Enthalpy recovery and free volume relaxation in a Zr44Ti11Ni10Cu10Be25 bulk metallic glass

We report on the free volume of the Zr₄₄Ti₁₁Ni₁₀Cu₁₀Be₂₅ bulk metallic glass in terms of its enthalpy recovery and volumetric relaxation below the glass transition temperature, T_g. Glassy samples are isothermally annealed below T_g using differential scanning calorimetry and the resulting enthalpy recovery, ΔH_r, is measured upon re-heating into the supercooled liquid region. Volumetric changes below T_g are measured isothermally using Thermo-Mechanical Analysis. The total changes in the relative free volume, Δν_f/ν_m, between the initially glassy state and the equilibrium liquid are calculated from the volumetric relaxation. The measured values of ΔH_r and Δν_f/ν_m correlate well within the framework of free volume theory and a linear relationship is found between the two.     

Atomic structure of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass alloy

Ab initio molecular dynamics (AIMD) calculations were performed on the atomic configuration of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass. The local structures were characterized in terms of structure factors (SF), pair correlation functions (PCF), coordinate numbers, bond pairs and Voronoi polyhedra. The glass transition temperature, generalized PCF and SF predicated by AIMD are in good agreement with the experimental data. Icosahedral short-range orders (ISRO) are found to be the most dominant, in view of the presence of the majority of bond pairs with 1551, 1541 and 1431, and Voronoi polyhedra with <0,3,6,1>, <0,2,8,1>, <0,0,12,0> and <0,2,8,4>. Icosahedral medium range orders (IMROs) are formed from icosahedra via the linkage of vertex-, edge-, face- and intercross-shared atoms. The glass structure on the nanometer scale is accumulated by polyhedra through an efficient packing mode. It is suggested that the extraordinary glass-forming ability of this alloy is essentially attributable to the formation of ISRO and IMRO, and the dense packing of atoms.     

Correlation between enthalpy change and free volume reduction during structural relaxation of Zr55Cu30Al10Ni5 metallic glass

The correlation between enthalpy change (ΔH) and free volume reduction (Δv_f) during structural relaxation of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glass was investigated by differential scanning calorimetry (DSC) and density measurements of thermally treated samples. A linear dependence between ΔH and Δv_f over a wide temperature range was found.     

Characterization of free volume in cold-rolled Zr55Cu30Ni5Al10 bulk metallic glasses

Cold-rolled Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glasses (BMGs) exhibited volume dilatation across the entire range of thickness reduction fractions from 5.7% to 63%. The dilatation was dominantly attributed to the free volume generated during cold-rolling, together with any open volumes, like the voids or microcracks, or both, which survived even after sufficiently annealing the rolled sample at the temperature in the supercooled liquid region. The relaxation of free volume formed during cold-rolling resulted in more heat evolution per unit volume than that of as-cast glass. The structural change during cold-rolling could be ascribed to the increased fluctuation of atomic-level hydrostatic stress, 〈p²〉, in the glass.     

Deformation behavior of Zr60.0Al15.0Ni25.0 metallic glass

The effect of thermal crystallization on viscous flow of supercooled liquid in Zr₆₀Al₁₅Ni₂₅ metallic glass was investigated. Zr₆Al₂Ni crystalline precipitates with ellipsoidal morphology appeared during thermal annealing and deformation at high temperatures. No significant difference in phase selection or morphology of crystalline precipitates was observed between non-deformed and deformed specimens. The viscous flow behavior is very sensitive to the strain rate and the stress–strain behavior can be classified into three types depending on the strain rate: stress overshoot mode, stable viscous flow mode with constant flow stress, and strain hardening mode. The strain hardening is caused by the precipitation of Zr₆Al₂Ni phase from supercooled liquid. The flow stress increased with increase in the crystallization ratio for specimens containing a volume fraction of Zr₆Al₂Ni phase higher than 10% although the stress showed no significant change with slight crystallization.     

Electron irradiation induced nano-crystallization in Zr66.7Ni33.3 amorphous alloy and Zr60Al15Ni25 metallic glass

Electron irradiation induced phase transformation behavior of an amorphous phase in Zr_66.7Ni_33.3 alloy, and an amorphous phase or supercooled liquid in Zr₆₀Al₁₅Ni₂₅ alloy was investigated. The amorphous phase could not maintain the original glassy structure under electron irradiation at 298 K, and f.c.c.-solid solution precipitated under electron irradiation in both alloys. The precipitation of C16-Zr₂Ni, big-cube (metastable f.c.c.-based Zr₂Ni intermetallic compound), Zr₆Al₂Ni and Zr₅AlNi₄ crystalline phases from an amorphous phase was not observed during electron irradiation induced crystallization. The amorphous phase in Zr₆₀Al₁₅Ni₂₅ metallic glass shows the highest phase stability against electron irradiation induced crystallization among Zr_66.7Cu_33.3, Zr_66.7Ni_33.3, Zr₆₅Al_7.5Ni_27.5, Zr₆₀Al₁₅Ni₂₅ and Zr₆₅Al_7.5Ni₁₀Cu_17.5 alloys. In Zr₆₀Al₁₅Ni₂₅ metallic glass, electron irradiation promoted the precipitation of f.c.c.-solid solution and Zr₆Al₂Ni crystalline phases from the supercooled liquid.     

Phases in the crystallization process of Zr_(65)Al_(7.5)Ni_(10)Cu_(17.5)metallic glass

The crystallization process of metallic glass Zr65Al7.5Ni10Cu17.5, (atom fraction in %) were studied by transmission electron microscopy and X-ray diffraction. Two stages of crystallization process, which are indicated by the two exothermic peaks in the differential scanning calorimetry diagram, were studied separately. It is found that the phases in the various stage of the crystallization are different, In the first stage, it consists mainly of tI-CuZr2 and tP-Al2Zr3 phases, between which a definite orientation relationship is found. While in the second stage, in addition to the above phases, phase hP2-Al2NiZr6 and phase hP3-α-Zr appear, between which another orientation relationship is also found. At the same time, a comparison is made based on the study of the phases exist in the arc-melted master ingot. The lattice parameters of the identified phases were determined and some interesting similarities were found. According to these similarities, all the phases were classified into two groups. The fi     

Laser welding of Zr53.7Ni9.4Cu28.5Al8.4 bulk metallic glass

采用激光焊接技术对Zr53.7Ni9.4Cu28.5Al8.4块体金属玻璃进行焊接试验,研究在激光焊接下不同工艺参量对焊接接头组织的影响,探讨了焊接热循环过程中焊缝和热影响区的晶化行为.结果表明,固定激光功率1200W,焊接速度从8m/min提高到30 m/min,均成功获得了无气孔和裂纹等缺陷的焊接接头.焊缝熔化区保持了非晶结构,热影响区则出现不连续的呈弧形分布的点状晶粒.随着焊接速度的提高,热影响区的晶粒尺寸逐渐减小.     

Nanocrystallization of Zr61Al7.5Cu17.5Ni10Si4 metallic glass

The primary nanocrystallization behavior and microstructural evolution 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 continuous heating of the 4Si and the base (contains no Si) amorphous alloys at a heating rate of 10 K/min, the saturation point of nucleation for the 4Si amorphous alloy occurs at a crystallization fraction of 78%, which is significantly higher than 65% for the base alloy, implying that these metalloid atoms would extend the nucleation stage and refine crystalline particles. The sequence of crystallization phase from the amorphous matrix for the isothermally annealed 4Si amorphous alloy at 703 K is observed to be Zr₂Cu and Zr₂Ni at the early stage, Zr₃Al at an intermediate stage, and Zr₂Si at the final stage. Moreover, enrichment of Si atoms at the interface between Zr₂Cu crystal and the amorphous matrix is detected. This may result in increasing the thermal stability of the remaining amorphous phase and retardation of the crystal growth of Zr₂Cu particles.     

Laser welding of annealed Zr55Cu30Ni5Al10 bulk metallic glass

Laser welding is one of the promising ways for manufacturing metallic glass products with complicated shape and geometry. In this work we focus on the effect of annealing treatment and welding parameters on laser welding of annealed Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glass as intended and unintended heat treatment occurs in the process. We find that laser welding can produce well welded specimen plates with no obvious welding defects in the joints and high welding speed may lead to better joints. Although higher annealing temperature or longer annealing time leads crystallization, bulk metallic glass material still remains largely amorphous in the heat affected zone. Compared with the welded joint without annealing, the micro-hardness and bending strength are enhanced due to the presence of the nanocrystals occurred in annealed welding joint. Therefore, appropriate annealing treatment with the annealing temperature near the glass transition temperature and annealing time as long as that in hot embossing of BMG parts may play a beneficial role in laser welding of metallic glasses.     

Zr55Al10Ni5Cu30大块非晶合金的超塑性挤压成形性能

通过示差扫描量热分析确定了Zr55Al10Ni5Cu30大块非晶合金的过冷温度区域范围,采用应变速率突变压缩实验分析了合金在450℃时的力学性能,研究了合金在不同挤压速度、不同真空度等工艺条件下的挤压成形性能.结果表明:Zr55Al10Ni5Cu30大块非晶合金的玻璃化转变开始温度Tg为422.4℃,晶化开始温度Tx为482.4℃;在450℃、应变速率小于5×10-3s-1的条件下,合金的流动应力小于40 MPa在挤压速度为0.002～0.004mm/s范围内挤压时,合金的最大挤压载荷变化较小;在挤压温度为450℃时,合金的最大挤压力随着真空度(2～2×10-3Pa)的提高而增加;大块非晶合金在超塑性成形时呈现出比一般金属材料更大的摩擦阻力.     

Deformation behaviors under tension and compression: Atomic simulation of Cu65Zr35 metallic glass

To clarify the difference in the plasticity of metallic glasses observed experimentally under tension and compression, the changes in the structural state induced during the deformation of Cu₆₅Zr₃₅ glassy alloy were analyzed by means of molecular dynamics (MD) simulation. Although mechanical responses were distinct, no clear difference was detected in the change in the short-range order of the deformed structures, except for the intensive collapse of the (0,3,6,4) bonding in the elastic regime seen in the extended sample. To explain the difference in the plasticity and particularly the brittleness of bulk metallic glass (BMG) in the tensile mode, we examine the degree of strain localization and discuss the role of normal stress on the activation of shear transformation zones (STZs) during tension and compression in the elastic regime.     

Friction joining of Ti40Zr25Ni3Cu12Be20 bulk metallic glass

Joints with fully amorphous structure can be obtained by selecting proper friction time and rotational speed. The optimum strength of the joint is close to that of the base bulk metallic glass (BMG), demonstrating a perfect bonding. Based on the analysis of the temperature distribution during the friction welding, an equation mentioning the critical friction time for retaining amorphous structure has been established. The experimental results prove the validity of the equation. Moreover, the temperature distribution in the friction interface has been studied by numerical simulation, aiming to provide a better understanding of the friction welding process and the microstructure in the interface. The simulation results further confirm the availability of the calculation results, thus can be helpful for designing the process parameters using other BMG systems.     

Crystallization kinetics of Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass

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Arrhenius activation of Zr65Cu18Ni7Al10 bulk metallic glass in the supercooled liquid region

In the current research, the dynamic mechanical spectrum and compressive deformation of Zr₆₅Cu₁₈Ni₇Al₁₀ bulk metallic glass in the supercooled liquid region (SLR) are investigated. The experimental results prove the existence of transition from Newtonian flow to non-Newtonian flow in the metallic glasses. In addition, we found that the characteristic stress σ_tc, which is obtained by a stretched exponential function based on the normalized viscosity, can be regarded as a transition point from Newtonian to non-Newtonian flow. The correlation between strain rate sensitivity exponent and corresponding strain rate was obtained at a certain temperature. It is noted that the variation of transition strain rate from Newtonian to non-Newtonian flow with different absolute temperatures follows the Arrhenius equation. The activation energy is in good accordance with that using the mechanical spectroscopy method.     

Ductile to brittle transition in the Zr41.2Ti13.75Cu12.5Ni10Be22.5 bulk metallic glass

The variation of impact toughness, Γ, of a Zr_41.2Ti_13.75Cu_12.5Ni₁₀Be_22.5 (Vitreloy-1) bulk metallic glass (BMG) within the temperature range of 123–423 K was evaluated by using an instrumented Charpy impact testing machine, in order to examine if the BMGs exhibit ductile-to-brittle transition (DBT) that is seen in rapidly quenched glasses. Results show an abrupt reduction in Γ when the testing temperature is lowered to below 150 K, implying that the BMGs are also prone to the DBT. Fractographic observations indicate a transition in the fracture mode; from ductile vein-like morphology above DBT to a cleavage-dominant fracture mode below it. Complimentary Vickers indentation measurements show no variation in hardness with temperature. However, the shear banded plastic regions that are typically seen around the indents were observed to be completely absent around the indents that were made at low temperatures, indicating that the inhomogeneous plasticity mediated by shear bands becomes inoperative below a critical temperature resulting in the DBT. This observation suggests that the minimum amount of free volume required for extensive plasticity (and hence high toughness) in metallic glasses is strongly dependent on the temperature. Testing of the structurally relaxed samples (through annealing at 530 K for 2.5 h that induces severe embrittlement at room temperature) at 423 K reveal almost complete recovery of Γ, supporting this hypothesis.     

Zr56.6Cu17.3Ni12.5Al9.6Ti4块体非晶合金的玻璃形成能力

采用低纯度的原料,通过电弧熔炼铜模铸造法制备了直径达10mm的Zr56.6Cu17.3Ni12.5Al9.6Ti4非晶合金圆棒.该合金玻璃转变温度tg＝385.8℃,晶化温度tx＝464.2℃,过冷液相区温差Δtx＝78.4℃,约化玻璃温度trg(tg/tmL)＝0.62.以基于DTA的合金凝固点偏移的方法确定该合金的临界冷却速度Rc=7.1℃/s,低于商业合金Vit.105合金的临界冷速(约为10℃/s).楔形试样对比结果显示:Zr56.6合金试样中的非晶组织区域明显大于Vit.105合金的,预示前者具有较好的实际玻璃形成能力.以上结果表明,Zr56.6Cu17.3Ni12.5Al9.6Ti4合金是Zr-Al-Ni-Cu-Ti系中玻璃形成能力最强的合金之一.     

Formation and properties of Zr48Nb8Cu14Ni12Be18 bulk metallic glass

Zr₄₈Nb₈Cu₁₄Ni₁₂Be₁₈ bulk metallic glass (BMG) with excellent glass-forming ability was prepared by water quenching method. The BMG exhibits high glass transition temperature T_g and onset crystallization temperature T_x, compared with Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 BMG. The crystallization processes, change of elastic constants, and density and hardness in the crystallization process were studied by using X-ray diffraction, differential scanning calorimetry and acoustic method. The shear modulus, Poisson ratio, density and hardness are found to be sensitive to the crystallization process. A striking softening of long-wavelength transverse acoustic phonons in the BMG relative to its crystallized state is observed. The linear expansion coefficient, determined by a dilatometer method, is α_TG=1.04×10⁻⁵ K⁻¹ (300–656 K) for the BMG and α_TC=1.11×10⁻⁵ K⁻¹ (356–890 K) for the crystalline alloy. The Mie potential function and the equation of state of the BMG are determined from the expansion coefficient and acoustic experiments.