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1.
《Journal of Non》2004,337(1):42-47
Milling up to 800 h causes amorphous Co70.3Fe4.7Si10B15 alloy, prepared in the form of thin ribbon, to partially crystallize thus forming a powder material consisting of an amorphous phase and fcc-Co nanocrystals with an average grain size of about 10 nm. A gradual increase of the nanocrystalline fcc-Co fraction, produced by ball milling, was detected. Prolonged milling results in destabilization of the fcc-Co phase and oxidation of the powder material (presence of CoO phase after 1500 h of milling). The thermal stability studies of as-quenched and milled Co70.3Fe4.7Si10B15 alloy emphasized a two step crystallization behavior. During the first crystallization event, cobalt rich phases, i.e., fcc-Co and hcp-Co crystallize, whereas after the second crystallization event, Co2B and Co2Si are formed. 相似文献
2.
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. 相似文献
3.
We have produced a series of bulk metallic glasses of composition (HfxZr1−x)52.5Cu17.9Ni14.6Al10Ti5 (with x=0-1) by an arc melting/suction casting method. The density of these alloys increases by nearly 67% with increasing Hf content from 6.65 g/cm3 (x=0) to 11.09 g/cm3 (x=1). Over the same composition range the glass-forming ability decreases, as demonstrated by the size of the largest amorphous ingots that can be cast without crystallization. Although both the glass transition temperature and the melting temperature increase linearly with increasing Hf content, the reduced glass transition temperature (Tg/Tm) decreases, from 0.64 (x=0) to 0.62 (x=1), which suggests that the `confusion principle' correlating increased glass-forming ability with increased number of components, does not apply in this case due to the chemical similarity between Zr and Hf. A different crystallization behavior is observed for Zr-based and Hf-based glasses. The final crystalline phases are CuZr2 and Zr2Ni for Zr-based alloys, and Al16Hf6Ni7 and CuHf2 for Hf-based alloys. 相似文献
4.
The decomposition of AlDy6Ni10 and AlDy8Ni6Co2 glasses prepared by melt spinning after different pre-aging regimes was investigated. The modelling of the transformation at the glass transition and the crystallisation was tested by a more general Johnson-Mehl-Avrami-Kolmogorov model and by the determination of the local kinetic parameter. The relaxation was tested with an lnt kinetic model after Gibbs. 相似文献
5.
The formation of the supercooled liquid region and devitrification behavior of Ni-based glassy alloys were studied by using X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and isothermal calorimetry. oC68 Ni10Zr7-type phase is primarily formed in the studied alloys in the initial stage of the devitrification process by nucleation and three-dimensional diffusion controlled growth. The replacement of Cu by Ni in Cu55Zr30Ti10Pd5 glassy alloy induces precipitation of oC68 Ni10Zr7 phase directly from the glassy phase. The reasons for such a behavior are discussed taking into account mixing enthalpy in a liquid state and the interval of the supercooled liquid region. 相似文献
6.
Zr70Cu30, Zr70Cu20Co10, Zr70Cu20Rh10 and Zr70Cu20Ir10 glassy alloys were prepared by the single roller melt-spinning method and the crystallization process was studied by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy, with an emphasis on the initial stage. The Zr70Cu30 metallic glass crystallizes through the direct precipitation of the stable crystalline phase from the matrix. The addition of Co, Rh and Ir to the Zr70Cu30 metallic glass induces the precipitation of metastable phases prior to the formation of the stable ones. The metastable phases are a Ti2Ni-type compound for Zr70Cu20Co10, a mixture of the Ti2Ni-type compound and an icosahedral quasicrystalline phase (I-phase) for Zr70Cu20Rh10, and the I-phase for Zr70Cu20Ir10 metallic glasses. The different effects of Co, Rh and Ir addition are explained based on their difference in atomic radius. 相似文献
7.
Crystallization was examined for glasses having chemical composition of 2(Ca,Sr,Ba)O-TiO2-2SiO2 in which the CaO/SrO/BaO molar ratio varied. Powdered glass samples were pelleted into disks and sintered at 950 °C for 2 h. The major phase precipitated in the sintered samples was (Ca,Sr,Ba)2TiSi2O8 and minor phase of perovskite such as CaTiO3 or SrTiO3 increased with CaO content in the samples containing more than 40 mol% of CaO in total CaO+SrO+BaO. Three regions having different slopes were found in linear relationships between SrO mol% and exothermal peak temperature on DSC curves or d[0 0 2] values determined by powder XRD method. These facts suggested that the major phase precipitated in each region was a solid solution containing a different amount of CaO, SrO, BaO and that these compositions varied depending on SrO content in the sample. The micro-crystalline structure, which could be useful in fabricating a dielectric dense body, was observed for samples containing 30-70 mol% of SrO. 相似文献
8.
The crystallization capability of a parent glass made from a mixture of coal ash (40 wt%) and soda-lime glass was investigated using differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy. Different glass particle size distributions were considered in the range 20-500 μm. Two crystallization exotherms in DTA were attributed to the formation of both pyroxenes (diopside Ca(Mg,Al)(Si,Al)2O6 and augite Ca(Mg,Fe)Si2O6) and plagioclase (Na,Ca)(Si,Al)4O8. These phases were confirmed by XRD analyses. Analysis of non-isothermal DTA data yielded values of 545 kJ/mol and 1.8 for the activation energy of crystallization and the Avrami exponent, respectively. This value for the Avrami exponent was consistent with a decreasing nucleation rate and the observed dendritic morphology. The data on crystallization kinetics obtained in this study are relevant for the production of glass-ceramic materials by a sintering/crystallization method from powder compacts made of this parent glass. 相似文献
9.
The effect of Sn substitution for Ni on the glass-forming ability was studied in Cu47Ti33Zr11Ni8−xSnxSi1 (x=0,2,4,6,8) alloys by using thermal analysis and X-ray diffractometry. With increasing x from 0 to 8, the glass transition temperature, Tg, of melt-spun Cu47Ti33Zr11Ni8−xSnxSi1 alloys increased gradually from 720 to 737 K. On the other hand, the crystallization temperature, Tx, increased from 757 K at x=0 to 765 K at x=2, being nearly same with further increase of x. Partial substitution of Ni by Sn in Cu47Ti33Zr11Ni8Si1 promotes the glass formation. Both amorphous Cu47Ti33Zr11Ni8−xSnxSi1 alloys prepared by melt spinning and injection casting showed similar crystallization process during continuous heating in DSC. Temperature range of undercooled liquid region exhibits good correlation with the critical diameter for the formation of an amorphous phase in injection casting. 相似文献
10.
H. Schlenz A. Kirfel K. Schulmeister N. Wartner W. Mader W. Raberg K. Wandelt C. Oligschleger S. Bender R. Franke J. Hormes W. Hoffbauer V. Lansmann M. Jansen N. Zotov C. Marian H. Putz J. Neuefeind 《Journal of Non》2002,297(1):37-54
The structures of four different Ba-silicate glasses were analysed applying a broad spectrum of diffraction and spectroscopic methods, atomic force microscopy and computer simulations. This collaborative study offers new insights into the influence of the network modifier BaO. 3 at.% carbon were incorporated into two of the glasses with the intention to obtain additional information about the effect of partial substitution of carbon for oxygen on the considered structures. A structure model for Ba-silicate glasses with compositions close to BaSi2O5 is discussed that differs with respect to other models proposed in previous studies. We propose that [SiO4] tetrahedra do not exclusively form six-membered rings, but also considerable numbers of smaller rings arranged in (slightly) folded layers which alternate with barium layers and/or barium chains. Additionally, the occurrence of tetrahedral chains, possibly isolated rings and of regions in which the [SiO4] tetrahedra form a silica-like network is assumed. 相似文献
11.
Following the recognition during the last decade that knowledge of the shift in the exothermic crystallization DTA peak as a function of pre-DTA isothermal heat-treatment times and temperatures, can provide quantitative information about the crystallization kinetics, there has been renewed interest in DTA investigations of crystallization of glasses. Most studies to date, however, have focussed on the kinetics of polymorphic crystallization (where the compositions of the crystal and the parent glass are the same). These studies have established that the DTA peak shifts to lower temperatures with increased pre-DTA heat-treatment times and JMAK-based formalisms have been developed to extract the steady state nucleation rate from the DTA peak shift data. In this paper, we report new results on the DTA peak shift in systems undergoing primary crystallization (where the compositions of the crystal and glass are different). The DTA results show that the exothermic peak temperature decreases initially but increases later on, becoming significantly larger than the initial value, with increase in the pre-DTA heat-treatment time at a fixed temperature. This increase at long times has not been reported previously and is qualitatively different than the monotonic decrease reported for polymorphic crystallization. To rationalize these new results, a model of primary crystallization has been developed which includes homogeneous nucleation, diffusion-controlled growth, Gibbs-Thomson effect, and a mean field soft-impingement correction during growth. Based on this model and experimental results, it is concluded that the initial shift to lower temperatures is due to an increase in the number of nuclei (as concluded previously by others for the case of polymorphic crystallization) and the later shift towards high temperatures in our experiments is due to diffusion-controlled growth during the pre-DTA heat treatment. 相似文献
12.
A metallic glass matrix composite (MGMC) reinforced by copper short fibers has been prepared by warm extrusion of powders, and its deformation behavior at room temperature and in the supercooled liquid region of the metallic glass has been investigated. A mixture of Ni59Zr20Ti16Si2Sn3 metallic glass powders and copper powders is extruded in the supercooled liquid region of the metallic glass with an extrusion ratio of 5. The volume fraction of the copper phase is 0.2. After extrusion, initially spherical powders are elongated along the extrusion direction; no pores are visible. The MGMC shows a high failure strength of around 1.85 GPa, slightly lower than that of the as-cast Ni59Zr20Ti16Si2Sn3 metallic glass, under uniaxial compression. However, due to the crack bridging mechanism produced by the randomly distributed copper short fibers, the MGMC does not catastrophically fail by a single shear band propagating across the whole monolithic sample. In the supercooled liquid region of the metallic glass, the MGMC shows large elongation to failure but fails by cavitation due to the preexisting Ni-based crystalline powders. 相似文献
13.
The crystallization behavior of glass with the composition: 55.6 mol% SiO2, 22.8 mol% Al2O3, 17.7 mol% ZnO and 3.84 mol% of TiO2 as nucleating agent and with different particle sizes has been studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and tranmission electron microscopy (TEM). In glass powders two crystalline phases: zinc-aluminosilicate s.s. with high-quartz structure, Znx/2AlxSi3−xO6, (x varies dependent on heat-treatment temperature) and gahnite are formed. The ratio of these phases depends on particle sizes. In bulk glass, however, gahnite is the sole crystalline phase. The composition of initially formed zinc-aluminosilicate s.s. was determined by Rietveld refinement of XRD patterns to be Zn0.69Al1.38Si1.62O6. With temperature increase, the amount of zinc-aluminosilicate s.s decreased with simultaneous reduce of zinc and aluminum incorporated in the structure. Eventually at 1423 K almost pure high-quartz structure was formed. The activation energies of zinc-aluminosilicate s.s. and gahnite crystallization were determined by non-isothermal method to be 510 ± 18 and 344 ± 17 kJ mol−1, respectively. The latter value matches well with those cited in literature for crystal growth of gahnite in similar glasses. That is attributed to the fact that the high-quartz structure acts as a precursor for gahnite crystallization. 相似文献
14.
Potassium niobium silicate (KNS) glasses the composition of which is characterized by the K2O/Nb2O5 molar ratio ranging from 0.85 to 1.2 and SiO2 50-54 mol% were examined in order to clarify the influence of chemical composition on formation of transparent nanostructured state of glasses. Differential thermal analysis, X-ray diffraction and scanning electron microscopy were used to study the non-isothermal crystallization of the KNS glasses as well as their morphological features. It was found that all glasses devitrify in three steps forming unidentified phases at the first two ones while at higher temperature (1000-1100 °C) the crystallization of K3Nb3O6Si2O7 takes place. For prolonged heat treatment time (more than 5 h) at high temperature (1050-1100 °C) the transformation of this phase into the KNbSi2O7 ferroelectric one occurs in some extent. Nanostructuring occurs at the first stage of the devitrification process. It results from two partially overlapped processes: amorphous phase separation and subsequent crystallization. It was shown that only for the glass with the K2O/Nb2O5 molar ratio equal to 0.85 and SiO2 50 mol% it is possible to separate the above processes by isothermal heat treatments at 680 °C obtaining fully transparent nanostructured samples. These samples contain nanocrystals 10 times smaller than the amorphous inhomogeneities of the phase separated matrix in which are dispersed. 相似文献
15.
Strontium is often substituted for calcium in order to confer radio-opacity in glasses used for dental cements, biocomposites and bioglass-ceramics. The present paper investigates the influence of substituting strontium for calcium in a glass of the following composition: 4.5SiO23Al2O31.5P2O53CaO2CaF2, having a Ca:P ratio of 1.67 corresponding to calcium fluorapatite (Ca5(PO4)3F). The glasses were characterized by magic angle spinning nuclear magnetic resonance (MAS-NMR), by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). The 29Si, 27Al and 31P NMR spectra for the glasses with different strontium contents were identical. The 19F spectra indicated the presence of F-Ca(n) and Al-F-Ca(n) species in the calcium glasses and in the strontium glasses F-Sr(n) and Al-F-Sr(n). It can be concluded that strontium substitutes for calcium with little change in the glass structure as a result of their similar charge to size ratio. The low strontium glasses bulk nucleated to a calcium apatite phase. Intermediate strontium content glasses surface nucleated to a mixed calcium-strontium apatite and the fully strontium substituted glass to strontium fluorapatite. 相似文献
16.
The crystallization behavior of Zr65.0Al7.5Ni10.0Cu17.5 metallic glasses by addition of Ni with 753 K annealing treatment and its effect on the oxidation resistance around the supercooled liquid region at 663 K were studied. By annealing at 753 K, the nanocrystalline phase of bct-Zr2Cu precipitates was observed in the Zr65.0Al7.5Cu27.5 specimen, while microstructures consisting of finer nanocrystalline bct-Zr2Cu, fcc-Zr2Ni and Zr6Al2Ni formed in the Zr65.0Al7.5Ni10.0Cu17.5 specimen. The oxidation resistance of the melt-spun Zr65.0Al7.5Ni10.0Cu17.5 specimen was improved by addition of Ni, which is evidenced by less mass gain and thin oxide scale. The microstructural refinement by the formation of numerous nanocrystalline phases of bct-Zr2Cu, fcc-Zr2Ni and Zr6Al2Ni from the matrix resulted in an improvement of the oxidation resistance, whereas a relative coarse nanocrystalline phase consisting of bct-Zr2Cu exhibited fast oxidation along grain boundaries. Although the oxide species for both specimens were composed of a large amount of CuO/Cu2O, some tetragonal and monoclinic-ZrO2 as well as a minor amount of the oxide state of Cu3+, the amount of oxides especially for ZrO2 in the Zr65.0Al7.5Ni10.0Cu17.5 specimen was lower, which was probably due to suppressed oxygen diffusion in ZrO2. 相似文献
17.
Sundeep Mukherjee 《Journal of Non》2004,337(1):21-28
The containerless high-temperature high vacuum electrostatic levitation (ESL) technique was used for the determination of thermo-physical properties of the binary eutectic alloy, Ni59.5Nb40.5, and the ternary alloy Ni60Nb34.8Sn5.2. The thermo-physical properties measured were density, specific heat over hemispherical total emissivity, surface tension and viscosity. The density for both the melts was found to be higher than that predicted by the rule of mixtures. This was attributed to strong attractive interaction between the constituents. The temperature dependent specific heat capacity of the binary alloy was determined by fitting with TEMPUS results. The hemispherical total emissivity was estimated to be 0.27. Using the specific heat data and the hemispherical total emissivity, the enthalpy and entropy of fusion were calculated for the binary alloy. The viscosities for both the alloys were fit with the Vogel-Fulcher-Tammann equation to obtain the fragility parameter. The surface tension for the binary alloy showed an anomalous positive gradient while that of the ternary alloy showed a small negative gradient with temperature. 相似文献
18.
The effects of nucleation temperature and time on the kinetics of non-isothermal glass crystallization have been re-examined to demonstrate the limitations of some approximate solutions used to extract kinetic parameters from differential thermal analysis (DTA) experiments. Those features were analyzed by numerical solutions of equations describing the dependence of fraction crystallized on the rates of nucleation and growth, and the corresponding transient time, reported for lithium disilicate. It was shown that the temperature of maximum nucleation rate varies on changing the nucleation time. Some guidelines were established to assist the selection of suitable conditions to perform crystallization studies by DTA, and to extract the values of activation energy and dimensionality of growth from the dependences of crystallization peak temperature on heating rate, and nucleation time. The main limitations of these methods were identified and discussed. 相似文献
19.
In this paper, the effects of annealing and nanocrystallization on the mechanical properties of a Zr55Al10Ni5Cu30 metallic glass have been studied. It has been shown that the high volume fractioned nanocrystals facilitate the formation of shear bands and thus decrease the yield stress. At the same time, the nanocrystals also facilitate the formation of interfacial voids during compression, resulting in substantial decrease in the plasticity of the metallic glass. 相似文献
20.
Mousa M.A ImranN.S Saxena Y.K VijayR Vijayvergiya N.B MaharjanM Husain 《Journal of Non》2002,298(1):53-59
Results of differential scanning calorimetry (DSC) under non-isothermal condition on Se96In4 semiconducting chalcogenide glass before and after slow neutron irradiation, for different exposure times, have been reported and discussed. Some of Sn atoms have been injected into the glass by nuclear transmutation processes and the binary glass is converted into a ternary. This is accompanied by an increase in the activation energy of crystallization, Ec, and in the glass transition temperature, Tg and a decrease in the glass transition activation energy, Et, in the onset crystallization temperature, Tc and in the peak temperature of crystallization Tp. Optical band gap measurements have also been carried out, before and after irradiation, on identical thin pellets of Se96In4 glass. The energy band gap, Eg, is found to decrease upon irradiation. These effects have been attributed to a structural change upon doping and to irradiation induced defects. 相似文献