Devitrification behaviour of rapidly solidified Al87Ni7Cu3Nd3 amorphous alloy prepared by melt spinning method

Rapidly solidified Al87Ni7Cu3Nd3 amorphous alloy was prepared by using melt spinning. Its calorimetric behavior was characterized by using differential scanning calorimeter in a continuous or isothermal heating mode. phase transformation was investigated, with a special interest in primary crystallization, by using an in-situ examination of X-ray diffractometry （XRD） and high resolution transmission electron microscopy （HRTEM）. The results show that, the whole devitrification of rapidly solidified Al87NiyCu3Nd3 amorphous alloy involves two main processes of primary crystallization and secondary crystallization that consist mainly of two reactions. For primary crystallization, the apparent activation energies, EIso and EKis and growth activation energies Eg are about 153, 166 and 288 kJ/mol, respectively. The interdiffusion of Al atoms is a rate-controlled step of formation of the a（Al） particles, but slow diffusion of Ni and Nd atoms plays a significant role in retarding growth of the α （Al） particles. For secondary crystallization, EIso, EKis and Eg of the first reaction are about 291,208 and 290 kJ/mol, and those of the second reaction are about 367, 269 and 372 kJ/mol. The two reactions of secondary crystallization are controlled mainly in an interface-controlled three-dimensional mode, depending mainly on slow diffusion of Ni and Nd atoms.     

Microstructure and mechanical properties of rapidly solidified Al-Cr alloys

The microstructure and mechanical properties of rapidly solidified Al-Cr alloys were investigated by XRD, TEM and microhardness testing instrument. The results indicate that the matrix of rapidly solidified Al-Cr alloys is α-Al solid solution when the Cr content is lower than 4 wt%. However, when the Cr content is above 4 wt%, the microstructures of rapidly solidified Al-Cr alloys are different along cross section. The microstructure of alloy contacting copper roller consists of α-Al and a few intermetallic compounds. With the increase of distance from copper roller, the matrix consists of α-Al and spherical intermetallic compounds which conglomerate in α-Al matrix. These intermetallic compounds are Al₇Cr, Al₁₁Cr and Al₄Cr. The tensile strength has the maximal value when the Cr content is about 8 wt%. The annealed microstructures show that supersaturated α-Al solid solution dissolved with increasing anneal temperature. The starting temperature of the second phase precipitated from the supersaturated α-Al solid solution desponds on the supersaturation. Meanwhile, the microhardness of rapidly solidified Al-Cr alloy reaches maximal value after annealing at 300 °C. Funded by the Innovation Fund for Outstanding Scholar of Henan Province (No. 0621000700)     

Crystallization kinetics of amorphous Nd3.6 Pr5.4 Fe83Co3B5 and preparation of α-Fe/Nd2Fe14B nanocomposite magnets by controlled melt-solidification technique

The crystallization kinetics of amorphous Nd3. 6 Pr5.4 Fe83 Co3 B5 and the preparation of α-Fe/Nd2 Fe14 B nanocomposite magnets by controlled melt-solidification of Nd3.6Pr5.4Fe83Co3B5 was investigated by employing DTA, XRD, and TEM. The results show that a metastable intermediate phase Nd8Fe27B24 prior to α-Fe and Nd2 Fe14 B phases is crystallized as the amorphous Nd3.6 Pr5.4 Fe83 Co3 B5 is heated to 1 223 K. The crystallization activation energy of α-Fe and Nd8 Fe27324 phases is larger at the beginning stage of crystallization, and then it decreases with crystallized fraction x for the former and has little change when x is below 70% for the latter, which essentially results in an α-Fe/Nd2 Fe14 B microstructure with a relatively coarse grain size about 20-60 nm and a non-uniform distribution of grain size in the annealed alloy. The a-Fe/Nd2 Fe14 B nanocomposite magnets with a small average grain size about 14 nm and a quite uniform grain size distribution were prepared by controlled melt-solidification of nealing the amorphous Nd3. 6 Pr5. 4 Fe83 Co3 B5 precursor alloy.     

Preparation of uitrafine α-Al2O3 powders by catalytic sintering of ammonium aluminum carbonate hydroxide at low temperature

The precursor of ammonium aluminum carbonate hydroxide was synthesized by using aluminum sulfate （Al2 （SO4）3 ） and ammonium carbonate（（NH4 ）2CO3 ）. The effects of α-Al2 O3 seeds and mixture composed of α- Al2O3 and ammonium nitrate, as well as multiplex catalysts （AT） on phase transformation of alumina in sintering process were investigated respectively. The results show that the α-Al2 O3 seeds and the mixture of α-Al2 O3 and ammonium nitrate can lower the phase transformation temperature of α-Al2O3 to different extents while the particles obtained agglomerate heavily. AT has great potential synergistic effects on the phase transformation of alumina and reduces the phase transformation temperature of α-Al2O3 and the trends of necking-formation between particles. Therefore the dispersion of powder particles is improved significantly.     

Si3N4-doped Zr50Al15Ni10Cu25 glassy alloy by mechanical alloying and sintering process

Zr₅₀Al₁₅Ni₁₀Cu₂₅ amorphous powder was synthesized by mechanical alloying. The effect of Si₃N₄ addition on the crystallization behavior of the alloy during sintering process was studied. Thermal stability of the powders was performed by differential scanning calorimetry (DSC). The phase and microstructure of the powders and bulk specimens sintered were determined by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results show that, introducing 0.05% (mass fraction) Si₃N₄ can enhance the crystallization activation energy of the Zr₅₀Al₁₅Ni₁₀Cu₂₅ amorphous powders, which indicates that Si₃N₄ addition has hindrance effect on forming crystals from Zr₅₀Al₁₅Ni₁₀Cu₂₅ amorphous powder. However, 0.10% Si₃N₄ results in the decrease of the crystallization activation energy, which makes its crystallization process easy to occur.     

Effect of cooling rate on the microstructure of ZA48 alloy

The effect of cooling rate on the microstructure of ZA48 alloy was investigated. The alloy was prepared using a relatively simple technique, i e, rapid cooling of the melt in a steel wedge mould. The dependence of microstructure on the cooling rate (about 40 to 103 K/s) was determined by the secondary dendrite arm space size measurement, optical microscopy(OM), and transmission electron microscopy (TEM). It is found that the matrix structure over a large cooling rate is composed of α-Al dendrite and eutectoid (α+η), the size of α-Al dendrite decreases with increasing cooling rate. The relationship between the cooling rate and the secondary dendrite arm space size has been established. TEM shows that a large number of small and dispersed precipitations can be seen in the primary α phase of tip region. Electron diffraction pattern shows that the precipitate phase is Zn3Mg2 phase.     

Cooling Rate Dependence of Structural Order and Energy Landscape in Zr55Cu35Al10 Glass

论冷却速率对Zr55Cu35Al10合金结构和能量的影响

赵静锋,谢祥续,陈锋,狄凯龙,周雪峰*

（常熟理工学院 材料工程学院,江苏 常熟 215500）

中文说明：

经典分子动力学模拟被广泛应用于非晶合金的快速冷却过程研究。然而,模拟冷却速率比实验冷却速率高几个数量级。本文以Zr55Cu35Al10合金为例,发现在略低于玻璃转变温度的温度下加入等温退火,延长等温退火时间可有效降低冷却速率。用这种方法获得的非晶样品具有显著的能量稳定性和更有序的中短程序结构。

关键词：经典分子动力学模拟,冷却速率,等温退火,能量稳定性,短程有序,中程有序

     

Effects of heating rate on the process parameters of superplastic forming for Zr<Subscript>55</Subscript>Cu<Subscript>30</Subscript>Al<Subscript>10</Subscript>Ni<Subscript>5</Subscript>

We investigated the effects of heating rate on the process parameters of superplastic forming for Zr₅₅Cu₃₀Al₁₀Ni₅₅ by differential scanning calorimetry. The continuous heating and isothermal annealing analyses suggested that the temperatures of glass transition and onset crystallization are heating rate-dependent in the supercooled liquid region. Then, the time-temperature-transformation diagram under different heating rates indicates that increasing the heating rate can lead to an increase of the incubation time at the same anneal temperature in the supercooled liquid region. Based on the Arrhenius relationship, we discovered that the incubation time increases by 1.08–1.11 times with double increase of the heating rate at the same anneal temperature, and then verified it by the data of literatures and the experimental results. The obtained curve of the max available incubation time reveals that the incubation time at a certain anneal temperature in the supercooled liquid region is not infinite, and will increase with increasing heating rate until this temperature shifts out of the supercooled liquid region because of exceeding critical heating rate. It is concluded that heating rate must be an important processing parameter of superplastic forming for Zr₅₅Cu₃₀Al₁₀Ni₅.     

Effect of Heat Treatment on Crystallization of Nd:YAG Ceramics

(Nd0.01 Y0.99 )3 Al5 O12 nano-sized powders were synthesized by low temperature combustion (LCS),using Nd2 O3 ,Y2 O3 ,Al(NO3)3 ·9H 2 O,ammonia water and citric acid as starting materials.The powders were characterized by TG-DTA,XRD,FT-IR,ICP and TEM,respectively.The grain sizes were calculated by the Scherrer’s formula using the full width at half maximum(FWHM)of YAG(420)crystal plane diffraction lines. The study focused on crystallization of ceramics at different heat treatment temperatures.The experimental results show that crystallizing temperature of YAG is 850℃,and the intermediate crystal phase YAP,appearing during heat treatment,transforms to YAG cubic crystal phase at the temperature of 1 050℃.The particle size of the powders synthesized by LCS is nano-sized.With the temperature increasing,the mean grain sizes raise,the stand deviations keep almost at the value of 2.0 and the lattice parameters decrease.The grains mainly grow by grain boundary diffusion.The lattice parameter expansion is caused by an increase of the repulsive dipolar interactions on surfaces of crystallites.     

Crystallization kinetics and magnetic properties of Fe73.5Si13.5B9Cu1Nb1V2 nanocrystalline powder cores

Amorphous ribbons of the alloy Fe_73.5Si_13.5B₉Cu₁Nb₁V₂ were prepared by the standard single copper wheel melt spinning technique in the air atmosphere. The crystallization kinetics of amorphous ribbons was analyzed by non-isothermal differential scanning calorimetry (DSC) measurements. The crystallization activation energies of amorphous ribbons calculated by using Kissinger model were 364 and 337 kJ/mol for the first and the second crystallization, respectively. The Avrami exponent n was calculated from the Johnson-Mehl-Avrami (JMA) equation. The value of the Avrami exponent showed that the crystallization mechanism in the non-isothermal primary crystallization of amorphous ribbons was all shapes growing from small dimensions controlled by diffusion at decreasing nucleation rate. The variation of soft magnetic properties of nanocrystalline Fe_73.5Si_13.5B₉Cu₁Nb₁V₂ alloy powder cores as a function of milling times has been investigated. It is found that the effective permeability of the cores shows high frequency stability and decreases with the increase of milling times. The quality factor increases with increasing frequency in lower frequency range, and reaches a maximum at the frequency of 80 kHz then decreases gradually with increasing frequency.     

Application of an analytical phase transformation model

An analytical phase transformation model has been used to study the kinetics of crystallization of amorphous alloys subjected to either isothermal or isochronal anneals. The model has been applied to Mg_82.3Cu_17.7 and Pd₄₀Cu₃₀P₂₀Ni₁₀, employing isothermal and isochronal differential scanning calorimetry. Applying different combinations of nucleation and growth mechanisms to the same experiments, the nucleation and growth modes dominating the crystallization and the values for the corresponding kinetic parameters, including the constant activation energies for nucleation and growth, have been determined. Further, the influence of isothermal pre-annealing on subsequent isochronal crystallization kinetics, involving a gradual change of nucleation mode up to site saturation with increase of pre-annealing, can be analyzed.     

Microstructure refinement and magnetic properties enhancement of nanocrystalline Nd<Subscript>12.3</Subscript>Fe<Subscript>81.7</Subscript>B<Subscript>6.0</Subscript> ribbons by addition of Zr

Effects of Zr addition and annealing treatment on the formation, microstructure and magnetic properties of Nd_12.3Fe_81.7−x Zr _x B_6.0 (x=0−3.0) ribbons melt-spun and annealed have been systematically investigated by means of vibrating sample magnetometer (VSM), differential scanning calorimeter (DSC), X-ray diffraction (XRD), and high resolution scanning electron microscopy (HRSEM). Phase analysis reveals that Nd₂Fe₁₄B is single-phase material. It has been found that the intrinsic coercivity H _ci of the optimally processed Nd_12.3Fe_81.7−x Zr _x B_6.0 ribbons increases monotonically from 751.7 kA/m for x=0 to 1005.3 kA/m for x=3.0. The remanence polarization J _r and maximum energy product (BH)_max increase first with Zr addition, then slightly decrease with further increasing Zr content. Optimum magnetic properties with J _r=1.041 T, H _ci=887.5 kA/m and (BH)_max=175.2 kJ/m³ have been achieved for the ribbons with x=1.5. The significant improvement of magnetic properties originates from the finer grains of the ribbons by introducing Zr.     

Fe3B添加量对Nd-Fe-B快淬薄带结构与磁性能的影响

通过在Nd2Fe14B合金中按Fe3B原子比同时添加Fe与B,研究了Nd15Fe77B8、Nd9Fe81B10、Nd4.5Fe77.0B18.5 3种合金薄带的结构与磁性能.试验发现,40 m/s的快淬速度使三种合金薄带中形成了较多的非晶相,而且随着合金中Fe3B添加量的增加,薄带的非晶形成能力增强、α-Fe与Fe23B6含量增加、Nd2Fe14B含量减少,磁性能单调降低.经680℃保温10 min退火处理后,3种薄带中的非晶相明显减少,Fe3B相消失,.α-Fe与Fe23B6含量随Fe3B添加量的增加而增加.其中,薄带中Nd2Fe14B晶粒约50 ～ 100 nm,而α-Fe晶粒一般小于50 nm.Nd15Fe77B8退火薄带更多地表现为单硬磁相特征;Nd9Fe81B10退火薄带中形成了α-Fe/Nd2Fe14B/Fe23B6三相交换耦合微结构,剩磁比达到了0.612,并获得最大的剩磁58.3 A· m2/kg;而Nd4.5Fe77.0B18.5薄带中Fe23B6与α-Fe含量太高,矫顽力太低而使剩磁增强效应不明显.     

Preparation of Sialon powder from coal gasification slag

X-ray fluorescence spectrometry(XRF),X-ray powder diffractometry(XRD) and scanning electron microscopy(SEM) were used to characterize the chemical composition,phase constituent and microstructure of the coal gasification slag.Sialon powders were synthesized by carbothermal reduction and nitridation using the coal gasification slag as raw materials.The experimental results showed that glass and amorphous carbon were the main phases,quartz and calcite as minor crystalline phases were also presented in porous coal gasification slag.Main constituents of coal gasification slag were SiO2,Al2O3,CaO and residual carbon.Sialon powder with Ca-α-Sialon as main crystalline phase can be synthesized when coal gasification slag powders were reduced and nitrided at 1500 ℃ for 9 h using nitrogen flow of 500 ml/min.The coal gasification slag is a valuable and economic starting material for preparing Sialon powders.     

Effects of Ta addition on the microstructure and mechanical properties of Ti40Zr25Ni8Cu9Be18 amorphous alloy

The effects of Ta addition on the microstructure and mechanical properties of Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ bulk amorphous alloy were investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), scan electron microscopy (SEM) and compressive testing. As a result, the addition of Ta (0-8at%) prompted the successive precipitation of quasicrystalline phase, CuTi₂ phase and bcc β-Ti solid solution. Additionally, the addition of less Ta content (3at%-5at%) led to the formation of amorphous matrix/nanoquasicrystal/CuTi₂ complex phase structure; and nanoquasicrystals, as reinforcement precipitates, improved the fracture strength of Ti-Zr-Ni-Cu-Be-Ta alloys, which led to the high compressive fracture strength 1856 MPa of Ta5 alloy. With increasing Ta content (5at%-8at%), although the ductile dendritic β-Ti solid solution was precipitated, the strength and plasticity decreased to a great extent resulting from the growth of quasicrystalline phase and CuTi₂ phase.     

Study on the isothermal oxidation behavior in air of Ti3AlC2 sintered by hot pressing

The isothermal oxidation behavior at 900–1300°C for 20 h in air of bulk Ti₃AlC₂ with 2.8 wt% TiC sintered by means of hot pressing was investigated in the work. The isothermal oxidation behavior generally followed a parabolic rate law. The parabolic rate constants increased from 1.39×10⁻¹⁰ kg²·m⁻⁴·s⁻¹ at 900°C to 5.56×10⁻⁹ kg²·m⁻⁴·s⁻¹ at 1300°C. The calculated activation energy was 136.45 kJ/mol. It was demonstrated that Ti₃AlC₂ had excellent oxidation resistance due to the continuous, dense and adhesive protect scales consisted of a mass of α-Al₂O₃ and a little of TiO₂ and/or Al₂TiO₅. In principle, the oxide scale was grown by the inward diffusion of O²⁻ and the outward diffusion of Ti⁴⁺ and Al³⁺. The rapid outward diffusion of cations usually resulted in the formation of cracks, gaps, and holes.     

Structure and Giant Magnetoresistance in Zinc Ferrite

The tunneling structure of ZnFe3-xO4 ferrite was confirmed by high resolation electron microscopy （HREM）, non-Ohmic Ⅰ-Ⅴ curve and kinetics for α-Fe2O3 isothermal phase transformation. The Zn0.41 Fe2.59O4/ α-Fe2O3 two-phase polycrystalline has a huge tunneliug magnetorsistance （ TMR ） mainly caused by the tummeliug structure. The Zn0.41Fe2.59O4 grains are separated by insulatiug α-Fe2O3 thin layer boundaries. The pattern of nanostructure uns verified by HREM.     

AFM research on Fe-based nanocrystal crystallization mechanism

The cross-section pattern of Fe-based alloy ribbon (Fe_73.5Cu₁Nb₃Si_13.5B₉) annealed at different temperatures was investigated by AFM (atomic force microscope), and the effect mechanism of Nb and Cu in Fe-based alloy ribbon annealing was analyzed with XRD diffraction crystal analysis technique and other research results. New concepts of encapsulated grain, Nb vacancy cluster, Nb-B atom cluster and so on were proposed and used to describe the formation mechanism of α-Fe (Si) nanocrystal. Finally, a three-phase (separation phase, encapsulated phase and nanocrystalline phase) interconnected structure model in Fe-based nanocrystalline alloy was established. Supported by the Natural Science Foundation of Zhejiang Province (Grant No. Y405021), Zhejiang Provincial Science and Technology Key Project (Grant No. 2006C21109) and Key Project of Science and Technology Research of China Ministry of Education (Grant No. 204059)     

Modeling and estimation of the kinetics of seeded solvent-mediated phase transformation in a batch crystallizer

Modeling the kinetics of the preparing process is necessary to produce a product with the appropriate particle properties and minimum production cost.Owing to the lackness of crystal size distributor (CSD) information,however,solvent-mediated phase transformation encounters difficulty in modeling the kinetics as compared to solution crystallization.Consequently,a model was established by making the product CSD to move along by horizontal translation to obtain the CSDs of the stable phase in the process of transformation.Then the moment method was used to solve the popular balance equation,and the least square nonlinear regression method was applied to estimate the kinetics parameters.The model has been successfully used to simulate the transformation of CaSO4?2H2O to α-CaSO4?1/2H2O in an isothermal seeded batch crystallizer with different stirring speeds,and it is beneficial to producing high performance α-CaSO4?1/2H2O crystals which have the right particle characteristics.     

Influence of cerium on microstructures and mechanical properties of Al-Zn-Mg-Cu alloys

Effect of element cerium (Ce) on microstructure and mechanical properties of Al-Zn-Mg-Cu alloys has been investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and hardness test. The results show that addition of Ce can remarkably refine the as-cast grains and eutectic microstructure. A transformation from Mg(Zn,Cu,Al)₂ phase to Al₂CuMg phase is observed during homogenization. Furthermore, the Ce addition introduces changes in the precipitation process and consequently in the age-hardening behavior of the alloy. Microstructural measurements reveal that the addition of Ce promotes the precipitation of η′ phase, but it also partly retards the precipitation of GP zones. The density of precipitates decreases in a certain degree and rod-like η′ precipitates increase when Ce content is from 0.2% to 0.4% (mass fraction).