Tianli Zhang

Shear band melting and serrated flow in metallic glasses

Scanning electron microscopy observations of shear steps on Zr-based bulk metallic glasses show direct evidence of shear band melting due to heat generated by elastic energy release. The estimated range of attained temperatures and the observed morphologies are consistent with shear steps forming at a subsonic speed limited by a required redistribution of local microscopic stresses. The calculations indicate that a 0.2μm layer melts in the vicinity of a shear band forming a 1μm shear step. The plastic part of the stress strain curve is serrated but a majority of shear events are not associated to serrations.

Giant magnetostrictive actuators for active vibration control

Giant magnetostrictive actuators are designed and fabricated with home-made TbDyFe magnetostrictive rods. The corresponding static and dynamic characteristics are tested. The total output displacement can be obtained up to 100 µm and the output force up to 1500 N. The dynamic responses of input and output are accordant and have a small hysteresis. Experiments on active vibration control are implemented in single-degree-of-freedom (DOF) and six-DOF platforms in a flexible space structure. The excellent damping effect, up to 30 dB, proves the good performance of the actuators, the feasibility of the control algorithms, and the reasonable design of the six-DOF platform.

Grain-(111)-oriented anisotropy in the bonded giant magnetostrictive material

A strong ⟨111⟩ preferred orientation is induced along the axis of the TbDyFe/epoxy bonded giant magnetostrictive rods by curing the epoxy under a moderate magnetic field. TbDyFe particles with a size distribution of 200–300 μm align in an epoxy matrix, showing an approximate chain structure. A high magnetostrictive performance resulting from the grain-⟨111⟩-oriented anisotropy is achieved with the longitudinal magnetostriction λ∥ up to 1360×10−6 under the prestress of 17 MPa.

Permanent-magnet longitudinal fields for magnetostrictive devices

This paper discusses a technique for improving the homogeneity of the magnetic field of tube-like permanent magnets designed for magnetostrictive devices. The high homogeneity of the magnetic field along the tube is achieved by gradually changing shape or remanence of combined short rings. The analysis of the field in the magnet is performed using the magnetic charge principle and axial magnetic field equations are deduced. An example of design of a longitudinal uniform field for a magnetostrictive rod with large length to diameter ratio is presented.

Ageing behaviour of Pd40Cu30Ni10P20 bulk metallic glass during long-time isothermal annealing

In this paper, positron annihilation spectroscopy and differential scanning calorimetry were applied to study the structural relaxation of Pd40Cu30Ni10P20 bulk metallic glass during long-time annealing at 523 K. It was found that the relaxation of the alloy follows a two-stage process. In the initial stage, with increasing annealing time, the height of the glass transition peak (namely Δcp) increased, while the average positron lifetime decreased. The isothermal relaxation kinetics obeys a Kohlrausch law with exponent of β = 0.5± 0.03. However, when the annealing time is beyond 32 h, the height of the glass transition peak (Δcp) decreased, and the average positron lifetime increased. This is possibly due to the enhancement of the topological short-range ordering, which results in the generation of new defects on the interfaces between the solid-like regions and the liquid-like regions in amorphous phase. The experimental results were tentatively discussed in terms of free-volume theory and percolation theory.

Dynamic magnetostrain properties of giant magnetostrictive alloy actuators for damping

Giant magnetostrictive alloy (GMA) actuators are tending to replace piezoelectric actuators in many applications owing to their giant magnetostrain and fast response. In an active vibration control system using GMA actuators, the investigation of dynamic magnetostrain can supply the basis for the optimization of actuator designs and control algorithms. Experiments have been carried out under varied operating conditions. The results show that there is a better dynamic output only at suitable prestress and magnetic bias values. For a fixed frequency, the output is basically proportional to the input amplitude but the inclination is slightly different for each input frequency. The natural frequency of the actuator is around 5 Hz and the output displacement decreases with frequency increase after 10 Hz. The dynamic magnetostrain at the natural frequency can be larger than the quasi-static value by up to 40%.

Exchange-coupled SmCo5/Co nanocomposites synthesized by a novel strategy

SmCo5/Co nanocomposites with exchange-coupling are synthesized by a reverse design where Co is decomposed from SmCo5 nanoparticles by hydrogen disproportionation process to fabricate two-phase nanocomposites. The novel design makes SmCo5 nanoparticles uniformly surrounded by Co nanoparticles, which avoids the alloying in traditional methods and achieves exchange-coupling.

2:17-type SmCo quasi-single-crystal high temperature magnets

2:17-type SmCo high temperature magnets have drawn lots of attentions for potential applications at 500 °C. Usually, 2:17-type SmCo magnets are prepared by powder metallurgy. In this letter, 2:17-type SmCo quasi-single-crystal magnet of Sm(CobalFe0.1Cu0.1Zr0.033)6.93 with [001] orientation is reported, prepared by zone melting directionally solidification. The domain structure and excellent intrinsic magnetic properties of the quasi-single-crystal magnet are investigated. The quasi-single-crystal magnet exhibits a high performance with coercivity of 3.5 T, remanence of 9.5 kG, and energy density of 21.7 MG Oe at room temperature. The coercivity of 1.1 T at 500 °C is achieved, which is the highest value compared with the previously reported SmCo high temperature magnets.

The effect of hydrogen charging on Ln-based amorphous materials

In present work, the effects of hydrogen charging on Ln-based (Ln=La,Ce) bulk-metallic glasses (BMG) are studied. The (La0.5Ce0.5)65Al10Co25 were charged with hydrogen by an electrochemical method in an alkali solution. The hydrogen concentration in the sample after a 36 h charge can reach as high as 1286 w-ppm. With the presence of hydrogen atoms, the hardness of specimen increased by 80% compared to the as-cast samples. The structural evolution of the amorphous matrix due to the hydrogen-uptake process was investigated by the high-energy x-ray scattering technique. The sample surface was crystallized after hydrogen charging. X-ray diffraction measurements revealed broad crystalline peaks superimposed on an amorphous-scattering pattern. The crystalline phase grew from the surface to at least one hundred microns deep into the amorphous matrix. The atomic arrangements of both amorphous and crystalline phases were characterized by the atomic pair-distribution function.

Evolution of phase and microstructure in anisotropic nanocrystalline SmCo6.1Si0.9 magnets

The evolution of phase and microstructure has been investigated for the spark plasma sintered anisotropic nanocrystalline SmCo6.1Si0.9 magnets. With the increasing of sintering temperature from room temperature to 680 °C, the phase evolves from 1:7H (TbCu7-type) single phase to 1:7H dominant phase plus 2:17 R (Th2Zn17-type) minor phase, and then to 2:17 R dominant phase with phase-transformation twins structure plus minor 1:5 H (CaCu5-type) precipitated phase with particulate morphology. The evolution of phase and microstructure in anisotropic nanocrystalline SmCo6.1Si0.9 alloys, as well as its effect on the coercivity is discussed and compared with that in nanostructured 2:17 type SmCo alloys.