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.     

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).     

Crystallization behavior of Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass under the action of high-density pulsing current

The crystallization behavior of Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 bulk metallic glass (Vit1 BMG) under the action of high-density pulsing current (HDPC) have been studied experimentally. It has been found that high-density pulsing current can directly induce the rapid nanocrystallization of Vit1 BMG. The multiple crystallization processes of Vit1 BMG induced by HDPC have been confirmed as Amorphous → Amorphous + i-phase → Be₂Zr + Zr₂Cu + Ni₇Zr₂ + FCC phase + other phases → Zr₂Cu + Ni₇Zr₂ + FCC phase + other phases. By comparing to the crystallization behavior of Vit1 BMG induced by isothermal annealing, the crystallization temperature is reduced and crystallizing process is significantly shortened, while the sequence of crystallization process in both cases is basically same. The present results show that the HDPC has significantly influenced the crystallizing kinetics of Vit1 BMG due to that it can greatly promote the movement and rearrangement of atoms, which will result in a rapid nanocrystallization. It suggests that HDPC treatment can be an effective way to induce the rapid nanocrystallization of BMGs.     

The effect of annealing on the mechanical properties of a ZrAlNiCu metallic glass

In this paper, the effects of annealing and nanocrystallization on the mechanical properties of a Zr₅₅Al₁₀Ni₅Cu₃₀ metallic glass have been studied. It has been shown that the high volume fractioned nanocrystals facilitate the formation of shear bands and thus decrease the yield stress. At the same time, the nanocrystals also facilitate the formation of interfacial voids during compression, resulting in substantial decrease in the plasticity of the metallic glass.     

Crystallization processes in amorphous Zr54Cu46 alloy

The crystallization of amorphous Zr₅₄Cu₄₆ alloy was investigated by using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques. The experimental results show that an endothermic peak in DSC traces for amorphous Zr₅₄Cu₄₆ alloy exists at about 1006 K, indicating following eutectoid reaction occurs, namely, Cu₁₀Zr₇+CuZr₂↔CuZr in amorphous Zr₅₄Cu₄₆ alloy during heating. With increasing the heating rate, the glass transition temperature T_g and onset crystallization temperature T_x of amorphous Zr₅₄Cu₄₆ alloy increase in parallel, and the supercooled liquid region ΔT_x (=T_x−T_g) holds almost constant with an average value of 44 K. Both XRD and TEM results prove that Cu₁₀Zr₇ and CuZr₂ are main crystallization products for amorphous Zr₅₄Cu₄₆ alloy under continuous heating conditions. No CuZr phase is identified because of its small precipitation amount. Finally, the crystallization processes of amorphous Zr₅₄Cu₄₆ alloy were summarized.     

Structural relaxation of Ti40Zr25Ni8Cu9Be18 bulk metallic glass

Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ bulk metallic glass has a unique quenched-in nuclei/amorphous matrix structure. The crystallization of quenched-in nuclei, when the experimental isothermal annealing time is within its incubation time, may not disturb the enthalpy relaxation, which makes it have the accordingly common enthalpy relaxation behavior with amorphous materials. The alloy's annealing time dependence of recovery enthalpy follows a stretched exponential function with the mean relaxation time obeying an Arrhenius law. The equilibrium recovery enthalpy ΔH_T^eq, mean relaxation time τ and stretching exponent β are all dependent on the annealing temperature, and generally, a higher annealing temperature comes with a lower value of ΔH_T^eq, τ and a higher value of β. Two parameters, β_g and τ_g, representing the stretching exponent and the mean structural relaxation time at the calorimetric glass transition temperature, respectively, are correlated with glass forming ability and thermal stability, respectively. For Ti₄₀Zr₂₅Ni₈Cu₉Be₁₈ BMG, the high value of β_g, which is much higher than 0.84 and approaches unity, reveals its good glass forming ability, while, on the other hand, the low value of τ_g indicates a worse thermal stability compared with typical BMGs.     

Elasticity, thermal expansion and compressive behavior of Mg65Cu25Tb10 bulk metallic glass

The existence of special covalently bonded short-range ordering structures in a Mg₆₅Cu₂₅Tb₁₀ bulk metallic glass (BMG) is confirmed by thermal expansion and compression behavior. Under ambient conditions the linear thermal expansion coefficient obtained is almost constant in the glassy state with a value of 4.0 × 10⁻⁵ K⁻¹. By fitting the static equation of state at room temperature under ambient conditions we find the value for bulk modulus B of 48.7 GPa, which is in excellent agreement with the experimental study by pulse-echo techniques of 44.7 GPa. Unlike many bulk metallic glasses, such as Zr- and Pd-based, which bulk modulus is much larger than 100 GPa, the value B of Mg₆₅Cu₂₅Tb₁₀ BMG falls into the range of SiO₂ and fluorozirconate glass ZBLAN. Moreover, the elastic constant of the Mg₆₅Cu₂₅Tb₁₀ BMG is almost the same as those of ZBLAN. No evidence for the high-pressure phase transitions of the Mg₆₅Cu₂₅Tb₁₀ BMG has been found up to 31.19 GPa at room temperature.     

Effect of microalloying of Nb on corrosion resistance and thermal stability of ZrCu-based bulk metallic glasses

Bulk metallic glasses (BMGs) of Zr_46−xNb_xCu_37.6Ag_8.4Al₈ with x = 0, 0.5, 1, 2 and 4 at.% were prepared by copper mould casting. The corrosion resistance of the ZrCu-based BMGs with different Nb contents was carefully examined by weight loss measurements and potentiodynamic polarization tests in 3 mass% NaCl, 1 N HCl and 1 N H₂SO₄ solutions, respectively. Nb addition improves the newly developed BMGs’ corrosion resistance in chloride-containing solutions and the alloys all exhibit excellent corrosion resistance in 1 N H₂SO₄. The corrosion behavior of the alloy containing 0 and 4 at.% Nb in phosphate-buffered solution was examined by electrochemical polarization tests. The influence of Nb addition on glass forming ability (GFA), thermal stability and mechanical property was investigated by X-ray diffraction, differential scanning calorimetry and compression tests, respectively. It is found that the addition of Nb can deteriorate the GFA and thermal stability of the base system, but little effect is observed on the mechanical properties, e.g., yielding strength and plasticity, of the ZrCu-based BMG alloys.     

Crystallization of amorphous Cu60Zr40 prepared by magnetron sputter deposition

The crystallization of amorphous Cu₆₀Zr₄₀ prepared by magnetron sputter deposition was studied by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. Calorimetric results were similar to those reported in the literature for liquid-quenched Cu₆₀Zr₄₀, including the manifestation of a glass transition. Crystallization above and below the glass transition temperature, T_g, occurred by nucleation and growth of the equilibrium phase, Cu₁₀Zr₇. This phase was characterized by convergent beam electron diffraction. With isothermal annealing below T_g, the time scale for crystallization indicated that the vapor-quenched alloy was kinetically more stable than the liquid-quenched alloy. This was interpreted as a difference in the quenched-in structures, produced by the different synthesis methods. During longer anneals, TEM analysis indicated that the structure was being contaminated by oxygen.     

Glass-forming ability and crystallization of bulk metallic glass (HfxZr1−x)52.5Cu17.9Ni14.6Al10Ti5

We have produced a series of bulk metallic glasses of composition (Hf_xZr_1−x)_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (with x=0-1) by an arc melting/suction casting method. The density of these alloys increases by nearly 67% with increasing Hf content from 6.65 g/cm³ (x=0) to 11.09 g/cm³ (x=1). Over the same composition range the glass-forming ability decreases, as demonstrated by the size of the largest amorphous ingots that can be cast without crystallization. Although both the glass transition temperature and the melting temperature increase linearly with increasing Hf content, the reduced glass transition temperature (T_g/T_m) decreases, from 0.64 (x=0) to 0.62 (x=1), which suggests that the `confusion principle' correlating increased glass-forming ability with increased number of components, does not apply in this case due to the chemical similarity between Zr and Hf. A different crystallization behavior is observed for Zr-based and Hf-based glasses. The final crystalline phases are CuZr₂ and Zr₂Ni for Zr-based alloys, and Al₁₆Hf₆Ni₇ and CuHf₂ for Hf-based alloys.     

Hf–Cu–Ni–Al bulk metallic glasses: Optimization of glass-forming ability and plasticity

In the Hf–Cu–Ni–Al quaternary system, the Hf₅₁Cu_27.75Ni_9.25Al₁₂ bulk metallic glass (BMG) exhibits the best combination of the large glass-forming ability (GFA) and compressive plasticity. Minor substitution of Nb for Hf in Hf–Cu–Ni–Al BMGs degrades not only the GFA but also plasticity, while the substitution of Ta does not have an appreciable effect on both properties. For the investigated Hf-based BMGs, the shear modulus G is a more sensitive indicator to correlate with their plasticity than the Poisson′s ratio. Meanwhile, the correlation between the G and the glass transition temperature T_g for the Hf-based BMGs can be expressed as G = 9.9 + 576 (T_g/V_m)[1 ? 4/9(T/T_g)^2/3].     

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.     

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.     

Phase competition of Cu64Zr36 and its effect on glass forming ability of the alloy

In the present paper, crystallization behavior and glass forming ability (GFA) of Cu₆₄Zr₃₆ bulk metallic glass (BMG) were studied based on the crystal phase competition. Electrical resistivity and X‐ray diffraction results indicate that Cu₆₄Zr₃₆ glass underwent a two‐stage crystallization process, during which Cu₁₀Zr₇ and Cu₅₁Zr₁₄ crystals precipitate at first and then there are only Cu₁₀Zr₇ and Cu₈Zr₃ phases at the end of the second stage. Intriguingly, it was found that it is the competing phase Cu₁₀Zr₇ that weakens GFA of the Cu₆₄Zr₃₆ alloy, because the Cu₁₀Zr₇ precipitate is fully restrained in the cast rod when substituting Zr with 7.5at%Ti and thereby its diameter with fully amorphous structure is enlarged from 1 mm to 2 mm.     

Glass formation and crystallization behavior in Mg65Cu25Y10−xGdx (x=0, 5 and 10) alloys

The glass forming ability and crystallization behavior of Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5 and 10) alloys have been investigated. The glass forming ability (GFA) is significantly improved when Y in Mg₆₅Cu₂₅Y₁₀ is substituted with Gd. Ternary Mg₆₅Cu₂₅Gd₁₀ bulk metallic glass (BMG) with diameter of at least 8 mm was successfully fabricated by conventional Cu-mold casting method in air atmosphere. Mg₂(Y, Gd) is the first competing crystalline phase against the glass formation in the Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5) alloys, while Mg₂Cu and Cu₂Gd are the competing crystalline phases in the Mg₆₅Cu₂₅Gd₁₀ alloy. Therefore, the suppression of the formation of Mg₂(Y, Gd) during cooling from the liquid improves the GFA significantly.     

Characterization of the free volume in a Zr45.0Cu39.3Al7.0Ag8.7 bulk metallic glass by reverse Monte Carlo simulation and density measurements

The absolute contents of free volume in the as-cast and annealed Zr_45.0Cu_39.3Al_7.0Ag_8.7 bulk metallic glass (BMG) samples were quantified by density measurements and reverse Monte Carlo (RMC) simulation using the total structural factors F(Q) determined by X-ray diffraction (XRD) experiments as fitting constraints. The densities of the as-cast, annealed and crystallized samples measured by Archimedes method are 7.319, 7.327 and 7.342 g/cm³ (precision ± 0.003 g/cm³), respectively. A new approach was used in the RMC simulation to define the free volume as the difference between the volume of the Voronoi polyhedron and the volume of the Wigner-Seitz cell of the constituent atoms. Two types of initial configurations were constructed: (1) in configuration A, all types of potential atomic pairs are allowed; (2) in configuration B, Al-Al and Ag-Ag atomic pairs are excluded. The contents of free volume in the as-cast and annealed samples were found to be 0.59% and 0.39% (configuration A), 0.55% and 0.34% (configuration B), respectively (precision ± 0.03%). The probability distribution of the as-calculated free volume can be well-fitted by the equation proposed by Turnbull and Cohen. Finally, it is shown that the contents of free volume determined by density measurements and RMC are comparable, while the discrepancy of the results is discussed.     

The thermal stability and activation energy of crystallization of (Cu61.8Zr38.2)1−xAlx bulk metallic glasses

The present paper reports on the thermal stability and activation energy of crystallization of bulk metallic glasses (BMGs) (Cu_61.8Zr_38.2)_1−xAl_x. The (Cu_61.8Zr_38.2)_1−xAl_x composition series, prepared by copper mould suction casting into bars with a diameter of 3 mm, form BMGs with an e/a range of 1.24–1.3. These BMGs manifest increased thermal stability with increased e/a ratios. The activation energies (ΔE) of crystallization as derived from thermal analysis at different heating rates follow a similar tendency to that of the thermal stability, indicating stronger short-range ordering with increasing e/a ratios. The optimum BMG Cu_58.1Zr_35.9Al₆ exhibits the highest thermal stability and the largest ΔE.     

Influence of structural relaxation on atomic mobility in a Zr41.2Ti13.8Cu125Ni10.0Be22.5 (Vit1) bulk metallic glass

The effect of an annealing at a temperature above or below the glass transition temperature in a Zr_41.2Ti_13.8Cu₁₂₅Ni_10.0Be_22.5 bulk metallic glass was investigated using dynamic mechanical analysis. Structural relaxation influences both the storage modulus (elastic component) and the loss modulus (viscoelastic component). Kinetics can be captured by a stretched exponential relaxation function. Experimental results are correctly described using a physical model based on the concept of defects for the mechanical response of amorphous materials and especially for the characteristic time relative to atomic mobility.     

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.     

Modeling and measurement of residual stresses in a bulk metallic glass plate

The recent advent of multi-component alloys with exceptional glass forming ability has allowed the processing of large metallic specimens with amorphous structure. The possibility of formation of thermal tempering stresses during the processing of these bulk metallic glass (BMG) specimens was investigated using two models: (i) instant freezing model, and (ii) viscoelastic model. The first one assumed a sudden transition between liquid and elastic solid at the glass transition temperature. The second model considered the equilibrium viscosity of BMG. Both models yielded similar results although from vastly different approaches. It was shown that convective cooling of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 plates with high heat transfer coefficients could potentially generate significant compressive stresses on the surfaces balanced with mid-plane tension. The crack compliance (slitting) method was then employed to measure the stress profiles in a BMG plate that was cast in a copper mold. These profiles were roughly parabolic suggesting that thermal tempering was indeed the dominant residual stress generation mechanism. However, the magnitude of the measured stresses (with peak values of only about 1.5% of the yield strength) was significantly lower than the modeling predictions. Possible reasons for this discrepancy are described in relation to the actual casting process and material properties. The extremely low residual stresses measured in these BMG specimens, combined with their high strength and toughness, serve to further increase the advantages of BMGs over their crystalline metal counterparts.