Functionally graded NiTi materials

[Truncated abstract] This PhD thesis explores the creation and characterization of functionally graded NiTi materials. These materials are designed to have gradual or abrupt microstructural or compositional variations within the body of one piece of material, hence exhibiting gradual or sudden martensite transformations from one part of the material to another. These materials have several distinctive advantages for application, including enlarged transformation temperature and stress intervals for the forward and reverse martensite transformations, thus offering better controllability for actuation design, complex shape memory mechanical behaviours, and property variations in different parts of a material to suit specific application requirements. This research on the design and development of functionally graded NiTi materials consists of four major parts: (1) fundamental study of the temperature interval of the B2-B19’ thermoelastic martensitic transformation in near-equiatomic NiTi, (2) fabrication of one-dimensional NiTi wires with discrete microstructural variation along the length, (3) creation of two-dimensional NiTi plates with microstructural gradient within the thickness, and (4) creation of two-dimensional NiTi plates with compositional gradient within the thickness. Part (1) focuses on a fundamental research that serves as theoretical guide for the other three parts. Parts (2), (3), (4) explore the creation of three novel forms of functionally graded NiTi materials. This thesis is written in the form of compilation of research publications, with two papers in each part. (1) Temperature intervals and athermal nature of thermoelastic martensitic transformations Transformation temperature interval is a fundamental thermodynamic parameter for thermoelastic martensitic transformations as well as an important technical parameter for the design of functionally graded NiTi.