The crystallization kinetics of Fe-Ni based metallic glasses

Differential scanning calorimetry is used to study the crystallization kinetics of two commerical Fe-Ni metallic galsses near their glass transition point. For 0.01 <x<0.85 the fraction transformed,x, as a function of time,t, satisfies the Johnson-Mehl-Avrami equation with exponentn varying from 2.8 to 4.3 as the annealing temperature is increased. The activation energies for the crystallization process are estimated from the time to 50% transformation as close to 100 kcal mol^–1 and are interpreted as arising from viscous flow.     

Nucleation kinetics of primary crystallization products in FeSiB metallic glasses

Three iron-based metallic glasses in the Fe_96–x Si₄B _x series were examined after various dynamic and isothermal annealing treatments. The number and type of the primary crystallization products formed in these alloys were determined as functions of time and temperature, and the results compared with theoretical models. It has been found that, for certain alloys, a proportion of the primary -iron crystals in the structure are nucleated by particles of the metastable Fe₃B phase. The transformation kinetics of these composite crystals are different from those which do not contain an Fe₃B core, suggesting different nucleation mechanisms for the two types of crystal.     

Surface crystallization in melt-spun metallic glasses

A series of hypoeutectic FeSiB metallic glasses produced by single-roller melt spinning were found to have substantial wheel-side surface crystallization. These crystals were shown to be associated with areas on the ribbon surface which were in good contact with the wheel rather than in the depressions formed by gas entrainment during casting, as has been previously suggested for similar effects in other alloys. Transmission electron microscopy revealed that the crystals have a fine cellular substructure with the intercellular spaces freezing as glass and that the structure becomes progressively finer during growth.     

Effect of high-energy heavy ion irradiation on the crystallization kinetics of Co-based metallic glasses

Differential scanning calorimeter (DSC) is employed to study the crystallization kinetics of irradiated (at three different fluences with high-energy heavy ion; Ni¹¹⁺ of 150 MeV) specimens of two Co-based metallic glasses. It is found that the crystallization process in both the glasses is completed in two phases. The DSC data have been analysed in terms of kinetic parameters viz. activation energy (E _c), Avrami exponent (n), dimensionality of growth (m), bdusing two different theoretical models. The results obtained have been compared with that of virgin samples. The lower activation energy in case of second crystallization occurring at higher temperature indicates the easier nucleation of second phase. The abnormally high value of Avrami exponent in Co-Ni glass indicates very high nucleation rate during first crystallization.     

Laser pulse crystallization of Pd-Si metallic glasses

A novel method of laser pulse crystallization (LPC) capable of revealing and quantifying crystallization in amorphous ribbons is presented. It is used to study the morphology of crystallization and, indirectly, surface crystallization (SC) of Pd-Si amorphous alloys in the composition range Pd₇₇Si₂₃-Pd₈₅Si₁₅. The laser-induced, frozen-in fronts of crystallization are studied by optical metallography and X-ray diffraction. The shapes of these fronts are determined by the initial degree of SC, which depends on alloy compositions, their thermal history and/or the surface of the ribbon being analysed. SC is inactive in alloys of eutectic composition (Pd₈₅Si₁₅) and is most pronounced for compositions near the border to the amorphous region on the silicon-rich side (Pd₇₇Si₂₃). For as-quenched ribbons, a stronger tendency to SC exists on the shiny surface of the ribbon. Low-temperature furnace annealing as well as room temperature ageing over a period of 2 years equalizes the degree of SC on both surfaces. The X-ray diffraction patterns taken from the regions containing frozen-in fronts of crystallization only reveal the presence of metastable phase II, independently of composition and/or thermal history of the given sample.     

Crystallization kinetics of some iron-based metallic glasses

The crystallization behaviour of a range of iron-based metallic glasses has been examined by isothermal differential scanning calorimetry, optical and transmission electron microscopy. It was found that the conventional procedure of applying the Johnson-Mehl-Avrami equation to determine reaction mechanisms, i.e. calculating a mean value of the Avrami exponent over a range of volume fraction transformed, can be misleading. The technique suggested by Calka and Radlinski, when used in conjunction with detailed metallography, is shown to be a more sensitive and reliable indicator of reaction mechanisms.     

Current views on the structure and crystallization of metallic glasses

An attempt has been made in this review to cover recent information on the structure of metallic glasses (amorphous metals) obtained by X-ray diffraction. Based on the experimental data, the partial structure factors of a binary metallic glass are given. Various characteristics such as crystallization processes, thermal effects and alloying effects are also discussed.     

Applicability of FMR for crystallization studies in metallic glasses

It has been shown theoretically that the peak-to-peak ferromagnetic resonance (FMR) line-width (H _pp) should be proportional to the volume fraction (f) of the crystalline phase formed during annealing in ferromagnetic glasses. However, in the case of zero-magnetostrictive cobalt-based glasses (=0), H _pp should remain constant due to very low values of anisotropy. In Co₆₈Fe₄Mo₁Si₁₇B₁₀ glass (=0), H _pp has been found to remain unaffected upon progressive crystallization. In Fe₆₅Cr₈B₂₇ glass, H _pp has been found to be linearly proportional tof. Using this dependence, the activation energy of crystallization (E _a) has been calculated. The value ofE _a obtained from the FMR technique (248 kJ mol^–1) agrees quite well with those from DTA studies using Kissinger's or Ozawa's technique.     

Non-isothermal crystallization kinetics of niobium-doped BGO70 glasses

Niobium-doped BGO glass of composition 85[3Bi₂O₃–7GeO₂ (BGO70)]–Nb15 was investigated for its kinetic parameters n and activation energies E _i : i = 1, 2, g of transformations via available models. Two crystallization exothermal peaks were observed for all heating histograms exploiting 5, 10, 15, and 20 K/min heating rates following the glass transition in non-isothermal DTA curves. An exothermal shoulder prior to first crystallization peak P₁ was identified as thermally favorable crystallizing metastable phase Bi₂GeO₅. P₁ was assigned to thermally stable crystal phase Bi₄Ge₃O₁₂. T _g lied between 729 and 731 K ± 4 K; T _p1 varied from 835–855 K ± 2 K, and T _p2 had values 965–986 K ± 2 K. Independent of the model exploited, activation energies E _g, E _p1, and E _p2 were 32.89 ± 3.1, 47.33 ± 0.72, and 62.74 ± 0.72 kcal/mol, respectively. P₁ corresponds to crystalline disks of Bi₄Ge₃O₁₂, and P₂ identified stable phase BGO crystal rods growing inward from the surface imbedded in glass matrix. Niobium doping modified the transformation kinetic parameters n and E _a of the composition by providing a control on growth mechanism for BGO platelets or rods.     

Studies on crystallization behaviour and mechanical properties of Al-Ni-La metallic glasses

Alloy ingots with nominal composition, Al_92−x Ni₈La _x (x = 4 to 6) and Al_94−x Ni₆La _x (x = 6, 7), were prepared by induction melting in a purified Ar atmosphere. Each ingot was inductively re-melted and rapidly solidified ribbons were obtained by ejecting the melt onto a rotating copper wheel in an argon atmosphere. The crystallization behaviour of melt-spun amorphous ribbon was investigated by means of differential scanning calorimetry (DSC), X-ray diffractometry and transmission electron microscopy. DSC showed that Al₈₆Ni₈La₆ alloy undergoes a three-stage and rest of the alloys undergo a two-stage crystallization process upon heating. The phases responsible for each stage of crystallization were identified. During the first crystallization stage fcc-Al precipitates for low La-containing alloys and for higher La-containing alloys a bcc metastable phase precipitates. The second crystallization stage is due to formation of intermetallic compounds along with fcc-Al. Microhardness of all the ribbons was examined at different temperatures and correlated with structural evolutions. Precipitation strengthening of nano-size fcc-Al is responsible for maximum hardness in these annealed alloys.     

Evaluation of crystallization kinetics of glasses by non-isothermal analysis

Based on Johnson-Mehl-Avrami transition equation, this paper proposes a new non-isothermal method for evaluating the crystallization kinetics of glasses. An equation relating the kinetic parameters of the crystallization activation energy, E, and the frequency factor, v, to the inflection-point temperature, T _f, and the heating rate, , of differential thermal analysis (DTA) experiment is established. The inflection-point temperature, T _f, can be easily determined from the derivative differential thermal analysis (DDTA) curves. The validity of the proposed method is ascertained by applying it to Li₂O·2SiO₂ glass. The acquired values of the crystallization kinetic parameters by this method are excellent agreement with the isothermal analysis results. In contrast, the traditional non-isothermal methods give much higher values.     

A study of the crystallization kinetics of some Cu-As-Te glasses

The crystallization kinetics of two amorphous alloys in the Cu-As-Te system was studied by differential scanning calorimetry, using continuous heating methods, and applying a new analysis procedure in order to calculate the kinetic parameters which define the crystallization reactions, in the Johnson-Mehl-Avrami model. In this analysis, the crystallized fraction interval at which the characteristic function of said model is constant was taken into account. The values obtained for these parameters made it possible to discuss the glass-forming ability of the compounds under study, and the types of crystalline growths in the alloys.     

Non-isothermal crystallization kinetics of Ag-As2S3 semiconductor glasses

Using a differential thermal analysis technique to give single scan thermograms, the transformation mode,n, and the activation energy of crystallization,E, were determined for the chalcogenide glasses of the system Ag _x (As₂S₃)_100–x' withx=6, 15 and 25. Bothn andE were found to be compositionally dependent. The value ofn varies between 2.1 and 3.6, and that ofE between 2.4 and 3.6 eV. The crystallization kinetic data were explained using X-ray diffraction results of amorphous and crystalline structures.     

Isothermal crystallization kinetics of Ni60Nb4o-xCrx,glasses

Isothermal crystallization kinetics of Ni₆₀Nb_40-xCr_x (x = 0, 5, and 13 at %) glasses was investigated by differential scanning calorimetry. It was possible to separate out the kinetics of formation of various phases which are obtained on crystallization of these glasses. The results are compared with the earlier investigations [1] in which the data were obtained by constant heating rate experiments. The crystallization of M-phase, Ni₃Nb and NbCr₂ phases could be described by Johnson-Mehl-Avrami kinetics. The activation energies for the formation of these phases were found to decrease in the same order. A decreasing activation energy with increasing transformed fraction (time) was also observed. The results are interpreted in light of the values obtained for Avrami exponents.     

The kinetics of crystallization of the metallic alloy Pd4Ge

The kinetic parameters for the crystallization of Pd₄Ge have been measured by differential scanning calorimetry (DSC). The effective activation energy was found to be 215 kJ mol^–1 (51 kcal mol^–1) and the heat of reaction was determined to be 13 kJ mol^–1 (3.1 kcal mol^–1). Preliminary investigation of the products of crystallization was undertaken using X-ray diffraction and electron microscope techniques. Some new phases for this alloy were observed.