Impact of anisotropic growth on kinetics of solid-state phase transformation

Based on the statistical analysis of blocking effect arising from anisotropic growth,the anisotropic effect on the kinetics of solid-state transformation was investigated.The result shows that the blocking effect leads to the retardation of transformation and then a regular behavior of varying Avrami exponent.Following previous analytical model,the formulations of Avrami exponent and effective activation energy accounting for blocking effect were obtained.The anisotropic effect on the transformation depends on two factors,non-blocking factor γ and blocking scale k,which directly acts on the dimensionality of growth.The effective activation energy is not affected by the anisotropic effect.The evolution of anisotropic effect with the fraction transformed is taken into account,showing that the anisotropic effect is more severe at the middle stage of transformation.

Isochronal and isothermal crystallization in Zr_(55)Cu_(30)Ni_5Al_(10) bulk metallic glass

Non-isothermal crystallization transformation kinetics and isothermal crystallization kinetics in super-cooled liquid region（SLR） in Zr55Cu30Ni5Al10 bulk metallic glasses were studied by differential scanning calorimetry（DSC） and X-ray diffraction（XRD）.In isochronal mode,the average values of activation energy in Zr55Cu30Ni5Al10 bulk metallic glass determined by different models（Kissinger method,Flynn-Wall-Ozawa method and Augis-Bennett method） are in good agreement with each other.In addition,the isothermal transformation kinetics in Zr55Cu30Ni5Al10 bulk metallic glasses was described by the Johnson-Mehl-Avrami（JMA） model.For Zr55Cu30Ni5Al10 bulk metallic glass,the Avrami exponent n ranges from 2.2 to 2.9,indicating that crystallization mechanism in the bulk metallic glass was mainly diffusion-controlled;crystal growth is controlled by long range ordering diffusion in three-dimensional growth during isothermal crystallization process.The average value of activation energy in Zr55Cu30Ni5Al10 bulk metallic glass is 469 kJ/mol in isothermal transformation process.

EFFECT OF PRE-ANNEALING ON CRYSTALLIZATION KINETICS OF Cu-Ti GLASS

The crystallization kinetics of the Cu_(40)Ti_(60) glasses,either as-quenched or pre-annealed,has been studied by differential scanning calorimetry.The crystallization temperature,activation energy for crystallization and Avrami exponent were found to be lowered by the pre-an- nealing.Both activation energies of nucleation and growth of the amorphous CuaoTi_(60) are ob- tained on the basis of experimental results and theoretical analysis.

THE NON—ISOTHERMAL CRYSTALLIZATION KINETICS OF A LIQUID CRYSTALLINE RANDOM COPOLYESTER WITH DIFFERENT COMPOSITIONS

The non-isothermal crystallization kinetics analysis of liquid crystalline random copolyestercomposed of p-oxybenzoate (B) and 2, 6-oxynaphthoate (N) monomers was carried out by meansof differential scanning calorimetry basing on Ziabicki and Jeziorny method. Although the compo-sition of 58/42 B-N copolyester had poorer thermodynamic crystallizability comparing with thoseof 30/70 and 75/25, its kinetic crystallizability Go was slightly larger. This fact was due to thepoorer match of sequences of 58/42 B-N in domains of nematic melt,and therefore better mobilityof chains during crystallization. The Avrami exponents of three composition species were allabout 2. 5, implying two dimensional growth in so-called non-periodic layer crystallites and a mixture of homogeneous and heterogeneous nucleation.

INVESTIGATION ON CRYSTALLIZATION KINETICS OF Pd-Cu-Si GLASS

The crystallization kinetics of Pd-Cu-Si glass was studied by means of diferential scanning calorimetry-Ⅱ.According to Kissinger peak shift methd and Arrhenius equation,the apparent activation energy was calculated.The crystallization kinetics follows Johnson- Mehl-Avrami equation with n=3.0 within 0.15<x<0.85.In isothermal treatment,the concepts of local Avrami exponent and local activation energy have been introduced into Pd-Cu-Si system for understanding the isothermal crystallization process.

Crystallization Kinetics of a High-zirconium-based Glassy Alloy：A DSC Study

The non-isothermal and isothermal crystallization kinetics of Zr_（72.5）Al_（10）Fe_（17.5） glassy alloy was investigated using differential scanning calorimeter（DSC）.Under non-isothermal heating condition,the primary phase in the initial crystallization is Zr_6Al_2Fe phase and the final crystallized products consist of Zr_6Al_2Fe,Zr_2Fe and a-Zr phases.The apparent activation energy for crystallization estimated using the Kissinger method is 342.1 ±8.1 kJ/mol.The local activation energy decreased with the increase in the crystallization volume fraction during nonisothermal crystallization.Under isothermal heating condition,the average Avrami exponent of about 2.76 implies a mainly diffusion-controlled three-dimensional growth with an increasing nucleation rate.The local activation energy for isothermal crystallization shows a different variation trend from that for nonisothermal crystallization,indicating different nucleation-and-growth mechanisms for the two crystallizaiton conditions.

Effects of Nb Alloying on Nano-Crystallization Kinetics of Fe_(55-x)Cr_(18)Mo_7B_(16)C_4Nb_x(x=0, 3) Bulk Amorphous Alloys

Crystallization kinetics of Fe55-xCrzsMo7B10C4Nbx（x= 0, 3） bulk amorphous alloys were analyzed using X-ray diffraction and differential scanning; calorimetric （DSC） tests. In practice, crystallization and growth mechanism were evaluated using DSC tests at four different heating rates （10, 20, 30, and 40 K/rain） and kinetic models. Two-step crystallization behavior was observed when Fe55Cr18MOTB16C4 and Fe52Cr18MoTB16C4Nb3 bulk amorphous alloys were annealed, where Fe36Cr12M010 phase was crystallized in the first step of crystallization. Results show that Fe36Cr12Mo10 and Fe3C phases were crystallized in the structures of the alloys after further annealing： process. Activation energy for the crystallization of Fe36Cr12Mozo phase was measured to be 543 kJ/mol in Fe52Cr18MoTB16C4Nb3 alloy and 375 kJ/mol for Fe55Cr18Mo7B16C4 alloy according to Kissinger-Starink model. Moreover, a two-dimensiona nucleation rate was found in Fe52Cr18Mo7B16C4Nb3 diffusion controlled growth mechanism with decreasing alloy whereas a three-dimensional diffusion controlled growth mechanism with decreasing nucleation rate was found in crystallization of Fe36Crz2Mo10 phase during annealing of Fe55Cr18MoTB16C4 alloy. TEM （transmission electron microscopy） observations reveal that crystalline Fe36Cr12M010 phase nucleated in the structures of the alloys in an average size of 10 nm with completely mottled morphology.

Nanocrystallization of α-Fe Crystals in Fe_(52)Cr_(18)Mo_7B_(16)C_4Nb_3 Bulk Amorphous Alloy

Crystallization kinetics of Fe52Cr18Mo7B16C4Nb3 alloy was evaluated by X-ray diffraction（XRD）,differential scanning calorimetric（DSC） tests and transmission electron microscopy（TEM） observations in this research work.In effect,crystallization and growth mechanism were investigated by using DSC tests at four different heating rates（10,20,30,40 K/min）.Results showed that a two-step crystallization process occurred in the alloy in which α-Fe and Fe3B phases were crystallized,respectively in the structure after heat treatment.Activation energy for the first step of crystallization,i.e.α-Fe was measured to be 421 and 442 kJ/mol according to Kissinger-Starink and Ozawa models,respectively.Further,Avrami exponent calculated from DSC curves was 1.6 and a two-dimensional diffusion controlled growth mechanism with decreasing nucleation rate was observed in the alloy.Moreover,it was known from the TEM observations that crystalline α-Fe phase nucleated in the structure of the alloy in an average size of 10 nm and completely mottled morphology.

Phase precipitation and isothermal crystallization kinetics of FeZrB amorphous alloy

The crystallization process of Fe78ZrTBls （at%） amorphous ribbon was investigated by Xray diffraction （XRD）, differential scanning calorimetry and scanning electron microscopy （SEM）. The fully amorphous structure of asquenched （Aq） ribbons was confirmed by XRD pattern. The saturation magnetization （Ms） and Curie tem perature of the Aq ribbon were measured as 124.3 （A.mZ）＆g and 305 ℃ with vibrating sample magnetometer （VSM）, respectively. When the ribbons was annealed at 550 ℃ near the first onset temperature （Txl = 564.9 ℃）, the Ms was increased by 17 %, which was caused by the formation of a dual phase structure. The isothermal crystallization kinetics and crystallization mechanism of primary ctFe phase in the dual phase structure were studied by Arrhenius and JohnsonMehlAvramiKolmogorov equations respectively. The results showed that the crystallization of Fe phase was a diffusioncontrolled surface nucleation growth process, and the nucleation rate decreased with longer crystallization time.

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.

Correlation between mechanical and thermodynamic properties for La-Ce-Ni-Cu-Al high-entropy metallic glasses

The mechanical properties, thermodynamic features and their correlation were studied for La-Ce-Ni-Cu-Al high-entropy bulk metallic glasses （HE-BMGs）. Compressive testing indicated that the HE-BMGs are ductile on a microscopic scale but brittle on a macroscopic scale, because of the low fragility index rn of the HE-BMGs. In the non-isothermal process, the activation energies for glass transition for these HE-BMGs are the lowest of the known HE-BMGs. Large values of the Avrami exponent n imply that the crystallization process proceeded through three-dimensional growth and with an increasing nucleation rate. The activation energy for glass transition （Eg） is almost proportional to the HE-BMG fracture strength, because a higher Eg is required to dislodge the molecules from the glassy configuration for the HE-BMGs with a high strength. The findings provide unambiguous evidence for the correlation between the mechanical and thermodynamic properties.

Isothermal crystallization in Ce_(70)Ga_6Cu_(24) bulk metallic glass

The isothermal crystallization behaviors in a newly developed CeGaCu bulk metallic glass have been investigated through the classic differential scanning calorimeter （DSC） method. It is found that the apparent activation energy （Ea） strongly depends on the fraction （x） of isothermal crystallization. Johnson-Mehl-Avrami （JMA） formula was used to analyze the mechanism of crystallization and the obtained Avrami exponent （n） was discovered to show an obvious correlation with the crystallization fraction x. With the help of the relation between Ea and n, the nucleation and growth activation energies, En and Eg, were estimated to be 214-304 kJ/mol and 91 kJ/mol, respectively. This result suggests that the main energy barrier against crystallization in the present glass should be the nucleation of nucleates, rather than the growth of crystals. Such a large E, is also believed to be responsible for the good glass forming ability of the CeGaCu alloy.