Hump peak formation and the crystallization in amorphous Al87Co10Ce3 alloy

A distinct hump peak was observed at the scattering angle 2θ ≈ 44° in the X-ray diffraction (XRD) pattern of rapidly quenched amorphous Al₈₇Co₁₀Ce₃ alloy. From the XRD patterns of amorphous Al-Co-Ce alloys with different compositions, it is found that the intensity of the hump peak increases as the Co/Ce atomic ratio increases, while the prepeak which characterizes the medium-range order (MRO) becomes weak. The hump peak has a close relationship with the glass forming ability in the Al-Co-Ce system. Furthermore, the crystallization behavior of Al₈₇Co₁₀Ce₃ alloy was investigated by the DSC and XRD methods. Compared with Al₈₇Ni₁₀Ce₃ alloy, Al₈₇Co₁₀Ce₃ alloy did not show a single process of grain growth for fcc-Al particle. The formation of a hump peak is presumed to be associated with the presence of pre-existing nuclei of fcc-Al and Co₂Al₉ in as-quenched amorphous Al₈₇Co₁₀Ce₃ alloy.     

Crystallization of amorphous Al84.2Ni10La2.1Ce2.8Pr0.3Nd0.6 alloy

The crystallization of amorphous Al_84.2Ni₁₀La_2.1Ce_2.8Pr_0.3Nd_0.6 alloy was investigated by using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) techniques. The amorphous Al_84.2Ni₁₀La_2.1Ce_2.8Pr_0.3Nd_0.6 alloy crystallizes through the primary precipitation of fcc-Al phase from the amorphous matrix in the range of 490–550K, followed by the precipitation of Al₃Ni and Al₁₁(La, Ce)₃ phases in the temperature range of 590–680 K. Comparing amorphous Al_84.2Ni₁₀La_2.1Ce_2.8Pr_0.3Nd_0.6 and Al₈₄Ni₁₀Ce₆ alloys, the addition of La can improve the precipitation of primary fcc-Al phase upon reheating the samples. On the basis of the Kissenger equation, the activation energy for crystallization of Al₃Ni phase is higher than that of the other two phases, fcc-Al and Al₁₁(La, Ce)₃ phases, implying that the thermal stability of the Al₃Ni phase is higher than that of the other two phases.     

Crystallization behaviour and mechanical properties of rapidly solidified Al<Subscript>87.5</Subscript>Ni<Subscript>7</Subscript>Mm<Subscript>5</Subscript>Fe<Subscript>0.5</Subscript> amorphous alloy

The crystallization behaviour and the mechanical properties of rapidly solidified Al_87.5Ni₇Mm₅Fe_0.5 alloy ribbons have been examined in both as-melt-spun and heat-treated condition using differential scanning calorimetry, X-ray diffractometry (XRD), transmission electron microscopy (TEM), tensile testing and Vicker’s microhardness machine. XRD and TEM studies revealed that the as-melt-spun ribbons are fully amorphous. The amorphous ribbon undergoes three-stage crystallization process upon heating. Primary crystallization resulted in the formation of fine nanocrystalline fcc-Al particles embedded in the amorphous matrix. The second and third crystallization stages correspond to the precipitation of Al₁₁(La,Ce)₃ and Al₃Ni phases, respectively. Microhardness and tensile strength of the ribbons were examined with the variation of temperature and subsequently correlated with the evolved structure. Initially, the microhardness of the ribbon increases with temperature followed by a sharp drop in hardness owing to the decomposition of amorphous matrix that leads to formation of intermetallic compounds     

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.     

Ti-based amorphous alloys with a wide supercooled liquid region

Ti-Cu-Ni-Co quaternary amorphous alloys produce by melt spinning were found to have a wide supercooled liquid region before crystallization, though no glass transition was observed in Ti-Cu binary amorphous alloys. The largest temperature interval of the supercooled liquid region (ΔT_x) is as large as 90 K for Ti₅₀Cu₂₅Ni₂₀Co₅.There is a tendency for ΔT_x to increase with an increase in storage modulus and with a decrease in loss modulus. It is therefore presumed that the increase in ΔT_x for the multicomponent amorphous alloy is due to the suppression of crystallization for the supercooled liquid resulting from the increase in viscosity.     

Crystallization study of amorphous Al87.5Ni7Mm5Fe0.5 alloy by electrical resistivity measurement

Electrical resistivity measurement has been used to study the crystallization in amorphous Al_87.5Ni₇Mm₅Fe_0.5 alloy. Differential scanning calorimetry (DSC) and resistivity measurement showed that the alloy undergoes a three-stage crystallization process compared to the two-stage crystallization behaviour in many metallic glasses. The first and second peaks around 425 and 609 K, respectively, correspond to the precipitation of fcc-Al and Al₁₁(La,Ce)₃ phases. The activation energy for the formation of Al₁₁(La,Ce)₃ phase has been found to be 153±5.3 kJ/mol. The Avrami exponent is in the range of 1.50–1.76 indicating a three-dimensional growth mechanism with a decreasing nucleation rate.     

Effects of liquid quenching and subsequent heating on the structure of Co-Al Alloys

The phase composition and structure of Co-Al alloys after liquid-quenching (LQ) and subsequent heating were determined by x-ray diffraction, differential thermal analysis, and differential scanning calorimetry. After LQ, the Co-19.5 at % Al alloy consisted of two phases: CoAl (B2 structure, a=0.2852 nm) and an fcc Co_{1 – x} Al_x solid solution (a=0.3573 nm). The phase composition of the LQ alloy and the lattice parameters of the intermetallic phase CoAl and solid solution corresponded to a solidification temperature of 1100°, rather than to the eutectic temperature (1400°C). Heating to 1000°C brought the alloy to the equilibrium state. In the range 25–28 at % Al, the LQ alloys were single-phase and consisted of Co-enriched CoAl (B2). Decomposition of this intermetallic phase during heating in the calorimeter gave rise to an exothermic peak at 680°C and led to precipitation of hcp Co, which converted to an fcc Co_{1 – x} Al_x solid solution on heating to 1000°C. The activation energy of the decomposition process, evaluated by the Kissinger method, was E _a=125 ± 10 kJ/mol. The LQ Co-71.4 at % Al alloy consisted of three phases: metastable quasicrystalline phase (similar in structure to Al₁₄Co₃Ni₃), Co₂Al₅, and CoAl. Heating brought the alloy to the equilibrium state (Co₂Al₅), with an exotherm near 690°C. The activation energy of this phase transformation was E _a=195 ± 15 kJ/mol. The LQ alloys containing 76.5 and 82 at % Al were single-phase and consisted of monoclinic Co₄Al₁₃ or Co₂Al₉ with reduced lattice parameters.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 420–426.Original Russian Text Copyright © 2005 by Portnoi, Tretyakov, Filipova, Latuch.     

Corrosion behavior of bulk amorphous Zr55Al10Cu30Ni5−xPdx alloys

Bulk amorphous Zr₅₅Al₁₀Cu₃₀Ni_5−xPd_x (x=0, 5at.%) alloys were produced by copper mould casting. The microstructure of samples was characterized by X-ray diffraction (XRD). The corrosion resistance of the bulk amorphous alloys was studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The Pd-free alloy showed an active–passive transition by anodic polarization in 3.5% NaCl solution, it was spontaneously passive and had a wide passive region with significantly low passive current density. The 5% Pd amorphous alloy shows a single active state even with a limited passive region. X-ray photoelectron spectroscopy (XPS) analysis revealed that the spontaneous passive films formed on the alloys after immersion in 0.6 M NaCl solution for 72 h were composed of the oxidation of Zr, Al and Cu element. The Pd-free and 5% Pd alloys show excellent corrosion properties in 1 M NaOH solution.     

Microstructure and magnetocaloric effects in partially amorphous Gd55Co15Al30−xSix alloys

In order to clarify the phase components and further improve the glass-forming ability of Gd₅₅Co₁₅Al₃₀ alloy, substitution of Al with Si was adopted. Although the X-ray powder diffraction experiment indicated an amorphous structure of the Gd₅₅Co₁₅Al_30−xSi_x (x = 1, 2, 3) alloys, precipitation of crystalline Gd₂Al phase was evident from the energy-dispersive spectroscopy, selected-area diffraction, and magnetization measurements. The magnetocaloric effect of Si substituted alloys is lower than that of Gd_52.5Co_16.5Al₃₁ alloy with a similar composition and full amorphous structure, which is ascribed to the presence of antiferromagnetic Gd₂Al phase whose magnetic entropy change is lower.     

The effect of Fe on the crystallization behavior of Al–Mm–Ni–Fe amorphous alloys

The crystallization behavior and thermal stability of Al₈₆Mm₄Ni_10–x Fe _x alloys were investigated as a function of Fe content. Alloys, produced by a single roll melt-spinner at a circumferential speed of 52 m/s, revealed fully amorphous structures. The thermal stability of the present amorphous alloys increased with the increase of Fe content. The activation energy for crystallization of -Al increased as the Fe content increased. This increase of activation energy resulted in the simultaneous precipitation of -Al and intermetallic phase observed especially in Al₈₆Mm₄Ni₅Fe₅ and Al₈₆Mm₄Ni₂Fe₈ alloys. The glass transition was observed in DSC thermogram only after proper annealing treatment. The effect of alloy composition on the thermal stability could be explained in terms of the atomic structure of the amorphous alloy.     

Formation of metastable solid solutions of boron in nickel under mechanochemical synthesis from Ni-B and Ni-Nb-B mixtures

Using high-energy ball milling of Ni₈₇B₁₃ and Ni_{87 ? x} Nb _x B₁₃ component mixtures (x = 7, 10, 12, 14) for 2 h, nanocrystalline alloys containing fcc solid solutions of Ni〈B〉 and Ni〈Nb,B〉 were obtained. According to the results of X-ray analysis, oversaturated solid solutions of Ni〈B〉 are interstitial solutions; those of Ni〈Nb,B〉 are substitutional solutions or mixed substitutional-interstitial solutions. Upon heating of the alloys, exothermal effects appear on differential scanning calorimetry (DSC) curves; they are attributed to decomposition of metastable solid solutions and to crystallization of the amorphous phase appearing in mechanical alloying. After heating to 720°C, the Ni₈₇B₁₃ alloy contained stable phases of Ni and Ni₃B; the Ni₇₅Nb₁₂B₁₃ alloy contained a τ phase (Ni₂₁Nb₂B₆) and a metastable solid solution of Ni〈Nb〉.     

Amorphous Ti–Cu–Ni–Al alloys prepared by mechanical alloying

Ti _x (CuNi)_90–x Al₁₀ (x = 50, 55, 60) amorphous powder alloys were synthesized by mechanical alloying technique. The evolution of amorphization during milling and subsequent heat treatment was investigated by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The fully amorphous powders were obtained in the Ti₅₀Cu₂₀Ni₂₀Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₆₀Cu₁₅Ni₁₅Al₁₀ alloys after milling for 30, 20 and 15 h, respectively. Differential scanning calorimetry revealed that thermal stability increased with the increasing (CuNi) content: Ti₆₀Cu₁₅Ni₁₅Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₅₀Cu₂₀Ni₂₀Al₁₀. Heating of the three amorphous alloys at 800 K for 10 min results in the formation of the NiTi, NiTi₂ and CuTi₂ intermetallic phases.     

Enhanced thermal stability of amorphous aluminum alloys through microalloying

Amorphous aluminum alloy powders with compositions of Al₈₅Y₇Fe₈, Al₈₃Y₇Fe₈Ti₂, and Al₇₉Y₇Fe₈Ni₃Ti₂Nd₁ were synthesized with crystallization temperatures of 342 °C, 446 °C, and 457 °C, respectively. In-situ high-temperature synchrotron diffraction results were correlated with differential scanning calorimetry studies to investigate the structural evolution during the crystallization. The results show that, through microalloying, the onset-of-crystallization temperature was increased by 115 °C, and the resulting crystalline products changed from a mixture of fcc-Al and an intermetallic phase in the case of Al₈₅Y₇Fe₈ to only intermetallic phases.     

Calorimetric studies of non-isothermal crystallization in amorphous Cu x Ti100−x alloys

The present paper reports the composition dependence of pre-exponential factor and activation energy of non-isothermal crystallization in amorphous alloys of Cu _x Ti_100?x system using differential scanning calorimeter (DSC) technique. The applicability of Meyer-Neldel relation between the pre-exponential factor and activation energy of non-isothermal crystallization for amorphous alloys of Cu-Ti system was verified.     

Crystallization kinetics of an amorphous Al<Subscript>75</Subscript>Ni<Subscript>10</Subscript>Ti<Subscript>10</Subscript>Zr<Subscript>5</Subscript> alloy

The formation and crystallization behaviors of a mechanically alloyed Al₇₅Ni₁₀Ti₁₀Zr₅ amorphous alloy were studied by X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry in the present study. The effective activation energy of the crystallization was determined by the Kissinger and Ozawa equations, respectively. The two equations yield close results and the average activation energy is 252 ± 13 kJ/mol. The resultant crystalline products were Al and Al₃Ni, and the crystallization mechanism is two- or three-dimensional nucleation and growth controlled by the diffusion of atoms. The thermal stability of the alloy was evaluated by a continuous transformation diagram obtained by the extended Kissinger equation.     

Crystallization of amorphous Ni60 Nb40−x Cr x alloys

Crystallization behaviour of amorphous Ni₆₀ Nb_40-x Cr _x (x = 0, 5, 10 and 13 at%) alloys was studied by differential Scanning calorimetry and X-ray diffraction measurements. It is shown that the addition of chromium reduces the crystallization temperature, stages of crystallization and activation energies associated with the crystallization sages of the Ni₆₀Nb₄₀ glass. Crystallization of the Ni₆₀Nb₄₀ glass occurred in three stages; in the initial stage a metastable M-phase formed in the amorphous matrix as reported earlier [1] . However, contrary to earlier observation [1], M -phase was not very stable and transformed together with some amorphous phase to the equilibrium Ni₃Nb phase in the second stage of crystallization. In the third stage, the remaining amorphous matrix transformed to the equilibrium NiNb phase. On addition of chromium the formation/stability of the M-phase was found to be suppressed and equilibrium NbCr₂ phase precipitated preferentially in the first stage. The second stage, corresponding to the formation of Ni₃Nb phase, remained almost unaltered. The third stage corresponding to the crystallization of NiNb phase disappeared completely at 13 at% Cr. In the fully crystallized samples the proportion of the NiNb phase decreased and that of NbCr₂ phase increased continuously with chromium concentration.     

Devitrification of Al-Ni-Rare earth amorphous alloys

The effect of rare earth (RE) elements on the devitrification behaviour of Al₈₇Ni₇RE₆ alloys (RE = La, Sm) is reported. Two crystallisation mechanisms are found as a function of the type of element and, therefore, size: when RE = La the transformation occurs in two steps, when RE = Sm the transformation has an additional step. The glass transition becomes manifest for both alloys when fast enough rates are used in Differential Scanning Calorimetry. The first crystallisation step implies precipitation of nanocrystalline Al in Al₈₇Ni₇Sm₆ and of a metastable intermetallic phase in Al₈₇Ni₇La₆, relatively coarse grained. Its formation is suppressed by substitution of 1% at La with Ti or Zr. Interestingly, the amorphous matrix in Al₈₇Ni₇La₆ is stabilised by annealing at temperatures below the glass transition so that the second crystallisation peak is shifted to higher temperature. In Al₈₇Ni₇Sm₆ the glass transition remains visible after partial crystallisation showing that the matrix is readily homogenised. As a consequence, it transforms abruptly with a polimorphic-like reaction. There is evidence of a calorimetric continuous background for all alloys which is attributed to diffusional homogenisation of the alloy and grain growth. The results are discussed in terms of nucleation and growth mechanisms.     

Influence of the substituting Ni with Fe on the cycle stabilities of as-cast and as-quenched La0.7Mg0.3Co0.45Ni2.55 − xFex (x = 0–0.4) electrode alloys

The electrode alloys La_0.7Mg_0.3Co_0.45Ni_2.55 ? xFe_x (x = 0, 0.1, 0.2, 0.3, 0.4) are fabricated by casting and rapid quenching techniques. The effects of the substitution of Fe for Ni on the cycle stabilities as well as the structures of the alloys have been investigated thoroughly. The results indicate that the substitution of Fe for Ni significantly enhances the cycle stability of the alloys. Furthermore, the positive impact of such substitution on the cycle stability has been observed to be more pronounced for the as-quenched alloy as compared to that for the as-cast one. Scanning electron microscopy (SEM) studies demonstrate that all the alloys exhibit a multiphase structure comprising of two major phases (La, Mg)Ni₃ and LaNi₅ along with a residual phase of LaNi₂. The substitution of Fe for Ni has been observed to facilitate the formation of a like amorphous structure in the as-quenched alloy. With an increase in Fe contents, a significant grain refinement of the as-quenched alloy and an obvious enlargement in the lattice constants and the cell volumes of the alloys have been noticed.     

A study of vacancy-type defects in pure and boron-doped Ni3Al alloys

Positron lifetimes have been measured for two sets of pure and boron-doped Ni₃Al alloys. The alloys were large-grain polycrystals and had compositions of Ni_75+xAl_25−x (x = −1, 0, + 1) with 0, 100 and 500 wt ppm boron added. Lifetime parameters for samples of composition Ni_75+xAl_25−x (x= ± 1) with 0 and 500 wt ppm boron added were measured after initial thermal conditioning and after a subsequent cold-work anneal treatment. Positron trapping (≈20%) was observed in all unprocessed alloys. The vacancy concentration was calculated to be ≈ 5 × 10⁻⁶ and showed little, if any, systematic dependence on either alloy composition or boron concentration. Cold-worked fully annealed samples contained no detectable vacancies, i.e. the trapped state intensity was observed to be zero. The results are at variance with previously published data. During the annealing procedure (> 350°C) carbon was observed to diffuse out of the cold-worked samples. It is therefore possible that carbon stabilizes vacancies in Ni₃Al alloys. There is, however, no evidence to suggest that boron interacts with constitutional vacancies in Ni₃Al.     

Glass formation and phase selection of melt-spun Al–Zn–Ce alloys

The glass-formation characteristics and phase-selection behavior of Al–Zn–Ce alloys have been studied by X-ray diffraction (XRD) and different scanning calorimetry (DSC). As the concentration of Ce increases, an intermetallic compound Al₂Zn₂Ce appears, which prevents the occurrence of phase separation, and improves the forming ability of the single amorphous phase. In Al₈₃Zn₁₀Ce₇ alloys, the precipitation of fcc-Al was accompanied with the Al₂Zn₂Ce phase and Al₄Ce phase. Moreover, the presence of fcc-Al appears to favor the nucleation and growth of the Al₂Zn₂Ce and Al₄Ce phase. However, it seems that the Al₂Zn₂Ce and Al₄Ce nucleate competitively with the fcc-Al phase and the growth of fcc-Al and Al₂Zn₂Ce prefers to that of Al₄Ce. The competitive nucleation and growth limitation of the various phases are favorable to the formation of Al–Zn–Ce amorphous alloys. For the amorphous Al–Zn–Ce alloys, the glass formation is not controlled by nucleation restrictions but largely by the suppression of growth of nuclei formed during rapid melt quenching.