research topics

Research and development of Ti-base biomedical shape memory alloys

Shape memory alloys are fascinating materials for biomedical applications. Ti-Ni shape memory alloys have been used successfully as biomedical materials owing to their superior shape memory property and superelasticity. However, the Ni-hypersensitivity and toxicity of Ni has been pointed out in Ti-Ni alloys. Although the Ti-Ni alloys have been successfully applied for many medical products, the development of Ni-free shape memory alloys is on the other hand strongly required in order to pursue absolute safety. The β-type Ti alloys reveal a martensitic transformation from β(disordered BCC) to two metastable martensite structures, either hexagonal martensite(α') or orthorhombic martensite(α''), by quenching. The martensite structure changes from α' to α'' above a critical alloying content. Transformation strains from the β to α'' is accommodated primarily by internal twinning. The reversion of α'' to β is related to the shape memory effect in β-type Ti alloys. The martensitic transformation start (Ms) temperature can be controlled by addition of alloying elements. But, for the biomedical application, alloy designs with combination of non-toxic elements are required. Our group has systematically investigated new Ti-base alloys such as Ti-Nb-X, Ti-Mo-X, Ti-Ta-X and Ti-Nb-Mo-Ta-X, in order to develop Ni-free biomedical shape memory alloys.

Research and development of Ti-Ni high temperature shape memory alloys

High temperature shape memory materials have attracted much interest in recent years because of their potential applications for high temperature devices. Conventional Ti-Ni binary alloys are limited due to the martensitic transformation start (Ms) temperature below 373K. Several candidates for high temperature shape memory alloys have been investigated. The addition of platinum, palladium, gold in replacement of Ni in Ti-Ni alloys can raise the Ms temperature higher than 800K. Besides, 20% addition of zirconium or hafnium increases the Ms temperature to about 550K. Our group has systematically investigated high temperature shape memory alloys, e.g., Ti-Ni-Pd, Ti-Ni-Zr, Ti-Ni-Hf.

Research and development of Ti-Ta high temperature shape memory alloys

Shape memory alloys (SMAs) exhibiting martensitic transformation temperatures above 373K are attractive for applications as advanced actuator and superelastic materials that can function in a hot environment, such as in home appliances (gas stove, heaters and many general appliances), transportation (automobile and aircrafts) and power generation systems (oil and gas energy plants). The martensitic transformation start temperature (Ms) of commercially available Ti-Ni-base alloys is below 373K so their applications are currently limited to below that temperature. For this reason the development of high-temperature shape memory alloys (HTSMAs), such as Ti-Ni-X (X = Zr, Hf, Pd, Pt, Au), Ni-Al and Ni-Mn-base HTSMAs, has been extensively pursued. However, the inherent poor cold workability of the existing HTSMAs makes them difficult to be fabricated into fine wires and thin plates with dimensions suitable for actual applications.
Recently the β-type Ti-base SMAs, which exhibit excellent cold workability, have been extensively investigated. For example, the shape memory behavior of β-type Ti-base alloys, such as Ti-Mo, Ti-Nb and Ti-Ta alloys, has been reported. The excellent cold workability of β-type Ti-base SMAs have made them attractive for practical applications as new smart materials, since they can be easily fabricated to fine wires and thin plates of dimensions suitable for actual applications. Our group has systematically investigated Ti-Ta-base high temperature shape memory alloys.

Development and characterization of shape memory thin films (Ti-Ni, Ti-Ni-X (X = Cu, Pd, Zr) )

Among several types of performance materials proposed for fabricating microactuators, Ti-Ni shape memory alloys have great advantages such as large deformation and recovery force. Since 1990, Ti-Ni and Ti-Ni-X (X=Cu, Pd, Zr) thin films have been made by sputtering. The motivation for fabricating sputter-deposited TiNi-base shape memory alloy thin films originates from the great demand for the development of powerful microactuators which can drive micromachines, because actuation force and displacement are greatest in shape memory alloys amongst many actuator materials. Stable shape memory effect and superelasticity, which are equivalent to those of bulk alloys, have been achieved in the sputter-deposited Ti-Ni thin films. Narrow transformation temperature hysteresis and high transformation temperatures were also achieved in Ti-Ni-Cu and Ti-Ni-(Pd or Zr) thin films, respectively. In the meantime, unique microstructures consisting of nonequilibrium nanoscale precipitates and nonequilibrium compositions in the matrix have been found in Ti-rich Ti-Ni thin films which were fabricated from amorphous condition by annealing at a considerably low temperature. Several micromachining processes have been proposed to fabricate some prototypes of microactuators utilizing Ti-Ni thin films.

Surface study of Shape memory alloys using STM

Internal twinned microstructure and surface relief are important characteristics of the martensitic transformation of shape memory alloys. However, quantitative studies of the surface relief and surface structure have seldom been reported in an atomic scale. Up to now, almost all the research concerning the surface relief has been done with optical microscopy and SEM which have not enough vertical and lateral resolution in an atomic scale. In our group, change of surface due to the martensite transformation has been observed in an atomic level using high resolution scanning tunneling microscopy (STM).

Research and development of Ti-Ni shape memory alloys