Crystallization kinetics of Zr-33 at % Ni amorphous alloy

The amorphous to crystalline transition in Zr-33 at% Ni amorphous alloy has been found to occur by polymorphic crystallization. The product of the cyrstallization process has been identified as the equilibrium Zr₂Ni intermetallic phase. The kinetics of crystallization have been studied independently using differential scanning calorimetry (DSC) and transmission electron microscopy. The activation energy of crystallization has been evaluated by isothermal and continuous heating in DSC. The isothermal anneals have revealed that the crystallization follows the Johnson-Mehl-Avrami kinetics with an Avrami exponent close to 4. The microstructural changes accompanying crystallization have been studied for an interpretation of the Avrami exponent. The nucleation and growth rates of crystals have been estimated at different temperatures in order to determine the activation energies of the two processes. It has been found that nucleation is thermally activated and growth is interface controlled.     

Co-Finemet非晶合金晶化动力学及其纳米晶粉芯的磁性

利用单辊熔体快淬法在大气环境中制备了Fe73.5-xCoxSi13.5B9Cu1Nb3(x=10,30,50)非晶薄带,利用差示扫描量热法(DSC)研究了非晶薄带的晶化动力学行为。采用Kissinger和Ozawa方法计算了非晶薄带的晶化表观激活能,计算结果表明:随着Co含量的增加,一次晶化的表观激活能降低而二次晶化的表观激活能升高。利用Johnson-Mehl-Avrami(JMA)模型计算了非晶薄带一次晶化的局域Avrami指数m,计算结果表明非晶薄带一次晶化的机理在不同的晶化阶段是不一样的,晶化初期为扩散控制的三维形核和晶粒生长的整体晶化,晶化中后期为一维形核和晶粒生长的表面晶化,形核率近似为零。研究了Fe63.5Co10Si13.5B9Cu1Nb3纳米晶粉芯的磁性与球磨时间之间的关系,结果表明:纳米晶粉芯的有效磁导率表现出较好的频率稳定性,而且随着球磨时间的增加而减小,品质因子在低频范围内随着频率的增加而增加,在约80 kHz达到峰值,然后随着频率的进一步增加而逐渐减小。     

Electron transport study of nanocrystallization in FeSiB based alloys

The influence of Cu, Nb and Ta additions on the crystallization of amorphous FeSiB based alloys was studied. For each alloy, coherent electrical resistivity and thermoelectric power behaviors were observed and related to the mean grain sizes. Anomalous electron transport parameters strongly influenced by the electron scattering at grain boundaries were explained using the Mayadas-Shatzkes model. From values of the local Avrami exponent, the nucleation rate and the growth kinetics were determined. The influence of relaxation processes on Avrami exponent was investigated.     

非晶软磁合金Co_(65)Fe_4Ni_2Si_(15)B_(14)的纳米晶化动力学研究

采用差示扫描量热分析(DSC)和X射线衍射技术(XRD)研究了非晶态合金Co_(65)Fe_4Ni_2Si_(15)B_(14)的非等温晶化动力学.结果表明,初始晶化的晶化峰值温度T_p与升温速率β呈线性关系:T_p=11.49lnβ+795.43.采用Kissinger和Doyle-Ozawa方法计算了表观晶化激活能E_a,分别为471.68kJ/mol和461.50kJ/mol.进一步研究发现,该非晶合金的晶化为多阶段的连续形核直至饱和的过程;当进入稳定晶化阶段时,剩余非晶的局域晶化激活能逐渐下降,非晶基体的热稳定性降低,这是由B原子的高温扩散导致的.同时,局域Avrami指数n(α)也反映了不同晶化阶段的形核长大机制.     

Mg-Zn-Ca-Ti非晶合金的非等温晶化动力学

采用机械合金化的方法制备Mg_(48)Zn_(40)Ca_5Ti_7和Mg_(58)Zn_(30)Ca_5Ti_7非晶合金。利用X射线衍射仪(XRD)对其物相进行检测;结合差示扫描量热分析方法(DSC),对试样在不同升温速率下的非等温晶化进行研究。结果表明:Mg_(48)Zn_(40)Ca_5Ti_7和Mg_(58)Zn_(30)Ca_5Ti_7基本形成非晶;随着加热速率的增加,合金结晶峰均向更高温度的方向移动;用Kissinger、Ozawa和Augis-Bennett方法分别计算出Mg_(48)Zn_(40)Ca_5Ti_7和Mg_(58)Zn_(30)Ca_5Ti_7非晶合金的表观激活能Eα,发现Mg_(48)Zn_(40)Ca_5Ti_7非晶合金的Eα在250kJ·mol~(-1)~270kJ·mol~(-1)范围内,Mg_(58)Zn_(30)Ca_5Ti_7非晶合金的Eα在180kJ·mol~(-1)~200kJ·mol~(-1)范围内;Mg_(48)Zn_(40)Ca_5Ti_7非晶合金的Avrami指数n在不同升温速率下均在1左右;Mg_(58)Zn_(30)Ca_5Ti_7非晶合金的Avrami指数n随着升温速率的增加,由2.7减小到1.9。     

Crystallization kinetic studies of CaBi<Subscript>2</Subscript>B<Subscript>2</Subscript>O<Subscript>7</Subscript> glasses by non-isothermal methods

Transparent glasses of CaBi₂B₂O₇ (CBBO) were fabricated via the conventional melt-quenching technique. The amorphous and the glassy nature of the as-quenched samples were, respectively, confirmed by X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC). The glass transition (T _g) and the crystallization parameters (crystallization activation energy (E _cr) and Avrami exponent (n)) were evaluated under non-isothermal conditions using DSC. The heating rate dependent glass transition and the crystallization temperatures were rationalized by Lasocka equation for the as-quenched CBBO glasses. There was a close agreement between the activation energies for the crystallization process determined by Augis and Bennet and Kissinger methods. The variation of local activation energy (E _c(x)) that was determined by Ozawa method increased with the fraction of crystallization (x). The Avrami exponent (n(x)) decreased with the increase in fraction of crystallization (x), suggesting that there was a changeover in the crystallization process from the bulk to the surface.     

Isothermal crystallization kinetics of the Ni63Cr18Si13B6 alloy

Isothermal crystallization kinetics of the Ni₆₃Cr₁₈B₁₃Si₆ alloy was carried out by differential scanning calorimetry. The JMA approach was used and the mean Avrami exponent values obtained were found to be consistent with a primary precipitation and a polymorphic crystallization, respectively, for the two crystallization stages. The novel concept of the local value of the Avrami exponent was also applied. The n _loc–x plot was seen to be independent of the temperature of isothermal crystallization for the two stages, indicating that the crystallization processes depend only on the volume fraction transformed. Finally, the time-scaling law in the kinetic plots was verified.     

Crystallization study of amorphous Fe40Ni40B20 by electrical resistivity measurement

The crystallization in amorphous Fe₄₀Ni₄₀B₂₀ alloy has been studied by electrical resistivity measurement. It shows a three stage compared to the more conventional two-stage crystallization behaviour in many metallic glasses. The Johnson-Mehl-Avrami equation was found to be operative only for 40% transformed crystallized volume for the highest measured isotherm (740 K). The Avrami exponent and activation energy were found to be 1.75 and 53 kcal mol^–1, respectively. The low activation energy in the amorphous alloy has been explained by the structural relaxation model.     

Influence of seeding on crystallization behaviour of BaNaB<Subscript>9</Subscript>O<Subscript>15</Subscript> glasses

Transparent BaNaB₉O₁₅ (BNBO) glasses were fabricated via the conventional melt-quenching technique. X-ray powder diffraction (XRD) followed by differential scanning calorimetric (DSC) studies confirmed the amorphous and glassy nature of the as-quenched samples, respectively. The effect of seeding on the crystallization of BNBO glasses was studied by non-isothermal DSC method and was modeled using the Johnson-Mehl-Avrami and Ozawa equations. The activation energy for seeded glasses decreased with the increase in fraction of crystallization. The values for the onset of crystallization and Avrami exponent were found to be lower for seeded samples which were associated with the heterogeneous nucleation and epitaxial processes.     

Influence of seeding on crystallization behaviour of BaNaB<Subscript>9</Subscript>O<Subscript>15</Subscript> glasses

Transparent BaNaB₉O₁₅ (BNBO) glasses were fabricated via the conventional melt-quenching technique. X-ray powder diffraction (XRD) followed by differential scanning calorimetric (DSC) studies confirmed the amorphous and glassy nature of the as-quenched samples, respectively. The effect of seeding on the crystallization of BNBO glasses was studied by non-isothermal DSC method and was modeled using the Johnson-Mehl-Avrami and Ozawa equations. The activation energy for seeded glasses decreased with the increase in fraction of crystallization. The values for the onset of crystallization and Avrami exponent were found to be lower for seeded samples which were associated with the heterogeneous nucleation and epitaxial processes.     

Fe45Co7Cr15Mo10Y2B6C15大块非晶合金的晶化动力学研究

为了明确Fe基块体非晶材料的晶化过程,通过铜模吸铸法制备了Fe45Co7Cr15Mo10Y2B6C15块体非晶合金,采用连续和等温差热扫描量热法对非晶合金在1 000 K以下的晶化动力过程进行了研究.研究表明:在973 K时效后的晶化产物为铁的碳化物;晶化过程的激活能达到了555.9 kJ/mol,非晶热稳定性良好;Avrami常数n值接近2,随着退火温度的变化,n值没有明显的变化,非晶晶化机理没有发生改变,而放热峰宽随着退火温度的降低明显宽化,说明非晶晶化过程是一个二维扩散控制的形核与长大过程.     

On the crystallization behavior of amorphous Fe-Cr-Mo-B-P-Si-C powder prepared by mechanical alloying

The crystallization behavior and thermal stability of Fe-Cr-Mo-B-P-Si-C amorphous alloy, prepared by mechanical alloying (MA), were investigated by using differential scanning calorimetry (DSC). One exothermic peak was observed on DSC traces, implying that the crystallization process undergoes only one stage. The crystallization kinetic parameters, including activation energy (E_a), Avrami exponent (n) were determined with non-isothermal analysis method based on the DSC data. The results suggest that the crystallization mechanism is governed dominantly by a three-dimensional diffusion-controlled growth. In addition a relatively high value of activation energy of crystallization (386.04 ± 10 kJ/mol) was found, indicating that this amorphous alloy has high thermal stability.     

Crystallization Study of Cu_(56)Zr_7Ti_(37) Metallic Glass by Electrical Resistivity Measurement

In this paper,the crystallization behaviour of amorphous Cu56Zr7Ti37 alloy using thermal electrical resistivity(TER) and differential scanning calorimetry(DSC) studies has been described.Isochronal TER and DSC measurements indicate that crystallization occurs in two stages.Isothermal crystallization studies of the alloy by TER show that the kinetics conforms to Johnson-Mehl-Avrami model.Avrami exponents derived from kinetics,between 1.1 and 1.2,imply that the crystallization processes are diffusion controlled with near zero nucleation.Activation energy has been found to increase with the transformed volume fraction.A plausible explanation has been presented by separating the contributions due to nucleation and crystal growth towards total activation energy.     

Crystallization kinetics of magnetron-sputtered amorphous CoNbZr thin films

For non-isothermal and isothermal annealing, the crystallization kinetics of magnetron sputtered Co_85.5Nb_8.9Zr_5.6 amorphous alloy thin films have been investigated by differential scanning calorimetry measurements. As a result, in the case of non-isothermal crystallization, one distinct exothermic peak is observed at 470 °C, which is due to the crystallization of hcp α-Co. With the Kissinger method, the apparent activation energy was obtained to be 99.82 kJ/mol. By using the Deloy-Ozawa method, the local activation energy of non-isothermal crystallization was calculated. For isothermal crystallization, the Avrami exponents were determined by means of the Johson-Mehl-Avrami equation, which is in the range of 1.19-1.37. Based on an Arrhenius relationship, the local activation energy was analyzed, which yields an average value E_c=88.51 kJ/mol. Finally, the local Avrami exponent was used for discussing the details of the nucleation and growth behaviour during the isothermal crystallization.     

非晶态合金条带高压复合法制备大块非晶态合金

介绍一种制备大块非晶态合金的新方法－快淬非晶态合金和条带高压复合法，其原理是利用高压抑制非晶态合金的晶化过程，从而在较宽的温区通过大量均匀的粘滞流变实现快淬非晶态合金条带的全致密复合。     

原位交联改性PP/POE共混物的等温结晶行为

采用热塑性聚烯烃弹性体(POE)作为增韧材料,以过氧化二异丙苯(DCP)为交联剂制备了PP/POE共混物。利用差示扫描量热仪(DSC)和偏光显微镜(PLM)研究了PP/POE共混物等温结晶行为,并用Avrami方程分析了PP/POE共混物结晶动力学。结果表明,POE的加入能起到异相成核的作用,加快了PP的结晶成核速率。Avrami指数在1.93～3.09,DCP的引入对聚丙烯(PP)的成核与生长机理影响不大,但其原位交联作用阻碍了PP分子链的运动,使得PP结晶速率降低,结晶时间延长。     

The kinetics of isothermal cold crystallization and tensile properties of poly(ethylene terephthalate)

The exothermic peak that is frequently observed during the heating scan of a differential scanning calorimetry (DSC) experiment of poly(ethylene terephthalate) (PET) is due to a cold crystallization process, originating from the rearrangement of amorphous regions into a crystalline phase. In this work the isothermal cold crystallization kinetics of PET was investigated by using DSC, X-ray diffraction and tensile experiments. The isothermal crystallization rate was determined as a function of temperature, and the Avrami analysis was conducted. The results showed that at low temperatures the cold crystallization is a two-regime process, whereas at high temperatures just one stage is observed. The rate constant for isothermal crystallization K increased and the half time of crystallization (t_½) decreased with increasing crystallization temperature. The Avrami exponent n was close to 2, and this corresponds to a disc-like morphology formed by heterogeneous nucleation. Cold crystallization increased the crystallinity and therefore the tensile properties of the samples were enhanced.     

Crystallization kinetics of orthorhombic paracetamol from supercooled melts studied by non-isothermal DSC

A simple and highly reproducible procedure was established for the study of orthorhombic paracetamol crystallization kinetics, comprising melting, quench-cooling of the melt and scanning the formed glass by DSC at different heating rates. Results were analyzed on the basis of the mean as well as local values of the Avrami exponent, n, the energy of activation, as well as the ?esták–Berggren two-parameter autocatalytic kinetic model. The mean value of the Avrami kinetic exponent, n, ranged between 3 and 5, indicating deviation from the nucleation and growth mechanism underlying the Johnson–Mehl, Avrami–Kolmogorov (JMAK) model. To verify the extent of the deviation, local values of the Avrami exponent as a function of the volume fraction transformed were calculated. Inspection of the local exponent values indicates that the crystallization mechanism changes over time, possibly reflecting the uncertainty of crystallization onset, instability of nucleation due to an autocatalytic effect of the crystalline phase, and growth anisotropy due to impingement of spherulites in the last stages of crystallization. The apparent energy of activation, E_a, has a rather low mean value, close to 81?kJ/mol, which is in agreement with the observed instability of glassy-state paracetamol. Isoconversional methods revealed that E_a tends to decrease with the volume fraction transformed, possibly because of the different energy demands of nucleation and growth. The exponents of the ?esták–Berggren two-parameter model showed that the crystallized fraction influences the process, confirming the complexity of the crystallization mechanism.     

Crystallization behavior of Ti50Ni25Cu25 amorphous alloy

Crystallization behavior of the Ti₅₀Ni₂₅Cu₂₅ alloy was studied by means of scanning and isothermal differential calorimetry, X-ray diffraction, conventional and high-resolution transmission electron microscopy. A single stage polymorphic-type transformation of the amorphous phase forming a Ti₂CuNi crystalline phase was observed. The activation energy for such a single stage crystallization of the amorphous phase was determined by Kissinger analysis. Kinetics of the crystallization was analyzed on the basis of Johnson-Mehl-Avrami equation and discussed regarding to the value of Avrami exponent obtained. Relatively high average value of Avrami exponent of 5.5 at the range from 702 to 709 K suggests nucleation with an increasing nucleation rate.     

Microstructure formation and electrical resistivity behavior of rapidly solidified Cu-Fe-Zr immiscible alloys

The immiscible Cu-Fe alloy was characterized by a metastable miscibility gap. With the addition element Zr, the miscibility gap can be extended into the Cu-Fe-Zr ternary system. The effect of the atomic ratio of Cu to Fe and Zr content on the behavior of liquid-liquid phase separation was studied. The results show that liquid-liquid phase separation into Cu-rich and Fe-rich liquids took place in the as-quenched Cu-Fe-Zr alloy. A glassy structure with nanoscale phase separation was obtained in the as-quenched(Cu_(0.5) Fe_(0.5))40Zr_(60) alloy sample, exhibiting a homogeneous distribution of glassy Cu-rich nanoparticles in glassy Fe-rich matrix. The microstructural evolution and the competitive mechanism of phase formation in the rapidly solidified Cu-Fe-Zr system were discussed in detail. Moreover, the electrical property of the as-quenched Cu-Fe-Zr alloy samples was examined. It displays an abnormal change of electrical resistivity upon temperature in the nanoscale-phase-separation metallic glass. The crystallization behavior of such metallic glass has been discussed.