Hui Bin Xu

Martensitic Transformations of Ni54Mn25Ga21-xAlx Shape Memory Alloys

The non-stoichiometric NiMnGa shape memory alloy with high Ni content has been developed as promising thermo-actuated materials applied at high temperature. A substitution of Al for Ga in the Ni54Mn25Ga21 alloys is performed. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements have been carried out to study the effects of the phase transformations and microstructures of the Ni54Mn25Ga21-xAlx shape memory alloys. The results show that the martensitic transformation temperatures almost linearly decrease with the increase of Al substitution for Ga, which can be explained considering the effect of the size factor, i.e. the lattice parameter. A structural transition from a non-modulated tetragonal type to a seven-layered 14M one has been found during the increase of Al substitution for Ga.

On Control of Six Freedom Magnetostrictive Smart Structure

A six degree-of-freedom (DOF) Stewart platform is constructed, which consists of six TbDyFe alloy magnetostrictive actuators, and applied to active vibration control. To control the smart structure, a real time computer control system is built. An improved adaptive filtering algorithm is proposed in this paper, which is used for the computer control system. The results of experiments show that the smart structure and the proposed algorithm are efficient for active vibration control. More than 30 dB of vibration attenuation is achieved in real-time experiments.

Effects of Nb on the High Temperature Mechanical Properties of TiNiAl Alloys

A series of NiTiAl based alloys with different amount of Nb addition were prepared and the effects of Nb on both the mechanical properties and microstructure evolution were investigated. The addition of Nb can remarkably enhance the high temperature yield strength of these alloys. The highest yield strength of the alloys with 10 at% Nb reaches 1237MPa at 873K and 875MPa at 973K, respectively. The Ti2Ni(Al,Nb) precipitates and the solution strengthening effect might be responsible for the high yield stress achieved at high temperature.

The Influence of NiCrAlY Coatings Prepared by LPPS and EB-PVD on the Damping Properties of 1Cr18Ni9Ti Alloy

The influence of NiCrAlY coatings prepared by low-pressure plasma-sprayed (LPPS) and electron beam physical vapor deposition (EB-PVD) respectively on the damping properties of 1Cr18Ni9Ti alloy were investigated. The results shown that, compared to uncoated sample, the damping capacity of NiCrAlY coated samples has been improved significantly. The Q -1 value of the coated samples was approximately 0.008~0.01 in range of 20~300 .The interface adhesion state has a distinct effect on the coatings’ damping characteristics. The dynamic Young’s modulus of the coated samples decreased compared to the uncoated substrate; and LPPS coated sample’s dynamic modulus was lower than that of EB-PVD coated sample. Such difference resulted from different coating microstructure and interface adhesion morphology.

Stress Induced and Magnetic Field Enhanced Twin Variants Reorientation in NiMnGa Single Crystal

The behavior of twin variants reorientation in Ni50Mn27.5Ga22.5 single crystal under the applied mechanical stress and magnetic field was investigated. Through three times of compression, the twinning stress was reduced from 4.4MPa to 0.4MPa, and 2.5% stress induced strain was obtained. During the succedent MFIS measurement, with the increase of measurement number of times, the critical field (μ0H)c at which the MFIS jumps became lower, and the corresponding jumping magnetostrain ε j, and the maximal magnetostrain εmax ascended. At last, a quasi single-variant state was realized, and the sample showed repeatable 6.21% MFIS, which is close to the theoretical value (1-c/a)*100%. The results are discussed from the point of martensitic microstructure evolution.

FEM Simulation Study on Relationship of Interfacial Morphology and Residual Stress in TBCs

It is generally believed that the failure of TBCs is attributed to the spallation occurred in the ceramic coat. The spallation is closed linked with sinuate morphology factors, including its amplitude and period, at the TGO/bond coat interface. In this work, dependence of the residual stress distribution on the sinuate morphology in the TBCs has been studied by means of Finite Element Method (FEM) simulation for isothermally annealed specimens. The simulation results indicated that the maximum value of residual stress existed inside the TGO layer. It was also found that the maximum residual stress occurred at different points, near the TGO/bond coat interface at the peak of the sinuate interface, while near the TGO/ceramic coat interface at the valley, respectively. And the maximum residual stress increased with increasing the ratio of the amplitude to period in the sine morphology, which has been proved by the thermal cycle experimental results.

The First-Principles Study of Electronic Structure of Fe/MgO/Fe Magnetic Tunnel Junctions Interface

In this work series of models were constructed in order to investigate the relationship between atomic and electronic structure and TMR property. Models with normal component interface of Fe/MgO/Fe magnetic tunnel junctions were calculated by first-principles discrete variational method (DVM) within the framework of local spin density functional theory. The SP and TMR ratio of Fe at interface of ferromagnetic layer as well as density of states are analyzed. Our research shows that the thickness of ferromagnetic layers effect much on electronic structure. The interface and surface Fe layers have different feature from that of interior.

A New Method to Identify the Preisach Distribution Function of Hysteresis

In this paper, a new method which combines the least square method with Tree-Structured fuzzy inference system is presented to approximate the Preisach distribution function. Firstly, by devising the input sequence and measure the output, discrete Preisach measure can be identified by the use of the least squares method. Then, the Preisach function can be obtained with Tree-Structured fuzzy inference system without any special smoothing means. So, this new method is not sensitive to noise, and is a universal approximator of the Preisach function. It collect the merit and overcome the deficiency of the existing methods.

Fuzzy Modeling of the Magnetostrictive Hysteresis for TbDyFe Actuator

A mechanical properties prediction model for cobalt-free maraging steel was built upon the experimental data by fuzzy identification method. A method of fuzzy identification based on fuzzy clustering and Kalman filtering is proposed. The results showed that good correlations between the predicted result and the experimental data. The technique proposed could be served as a reliable tool for cobalt-free maraging steel mechanical properties control and design.

First-principles study on initial oxidation of NiAl (110)

The oxygen atom adsorption at Al-Al bridge, Ni-Ni bridge, Al top and Ni top site on the NiAl(110) surface by first-principles method within density functional theory has been studied in this paper. It has been found that the preferred adsorption position of the oxygen was at the Al-Al bridge site then the Ni-Ni bridge site. The charge transfer took place obviously between the O atom and the nearest Al atoms, but no charge transferred from the nearest Ni atoms to O atom. For the Al-Al (Ni-Ni) bridge adsorption site, the bond lengths of Al-O and Ni-O were about 1.741 Å (1.700Å) and 2.369Å (2.012Å), respectively, which means that the Al atom is easier to be oxidized than the Ni atom. It is revealed that the Al atom oxidized selectively and the chemical bond formed between the O ion and the nearest Al ions during the initial oxidation stage.