Effects of the addition of SiC on the crystallization of Al84Ni8Co4Y3Zr1 (at.%) amorphous ribbons

Amorphous alloys present exceptional values of mechanical strength but lack any significant plasticity at room temperature. Deformation of amorphous alloys occurs in shear transformation zones that connect to form shear bands, which are easier to deform than the surrounding matrix, thus facilitating further deformation in the same location of the specimen. However, the presence of particles dispersed in the amorphous matrix can modify such strain softening behavior, resulting in real plastic deformation before fracture. Also, depending on the type of particles and how they are introduced, they can modify the crystallization behavior of the amorphous matrix by acting as heterogeneous nucleation sites. In this context, this paper reports on the effects of the addition of SiC particles on the crystallization of Al₈₄Ni₈Co₄Y₃Zr₁ amorphous ribbons. Pre-alloyed ingots with and without added SiC particles were melt-quenched into amorphous ribbons by the single-roller melt-spinning technique and then selectively and partially crystallized at the first and second crystallization temperatures, as determined by differential scanning calorimetry. Primary crystallization of nanometric-sized fcc-Al crystals was found to occur in both ribbons (with and without added SiC), confirming that crystallization reactions were not altered by the ceramic particles. Aluminum carbide (Al₄C₃) crystals resulting from high-temperature liquid metal/SiC reactions were observed as coatings on the SiC particles and as isolated particles dispersed in the amorphous solid matrix. In both cases, the Al₄C₃ particles also did not change the crystallization behavior of the amorphous Al₈₄Ni₈Co₄Y₃Zr₁ matrix, since no heterogeneous nucleation of fcc-Al crystals was observed.     

The Al nano-crystallization process in amorphous Al85Ni8Y5Co2

The primary nano-crystallization of fcc Al in initially amorphous Al₈₅Ni₈Y₅Co₂ has been studied by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) in combination with energy electron loss spectroscopy (EELS), high-resolution transmission electron microscopy (HRTEM) and X-ray diffractometry (XRD). TEM in combination with EELS after both isochronal and isothermal annealing allowed the determination of the change of the crystalline particle density and particle density/size distribution. The crystallization in Al₈₅Ni₈Y₅Co₂ was found to take place in three sequences. In the first step of the first sequence spherical fcc Al nano-particles develop with a very high particle density. In the second step of the first sequence the more or less spherical Al particles develop protrusions without significant further nucleation of fcc Al particles. In the second sequence nucleation of new fcc Al particles takes place. Comparing the crystallization behavior of Al₈₅Ni₈Y₅Co₂ with that of Al₈₅Ni₅Y₈Co₂ it follows that the yttrium solute level has a strong influence on the nucleation and growth behavior during the fcc Al primary nano-crystallization.     

Crystallization behaviour of Al-based metallic glasses below and above the glass-transition temperature

The present paper aims to report an effect of a supercooled liquid region on crystallization behaviour of the Al₈₅Y_8−xNd_xNi₅Co₂ metallic glasses produced by rapid solidification of the melt. The paper describes the crystallization process at different regimes of heat treatment. It is found that crystallization behaviour of the above-mentioned Al-based metallic glasses above the glass-transition temperature and below it follows different transformation mechanisms. Formation of the primary nanoscale α-Al particles was observed during continuous heating or after isothermal annealing above the glass-transition temperature. During isothermal annealing below the glass-transition temperature an unknown metastable phase is formed conjointly with α-Al. The metastable phase formed in the Nd-free alloy varies from that in the Nd-bearing alloys. Al₈₅Nd₈Ni₅Co₂ amorphous alloy exhibiting no glass transition crystallizes equally during isothermal calorimetry at different temperatures and during continuous heating.     

Evaluation of the volume fraction of nanocrystals devitrified in Al-based amorphous alloys

An evaluation of fcc-Al volume fractions (V_f) in Al₉₀Ni₅Nd₅ and Al₈₅Ni₅Y₆Co₂Fe₂ nanostructured composites has been made by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) methods. For the Al₉₀Ni₅Nd₅ alloy, there exhibited lower V_f values obtained by DSC than that by XRD due to increased interface energy and the overlapping of primary crystallization and structural relaxation features during nanocrystallization process. For the Al₈₅Ni₅Y₆Co₂Fe₂ alloy, the V_f values calculated by both methods showed good agreements at low crystallinities, but apparent differences are observed at high crystallinities. This is due to compositional variations in the remaining amorphous matrix during devitrification.     

Crystallization kinetics in Cu46Zr45Al7Y2 bulk metallic glass by differential scanning calorimetry (DSC)

Crystallization transformation kinetics in isothermal and non-isothermal (continuous heating) modes were investigated in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass by differential scanning calorimetry (DSC). In isochronal heating process, activation energy for crystallization at different crystallized volume fraction is analyzed by Kissinger method. Average value for crystallization in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass is 361 kJ/mol in isochronal process. Isothermal transformation kinetics was described by the Johnson-Mehl-Avrami (JMA) model. Avrami exponent n ranges from 2.4 to 2.8. The average value, around 2.5, indicates that crystallization mechanism is mainly three-dimensional diffusion-controlled. Activation energy is 484 kJ/mol in isothermal transformation for Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass. These different results were discussed using kinetic models. In addition, average activation energy of Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass calculated using Arrhenius equation is larger than the value calculated by the Kissinger method in non-isothermal conditions. The reason lies in the nucleation determinant in the non-isothermal mode, since crystallization begins at low temperature. Moreover, both nucleation and growth are involved with the same significance during isothermal crystallization. Therefore, the energy barrier in isothermal annealing mode is higher than that of isochronal conditions.     

Comparative study of the effect of cold rolling on the structure of Al–RE–Ni–Co (RE = rare-earth metals) amorphous and glassy alloys

The present paper aims to compare the deformation-induced crystallization behavior of the rapidly solidified ribbon samples of Al₈₅Y₈Ni₅Co₂, Al₈₅Nd₈Ni₅Co₂, Al₈₅Gd₈Ni₅Co₂ and Al₈₅Mm₈Ni₅Co₂ non-crystalline alloys subjected to cold rolling with 33% reduction. It is found that amorphous Al₈₅Y₈Ni₅Co₂ alloys exhibiting glass-transition and Al₈₅Nd₈Ni₅Co₂ alloys exhibiting no glass-transition have somewhat different stabilities against crystallization under deformation at room temperature though their crystallization temperatures under heating without load or deformation are almost equal. Al₈₅Gd₈Ni₅Co₂ and Al₈₅Mm₈Ni₅Co₂ alloys (Mm = Mischmetal) did not exhibit deformation-induced crystallization at the reduction ratio used. Although, the formation of the primary nanoscale α-Al particles was observed in both Al₈₅Y₈Ni₅Co₂ and Al₈₅Nd₈Ni₅Co₂ alloys after the cold rolling, the size and volume fraction of the particles are larger in the Nd-bearing amorphous alloy as compared to the Y-bearing alloy. It is found that contrary to crystallization on heating no intermetallic compounds but only α-Al nano particles were formed during the deformation-induced crystallization of Al₈₅RE₈Ni₅Co₂ amorphous alloys. The local heating on deformation is hypothesized to affect the crystallization.     

Complex crystallization mode of amorphous/nanocrystalline composite Al86Ni2Co5.8Gd5.7Si0.5

Kinetics of the first crystallization stage of Al₈₆Ni₂Co_5.8Gd_5.7Si_0.5 amorphous alloy and structure of the partially crystallized specimens were investigated by measurements of electrical resistance, X-ray diffraction and transmission electron microscopy. The presence of Al nanocrystals and eutectic colonies consisted of mutually oriented crystals of Al and metastable phase was found. The transient nucleation and slowing-down diffusion-limited growth and the interface-controlled growth of the quenched-in nuclei were identified as mechanisms of formation of the Al nanocrystals and eutectic colonies, respectively.     

Thermal stability and crystallization phenomena of low cost Ti-based bulk metallic glass

The thermal stability, kinetics of crystallization, and glass forming ability of a Ti₄₈Ni₃₂Cu₈Si₈Sn₄ bulk amorphous alloy have been studied by differential scanning calorimetry using both isothermal and non-isothermal experiments. The activation energy, frequency factor, and reaction rate for the crystallization cascade were determined via the Kissinger method. X-ray diffractometry and transmission electron microscopy studies revealed that crystallization starts with the primary precipitation of Ti(Ni,Cu), followed by the nucleation of Cu₃Ti from the amorphous precursor. The kinetics of nucleation of the primary crystalline phase was also investigated using the Johnson–Mehl-Avrami method and the Avrami exponent, n, was determined. This new alloy possesses a significantly larger supercooled liquid region than any other non beryllium- or non rare earth – containing titanium-based bulk metallic glass to date.     

Investigation of viscosity and crystallization in supercooled-liquid region of Zr-based glassy alloys

Using viscosity measurement method and in-situ heating synchrotron radiation, the viscosity of the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)_100 ? xY_x (x = 0, 0.5, 1, 2) bulk metallic glasses (BMGs) in their supercooled liquid regions (SLRs) and the in-situ heating nucleation were investigated, respectively. In the SLR, the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)₉₉Y₁ metallic glass which shows distinct plastic strain in compression exhibits higher viscosity than the other three BMGs, however their Poisson's ratios are almost the same. The synchrotron diffraction results show that crystallization happened in the SLR of the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)₉₉Y₁ glassy alloy, which could be the reason for the higher viscosity and larger plastic strain in compression compared to the other three alloys. The fracture surfaces of the glassy alloys were observed and analyzed.     

Crystallization kinetics of the Zr61Al7.5Cu17.5Ni10Si4 alloy using isothermal DSC and TEM observation

The crystallization behavior and microstructure development of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy during annealing were investigated by isothermal differential scanning calorimetry, X-ray diffractometry and transmission electron microscopy. During isothermal annealing of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy at 703 K, Zr₂Cu crystals with an average size of about 5 nm were first observed during the early stages (30% crystallization) of crystallization by TEM. The Zr₂Cu crystal size increased with annealing time and attained an average size of 20 nm corresponding to the stage of 80% crystallization. In addition, the change in particle size with increasing annealing time exhibited a linear relationship between grain growth time and the cube of the particle size for the Zr₂Cu type crystalline phase. This indicates that the crystal growth of the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ alloy belongs to a thermal activated process of the Arrhenius type. The activation energy for the grain growth of Zr₂Cu is 155 ± 20 kJ/mol in the Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ amorphous alloy. The lower activation energy for grain growth in compared to that for crystallization in Zr₆₅Cu₃₅ 440 kJ/mol crystal corresponds to the rearrangement of smaller atoms in the metallic glass, Al or Si (compare to Zr).     

Determination of activation energies for nucleation and growth from isothermal differential scanning calorimetry data

Differential scanning calorimetry (DSC) is usually adopted to analyze solid-state phase transformation incorporating nucleation, growth and impingement. Then, for isothermal transformation, time-dependent Avrami exponent and overall effective activation energy can always be deduced using recipes, which are derived from an analytical phase transformation model. On this basis, a concise and reliable approach to determine time-independent activation energies for nucleation and growth is proposed. Numerical calculations have demonstrated that the new approach is sufficiently precise under different conditions of transformation (e.g. nucleation: mixed nucleation and Avrami nucleation; growth: interface-controlled growth and diffusion-controlled growth; impingement: randomly nuclei dispersed, anisotropic growth and non-random nuclei distributions). Application of the approach in crystallization of Zr₅₅Cu₃₀Al₁₀Ni₅, Zr₅₀Al₁₀Ni₄₀ and Cu₄₆Zr₄₅Al₇Y₂ bulk amorphous alloys as measured by isothermal DSC was performed.     

Nanocrystallization and structure of Fe78.5Ni1.0Mo0.5Si6.0B14.0 amorphous alloy

The effect of Ni and Mo alloying additions on crystallization of an Fe–Si–B based amorphous alloy was studied by applying various experimental techniques – DSC, XRD and TEM. It was shown that both alloying additions Ni and Mo change the crystallization temperature as well as the activation energy of primary crystallization. The phases formed during primary crystallization for the Fe₈₀Si₆B₁₄ and Fe_78.5Ni₁Mo_0.5Si₆B₁₄ alloys were the same, however the morphologies were significantly different. The addition of 1.0 at.% of Ni and 0.5 at.% Mo changed the crystallization mechanism and the type of formed phases. Such additions also resulted in formation of nanocrystals. The kinetic and thermodynamic characteristics of annealed specimens of amorphous metallic Fe₈₀Si₆B₁₄ and Fe_78.5Ni₁Mo_0.5Si₆B₁₄ alloys were established. These characteristics were determined based on measurements of instantaneous changes of electrical strength. It was shown that the method of electromotive force measurements was more sensitive to structural changes and the phase composition of amorphous metallic electrodes in comparison with the X-ray method.     

A method for evaluating viscosities of metallic glasses from the rates of thermal transformations

The apparent activation energies E(T) for the glass transition and crystallization in Pd_77.6Cu₆Si_16.5 and Pd₄₈Ni₃₂P₂₀ glass are seen to coincide with those for the viscous flow. This implies that both the rates of glass transition and crystallization in metallic glasses scale as the viscosity. Based on this proposition, the viscosity of a Pt₄₅Ni₃₀P₂₅ glass, for example, has been evaluated, by means of thermal measurements, from the glass transition far into the crystallization temperatures. The viscosity η decreases rapidly from 10¹³ P at 480 K to 10⁷ P at 580 K and is described by a Fulcher's expression as η = 10^?3 exp[5950/(T ? 320)].     

Effects of Sn addition on the glass forming ability and crystallization behavior in Ni-Zr-Ti-Si alloys

The effect of Sn substitution for Si on the glass forming ability (GFA) and crystallization behavior has been studied in Ni₅₉Zr₂₀Ti₁₆Si_5 − xSn_x (x=0, 3, 5) alloys. A bulk amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy with diameter up to 3 mm can be fabricated by injection casting. Partial substitution of Si by Sn in Ni₅₉Zr₂₀Ti₆Si_5 − xSn_x alloys improves the glass forming ability. The improved GFA of the Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy is can be explained based on the lowering of liquidus temperature. The crystallization sequence becomes completely different with addition of Sn. The amorphous Ni₅₉Zr₂₀Ti₁₆Si₅ alloy crystallizes via precipitation of only a cubic NiTi phase in the first crystallization step, whereas the amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy crystallizes via simultaneous precipitation of orthorhombic Ni₁₀(Zr,Ti)₇ and cubic NiTi phases. Addition of Sn in the Ni₅₉Zr₂₀Ti₁₆Si₅ alloy suppresses the formation of the primary cubic NiTi phase. The bulk amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy exhibits high compressive fracture strength of about 2.7 GPa with a plastic strain of about 2%.     

Change in environmental structure around Al in Zr60Ni25Al15 metallic glass upon crystallization studied by nuclear magnetic resonance

In order to investigate the structural evolution around Al, pulse NMR experiments were carried out on ²⁷Al in the Zr₆₀Ni₂₅Al₁₅ metallic glass and the related crystalline compound, Zr₆NiAl₂. Different chemical shift peaks were observed around 2750 and 3000 ppm in the as-quenched Zr₆₀Ni₂₅Al₁₅ and crystalline compound, Zr₆NiAl₂, respectively. Considering that the capped triangular prism of Zr₉Al₃ is formed around Al in the Zr₆NiAl₂ crystal, chemical correlation pairs of Al–Zr and/or Al–Al are fairly faint while that of Al–Ni may be dominant instead in the as-quenched state. These results suggest an inhomogenous chemical bonding nature in the Zr₆₀Ni₂₅Al₁₅ metallic glass. The resonant peaks around 3000 ppm, which were distinctive in the Zr₆NiAl₂ crystal, appeared and became stronger upon crystallization through the relaxed state. Thus, drastic change in the local atomic configuration around Al was confirmed so as to form the unlike chemical correlation pairs of Al–Zr upon crystallization. The high glass-forming ability of the Zr₆₀Ni₂₅Al₁₅ metallic glass should be attributed to the difficulties of significant atomic redistribution of the constituents around Al.     

Isothermal and non-isothermal crystallization kinetics in amorphous Ni45.6Ti49.3Al5.1 thin films

The crystallization kinetics in Ni_45.6Ti_49.3Al_5.1 film were studied by differential scanning calorimetry through isothermal and non-isothermal approaches. The activation energy for crystallization was determined to be 374 and 280 kJ/mol by the Kissinger and the Augis & Bennett method, respectively, in non-isothermal methods. In the isothermal annealing study, the Avrami exponents were in the range of 2.78-3.80 between 793 and 823 K, suggesting that the isothermal annealing was governed by three dimensional diffusion-controlled growth for Ni_45.6Ti_49.3Al_5.1 thin films, in which the activation energy of nucleation is higher than that of growth. In addition, the transformation rate curves of Ni_45.6Ti_49.3Al_5.1 film were also constructed by isothermal methods. The crystallization kinetics of amorphous Ni_45.6Ti_49.3Al_5.1 film can thus be appreciated and the transformation rate also can be employed to control the degree of crystallization.     

Fabrication and characterization of metallic glasses with a specific microstructure for micro-electro-mechanical system applications

Metallic glass microstructures with high aspect ratios for micro-electro-mechanical system applications have been fabricated by micro-electro-discharge machining and selective electrochemical dissolution methods. Micro-holes and three-dimensional microstructures machined on the La₆₂Al₁₄Ni₁₂Cu₁₂, Zr₅₅Al₁₀Ni₅Cu₃₀ and Cu₄₆Zr₄₄Al₇Y₃ bulk metallic glasses by micro-electro-discharge machining are evaluated by using X-ray diffraction, scanning electron microscopy, and nanoindentation. The experimental results demonstrate that the machined samples kept their amorphous structure without devitrification, and their machining characteristics are related to the thermo-physical properties of the alloys and the electrode diameters. Porous, single-pore and thin-walled Zr-based metallic glass tubes with micro-pore structures can be prepared by selective electrochemical dissolution method. The high aspect ratio microstructures fabricated by the two methods have the potential applications as micro-nozzles, polymer micro-injection molding tools, micro-channels or micro-flow meters in micro-electro-mechanical system devices.     

Synthesis and characterization of a multicomponent Fe-based bulk amorphous alloy

A new bulk metallic glass (BMG) Fe₆₀B₁₅Zr₁₀Co₇Mo_5.5Y₂Si_0.5 was synthesized by Cu mold casting. The alloy was characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) techniques. The thermal stability and glass-forming ability (GFA) of the alloy have been discussed by evaluating a number of thermal parameters. The maximum values of the key thermal parameters like T_rg (T_g/T_m, T_g/T_l), γ, δ and β parameters were found to be ～(0.66, 0.64), 0.407, 1.84 and 3.83, respectively. The alloy showed double stage crystallization process. The activation energies for crystallization were found to be 606.6 and 623.1 kJ/mol by Kissinger and Ozawa equations, which indicate the high thermal stability. Crystallization behavior of the alloy was explored by XRD. Mechanical properties like Vicker’s hardness, nanohardness and elastic modulus are found to be very promising.