similar structure evolution with annealing to the initially amorphous ribbon. The initially crystalline ribbon contains a mixed structure of B2 austenite and B19 martensite at RT. The two-phase microstructure did not change except that the amount of ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library

Effects of Cu content and heat treatment on the transformation behaviour and shape memory property of Ti-Ni-Cu alloys