Crystallization embrittlement of Ni-Ti-B glasses

While metallic glasses have excellent toughness and ductility in the as-cast and unrelaxed state, the process of crystallization leads to a nearly continuous and significant embrittlement. This change is examined on three Ni-Ti-B glasses and related to the morphology and distribution of the crystals obtained. For the partially crystallized materials, failure still occurs after intense shear on one well-defined shear plane, and it is shown how the crystals act as stress and strain incompatibilities causing localized crack or cavity formation. The relationships between sample mechanical properties, fracture surface characteristics and crystal distributions allow an analysis of cavity nucleation and growth rates, and thereby make it possible to suggest microstructures which may maintain reasonable toughness.     

Structural relaxation and embrittlement in Fe-Ni based metallic glasses

The fracture strain, changes in electrical resistivity and Curie temperature, and the volume change (the amount of annealed-out excess volume) were measured as a function of annealing temperature in some Fe-Ni based metallic glasses (Fe₂₇Ni₅₃P₁₄B₆, Fe₂₉Ni₄₉P₁₄B₆Si₂, Fe₄₀Ni₄₀P₁₄B₆, Fe₄₀Ni₃₈Si₈B₁₄ and Fe₆₃Ni₁₅Si₈B₁₄), in order to clarify the embrittlement behaviour during structural relaxation. A close relationship between the ductile-brittle transition temperature and the resistivity change was observed in these metallic glasses. Particularly, in Fe₂₇Ni₅₃P₁₄B₆ metallic glass, it was found that the ductile-brittle transition temperature is well consistent with the annealing temperature at which the changes in resistivity and Curie temperature are maximum. The results obtained in the present study indicate that the embrittlement behaviour during structural relaxation in these Fe-Ni based metallic glasses is closely related to the formation of more stable short range ordered structure.     

Compositional effect on the crystallization of the cordierite-type glasses

The determination of apparent activation energies for the crystallization of P₂O₅/B₂O₃-containing cordierite-type glasses was conducted by non-isothermal differential thermal analysis (DTA). Glass with excesses both of SiO₂ and Al₂O₃ has a higher activation energy for crystallization. In addition to the main phase of -cordierite, several other phases (farringtonite, clinoenstatite, and forsterite) were detected in the fully crystallized samples by the X-ray powder diffraction technique. The crystallization behavior for the powdered glass was governed by the surface crystallization mechanism, which would not change with composition.     

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.     

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.     

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.     

The crystallization behaviour of bismuth germanate glasses

Glassy samples containing Bi₂O₃ and GeO₂ in different ratios were prepared and absorption measurements were performed on the samples in the ultraviolet region in order to determine the optical behaviour. Thermal analysis performed on the vitreous samples gave the glass transition temperature and the crystallization temperature. The main crystalline phases separated during the heat treatment were Bi₄Ge₃O₁₂ (BGO) and Bi₂GeO₅. The activation energy of the crystallization process of the sample BGO (1∶3) was measured using the isothermal method, and the value obtained was 47.1 kcal mol⁻¹; the reaction order was equal to 1.1.     

Liquid immiscibility and crystallization in refractory phosphate glasses

The nature and evolution of liquid immiscibility and essentially complete crystallization in an optimized glass composition containing Al₂O₃, SiO₂ and P₂O₅ are examined from the results of electron microscopy, X-ray diffraction and density changes. Phase separation occurs during cooling from the melt; subsequent reheating to 850° C causes crystal nucleation followed by growth of the refractory phases of mullite (nominally 3Al₂O₃-2SiO₂) and AlPO₄ at approximately 1050 and 1160° C, respectively. Additional crystallization of these phases also occurs at 1400° C. Similar materials containing additions of ZrO₂ and Y₂O₃ are also discussed.When this work was conducted, the writers were Graduate Research Assistant and Associate Professor of Materials Engineering, respectively. Mr Lee is currently at Ohio State University.     

Time-scaling and crystallization kinetics of three Fe-B-based metallic glasses

Time-scaling properties of the isothermal transformation kinetics have been tested for three Fe-B-based metallic glasses exhibiting two crystallization stages. The time scale was defined as the time at which crystallization has reached half completion and is derived from calorimetric (DSC) data. The temperature dependence of the time-scaling parameter shows a lowtemperature freezing behaviour and can be described by empirical functions based on free-volume considerations.     

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.     

Formation and crystallization of chlorine- and bromine-containing hafnium fluoride-based glasses

We have studied glass-forming regions for ZBLAN-type hafnium fluoride-based glasses with the compositions (63–53)HfF₄ · 20BaF₂ · 4LaF₃ · (1–3)InF₃ · (0–20)NaF in which fluorine anions are partially replaced by chlorine and bromine anions. The crystallization mechanism of the glasses has been investigated by differential thermal analysis and X-ray diffraction. The results demonstrate that characteristic features of the glasses are a significant decrease in glass transition temperature (t _g) and the precipitation of fine-particle crystalline chloride and bromide phases at temperatures well below the crystallization temperature (t _x) of their fluoride analogs. We have studied the effect of the Cl/F and Cl/Br ratios in the glass batch and the melt cooling rate after glass preparation on the crystallization behavior of the glasses and determined the heat treatment temperature and time for the formation of transparent glass-ceramic samples upon heat treatment in the temperature range where chloride and bromide phases precipitate. Doping of the glasses with 1 at % or smaller amounts of rare-earth metals is found to have no significant effect on the phase segregation process during heat treatment.     

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.     

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.