Ti-based shape memory alloys (SMAs) have attracted extensive attentions and are playing important roles in a rich variety of industrial and biomedical applications due to their superelasticity, shape memory effect, high strength, low Young’s modulus, high power density, etc. For example, Ti-based SMAs are used as sensors and actuators, antennae, orthopedic implants, self-expanding vascular stents, active catheters, and orthodontic archwires, to name a few. However, it should be pointed out that there exist some challenges on applications of Ti-based SMAs althought they are the most commonly used commercial SMAs at present. Firstly, SMAs suffer from functional fatigue and dimensional instability during cyclic back and forth martensitic transformation (MT), which limits the service life o...[ Read more ]

Ti-based shape memory alloys (SMAs) have attracted extensive attentions and are playing important roles in a rich variety of industrial and biomedical applications due to their superelasticity, shape memory effect, high strength, low Young’s modulus, high power density, etc. For example, Ti-based SMAs are used as sensors and actuators, antennae, orthopedic implants, self-expanding vascular stents, active catheters, and orthodontic archwires, to name a few. However, it should be pointed out that there exist some challenges on applications of Ti-based SMAs althought they are the most commonly used commercial SMAs at present. Firstly, SMAs suffer from functional fatigue and dimensional instability during cyclic back and forth martensitic transformation (MT), which limits the service life of devices. Even for the most widely used commercial SMA, NiTi, the irrecoverable strain may approach 10% after 100 thermal cycles under a 150 MPa bias load, and failure by fracture would occur after a few thousand cycles. It is generally believed that this is caused by the generation and accumulation of crystalline defects, e.g. dislocations, during martensitic transformation. To solve this problem, improving geometric compatibility by tailoring the stress-free transformation strain via adjusting the composition is performed under the guidance of the cofactor condition. It is noteworthy that the cofactor condition is such a strict geometrical condition that only limited marterials can approximately satisfy although it is proven to be useful in enhancing the fatigue resistance of SMAs. Secondly, the efficiency of SMA actuators is often less than 1% in practical applications due to their large hysteresis, which is lower by several tens of times than that of piezoceramic actuators and hydraulic actuators. It is reported that cold working and grain nanocrystallization can reduce the hysteresis. But it should be noted that cold working will introduce various defects that adverse to the fatigue resistance and that it is challenging to refine grains to several nano meters. Thirdly, precise position control of actuators is required in some applications, e.g., robotics and active catheters, but the strong nonlinear pseudo-elasticity of SMAs makes such control difficult. People have tried to employ position feedback systems and designed a segmented SMA actuators which divided SMA wires into many segments and controlled their thermal states (i.e. heating for austenite or cooling for martensite) individually. However, it is unavoidable for these methods to complicate the design of SMA actuators. Last but not the least, the much higher elastic modulus of joint implants than human’s bone leads to the long standing “stress shielding” effect, which is the main life-limiting factor for orthopedic implants. β titanium alloys and GUM metals are designed to lower the elastic modulus and to make it match with that of human bones. However, the lowest modulus achieved is 42 GPa which is still higher than the elastic modulus of human bones (20~30 GPa).

The objective of this dissertation is to develop a general approach, which is able to provide a single SMA integrated properties of ultralow modulus, hysteresis-free and linear pseudo-elasticity, and high fatigue resistance. MT is a unique and important deformation mode for SMAs. However, its first-order nature makes it difficult to be used to tune the thermal and mechanical properties of SMAs. If we can control the kinetic process of MT, we will get a powerful means of manipulating the microstructure evolution to get desired properties. The MT of a GUM metal, Ti-24Nb-4Zr-8Sn-0.10O in wt.%, is studied, employing a combination of phase field simulations and the phenomenological theory of martensite crystallography (PTMC). A novel and general approach, concentration modulation (CM), is introduced to tune the kinetic process of MT, and the influence of CM on MT and mechanical properties of TiNb-based SMAs is studied systematically. Furthermore, a novel transformation pathway of martensities is developed to provide unprecedented properties in TiNb-based CM SMAs, including ultralow modulus, hysteresis-free and linear super-elasticity. In addition, CM TiNi SMAs with a sandwich structure is designed as well to tune the MT and achieve unprecedented properties in TiNi SMAs.