Papers by Author: Wei Cai

Abstract: The effect of rare earth element Ce addition on the microstructure and martensitic transformation behavior of Ti₅₁Ni₄₉ shape memory alloy was investigated by scanning electronic microscope (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results show that the microstructure of TiNiCe ternary alloy consists of Ti₂Ni phase, CeNi phase and the matrix. One-step martensitic transformation is observed in quenched TiNiCe ternary alloys, which is the same as that in quenched TiNi binary alloys. The martensitic transformation temperatures of Ti-rich TiNi alloy hardly increase with Ce addition.

Abstract: The tensile and low cycle fatigue tests were carried out on alumina short fibers reinforced Al-Si piston alloy composites (Al-Si MMCs). Three Al-Si MMCs reinforced with 10, 17 and 25 vol.% of alumina short fibers were prepared to investigate the effects of volume fraction on tensile and low cycle fatigue properties at room temperature (RT) and 350°C. The results showed that the tensile strength decreased with the increasing of volume fraction of fibers at RT and was slight different at 350°C. Among the three MMCs, the 17%-MMCs showed highest stress level under the low cycle fatigue tests. The fatigue cracks were usually initiated from the clustered and large size fibers near the surface of specimen, propagated along the fiber/matrix interface at RT and grew rapidly by means of broken the fibers at 350°C.

Abstract: The effect of La addition on the microstructure and phase transformation behavior of Ti _49.3Ni_50.7 alloy is investigated by optical microscope (OP), scanning electron microscope (SEM), X-ray diffraction analysis (XRD) and differential scanning calorimeter (DSC). The results show that the microstructure of Ti-Ni alloy is changed obviously with La addition, and the microstructure of Ti-Ni alloy containing La at room temperature is made up of the matrix and LaNi phase. La addition firstly increases the martensitic transformation temperatures evidently and the increase of Ms is about 62°C with 1at.% La. Secondly Adding La changes the type of phase transformation behavior of Ti-Ni alloys, and R phase transformation with Rs temperature about room temperature occurs in La added Ti-Ni alloy.

Abstract: Poly (glycerol-sebacate) (PGS) is a recently synthesized elastomer with superior mechanical property, biocompatibility and biodegradation, and serves as soft tissue regeneration and engineering materials or contact guidance materials. The samples for shape memory measurements were prepared by a two steps method. The microstructure and thermal properties of PGS are studied by using Fourier transform infrared (FTIR), differential scanning calorimetry (DSC) and Dynamic-mechanical analysis (DMA) methods. The shape memory effect of PGS is recorded by bending test. It was found that a crosslinked, three-dimensional network of the PGS acting as fixed phase and the amorphous phase of the PGS acting as reversible phase are the two necessary conditions for PGS with shape memory behavior. The response temperature of shape memory is dependent on the glass transition temperature of PGS. The PGS polymer with a high elasticity and a shape-memory ratio of almost 100% showed excellent shape memory effect.

Abstract: A biodegradable poly (glycerol-glycol-sebacate) terpolymer (PGGS) with different composition is prepared by a three-step method. The structure and properties of the PGGS terpolymers were characterized by means of attenuated total reflectance-Fourier transform infrared (ATR-FTIR), differential scanning calorimeter (DSC) and X-ray diffraction (XRD) methods. The experiment results indicate that the the composition of terpolymers have an obvious influence on the structure of PGGS terpolymers. The degradation rate increases with the increasing crosslink degree of PGGS.

Abstract: The Ni_54.75Mn_13.25Fe₇Ga₂₅ (at.%) ferromagnetic shape memory thin film was deposited onto silicon substrates using radio-frequency magnetron sputtering. The martensitic transformation, crystallographic structure, microstructure and magnetic-field induced strain were investigated by means of Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), Transmission Electron Microscope (TEM) and metal strain gauges. The results show that the martensite transformation temperature M_s is 296.6 K, the film with typical self-accommodated morphology is orthorhombic structure at room temperature. The field-induced strain of 52 ppm is obtained in this shape memory thin film.

Abstract: The Ni-Mn-Fe-Ga shape memory alloy thin film was deposited onto silicon substrates by using radio-frequency (R.F.) magnetron sputtering technique. Chemical composition, surface morphology and crystallographic structure were systematically investigated by means of X-ray fluorescence (XRF), atomic force microscope (AFM) and X-ray diffraction (XRD). The experimental results show that the magnetron sputtering process has remarkable influence on the chemical compositions and surface characteristics of Ni-Mn-Fe-Ga alloy thin films. As the sputtering power ranging between 245W and 405W, Ni content of the thin films decreases with the sputtering power increasing, whereas Mn and Fe contents increase with increasing the sputtering power and Ga content almost keep a constant. The surface roughness and the average particle size of thin films increase with the increase of Ar working pressure and sputtering power. The film deposited at room temperature has a cubic L2₁ structure.

Abstract: The microstructure evolution of sputtered polycrystalline Ni54.75Mn13.25Fe7Ga25 ferromagnetic shape memory thin film annealed under different conditions is studied. Microstructure of different annealed films was studied using Transmission Electron Microscope (TEM) and corresponding selected area electron diffraction (SAED) patterns. The result shows that in the microstructure of as-deposited Ni54.75Mn13.25Fe7Ga25 free-standing film, after annealed at 1073 K for different time, the crystalline grain grows up with the increase of the annealing time. By analysis of the SAED patterns, the structure of the thin films change from face-centered cubic austenite to orthorhombic structure martensite compared between the film annealed at 1073 K for 10 mins, 1hr, 4 hrs, and 24 hrs respectively. It indicated that the heat treatment is an effective method of crystallizing behavior for the thin film.

Abstract: The martensitic transformation behavior and shape memory effect (SME) have been investigated in a Ni-rich Ti29.6Ni50.4Hf20 high temperature shape memory alloy (SMA) in the present study. After aging, the transformation temperatures of Ti29.6Ni50.4Hf20 alloy increase obviously due to the precipitation of (Ti,Hf)3Ni4 particles. And the transformation sequence changes from one-step to two-step. When the experimental alloy is aged at different temperatures for 2h, the transformation temperatures increase rapidly with increasing the aging temperature and then change slightly with further increasing the aging temperature. Most of the martensite variants preferentially oriented in the aged Ti29.6Ni50.4Hf20 alloy. The aged Ti29.6Ni50.4Hf20 alloy shows the better thermal stability of transformation temperatures than the solution-treated one because the precipitates depress the introduction of defects during thermal cycling. In addition, the proper aged Ti29.6Ni50.4Hf20 alloy also shows the larger SME than the solution-treated one since the precipitates strengthen the matrix strongly.

Abstract: In: Eu: Fe: LiNbO3 crystals were firstly grown from a congruent melt according to the Czochralski method, with the various ratios of Li/Nb =0.885, 0.946, 1.051. The new crystal composition and defect structure were analyzed by the UV-Vis spectroscopy. In the two-wave coupling experiments, writing and erasure time, maximum diffraction efficiency, photorefractive sensitivity and dynamic range were measured and calculated. The dependence of the photorefractive properties on the defect structure of In: Eu: Fe: LiNbO3 Crystals were discussed.