HRTEM study of crystallization of Zr–Cu–Ni metallic glass

Employing differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy (HRTEM), the micromechanism for crystallization of Zr₇₀Cu₂₀Ni₁₀ metallic glass under isothermal annealing conditions has been investigated. It is found that the relationship between the annealing temperature and the peak position, incubation time and ending time in the isothermal annealing DSC traces of Zr₇₀Cu₂₀Ni₁₀ metallic glass obeys a first-order exponential function. However, the time–temperature transformation curves of Zr₇₀Cu₂₀Ni₁₀ metallic glass at different crystallized volume fractions can be well fitted by a second-order exponential function. It is observed that at the initial crystallization stage some ordered atomic clusters precipitate first, acting as nucleation sites and facilitating the subsequent crystallization process, and the crystal growth process mainly proceeds through the atomic depositing on the previously formed crystals. This behavior confirms that the new micromechanism for crystallization of amorphous alloys proposed by Lu and Wang can also be applied to the new series of zirconium based amorphous alloys.     

Pressure effect on the structural relaxation and glass transition in metallic glasses

Two different enthalpy recovery methods were applied to investigate the effects of pressure on structural relaxation and glass transition temperature (T_g) in two metallic glasses (Pd₄₀Ni₁₀Cu₃₀P₂₀ and Zr₆₅Al_7.5Cu_27.5). The pressure-induced enhancement of T_g (dT_g/dP) was derived to be 6 K/GPa in Pd₄₀Ni₁₀Cu₃₀P₂₀ glass, while an increment of 50 K/5 GPa was observed in Zr₆₅Al_7.5Cu_27.5 glass. Activation (formation and migration) volumes of the flow defect were used to interpret the pressure dependence of the structural relaxation and T_g in terms of the free volume model. According to the measured results, the activation volume of relaxation was derived to be 16.7 ų for Pd₄₀Ni₁₀Cu₃₀P₂₀, which is much smaller than that of the polymers.     

Structure relaxation and crystallization of Al83Ni10Ce7 metallic glass

Structure relaxation and crystallization of Al₈₃Ni₁₀Ce₇ metallic glass were studied by different scanning calorimetry (DSC) and X-ray diffraction (XRD). According to the DSC scan, it is interesting to find that the second exothermic peak changes with pre-annealing temperatures (below glass transition temperature), suggesting a change in the amorphous structure upon relaxation. Continuous heating crystallization and isothermal crystallization exhibit different crystallization mechanism of the present alloy. fcc-Al and a metastable phase precipitate simultaneously in the first stage crystallization during continuous heating; however, only a metastable phase precipitates during isothermal annealing below glass transition temperature (T_g).     

Avrami exponent and isothermal crystallization of Zr/Ti-based bulk metallic glasses

The isothermal crystallization kinetics of Zr/Ti-based bulk metallic glasses (BMGs) have been investigated by using Avrami exponent. It is a constant nucleation rate and a constant number of quenched-in nuclei that cause the linear John–Mehl–Avrami (JMA) modes in Avrami exponents of Zr₆₂Al₈Ni₁₃Cu₁₇ and Zr₆₅Al₈Ni₁₀Cu₁₇ BMGs, respectively. However, sub-T_g pre-annealing of Zr₆₂Al₈Ni₁₃Cu₁₇, and multiple-step phase transition in Ti₄₃Cu₄₃Zr₇Ni₇ and Ti₄₅Zr₅Cu₄₀Ni_7.5Sn_2.5 make their Avrami exponents deviate from the linear JMA mode. The difference in Avrami exponents maybe provides useful information for performing the microstructure control of BMG composites upon isothermal annealing.     

Superplastic deformation and crystallization behavior of Cu54Ni6Zr22Ti18 metallic-glass sheet

Superplastic deformation and crystallization behavior of a Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass were investigated. A maximum elongation of 650% was obtained at 733 K at 1 × 10⁻² s⁻¹ from the sheet fabricated by squeeze copper-mold casting method. At low strain rates, the strain-rate-sensitivity exponent value was close to 1, suggesting that Newtonian-like behavior governed the plastic flow. At a high strain rate around 10⁻² s⁻¹, a transition from Newtonian to non-Newtonian behavior took place with decrease in m value. Large strain hardening by crystallization occurred during the course of deformation. The strain hardening was found to be caused by crystallization according to the analyses of the relation of true stress vs. testing time, T-T-T diagram and DSC characteristics. The time periods up to the strain before strain hardening at 733 K for the Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass were similar to that of the Zr₆₅Al₁₀Ni₁₀Cu₁₅ metallic glass at 696 K as 180–300 s (3–5 min). This coincidence could be explained by comparison of their T-T-T diagrams showing that the incubation times for crystallization of the Cu BMG at 733 K and for Zr BMG at 696 K are similar.     

Effect of ball-milling and shot-peening on Zr55Al10Ni5Cu30 alloys

Effect of ball-milling and shot-peening on a metallic glass Zr₅₅Al₁₀Ni₅Cu₃₀, which possesses a large supercooled liquid region, has been investigated by means of differential scanning calorimetry, x-ray diffractometry and transmission electron microscopy. Metallic glassy ribbons, powders and plates were prepared by melt-spinning, gas-atomizing and mold-clamp casting techniques, respectively. No structural changes were observed in both the ribbon and powder specimens by ball-milling for around 100 h; however, the powder specimens were crystallized by Fe contamination when they were ball-milled for 540 h. No structural evolution was also observed when the plate specimens were subjected to shot-peening, while crystallized plate specimens were easily amorphized by mild and short period shot-peening. These results imply high phase stability of the Zr₅₅Al₁₀Ni₅Cu₃₀ metallic glass against deformation.     

Ball indentation tests for a Zr-based bulk metallic glass

Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass was characterized using ball indentation tests. Comparison of the data with the expanding cavity model revealed that the deformation is pressure insensitive for compressive loading. The plastic flow curves obtained from indentation tests showed perfectly plastic response and no strain rate sensitivity up to 15% strain.     

Correlation Between Dynamic Compressive Strength and Fracture Behaviors of Bulk Metallic Glasses

The mechanical behaviors of Zr_43.5Cu_43.5Ni₄Al₈Nb₁, Zr_55.4Cu_31.6Ni₄Al₈Nb₁, Ti_32.8Zr_30.2Ni_5.3Cu₉Be_22.7 (at.%) metallic glass at different strain rates were studied. For all the present alloys, the dispersion over 700 MPa was observed on the strength in the repeated dynamic compressions, which was much stronger than that of the quasi-static compressive strength. Such the dispersion of the dynamic compressive strength was well correlated with the corresponding fracture behaviors. The area of fracture surface was calculated and also showed a strong dispersion for all the fractured specimens tested at the strain rate of 500 s^-1 and 1000 s ^-1. All the specimens showed a linear relationship between the square of dynamic compressive strength and the area of fracture surface in the dynamic compression tests. This phenomenon was mainly thought to be related to the difference of mean initial free volume concentration of different samples, stress concentration caused by the split Hopkinson pressure bar experimental setup and high sensitivity of defects under dynamic deformation. These findings were beneficial to deeply understand the effect of strain rate on the mechanical properties of the metallic glass.     

Mechanically induced crystalline–glassy phase transformations of mechanically alloyed Ta55Zr10Al10Ni10Cu15 multicomponent alloy powders

New multicomponent Ta-based glassy alloy powder was synthesized by mechanical alloying (MA) the elemental powders of Ta₅₅Zr₁₀Ni₁₀Al₁₀Cu₁₅ at room temperature, using a low-energy ball milling technique. During the early stage of milling the agglomerated crystalline powders are mechanically crushed and fresh surfaces are rapidly created. Kneading of such ground powders enhances the atomic diffusion and leads to local alloying. As the MA time increases, the number of vacancies in the Ta lattice (base material) increases so that the atoms of the alloying elements for Zr, Al, Ni and Cu tend to migrate to the open defected lattice of metallic Ta. The number of atoms of the alloying elements that migrate to the bcc lattice of the base material are increasing with increasing MA time and this leads to a monotonic expansion of the Ta lattice. Further milling time (86–130 ks) plays an important role in increasing the rate of diffusion and this leads to an increase in the number of migrated atoms of the alloying elements that pass into the Ta lattice. The a₀ of the yielded solid solution at this stage does not change anymore with increasing MA time and a homogeneous supersaturated bcc-solid solution is obtained after 130 ks of MA time. This solid solution, which is subjected to continuous imperfections, is gradually transformed into a glassy phase upon increasing the MA time. The glassy powders of the final-product (1080 ks) in which its glass transition temperature (T_g) lies at a high temperature (834 K), crystallize through a single sharp exothermic peak at 1004 K (T_x). The total enthalpy change of crystallization (ΔH_x) is −10.32 kJ/mol. The width of the supercooled liquid region before crystallization (ΔT_x) of the synthesized glassy powder shows the largest value (170 K) of any reported metallic glassy system.     

Microstructure studies of Zr65Cu17.5Al7.5Ni10 and Zr65Cu15Al10Ni10 glass forming alloys: Phase morphologies and undercooled melt solidification

Zr₆₅Cu_17.5Al_7.5Ni₁₀ (at.%) and Zr₆₅Cu₁₅Al₁₀Ni₁₀ (at.%) glass forming alloy microstructures have been investigated by means of optical and electron microscopies. They are composed of a fine eutectic matrix with eutectic dendrites (EDs) that have peculiar morphologies. Al and Cu concentrations, in these alloys, favour primary dendrites and determine the ED morphologies and compositions. Their locations within the microstructures suggest a two-step solidification process of the two undercooled melts. The identified crystalline phases indicate the occurrence of solid state phase transformations in agreement with the structural defects observed in the grains. The crystalline phases can be classified into Zr-rich, Cu-rich, Ni-rich and Al-rich compounds resulting from competing diffusion between Cu, Ni, and Al in the melts.     

Glass formation ability and isothermal crystallization of melt-spun NdNiCuAl ribbons

On the basis of the previous work on (Nd_62.5Ni_37.5)₈₅Al₁₅ alloy, Cu is selected to partially substitute Ni to form (Nd_62.5Ni_37.5−xCu_x)₈₅Al₁₅ (x = 0, 10, 20, 30) melt-spun alloys. The glass-forming ability (GFA) of the as-prepared alloys is evaluated by the isochronal differential scanning calorimeter (DSC) measurement. The results show that GFA increases with Cu content according to several different criterions. The isothermal crystallization behaviors in the corresponding supercooled liquid region is discussed by both Johnson–Mehl–Avrami (JMA) equation and some nucleation and growth models. The fitting shows that it is reasonable to divide the whole crystallization processes into two stages. And the fittings with the nucleation and growth models infers that with increasing Cu content, the nucleation mechanism of the primary stage changes from the quenched-in and steady-state nucleation for (Nd_62.5Ni_37.5)₈₅Al₁₅, to the steady-state nucleation for (Nd_62.5Ni_27.5Cu₁₀)₈₅Al₁₅ and (Nd_62.5Ni_17.5Cu₂₀)₈₅Al₁₅, then to the time-dependent nucleation for (Nd_62.5Ni_7.5Cu₃₀)₈₅Al₁₅. And the dependence of crystallization mechanisms on Cu substitution agrees well with the change of their GFAs.     

Electron microscopy and X-ray diffraction studies of rapidly quenched Zr---Ni and Hf---Ni ribbons with about 90 at.% Ni

The structure of melt-spun ribbons of the alloys Zr₉Ni₉₁, Zr₁₀Ni₉₀ and Hf₁₁Ni₈₉ was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Ribbons of the Zr₉Ni₉₁ and Zr₁₀Ni₉₀ alloys prepared at a high cooling rate (ribbon thickness d=11 μm) were characterized by an amorphous matrix with a few per cent of quenched-in crystallites. The ribbon of the Hf₁₁Ni₈₉ alloy prepared with the same thickness (i.e. at the same cooling rate) exhibited a nanocrystalline grain structure of the HfNi₅ phase. Thicker ribbons of the Zr₉Ni₉₁ alloy (d=17–22 μm), for which the quench rate was correspondingly lower, were obtained as a b.c.c. Ni(Zr) solid solution phase with a grain size of nearly 1 μm. A high resolution (HR) TEM study of one of the Zr₉Ni₉₁ crystalline ribbons revealed a fine structure of the interior of the crystallites which can be attributed to an ordering on the Zr sublattice over distances of several nanometres within the b.c.c. grains.     

Novel Crystallization Behaviors of Zr-Based Metallic Glass Under Thermo-Mechanical Coupled Fatigue Loading Condition

Novel crystallization behaviors of Zr_(55)Cu_(30)Al_(10)Ni_5 bulk metallic glass are investigated.On the one hand,mixed oxides,including CuO,CuAlO_2,CuAl_2O_4 and ZrO_2,show sequential oxidation process determined by coupling effects of specific cyclic load and temperature.On the other hand,at a temperature(100 ℃)by far lower than Tg of 412 ℃,under cyclic loading condition,non-oxidized binary alloy CuZr_2 is precipitated;the thermo-mechanical coupled effects of temperature below Tg and fatigue accumulation on the non-oxidized crystallization behaviors are revealed.Meanwhile,at a constant temperature of 400℃, by comparing among the XRD patterns,respectively,obtained from tensile,creep and fatigue fractures,the dominating effect of cyclic load on the generation of non-oxidized CuZr_2 is verified.Furthermore,the crystallization behavior of amorphous phases under cyclic loading condition is observed through TEM micrograph and diffraction pattern at 100 ℃.     

Preparation, thermal stability, and magnetic properties of Fe---Co---Zr---Mo---W---B bulk metallic glass

A bulk metallic glass (BMG) cylinder of Fe₆₀Co₈Zr₁₀Mo₅W₂B₁₅ with a diameter of 1.5 mm was prepared by copper mould casting of industrial raw materials. The amorphous state and the crystallization behavior were investigated by X-ray diffraction (XRD). The thermal stability parameters, such as glass transition temperature (T_g), crystallization temperature (T_x), supercooled liquid region (ΔT_x) between T_g and T_x, and reduced glass transition temperature T_rg (T_g/T_m) were measured by differential scanning calorimetry (DSC) to be 891, 950, 59 K, and 0.62, respectively. The crystallization process took place through a single stage, and involved crystallization of the phases -Fe, ZrFe₂, Fe₃B, MoB₂, Mo₂FeB₂, and an unknown phase, as determined by X-ray analysis of the sample annealed for 1.5 ks at 1023 K, 50 K above the DSC peak temperature of crystallization. Mössbauer spectroscopy was studied for this alloy. The spectra exhibit a broadened and asymmetric doublet-like structure that indicated paramagnetic behavior and a fully amorphous structure. -Fe was found in the amorphous matrix for a cylinder with a diameter of 2.5 mm. The success of synthesis of the Fe-based bulk metallic glass from industrial materials is important for the future progress in research and practical application of new bulk metallic glasses.     

A mechanical spectroscopy study of the Zr69.5Cu12Ni11Al7.5 alloy

The vibrating reed technique has been used to investigate the Zr_69.5Cu₁₂Ni₁₁Al_7.5 glass forming alloy. The phase transformations of the material, as can be seen through the changes of the resonance frequency of the sample as a function of temperature, have been studied and the results have been compared with TEM and DSC measurements confirming the development, above T_g, of a metastable quasicrystalline state prior to crystallisation. Hydrogen-induced damping peaks were then observed in the different phases of the material with particular attention to the quasicrystalline state, where it seems that the microscopic reorientation mechanism responsible for the internal friction peaks should be similar to the one already known in the amorphous phase (hydrogen jumps between tetrahedral sites). Shortly after crystallisation, three damping peaks were observed, two of which are attributed to relaxation processes (Zener-type or intercrystalline Gorsky effect) taking place in the tetragonal CuZr₂ phase.     

Magnetic properties of Al–Gd–Ni orthorhombic compounds

The magnetic properties of the Al₄GdNi, Al₂GdNi and AlGd₂Ni₂ compounds have been investigated using magnetic measurements in the temperature range 4–800 K and magnetic fields up to 9 T and X-ray photoelectron spectroscopy at room temperature. The compounds Al₄GdNi and AlGd₂Ni₂ order antiferromagnetically at T_N = 20 and 25 K, respectively, and Al₂GdNi orders ferromagnetically at T_C = 39 K. The results are discussed in terms of the Ruderman–Kittel–Kasuya–Yosida theory, the polarization of 5d electrons by the local exchange interaction 4f–5d and the spin fluctuations on Ni sites.     

Thermal stability and magnetic properties of Gd–Fe–Al bulk amorphous alloys

Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous alloys were produced by copper mold casting method with the maximum diameters of 2, 1 and 1 mm, respectively. The crystallization temperature (T_x) and melting temperature (T_m) of the Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloy are 808 and 943 K, respectively. Accordingly, the temperature interval of T_m and T_x, ΔT_m (=T_m − T_x), is as small as 135 K and the reduced crystallization temperature (T_x/T_m) is as high as 0.86. The small ΔT_m and high T_x/T_m values are presumed to be the origin for the achievement of the high amorphous-forming ability of the Gd–Fe–Al bulk amorphous alloy. The Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous cylinders with a diameter of 1 mm exhibit superparamagnetism at room temperature, while the amorphous ribbon shows the paramagnetism at room temperature. Finally, the mechanical properties of Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloys are investigated.     

Types of mechanical relaxation by hydrogen in intermetallic compounds

Hydrogen in polycrystalline intermetallic compounds generally produces extremely broad damping spectra, indicating the presence of short-range as well as long-range relaxation mechanisms. These are discussed on the basis of vibrating-reed results on Zr₆₅Cu_27.5Al_7.5, Zr₆₅Cu_17.5Ni₁₀Al_7.5, and CuZr₂ alloys. A relaxation peak at 270 K observed in all three cases is interpreted as a Zener-type relaxation of hydrogen in the CuZr₂ lattice. The differences found at higher temperatures — a second relaxation peak in the ternary and quaternary alloys but an exponential increase of damping in CuZr₂ — are related to different grain sizes and can be attributed to an ‘intercrystalline Gorsky effect’ due to elastic anisotropy mismatch strains.     

Effect of Si addition on the glass-forming ability of a NiTiZrAlCu alloy

The effect of Si addition on the glass-forming ability (GFA) of a NiTiZrAlCu alloy was investigated by using differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The maximum diameter of glassy rods increased from 0.5 mm for the Ni₄₂Ti₂₀Zr₂₅Al₈Cu₅ alloy (the base alloy) to 2.5 mm for the Ni₄₂Ti₂₀Zr_21.5Al₈Cu₅Si_3.5 alloy and to 3 mm for the Ni₄₂Ti₁₉Zr_22.5Al₈Cu₅Si_3.5 alloy, when prepared by using the copper mould casting. The GFA of the alloys can be assessed by the reduced glass transition temperature T_rg(=T_g/T_l) and a newly proposed parameter, δ(=T_x/T_l − T_g). An addition of a proper amount of Si and a minor substitution of Ti with Zr can enhance the GFA of the base alloy by suppressing the formation of primary Ni(TiZr) and (TiZr)(CuAl)₂ phases and inducing the composition close to eutectic.     

Crystallisation of oxygen-stabilised amorphous phase in a Zr50Cu50 alloy

The crystallisation of the oxygen-stabilised amorphous phase in a Zr₅₀Cu₅₀ alloy has been investigated by means of neutron diffraction and electron microscopy. The crystallisation microstructure consists of ZrO₂, Zr₂Cu and Zr₇Cu₁₀. A two-stage crystallisation mechanism is suggested: (i) primary crystallisation of Zr₂Cu and (ii) formation of nanocrystals ZrO₂ and Zr₇Cu₁₀. In (i), it is proposed, Zr₂Cu crystallises from the oxygen-stabilised amorphous phase, leaving an oxygen- and copper-enriched matrix ; Zr₂Cu rapidly grows and eventually attains a grain size of about 100 nm. In (ii), it is suggested, the residual amorphous matrix crystallises into nanocrystals ZrO₂ and Zr₇Cu₁₀ due to the sluggish growth of ZrO₂ and to the already formed ZrO₂ which acts as a growth barrier to Zr₇Cu₁₀. In this case there is no particular orientation relationship between Zr₂Cu and Zr₇Cu₁₀.