Zhizhi Wang

Different solidification behaviours of Bi–10wt%Te alloy induced by liquid structural change

By means of DC four-probe technique, the temperature dependence of the electrical resistivity (ρ–T) of liquid Bi–10wt%Te alloy has been measured. Two abnormal changes on the ρ–T curve within the temperature ranges of 497–550°C and 639–708°C suggested that two irreversible liquid–liquid structural changes (LLSC) occurred. To explore the effects of the LLSC on the solidification behaviour of the alloy, solidification experiments were carried out. The results show that after experiencing LLSC, there will be enlarged undercooling, much finer microstructures and larger quality of Bi2Te phase. Moreover, the morphology of solidified Bi2Te is changed from dendrite to equiaxed.

Morphology and mechanical characteristics of monolayer and multilayer Ir coating by double glow plasma

Multilayer iridium coating was manufactured on tungsten carbide substrates by a double glow plasma process. As comparison, monolayer was also produced. The microstructure and morphology were observed using scanning electron microscopy. Grain orientation and phase were determined using X-ray diffraction. The residual stress of the coating was studied by glancing incidence X-ray diffraction. The adhesive force of the coating was measured by a scratch tester. The results showed that both monolayer and multilayer had a polycrystalline phase with a strong (110) reflection. The coating had an excellent adhesion with no evidence of delamination. The adhesive force of the monolayer and multilayer was about 50 and 43 N, respectively. The interfacial reaction between the substrate and the layer occurred and a new WIr phase was found due to the high-temperature deposition process. The residual stress in the monolayer and multilayer was -1.6 and -1.1 GPa, respectively.

Electrical resistivity of Sn–3.5Ag–xBi solder melts

This article reported the temperature dependence of the electrical resistivity (ρ) of liquid Sn–3.5Ag lead-free solder alloy in continuous heating and cooling processes with varying Bi content in the range of 0, 2, 3.5, 5, and 7 wt.% Abnormal transitions can be observed on ρ–T curves, which indicate liquid–liquid structure transition (LLST) occurs in Sn–3.5Ag–xBi melts. Interestingly, unlike the pattern at first heating cycle, the LLST is reversible during subsequent cooling and heating cycles. Sn may play an important role on the reversibility, and Bi has a noticeable influence on the turning temperatures and characteristics during first heating cycle. The transition mechanism is analyzed from the viewpoint of short-range orders.

Investigation on structural instability induced relaxation and crystallization in ZrCuAlNi bulk metallic glass

The nature of internal friction peak associated with the first-order phase transformation remains an unsolved fundamental problem in many fields of science and technology. Here, the internal friction behaviors of crystallization process of Zr55Al10Ni5Cu30 bulk metallic glass were investigated. With increasing temperature, the internal friction spectra exhibit a peak, which shows a crossover from a relaxation behavior to a phase transformation behavior. The combined differential scanning calorimetry results show that the phase transformation behavior at higher temperature is related to the crystallization process of amorphous alloy, while the relaxation behavior at lower temperature is composed of two processes: One originates from the glass transition and the other from the relaxation due to the structural instability before phase transformation. According to the coupling model and the compensation effect, the primitive activation energies for the glass transition and the structural instability before crystallization are about 1.0 eV and 1.4 eV, respectively. Therefore, it is suggested that the mechanism of internal friction peak associated with the first-order phase transformation is due to the structural instability before phase transformation and the sudden change of the relaxation parameters when phase transformation happens.The nature of internal friction peak associated with the first-order phase transformation remains an unsolved fundamental problem in many fields of science and technology. Here, the internal friction behaviors of crystallization process of Zr55Al10Ni5Cu30 bulk metallic glass were investigated. With increasing temperature, the internal friction spectra exhibit a peak, which shows a crossover from a relaxation behavior to a phase transformation behavior. The combined differential scanning calorimetry results show that the phase transformation behavior at higher temperature is related to the crystallization process of amorphous alloy, while the relaxation behavior at lower temperature is composed of two processes: One originates from the glass transition and the other from the relaxation due to the structural instability before phase transformation. According to the coupling model and the compensation effect, the primitive activation energies for the glass transition and the structural instability before cry...

Low-frequency internal friction behaviour of Zr 55 Al 10 Ni 5 Cu 30 metallic glass with different quenching temperatures

The correlation between the internal friction behaviour of Zr55Al10Ni5Cu30 BMG samples and their quenching temperatures was investigated. It was found that, below the glass transition temperature, the activation energy decreased with increasing quenching temperature, but in the surpercooled liquid region the activation energy tended to be enhanced with a further increase in the quenching temperature. Besides, there were both anelastic and viscoelastic relaxation for the amorphous alloys. The anelastic behaviour would change into viscoelastic relaxation easily for the samples prepared at higher temperature.

Structural relaxation of Zr60Al15Ni25 amorphous ribbon: experimental evidence of the electrical resistivity

In order to study the structural relaxation of the Zr60Al15Ni25 amorphous ribbon, the electrical resistivity was experimentally investigated. The changes in the resistivity before glass transition temperature were observed. Two temperature points (423 K and 573 K) were chosen for cyclic heating experiments. The results showed that both irreversibility and reversibility of structural changes existed in amorphous alloys, which were related to the selected temperature and cycle times. Based on the scattering mechanism of electron conduction in metal, the structural defects model was used to explain the changes of electrical resistivity. The sample was in a highly disorder state after experienced thermal cycling of high temperature (573 K). The number and kind of atoms may be changed to increase the crystallisation range.

Exploration of crystallization behavior of Cu_(36)Zr_(48)Al_8Ag_8 amorphous alloy and melt overheating on its GFA and thermal stability by electrical resistivity measurement

The crystallization behavior of Cu36Zr48Al8Ag8 amorphous alloy and the melt overheating on its glass-forming ability(GFA) and thermal stability are explored by the high-temperature four-electrode electrical resistivity method. The results show that this amorphous alloy has a high-temperature crystallization stage which has not been revealed in the previous literatures besides the lowtemperature crystallization stage. The crystallization products of the first stage are AgZr and Cu10Zr7 metastable phases by using differential scanning calorimetry(DSC) and X-ray diffractometry(XRD). They transform into equilibrium phases CuZr2, AlCu2Zr and CuAl2 in the second stage crystallization. On one hand, when the overheating temperature is greater than 430 K, the thermal stability of Cu36Zr48Al8Ag8 amorphous alloy decreases significantly, and the first stage crystallization behavior changes and the quasicrystals precipitate firstly. On the other hand, its glass-forming ability firstly increases and then decreases with the melt overheat increasing.The effect of the melt overheating on the glass-forming ability and the thermal stability of amorphous are analyzed and discussed from two aspects of melt structure homogeneity and the degree of oxidation.