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Zhao Ming Li Jianjun Jiang Qing School of Materials Science Engineering Jilin University of Technology Changchun P. R. China 《中国有色金属学会会刊》1999,(1)
1INTRODUCTIONToexaminethethermodynamicsanddynamicsofglasstransitionofamorphousaloy,itisnecessarytomeasurespecificheatoftheal... 相似文献
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HEATCAPACITIESOFFe_(40)Ni_(40)B_(20)GLASSYALLOYMEASUREDBYENTHALPYMETHOD¥JIANGQing;ZHAOMing;LIJianchen(JilinUniversityofTechnol?.. 相似文献
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Proper understanding of glass formation implies the knowledge of the thermodynamics of the undercooled melts. Specifically, high values of the excess specific heat of the liquid are expected for good glass-formers. Extending the work of Gillessen and Herlach [F. Gillessen, D.M. Herlach, J. Non-Cryst. Solids 117–118 (1990) 555–558], we re-propose a calculation of the temperature dependence of entropy difference between amorphous-liquid and crystal states.An amorphous Pd77.5Cu6Si16.5 alloy has been produced by injection casting in a cylindrical copper mould. DSC measurements in the liquid, amorphous and crystalline states were performed with samples sliced from the cylinder to determine the heat of fusion, of crystallization and the difference in specific heat capacity between amorphous-liquid and crystal phases. These thermodynamic quantities are used to calculate the thermodynamic functions of the liquid-glass with reference to the equilibrium crystal mixture. The data are compared to those of other bulk glass-formers in terms of fragility plots. 相似文献
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《Intermetallics》2000,8(5-6):477-480
The onset and offset melting temperatures Tm and Tl, glass transition temperature Tg and heat of fusion ΔHm of six La based La–Al–Cu–Ni–(Co) alloys were measured by DTA or DSC at a heating rate of 20 K/s. The absolute values of specific heat capacity for the undercooled liquid and the corresponding crystalline of these six alloys were obtained by means of temperature-modulated DSC (TMDSC). Entropy, enthalpy and Gibbs free energy differences between the undercooled liquid and crystalline of these alloys as a function of temperature have been calculated. Their glass forming ability was discussed from a thermodynamic point of view. 相似文献
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大块非晶合金淬态相等温长大过程 总被引:2,自引:0,他引:2
利用石英管水淬法制备了ZR41.2Ti13.8Cu12.5Ni10Be22.5大块非晶合金,在真空度不小于10-3Pa和原材料纯度99.9%的情况下,当制备样品直径大于10mm时,经常发现非晶基体中含有一定数量的淬态相,该相的数量不在X衍射仪检测范围内,同时DSC分析表明,这些淬态相对非晶合金的过冷液相区、晶化温度和玻璃转变温度影响也较小.非晶合金过冷液相区内的等温晶化过程表明,晶化初期淬态相具有优先长大的趋势,并有亚稳过渡相的生成.而在晶化的后期,形核和长大同步进行.利用经典形核和长大理论分析了非晶合金的形核和长大过程. 相似文献
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Sven Bossuyt 《Scripta materialia》2001,44(12):2781-2787
The microstructure in Vit101 and related alloys cooled close to the critical cooling rate for glass formation can be explained by considering the effect of the heat of crystallization on the temperature dependent nucleation and growth rates. The localized heat release from crystals nucleated early in the crystallization process leads to localized recalescence events, involving the formation of a high density of nanocrystals, in the deeply undercooled liquid. The critical cooling rate for glass formation is determined by the interaction of these localized recalescence events. 相似文献
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Yong Jin Kim 《Metals and Materials International》1995,1(2):85-98
Brief overview of current containerless electrostatic levitation processing technique and research progress of the area of bulk metallic glass formation is introduced. Undercooling behavior during solidification of the bulk metallic glass forming Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy has been studied using the containerless electrostatic levitation processing technique. The melt is successfully undercooled to the glass transition temperature forming the amorphous phase with the proper thermal treatment. Differential scanning calorimetry (DSC) is used to determine the Gibbs free energy difference between the crystal and the undercooled liquid. The results indicate that the Gibbs free energy difference between the metastable undercooled liquid and the crystalline solid of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy is relatively small compared to that of conventional metallic glass forming binary alloys even for large undercoolings. The hemispherical total emissivity of undercooled liquid is measured in the whole region of undercooled liquid state. Due to the combining effects of excellent thermal stability of the undercooled liquid in the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy with unique experimental technique of the containerless electrostatic levitation processing, it is possible to construct the complete time-temperature-transformation (TTT) diagram. The measured TTT diagram exhibiting the expected “C” shape can not be satisfactorily explained by the existing models due to the complex crystallization mechanisms. 相似文献
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Undercooling was achieved directly in the cell of a high temperature calorimeter (Setaram HTDSC) for Ni, Fe, Cu, Pd and several alloys, using cooling rates between 1 and 15 K min−1. The samples were immersed in alumina powder inside a standard alumina crucible under flowing helium. Ag, Au and Al were not undercooled significantly. The reproducibility of the measurements was within 1.5%. The heat of solidification of Ni at an undercooling of ΔT = 220 K was −17.5 ± 0.2 kJ mol−1, which is the same absolute value of the heat of fusion at the equilibrium melting point Tm. This implies that the specific heat of the undercooled liquid is very close to that of the crystalline solid in this temperature range. Fe appears to display a similar behavior at ΔT = 220 K. The difference between the heat of fusion at Tm and the heat of solidification at an average value of ΔT = 95 K is significant for a Pd77.5Cu6Si16.5 glass-forming alloy. From these data, we calculated an average specific heat difference between the liquid and crystal phases of 7 ± 5 J mol−1 K−1. The enthalpy data for Pd82Si18 comply with those of the ternary Pd---Cu---Si. 相似文献
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The energy saving produced by the equalization of Fermi energies of a crystal and a melt has been associated to the crystal formation in undercooled melts to determine the new homogeneous-nucleation critical-temperature T2 and the new nucleation critical barrier as a function of the temperature T. Small unmelted clusters act as nuclei by reducing the critical energy barrier and the nucleation times; the glass transition temperature Tg occurs near T = T2. The temperature dependence of the specific heat difference of the undercooled melt with the glass, the viscosity weakening below Tg and the nose of the temperature–time–transformation diagrams are successfully predicted. 相似文献
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WEI Bingbo YANG Gencang ZHOU Yaohe Northwestern Polytechnical University Xi''''an China 《金属学报(英文版)》1991,4(8):115-120
Undercoolings up to 397 K(0.283 T_E)have been obtained for Ni-32.5% Sn eutectic alloymelted by superheating-cooling cycles and denucleating with inorganic glasses.The predomi-nant dissipation of heat for highly undercooled alloy melt is through radiation.An approxi-mate method is consequently derived to calculate its mean specific heat from measured coolingcurves.With the aid of high speed cinematography,it is revealed that the surface or interfaceheterogeneous nucleation takes place in preference to homogeneous nucleation even though theundercooling has exceeded 0.2 T_E. 相似文献
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The curve of crystallization transition during continuous heating for the Zr41Ti14 Cu12. 5Ni10Be22.5 bulk amorphous alloy was measured by means of dilatation(Fully automatic transformation recording/measuring instrument) and X-ray diffraction(XRD) method. The deformation behavior of the alloy at various heating rates in the supercooled liquid region was studied. The results show that the glass transition temperature of the alloy increases slightly and the supercooled liquid region(SLR) increases significantly with increasing heating rate. The deformation amount under isobaric pressure of 1 N for the alloy in the SLR increases with increasing heating rate. As the heating rate of the alloy increases from 5 to 100℃/rain, the amount of deformation of the alloy increases from 8.3% to 45% 相似文献
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1 INTRODUCTIONMetallicglassisregardedasastatethatisofdis orderunlikecrystalalloyswith periodicatomstruc ture.Soitshowsexcellentcapabilitiesofsoftmag netism ,mechanics ,corrosionresistance ,etc .How ever ,mostofamorphousalloyswereproducedbyus ingrapidsolidificationmethodssuchassplatquench ing ,meltspinning ,andsoon ,withcharacteristiccoolingratesof 10 4 10 6 K/s .Becauseamorphousal loysarepreparedwithsilk ,powderandribbon ,itisgreatlylimitedinengineeringapplication .Recently ,severalbulk… 相似文献
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Joachim Bokeloh Gerhard Wilde 《JOM Journal of the Minerals, Metals and Materials Society》2014,66(8):1512-1519
Nucleation of a crystal in undercooled melts of higher melting face-centered-cubic-metals has often been studied in the past. However, the data available were not of sufficient accuracy and only covered nucleation rates in too small of a range to allow precise conclusions concerning the nature of the underlying process as well as concerning important parameters such as the solid–liquid interface free energy that can in principle be deducted from such analyses. One way to circumvent ambiguities and analyze nucleation kinetics under well-defined conditions experimentally is given by performing statistically significant numbers of repeated single droplet experiments. Application of proper statistics analyses yields nucleation rates that are independent of a specific nucleation model. The first studies that were conducted in accordance with this approach on pure model materials revealed that the approach is valid. The results are comparable to those obtained by classic nucleation theory applied to experimental data, and it was shown that one might need to rethink the common assumption that heterogeneous nucleation is almost always responsible for solidification initiation. The current results also show that often-used models for the solid–liquid interface free energy might lead to overestimated values. 相似文献
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Surface Tension Calculation of Undercooled Alloys 总被引:2,自引:0,他引:2
1 IntYOductionDue to the experimental difficulties inVolVedand time-money constuning, several modelshave been developed towards obtaining sufficiellt and reliable stirface tension dare for practical use. The STCBE computer Program, apowerful surface tension calculation Program[l] based on Butler's original treatment [2] inconjunction with CALPHAD (CALculation ofPHase Diagram) technique, has been developed.A series of surface tension calculation of hightemper~ melts including liquid a… 相似文献
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Expressions for the Gibbs energies of interfaces occurring in particular for solid and/or liquid/amorphous metals or alloys in contact with each other have been developed. To consider its energetics, an amorphous alloy has been modelled as a mixture of the undercooled liquid metal components near to the glass transition temperature making use of the enthalpy of melting, the entropy of melting and the temperature-dependent contribution of the heat capacity of the undercooled melt. Gibbs surface and interface energies have been obtained on the basis of the “macroscopic atom” Miedema model, where the entropy contributions of alloys have been derived applying a recently developed formalism. The Gibbs energy of a crystalline interface phase has been formulated. The molar fractions of the components of the alloy at the surfaces have been determined by minimising the surface energy. These results provide a thermodynamic basis for unusual phenomena observed in nano-sized systems. The formalism has been applied to calculate the thermodynamic stability of interface phases in a nano-sized, multi-layered system of iron and zirconium and to explain the aluminium-induced crystallisation of amorphous silicon and the layer exchange occurring in bi-layers of crystalline aluminium and amorphous silicon. 相似文献
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Bulk amorphous metal—An emerging engineering material 总被引:1,自引:0,他引:1
During the last two decades, researchers have developed families of metal alloys that exhibit exceptional resistance to crystallization
in the undercooled liquid state. Upon cooling, these alloys readily form glass or vitrify to form bulk amorphous alloys or
bulk metallic glasses. The stability of the undercooled molten alloys with respect to crystallization has enabled studies
of liquid thermodynamics, rheology, atomic diffusion, and the glass transition previously not possible in metallic systems.
Bulk amorphous alloys exhibit very high strength, specific strength, and elastic strain limit, along with unusual combinations
of other engineering properties. These factors, taken together, suggest that bulk amorphous metals will become widely used
engineering materials during the next decade. 相似文献