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.     

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.     

Influence of Gd and Fe on crystallization of Al87Y5Ni8 amorphous alloy

Crystallization of amorphous Al-based alloys (Al-Y-Gd-Ni-Fe) was investigated by applying differential scanning calorimetry (DSC), X-ray diffraction (XRD) and high resolution electron microscopy (HREM). It was shown that the crystallization in the examined alloys proceeds in three stages (DSC maxima). The two first stages are attributed to formation of solid solution of fcc Al(RE) nanograins in amorphous matrix. In the third stage the precipitation of ternary compound Al₁₉Ni₅RE₃ of the orthorhombic Al₁₉Ni₅Gd₃-type structure was observed. A partial substitution of Ni by Fe causes a change of stoichiometry and crystal structure of the ternary compounds: Al₈TM₄RE (TM = Fe, Ni; RE = Y, Gd) of the tetragonal ThMn₁₂ (Al₈Mn₄Ce)-type structure. A partial replacing of Y atoms by Gd in the Al₈₇Y₅Ni₈ based alloy shifts the Al(RE) nanocrystallization to lower temperatures. In contrast to this a partial replacing of Ni by Fe shifts the nanocrystallization to higher temperatures.     

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.     

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.     

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.     

X-ray diffraction study of amorphous alloys Al–Ni–Ce–Sc with using Ehrenfest’s formula

The effect of replacement of Ce by Sc in the amorphous Al₈₅Ni₁₀Ce₅ alloy on its structure has been studied. The replacement was shown to result in increasing the coordination number in the first coordination sphere, changing the type of short-range order. A splitting the first peak of the function of radial atom distribution into two subpeaks made it possible to determine separately the coordination number for the atom pair groups Ce–Al, Sc–Al, Al–Al and Ni–Al, Al–Al. The structure of the amorphous alloys Al₈₅Ni₁₀Ce_5−xSc_x (x = 0, 1, 5) was determined using X-ray diffractometry combined with application of Ehrenfest’s formula, which allowed us to directly determine the first coordination sphere radius from data on the structure factor and some cluster parameters obtained from the prepeak on the structure factor curve.     

Ductility improvement of Zr-Cu-Ni-Al glassy alloy

We studied the relationship between the phase diagrams and formation of crystalline inclusions in cast samples in order to improve the ductility of Zr-Cu-Ni-Al bulk amorphous alloy. A Zr₅₀Cu₄₀Al₁₀ bulk amorphous alloy with a composition close to the ternary eutectic point has no crystalline inclusions and possesses superior mechanical properties of tensile strength σ_B=2000 MPa, Young’s modulus E=107 GPa and Vickers hardness HV=580. Oxygen embrittlement of Zr-Cu-Ni-Al bulk amorphous alloys can be avoided in the Zr₅₀Cu₃₀Ni₁₀Al₁₀ bulk amorphous alloy, which exhibits superior ductility and resistance to oxidation. Furthermore, we tried to improve ductility by cold rolling by making use of the apparent work-softening phenomenon of Zr-Cu-Ni-Al bulk amorphous alloys. Preliminary studies indicated that cold rolling to produce a fine slip-band structure, which enabled uniform deformation and superior deformability, is an important procedure for improving the ductility of Zr-Cu-Ni-Al bulk amorphous alloys.     

Studies on the formability of Al-based metallic glasses/nanocomposites based on isochronal DSC analysis

The Al₈₆Si_0.5Ni_4.06Co_2.94Y₆Sc_0.5 metallic glass of highly improved glass-forming ability (GFA) has been investigated by isochronal differential scanning calorimetry measurements, as well as the Al₈₅Ni₅Co₂Y₈ for comparison. The experimental results indicate that the Al₈₆Si_0.5Ni_4.06Co_2.94Y₆Sc_0.5 exhibits enlarged temperature interval between the first and second crystallization onsets (termed as primary fcc-Al/glass region), as well as enlarged second activation energy (E_p2) against the nucleation and the growth of intermetallic compounds besides fcc-Al. The variation of the Avrami exponent demonstrates that the primary crystallization process is a rapid two- and three-dimensional diffusion-controlled nucleation and growth mechanism initially, and the whole process is strongly controlled by the growth of fcc-Al crystals. According to the analysis on the basis of atomic mobility of alloy components, it demonstrates that the enlarged primary fcc-Al/glass region obtained through proper coexistence of dissimilar and similar elements would be in favor of preparing Al-based metallic glasses or nanocomposites in greater size.     

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.     

Influence of outphase Cu50Ti50 amorphous alloy addition on microstructural evolution of mechanically alloyed Zr66.7Ni33.3 amorphous alloy

The influence of outphase Cu₅₀Ti₅₀ amorphous alloy addition on microstructural evolution of Zr_66.7Ni_33.3 amorphous alloy has been investigated using a mechanical alloying method. It has been found that the milling induced microstructural evolution is related to the change of peak positions of the first maximum on X-ray diffraction patterns of the as-obtained amorphous alloys. With increasing milling time, the 3 wt.% Cu₅₀Ti₅₀ addition can give rise to the cyclic amorphization transformation of the as-milled alloy. The mechanical stability of the mixing amorphous phase can be greatly enhanced with increasing Cu₅₀Ti₅₀ addition up to 10 wt.%. Moreover, the addition of outphase Cu₅₀Ti₅₀ amorphous alloy not only increases the onset crystallization temperature of Zr_66.7Ni_33.3 amorphous alloy but also alters its crystallization mode. The effect of outphase amorphous addition on the mechanical stability of the Zr_66.7Ni_33.3 amorphous phase has been discussed based upon the bond order theory.     

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.     

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.     

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.     

Atomic structure and hydrogen storage properties of amorphous–quasicrystalline Zr–Cu–Ni–Al melt-spun ribbons

A structural state of the Zr–Cu–Ni–Al melt-spun ribbons has been investigated by means of X-ray diffraction analysis. It was established that conditions of ribbon production have a very strong effect on their structural state. The Zr_69.3Cu_9.7Ni_15.1Al_5.9 ribbons produced at the surface wheel velocity of 44 m/s have an amorphous state at the contact side and mixed amorphous–quasicrystalline state at the free side. At the same time the Zr_67.5Cu_12.5Ni₁₂Al₈ ribbons are fully amorphous in case of producing velocity of 44 m/s and have only small features of quasicrystalline peaks on the amorphous halo on the free side in case of producing velocity of 30 m/s in contrast to an amorphous structure on the contact side. The uncoated by Pd amorphous and amorphous–quasicrystalline Zr–Cu–Ni–Al ribbons have a property of absorbing a large amount of hydrogen. Parameters of the amorphous state were calculated for the ribbons as-prepared and hydrogenated. The temperature coefficient of resistivity for all ribbons is negative in range of 20–380 °C, what as well as the high resistivity values is typical to the Zr–Cu–Ni–Al based systems in amorphous and quasicrystalline states.     

Amorphization and crystallization processes of the ball-milled Al–Y–Fe–TM alloys (TM = Ni,Co, Cu,and Fe)

High-energy ball milling was used to synthesize aluminum-based alloys containing amorphous and nanocrystalline phases to investigate the compositional effects of transition metals (TM) on the amorphization and crystallization processes of the ball-milled Al₈₅Y₇Fe₅TM₃ alloys (TM = Ni, Co, Cu, and Fe) were investigated. The crystallization kinetics of the ball-milled Al–Y–Fe–TM nanocomposite powders were studied using differential scanning calorimetry (DSC). The DSC results of Al₈₃Y₇Fe₅Ni₅ show that the crystallization temperature and the activation energy of crystallization are 668 K and 310 kJ/mol, respectively. In-situ high-temperature X-ray diffraction showed that the crystallization was a complex process involving growth of the nanocrystalline phase along with crystallization of the amorphous matrix phase.     

Synthesis and mechanical properties of Zr-based bulk amorphous alloys

Design and characterization of bulk amorphous alloys has been an active area of research due to their promising thermal and mechanical properties. An alloy composition Z₆₅Cu₁₇Ni₁₀Al₈ was designed and synthesized by Cu mold casting. In the base alloy 2 at.% Gd was added to study its effect on thermal and mechanical properties. Characterization of the alloys was done by techniques of X-ray diffraction (XRD), differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM). Many thermal parameters were evaluated to investigate the thermal stability and glass-forming ability of the alloys. In addition, the mechanical properties like nanohardness, elastic modulus and elastic recovery were measured. Thermal properties and activation energy reduced while mechanical properties like nanohardness, elastic modulus and percentage elastic recovery (% R) increased with Gd addition.     

Potentiodynamic polarization studies on amorphous Zr46.75Ti8.25Cu7.5Ni10Be27.5, Zr65Cu17.5Ni10Al7.5, Zr67Ni33 and Ti60Ni40 in aqueous HNO3 solutions

Potentiodynamic polarization studies were carried out on virgin specimens of Zr-based bulk amorphous alloys Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 and Zr₆₅Cu_17.5Ni₁₀Al_7.5, and conventional-type binary amorphous alloys Zr₆₇Ni₃₃ and Ti₆₀Ni₄₀ in solutions of 0.2 M, 0.5 M and 1.0 M HNO₃ at room temperature. The values of the corrosion current density (I_corr) for the bulk amorphous alloy Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 were found to be comparable with those of Zr₆₅Cu_17.5Ni₁₀Al_7.5 in 0.2 M and 0.5 M HNO₃, but the value of I_corr for the former was almost three times more than that of the latter in 1.0 M HNO₃. In the case of conventional binary amorphous alloys, Ti₆₀Ni₄₀ showed lower value of I_corr as compared to Zr₆₇Ni₃₃ in 0.5 M and 1.0 M HNO₃ and a comparable value of I_corr in 0.2 M HNO₃. In general, the binary Ti₆₀Ni₄₀ displayed the best corrosion resistance among all the alloys in all the cases and the corrosion current density (I_corr) for all the alloys was found to increase with the increasing concentration of nitric acid. It is noticed that the bulk amorphous alloys do not possess superior corrosion resistance as compared to conventional binary amorphous alloys in aqueous HNO₃ solutions. The observed differences in their corrosion behavior are attributed to different alloy constituents and composition of the alloys investigated.     

Estimation of critical cooling rates for formation of amorphous alloys from critical sizes

The critical cooling rate for marginal glass formers, like Al-based alloys, is difficult to measure experimentally. In this work, we acquired the critical cooling rate for formation of amorphous alloys by estimating the cooling rates of the specimens with critical dimensions. Analytical solutions were given to estimate the cooling rates for the gas-atomized powders and melt-spun ribbons, and as an example the critical cooling rate to form an amorphous Al₈₂Ni₁₀Y₈ alloy was estimated to be ~ 1.0 × 10⁶ K s^? 1. The effect of melt temperature on the cooling rate was quantitatively evaluated and its effect on the glass forming ability was also discussed.     

Bulk metallic glass formation: The positive effect of hydrogen

Zr₅₅Cu₃₀Ni₅Al₁₀ alloys with different amounts of hydrogen have been prepared by arc melting under the gaseous mixture of hydrogen and argon. Proper additions of hydrogen have been proved to effectively increase the glass-forming ability (GFA) of this alloy. Positive effect of hydrogen on GFA has been interpreted from the thermodynamic and structural points of view. Proper additions of hydrogen can decrease the liquidus temperature, which leads to more stable glass-forming liquid. Structure analysis by positron annihilation lifetime spectroscopy shows that proper additions of hydrogen can increase the concentration of shortest open volume and decrease the concentration of intermediate and largest open volumes. This leads to formation of a denser random packed structure, and thus increases the GFA of Zr₅₅Cu₃₀Ni₅Al₁₀ alloys.