Crystallization Of Metallic Glass Research Articles

Prediction of maximum homogeneous nucleation temperature for crystallization of metallic glasses

A graphical method is used to find out the viscosity at any temperature between the melting temperature and glass transition temperature, which in turn is used to predict the temperature corresponding to the maximum in homogeneous nucleation rate. The present approach does not need any critical experimental data and the model predictions are very close to those experimentally measured in known metallic glasses. This model can be applied for both pure metals and glass forming alloys.

Kinetics of crystallization of metallic glasses studied by non-isothermal and isothermal DSC

A method for describing the lengths of induction periods at linear-heating measurements, is employed for the study of induction periods in the crystallisation of metallic glasses. For Fe75Si15B10 glass, close values of the related kinetic parameters were obtained from isothermal and nonisothermal measurements. On the basis of the results obtained, the absence of induction period in the first crystallisation step of Al90Fe7Nb3 glass in the isothermal DSC measurement has been elucidated.

Low-Temperature Crystallization of Metallic Glass Induced by Electropulsing

Low-Temperature Crystallization of Metallic Glass Induced by Electropulsing

Temperature of nanocrystallisation of magnetically soft alloys for high-temperature applications

The newest class of magnetically soft materials are the nanocrystalline iron alloys obtained by partial crystallisation of metallic glasses. Within this group, FINEMET (Fe–Si–Nb–Cu–B) and NANOPERM (Fe–Zr–Cu–B) alloys exhibit the unique combination of interesting magnetic properties. These materials may be used at temperature below 230 °C. For high-temperature applications, the HITPERM alloys (Fe–Co–Zr–Cu–B) are used. Their optimisation should lead to good (i) magnetic properties at working temperature and (ii) thermal stability of structure and properties. The alloys’ properties are the function of chemical composition and heat treatment. This work is related to the optimisation of heat treatment conditions of HITPERM alloys. High operating temperature may induce further crystallisation of amorphous matrix. It is suggested that the nanocrystallisation temperature should be as high as possible, and the optimisation of HITPERM-type alloys should focus on their chemical composition.

Modelling of primary bcc-Fe crystal growth in a Fe 85B 15 amorphous alloy

A kinetic modelling of primary crystallization in metallic glasses, based on the CALPHAD approach and the moving boundary model, has been applied to the Fe–B system. The DICTRA software has been used to perform numerical calculations. Kinetic and thermodynamic parameters (atomic mobilities and thermodynamic factors) are required and they have been obtained from the literature. Various simulations have been performed in order to evaluate the influence of different parameters choice. The soft impingement effect has been discussed. Furthermore, amorphous Fe 85B 15 samples have been prepared and examined by differential scanning calorimetry. Calculated and experimental results, both on continuous heating and isothermal conditions, have been compared.

Comparative studies of crystallization of a bulk Zr–Al–Ti–Cu–Ni amorphous alloy

The recrystallization and associated nanoindentation behavior of a Zr–10Al–5Ti–17.9Cu–14.6Ni (at.%) bulk amorphous alloy was investigated using in situ transmission electron microscopy and high-temperature nanoindentation. The onset of recrystallization occurs around 710 K, with the formation of Zr 2Cu, Al 3Zr 4, and Ni 10Zr 7. Nanoindentation hardness and elastic modulus were obtained as a function of temperature from 298 to 723 K. The hardness was measured to be ∼7.5 GPa and found unchanged at temperatures below 673 K, but softened rapidly and decreased to 5 GPa at 723 K. These results can be correlated very well with the onset of recrystallization of this alloy at ∼700 K. The present study demonstrates that transmission electron microscopy, hot-stage nanoindentation, and X-ray diffraction are supplementary techniques for the study of glass relaxation and crystallization of metallic glasses.

Comparative studies of crystallization of a bulk Zr?Al?Ti?Cu?Ni amorphous alloy

The recrystallization and associated nanoindentation behavior of a Zr–10Al–5Ti–17.9Cu–14.6Ni (at.%) bulk amorphous alloy was investigated using in situ transmission electron microscopy and high-temperature nanoindentation. The onset of recrystallization occurs around 710 K, with the formation of Zr2Cu, Al3Zr4, and Ni10Zr7. Nanoindentation hardness and elastic modulus were obtained as a function of temperature from 298 to 723 K. The hardness was measured to be ∼7.5 GPa and found unchanged at temperatures below 673 K, but softened rapidly and decreased to 5 GPa at 723 K. These results can be correlated very well with the onset of recrystallization of this alloy at ∼700 K. The present study demonstrates that transmission electron microscopy, hot-stage nanoindentation, and X-ray diffraction are supplementary techniques for the study of glass relaxation and crystallization of metallic glasses.

Calculations of dominant factors of glass-forming ability for metallic glasses from viscosity

Abstract The calculation models in which the viscosity is used as a main parameter are proposed for the estimation of the three glass-forming ability (GFA) factors: critical cooling rate ( R c ) for formation of glassy phase, reduced glass transition temperature ( T g / T m ) and supercooled liquid range (Δ T x ≡ T x − T g ), where T g , T m and T x are the glass transition, melting and crystallization temperature, respectively. The R c and T g / T m were analyzed on the basis of the homogeneous nucleation and growth theory by the construction of a time-transformation diagram for the crystallization of metallic glass. On the other hand, Δ T x was calculated on the basis of a free volume theory proposed by Beukel and Sietsma, and then was related with R c calculated from the time–transformation curve. The calculations were carried out for Fe-, Pd-, Pt-, Zr-, Mg-based metallic glasses for which the viscosity is expressed by Vogel–Fulcher–Tammann equation or the Doolittle equation. The calculated results summarized in R c − T g / T m and R c −Δ T x diagrams agree with the experimental data qualitatively. It was shown that all the GFA factors of metallic glasses can be calculated from viscosity. These results indicate that the viscosity is the key parameter for the determinations of R c , T g / T m and Δ T x .

Crystallization of metallic glasses under the influence of high density dc currents

The effect of a dc current on the crystallization of Vit1A (Zr42.6Ti12.4Cu11.25Ni10Be23.75) and PCNP (Pd40Cu30Ni10P20) metallic glasses was investigated. Samples were isothermally annealed with and without the current, at 623 and 577 K for the two glasses, respectively. Small-angle neutron scattering analyses showed that in the absence of a current, the annealed Vit1A samples were amorphous, but the imposition of a current enhanced the crystallization process, increasing both the size and volume fraction of the crystallites. Similar general observations were seen for the PCNP glass. Differential scanning calorimetry patterns of Vit1A samples indicate a lower thermal stability of samples annealed with a current.

Microstructural implications of non-random nucleation protocols in nanocrystallized metallic glasses

Macroscopic properties of nanocrystallized metallic glasses are dependent of their nanostructure. The knowledge of the crystallization kinetics and its effect on the nanostructure are then essential in designing production and annealing protocols. Deviations from the Avrami kinetics in many of such systems are often interpreted in terms of either non-random nucleation or soft impingement due to overlapping concentration gradients. In this work, simple simulations of non-random nucleation processes allow us to evaluate the main features of both kinetic parameters and the nanostructure. It is shown that although non-random nucleation highly affects the nanostructure, it has a small effect on the transformed fraction evolution. Consequently, the decreasing Avrami exponents often reported in primary crystallization of metallic glasses have to be associated to a time dependent growth rate. Moreover, as similar kinetic behaviors are observed for different nucleation and growth protocols the understanding of the kinetics-nanostructure relationship becomes fundamental in studying such systems.

Microstructural Design by Controlled Crystallization of Metallic Glasses

ABSTRACTNanocrystalline materials can be produced e.g. by high energy ball milling or vacuum condensation; these methods require powder compaction as a final step. In another route - the nano-crystallization - metallic glasses are used as precursors for nanocrystalline materials without any porosity. The conditions for achieving an ultra-fine grained material by crystallization are small growth, but large nucleation rates. Whereas in Fe-Ni-B glasses the finest microstructure is produced at annealing temperatures above the glass transition close to the maximum of the nucleation rate, in Zr-based metallic glasses nanocrystallization was found to proceed only at relatively low temperatures below the glass transition. The aim of this contribution is to study systematically the micromechanisms involved in the microstructural design.Crystallization was studied in detail in Fe-Ni-B and Zr-based metallic glasses by means of TEM, X-ray diffraction and DSC. Nucleation and growth rates were estimated from crystallization statistics. By modeling the obtained time-dependent nucleation rates in the framework of diffusion controlled classical nucleation all relevant crystallization parameter could be derived. Using these data TTT-diagrams could be drawn and annealing conditions deducted, e.g. for the formation of a nanocrystalline alloy.Isothermal DSC plots for polymorphic crystallization can only be explained with a very significant decrease in the growth rate at later stages. Such a decrease is assumed to result from stresses building up during crystallization beyond the percolation limit for the crystalline phase; under this condition stresses resulting from the volume change during crystallization cannot be compensated by viscous flow as in the case of isolated crystals in an amorphous matrix.

Primary crystallization in amorphous Al 84Ni 8Co 4Y 3Zr 1 alloy

Understanding the crystallization of metallic glasses is essential for the consolidation of amorphous ribbons and powders into larger bodies. In this paper, we describe the evolution of crystalline phases formed during thermal induced crystallization of amorphous Al 84Ni 8Co 4Y 3Zr 1 melt-spun ribbons. Partially and fully crystallized samples were prepared either by heating in a differential scanning calorimeter to temperatures corresponding to each crystallization stage or by isothermal annealing in a heated oil bath at 473 and 628 K (±1 K). Isothermal annealing time ranged from 1 to 120 min. Thermal analysis, X-ray diffraction and transmission electron microscopy were used to determine both the reactions involved and the phases formed upon crystallization. Crystallization occurred in multiple stages, with nanometer-sized crystals of fcc-Al and Al 9Co 2 phases formed in the first and second initial stages, respectively, and formation of Al 16Ni 3Y-like, Al 3Y and Al 3Zr phases upon the final reactions. Both fcc-Al and Al 9Co 2 were found to primarily crystallize from the amorphous material. An Al 16Ni 3Y-like phase was formed by polymorphous crystallization of the amorphous phase remaining after the second stage.

Effect of the Supercooled Liquid Region on Al85Ni7Gd8 Metallic Glass Crystallization Products

ABSTRACTThe effect of the supercooled liquid region on the primary crystallization of Al85Ni7Gd8 metallic glass, which exhibits a clear glass transition before its primary crystallization, was evaluated systematically under different annealing conditions, including isothermal annealing at different temperatures and non-isothermal annealing employing different heating rates. It was found that isothermal annealing within the supercooled liquid region and non-isothermal annealing at small heating rates (≤ 5 °C/min) result in the co-precipitation of fcc-Al and Al compound(s). When isothermal annealing is done at temperatures where partial crystallization is involved, or non-isothermal annealing is carried out at a larger heating rate, the primary crystallization product is a single phase of fcc-Al. The effect of the supercooled liquid region on the crystallization product is discussed in detail.

Higher order analysis of the distribution of crystallization processes in metallic glasses

Metallic glassy alloys prepared by rapid quenching are expected to exhibit, unlike conventional polycrystalline matter, several specific features. The existence of disorder (frozen metastable state), chemical SRO, structural SRO (clusters), local heterogeneity, occurrence of ordered, yet noncrystalline, groups of atoms in the amorphous matter necessarily must be reflected in the complexity of processes controlling crystallization. Experimental observations on crystallization of amorphous alloys interpreted via classical thermodynamic analyses (which result usually in a single value, sum of values or prescribed activation energy distribution) provide further indication that these processes are numerous and should be more realistically described by a distribution reflecting the thermodynamic distribution of the initial state of the structure. These processes are by no means sufficiently described by standard simple theoretical distributions. Using a numerical Laplace inversion method, it has been possible to determine the distribution of processes controlling crystallization of metallic glasses in time and parametrically against temperature without assumptions concerning the form of the distributions. The results obtained from the changes of electrical resistivity in the course of isothermal crystallization of Fe–Co–B metallic glass have been analysed with respect to the expected inherent complexity of the system.

Crystallization of Metallic Glasses by Continuous Distribution Analysis

Crystallization of Metallic Glasses by Continuous Distribution Analysis

Crystallization of Metallic Glasses by Continuous Distribution Analysis

Crystallization of Metallic Glasses by Continuous Distribution Analysis

Evaluation of Glass-Forming Ability for Metallic Glasses from Time-Reduced Temperature-Transformation Diagram

An equation expressing the transformation curve for crystallization of metallic glasses has been proposed by using two parameters, viscosity and melting temperature (T m ). The Vogel-Fulcher-Tammann (VFT) equation, η = η 0 exp(B/(T - T 0 )) where η 0 , B and To (ideal glass transition temperature) are empirical parameters, was used for expressing the viscosity. A Time-reduced Temperature-Transformation (T-T r -T) diagram was calculated using the following five quantities: reduced temperature (Tr = T/T m ), three reduced viscosity parameters (η 0r = η 0 /T m , B r = B/T m and T 0r = T 0 /T m ), and reduced critical cooling rate (R cr = R c /T m ) for formation of the glassy phase. The R cr in the T-T r -T diagram was calculated for Ni, metallic glasses and SiO 2 The glass-forming ability (GFA) was estimated from T 0r -R cr (R cr = R c / T m ) diagrams corresponding to T g / T m - R c diagrams obtained experimentally. The metallic glasses with T 0r of 0.55 are calculated to have R cr ranging from 10 -5 to 10 0 s -1 , which agrees with the experimental data that metallic glasses with T g /T m of 0.6 or more have R c of less than 10 3 K/s. The following three points are clarified: (1) the higher GFA of metallic glass is achieved because of higher viscosity, (2) higher viscosity causes both the homogeneous nucleation frequency (I hom v ) and the ratio I N /I max ), at reduced nose temperature against the maximum of I hom v , to decrease, and (3) the R cr is numerically derived from reduced viscosity parameters.

On the determination of the crystallization activation energy of metallic glasses

The dependence of the crystallization temperature on the heating rate was measured for five conventional melt spun metallic glasses. The results obtained, together with similar data taken from the literature, were analyzed using the Kissinger method. It is shown that application of this method to glass transition and crystallization of metallic glasses leads to unreasonably high apparent attempt frequencies, by many orders of magnitude above the Debye frequency. It is concluded, in accordance with some remarks available in the literature, that this method gives obscure values of the activation parameters of glass transition and crystallization of metallic glasses. A simple equation for approximate estimate of the crystallization onset activation energy of metallic glasses is proposed.

Controlled Crystallization of Metallic Glasses Through Joule Heating

AbstractThis work reviews the basic aspects of an alternative thermal treatment of metallic glasses by Joule heating, which explores the heat produced by an electrical current flowing through samples. The advantages of this annealing method are discussed, and a brief description of available models in a variety of materials is given. The experimental procedures are discussed in detail, with emphasis in a new variant of Joule heating, known as “linear varying current Joule heating”. This method allows one to precisely monitor the structural transformations that take place during annealing, by interrupting the thermal treatment at any desired point, leading to samples with controlled micro and/or nanostructures. Some examples of Joule heating curves in novel bulk amorphous samples are shown, to illustrate the excellent perspectives of this annealing technique.

Nanocrystalline materials for high temperature soft magnetic applications: A current prospectus

Conventional physical metallurgy approaches to improve soft ferromagnetic properties involve tailoring chemistry and optimizing microstructure. Alloy design involves consideration of induction and Curie temperatures. Significant in the tailoring of microstructure is the recognition that the coercivity, (H c) is roughly inversely proportional to the grain size (D g) for grain sizes exceeding ∼0·1−1 µm (where the grain size exceeds the Bloch wall thickness,δ). In such cases grain boundaries act as impediments to domain wall motion, and thus fine-grained materials are usually harder than large-grained materials. Significant recent development in the understanding of magnetic coercivity mechanisms have led to the realization that for very small grain sizesD g<∼100 nm,H c decreases sharply with decreasing grain size. This can be rationalized by the extension of random anisotropy models that were first suggested to explain the magnetic softness of transition-metal-based amorphous alloys. This important concept suggests that nanocrystalline and amorphous alloys have significant potential as soft magnetic materials. In this paper we have discussed routes to produce interesting nanocrystalline magnets. These include plasma (arc) production followed by compaction and primary crystallization of metallic glasses. A new class of nanocrystalline magnetic materials, HITPERM, having high permeabilities at high temperatures have also been discussed.