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.     

Effect of Pd additions on phase stability and hydrogen-induced internal friction of Ti34Zr11Cu47Ni8 glassy alloys

The effects of additions of Pd below 10 at.% on the stability and hydrogen-induced internal friction behavior of Ti₃₄Zr₁₁Cu₄₇Ni₈ glassy alloys have been investigated. Thermal analyses indicate that the supercooled liquid region decreases distinctly with increasing Pd content. It was found that internal friction peak temperatures of Ti₃₄Zr₁₁Cu₄₇Ni₈–Pd hydrogenated glassy alloys (HGAs) in the hydrogen content range below approximately 30 at.% H were higher than those of the original Ti₃₄Zr₁₁Cu₄₇Ni₈ HGAs, especially at hydrogen contents below 10 at.% H. It was also found that the internal friction peaks of Ti₃₄Zr₁₁Cu₄₇Ni₈–Pd HGAs increased with increasing hydrogen content below 15 at.% H, after which they tended to saturate. These results are in contrast to the effects of Si addition as previously reported. The effects of Pd are discussed from the viewpoint of the interstitial site distribution for hydrogen and local atomic structure of the glassy alloy.     

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.     

Non-isothermal Crystallization Kinetics and Isothermal Crystallization Kinetics in Supercooled Liquid Region of Cu–Zr–Al–Y Bulk Metallic Glass

The crystallization kinetics of Cu_(43)Zr_(48)Al_9and(Cu_(43)Zr_(48)Al_9)_(98)Y_2bulk metallic glasses in non-isothermal and isothermal conditions was studied by differential scanning calorimetry.In the non-isothermal and isothermal modes,the average activation energy of(Cu_(43)Zr_(48)Al_9)_(98)Y_2is larger than that of Cu_(43)Zr_(48)Al_9,meaning the higher stability against crystallization of(Cu_(43)Zr_(48)Al_9)_(98)Y_2.In addition,the average activation energies for Cu_(43)Zr_(48)Al_9and(Cu_(43)Zr_(48)Al_9)_(98)Y_2calculated using Arrhenius equation in isothermal mode are larger than the values calculated by Kissinger–Akahira–Sunose method in non-isothermal mode,indicating that the energy barrier is higher in isothermal mode.The Johnson–Mehl–Avrami model was used to analyze the crystallization kinetics in the non-isothermal and isothermal modes.The Avrami exponent n for Cu_(43)Zr_(48)Al_9is above 2.5,indicating that the crystallization is mainly determined by a diffusion-controlled three-dimensional growth with an increasing nucleation rate,while the Avrami exponent n for(Cu_(43)Zr_(48)Al_9)_(98)Y_2is in the range of 1.5–2.5 in the non-isothermal mode,implying that the crystallization is mainly governed by diffusion-controlled three-dimensional growth with decreasing nucleation rate.Finally,the Avrami exponents n for Cu_(43)Zr_(48)Al_9and(Cu_(43-)Zr_(48)Al_9)_(98)Y_2are different in the non-isothermal and isothermal conditions,which imply different nucleation and growth behaviors during the crystallization processes.     

Effect of cooling rate on crystallization of metallic Zr–Cu–Ni glass

X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were employed to investigate the effect of cooling rate on crystallization behavior of metallic Zr₇₀Cu₂₀Ni₁₀ glass. It is found that all DSC traces of the metallic Zr₇₀Cu₂₀Ni₁₀ glasses under different cooling rates exhibit two exothermic peaks, indicating that the crystallization of metallic Zr₇₀Cu₂₀Ni₁₀ glass proceeds through a double-stage mode. In our previous studies, we have concluded that the first exothermic reaction mainly corresponds to the precipitation of the Zr₂Cu phase, and the second one is mainly due to the formation of nanoscale Zr₂Ni particles. It is observed that there exists a close relationship between the cooling rate and thermodynamic parameters of metallic Zr₇₀Cu₂₀Ni₁₀ glass, such as the onset crystallization temperature T_x, the first peak temperature T_p1 and the second one T_p2. The above three thermodynamic parameters reach a maximum when the surface velocity is 30 m/s. This effect is just similar to that of the Ni concentration, which has been discussed in our previous works. The activation energy for crystallization and the local Avrami exponent of metallic Zr–Cu–Ni glass under isothermal annealing conditions also exhibit a similar tendency with the cooling rate.     

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.     

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.     

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.     

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 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.     

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.     

A comparative study on the hydriding kinetics of Zr-based AB2 hydrogen storage alloys

Two kinetic models (Jander model and Chou model) are used to investigate the hydrogen absorption kinetic mechanism of Zr-based AB₂ type Laves phase alloys (Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5, Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55). The analysis shows that the rate-controlling step is the diffusion process at high temperatures in the range from 673 K to 923 K with a low hydrogen concentration (solid solution phase). Both models can well describe the experimental data but Chou model is preferred. Chou model is simpler and easier to use for analyzing the experimental results. The activation energies calculated using Chou model with the least square method are 29.3 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5, 43.8 kJ/mol H₂ for Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and 48.5 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55, which are close to the values reported in the literature (28.3 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5 and 40.3 ± 1.5 kJ/mol H₂ for both Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55).     

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.     

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.     

Al25Nb20Ti30Zr25低密度高熵合金的组织和性能

通过Thermo-Calc软件计算、微观组织多尺度表征以及热模拟试验等研究了Al₂₅Nb₂₀Ti₃₀Zr₂₅合金的组织结构、高温组织稳定性和热加工性能。结果表明,Al₂₅Nb₂₀Ti₃₀Zr₂₅合金的铸锭组织主要由BCC基体相和Zr₅Al₃析出相组成,Zr₅Al₃相在BCC晶界连续析出,晶粒内部的Zr₅Al₃相呈块状分布,平均尺寸在750 nm左右;合金在750~1000 ℃保温24 h后,基体中的晶粒尺寸并未发生明显变化;随着温度的增加,Zr₅Al₃相含量小幅度降低,合金的高温组织稳定性较好。建立了合金的本构方程为$\dot{ε}$=4.5×10¹⁴×[sinh(0.0063σ_p)]^2.8exp(-419/RT),并绘制了合金的能量耗散系数图;在1050 ℃/1 s^-1变形条件下,能量耗散系数达到峰值0.69,在该变形条件下等温锻造出尺寸为$\phi$ 180 mm×20 mm完整无开裂的圆形块体材料。锻造消除了原始晶界处连续分布的Zr₅Al₃相,使其分解成短杆状均匀分布于合金基体中,BCC基体组织发生了动态回复和部分再结晶。     

内生枝晶相体积分数可控的钛基非晶复合材料

采用直接调控增塑相摩尔分数的方法成功制备了系列含有bcc枝晶相的(Ti32.8zr30.2Ni5 3Cu9Be22 7)100-s(Ti61 5-Zr36 4Cu2.1)x(x=10—95)非晶复合材料.采用xRD,SEM和DSC等方法研究了不同x值下枝晶相的体积分数以及形貌的变化规律,并利用TEM研究了内生枝晶相的结构和两相界面结构.结果表明,在Ti32 8Zr30 2Ni5 3Cu9Be22 7合金中直接添加不同量的Ti61 5Zr36.4Cu2.1合金能够有效地控制内生枝晶相的析出量,获得界面结合良好的非晶复合材料,且枝晶相的尺寸及体积分数与x值成正比.室温压缩实验结果表明,当x>30时,复合材料的塑性变形能力可得到明显改善.x值越大,枝晶相体积分数越高,复合材料屈服强度越低,塑性改善效果越明显.不同体积分数的复合材料均表现出明显的加工硬化现象.当x=90时,复合材料的塑性变形量达到14.4%,断裂极限强度达1917 MPa.     

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 ℃.     

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.     

The effect of Hf substitution for Zr on glass forming ability and magnetic property of FeCoZrMoBAlY bulk metallic glasses

Bulk metallic glasses (BMGs) Fe₆₁Co₆Zr_8−xHf_xMo₇B₁₅Al₁Y₂ (x = 0–8) have been produced by copper mold casting technique using industrial raw materials. The effect of substitution of Hf for Zr on the glass forming ability (GFA) and the magnetic property has been studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and superconducting quantum interference device (SQUID). It was found that the substitution of an appropriate amount of Hf for Zr can improve the GFA of the base alloy Fe₆₁Co₆Zr₈Mo₇B₁₅Al₁Y₂, as demonstrated by the increase in reduced glass transition temperature T_rg (=T_g/T_l) and GFA parameters of γ (=T_x/T_g + T_l) and δ (=T_x/T_l − T_g). The Fe₆₁Co₆Zr₅Hf₃Mo₇B₁₅Al₁Y₂ alloy exhibits the highest GFA with the largest T_rg (0.612) and δ (1.633), and can cast a fully amorphous rod in 3 mm diameter. The substitution of Hf for Zr also enhances the magnetic properties, as verified by the increase in saturation magnetization (M_s) in the alloy of Fe₆₁Co₆Zr₃Hf₅Mo₇B₁₅Al₁Y₂, whose M_s is approximately 1.5 times higher than that of the base alloy (x = 0) at room temperature. Finally, the effect of the substitution of Hf for Zr on glass forming ability and magnetic properties is discussed.     

Analysis of the internal friction spectrum of TiNiCu alloy

The internal friction spectra of TiNiCu shape memory alloy (SMA), which involves thermoelastic martensitic transformation, can be divided into three different terms: intrinsic, transitory and phase transition. In this paper, the damping behavior during the reverse martensitic transformation in Ti₅₀Ni₂₇Cu₂₃ SMA has been investigated using dynamic mechanical analyzer instrument. The internal friction spectra of Ti₅₀Ni₂₇Cu₂₃ SMA have been analyzed quantitatively and divided into intrinsic and transitory contributions using an iterative method. The internal friction spectra with different oscillation frequency have been predicted based on the quantitative analysis.