Huan-Rong Wang

Crystallization kinetics of an amorphous Zr–Cu–Ni alloy: calculation of the activation energy

Abstract The activation energies of amorphous Zr–Cu–Ni alloy under continuous heating conditions have been calculated using different methods based on differential scanning calorimetry (DSC) data. The activation energy for crystallization of amorphous Zr 70 Cu 20 Ni 10 alloy are determined as 313, 411, 293, 253 and 286 kJ/mol by means of the Kissinger, Ozawa, Cheng, Gao and Wang and Matusita equations, respectively. However, under isothermal crystallization the average value of activation energy of amorphous Zr 70 Cu 20 Ni 10 alloy is 355 kJ/mol. As for the amorphous Zr 70 Cu 20 Ni 10 alloy, the Ozawa equation gives the largest value of activation energy and the Gao and Wang equation shows the smallest one. The difference in the activation energy for crystallization of an amorphous alloy may be attributed to the different approximations in these models.

Primary crystallization in rapidly solidified Zr70Cu20Ni10 alloy from a supercooled liquid region

Abstract The primary crystallization of amorphous Zr70Cu20Ni10 alloy has been investigated by employing the differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). It is interesting to note that the face-centered cubic Zr2Ni (fcc-Zr2Ni) phase precipitates first from the amorphous matrix when the amorphous Zr70Cu20Ni10 alloy is annealed at the supercooled liquid region. At the late crystallization stage, the Zr2Cu particles begins to precipitate. It is thought that at the initial stage the amorphous Zr70Cu20Ni10 alloy undergoes a phase decomposition process, accompanying with a compositional change, which is regarded as a preparation for the precipitation of the fcc-Zr2Ni particles. The whole crystallization process of amorphous Zr70Cu20Ni10 alloy can be divided into two parts, i.e., the interface-controlled transient nucleation and growth process to a critical size at the early stage, and the diffusion-controlled grain growth process at the late stage. This behavior has been analyzed from the viewpoints of thermodynamics and kinetics.

Crystallization processes in amorphous Zr54Cu46 alloy

Abstract The crystallization of amorphous Zr54Cu46 alloy was investigated by using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques. The experimental results show that an endothermic peak in DSC traces for amorphous Zr54Cu46 alloy exists at about 1006 K, indicating following eutectoid reaction occurs, namely, Cu10Zr7+CuZr2↔CuZr in amorphous Zr54Cu46 alloy during heating. With increasing the heating rate, the glass transition temperature Tg and onset crystallization temperature Tx of amorphous Zr54Cu46 alloy increase in parallel, and the supercooled liquid region ΔTx (=Tx−Tg) holds almost constant with an average value of 44 K. Both XRD and TEM results prove that Cu10Zr7 and CuZr2 are main crystallization products for amorphous Zr54Cu46 alloy under continuous heating conditions. No CuZr phase is identified because of its small precipitation amount. Finally, the crystallization processes of amorphous Zr54Cu46 alloy were summarized.

High stability of Zr2Ni nanocrystals in metallic Zr-Cu-Ni glass

Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed to investigate the origin of high stability of the fcc-Zr2Ni nanocrystals during crystallization of metallic Zr70Cu20Ni10 glass. Isothermal analysis for crystallization kinetics of metallic Zr70Cu20Ni10 glass indicates that the crystallization process is mainly controlled by three-dimensional nucleation and growth with increasing nucleation rate, which has been evidenced by the TEM observations. The fcc-Zr2Ni nanoparticles formed during crystallization of metallic Zr70Cu20Ni10 glass are very stable when annealed in the supercooled liquid region. XRD results seem to illustrate that the grain sizes of the Zr2Ni nanoparticles are independent of the annealing temperature and annealing time. Ni plays an important role for the stability of the fcc-Zr2Ni nanocrystals. The origin of high stability of the fcc-Zr2Ni nanoparticles during crystallization of metallic Zr70Cu20Ni10 glass is also analyzed.

Effect of Ni content on crystallization of metallic Zr–Cu–Ni glass

Abstract Employing the X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques, the effect of Ni content on crystallization behavior of metallic Zr 70 Cu 30− x Ni x ( x =5, 10 and 15) glasses has been investigated. It is found that all the DSC traces of metallic Zr 70 Cu 30− x Ni x ( x =5, 10 and 15) glasses display two exothermic peaks, indicating that the crystallization processes of metallic Zr 70 Cu 30− x Ni x ( x =5, 10 and 15) glasses proceed via a double stage mode. It is observed that the two exothermic peaks in the DSC scans of metallic Zr 70 Cu 25 Ni 5 and Zr 70 Cu 15 Ni 15 glasses overlap, implying that the crystallization processes of these two metallic glasses proceed through the almost simultaneous precipitation of crystalline phases. As for the metallic Zr 70 Cu 20 Ni 10 glass, the exothermic peaks in the DSC curves are more separated than those of the metallic Zr 70 Cu 25 Ni 5 and Zr 70 Cu 15 Ni 15 glasses, indicating that the crystallization products of metallic Zr 70 Cu 20 Ni 10 glass precipitate in sequence. To explain these experimental results, a micromechanism for crystallization of metallic Zr 70 Cu 30− x Ni x ( x =5, 10 and 15) glasses is suggested. In addition, the origin of the high stability of the f.c.c.-Zr 2 Ni phase is also interpreted from the viewpoint of local activation energy for crystallization.

Formation of nanocrystals in metallic Zr-Cu-Ni glass

Abstract Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) are employed to investigate the crystallization behavior in metallic Zr70Cu20Ni10 glass during continuous heating. It is found that the DSC curves of metallic Zr70Cu20Ni10 glass exhibit two exothermic peaks, indicating that the crystallization process proceeds via a double stage mode. TEM microstructural analysis confirms that the first exothermic reaction in the DSC traces of metallic Zr70Cu20Ni10 glass mainly corresponds to the precipitation of the Zr2Cu phase, while the second one corresponds to the formation of nano-scale Zr2Ni particles. Ni plays an important role in the stability of the nanocrystals, and its effect is also discussed.

RESrAlO4 (RE=Nd and La): a new type of perovskite ceramic substrate for Bi–Sr–Ca–Cu–O superconducting thick films

Abstract A suitable new type of substrate RESrAlO 4 (RE=Nd and La) for making Bi2212 thick films has been synthesized by solid state reaction. X-ray diffraction indicated that both compounds are single phase when synthesized at 1350–1500°C. The compounds are both of tetragonal structure and have high melting points together with low dielectric constants and energy loss. They are chemically compatible with Bi2212. Strongly c -axis oriented Bi2212 thick films with a T co of 85 K were successfully grown on the polycrystalline substrates. RESrAlO 4 is suitable for use as a substrate material for growing Bi–Sr–Ca–Cu–O (BSCCO) thick films for microwave devices.

HRTEM study of crystallization of Zr-Cu-Ni metallic glass

Abstract Employing differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy (HRTEM), the micromechanism for crystallization of Zr 70 Cu 20 Ni 10 metallic glass under isothermal annealing conditions has been investigated. It is found that the relationship between the annealing temperature and the peak position, incubation time and ending time in the isothermal annealing DSC traces of Zr 70 Cu 20 Ni 10 metallic glass obeys a first-order exponential function. However, the time–temperature transformation curves of Zr 70 Cu 20 Ni 10 metallic glass at different crystallized volume fractions can be well fitted by a second-order exponential function. It is observed that at the initial crystallization stage some ordered atomic clusters precipitate first, acting as nucleation sites and facilitating the subsequent crystallization process, and the crystal growth process mainly proceeds through the atomic depositing on the previously formed crystals. This behavior confirms that the new micromechanism for crystallization of amorphous alloys proposed by Lu and Wang can also be applied to the new series of zirconium based amorphous alloys.

Effect of cooling rate on crystallization of metallic Zr-Cu-Ni glass

Abstract X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were employed to investigate the effect of cooling rate on crystallization behavior of metallic Zr70Cu20Ni10 glass. It is found that all DSC traces of the metallic Zr70Cu20Ni10 glasses under different cooling rates exhibit two exothermic peaks, indicating that the crystallization of metallic Zr70Cu20Ni10 glass proceeds through a double-stage mode. In our previous studies, we have concluded that the first exothermic reaction mainly corresponds to the precipitation of the Zr2Cu phase, and the second one is mainly due to the formation of nanoscale Zr2Ni particles. It is observed that there exists a close relationship between the cooling rate and thermodynamic parameters of metallic Zr70Cu20Ni10 glass, such as the onset crystallization temperature Tx, the first peak temperature Tp1 and the second one Tp2. The above three thermodynamic parameters reach a maximum when the surface velocity is 30 m/s. This effect is just similar to that of the Ni concentration, which has been discussed in our previous works. The activation energy for crystallization and the local Avrami exponent of metallic Zr–Cu–Ni glass under isothermal annealing conditions also exhibit a similar tendency with the cooling rate.

Effect of Al on the Glass Forming Ability of Zr-Ni-Cu-Al Alloys

Abstract Employing X-ray diffraction, scanning electron microscopy and differential scanning calorimetry techniques, the effect of the aluminium concentration on the glass forming ability (GFA) of Zr65Ni10Cu25-xAlx alloys were systematically studied. The experimental results show that with increasing Al concentration the supercooled liquid region of the as-quenched ribbons of Zr65Ni10Cu25-xAlx alloys increases and the critical cooling rate required to avoid crystallization during the rapid solidification for these alloys decreases, indicating that the GFA of Zr65Ni10Cu25-xAlx alloys is significantly improved by the addition of Al. The effect of Al on the GFA of Zr65Ni10Cu25-xAlx alloys is discussed from the micromechanism point of view.