Enthalpy relaxation and its relation to the thermodynamics and crystallization of the Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 bulk metallic glass-forming alloy

This paper reports on the thermodynamics, on the enthalpy relaxation kinetics, and on the crystallization kinetics of the Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8 bulk metallic glass-forming alloy. The thermodynamics functions were determined from heat of fusion and specific heat capacity measurements. A novel differential scanning calorimetric method was optimized for the measurement of the enthalpy relaxation in the vicinity of the glass transition. The residual enthalpy frozen-in the glass is found to relax with stretching exponents approaching unity, which indicates rather strong glass behavior, reflected also by the high value of the fragility parameter and the small driving force for crystallization. The apparent activation energy for both, enthalpy relaxation and primary crystallization is the same as for the diffusion of the midsize atoms suggesting that their mobility controls both processes, but on different time and length scale.     

On the crystalline equilibrium phases of the Zr57Cu15.4Ni12.6Al10Nb5 bulk metallic glass forming alloy

The concentration and structure of the five crystalline equilibrium phases of the Zr₅₇Cu_15.4Ni_12.6Al₁₀Nb₅ bulk metallic glass forming alloy were determined. All phases can be derived from binary compounds that contain the other elements as solutes. Large linked fluxes of Al and Nb are necessary to nucleate any of the equilibrium phases.     

Corrosion behavior of Zr–Cu–Ni–Al bulk metallic glasses in chloride medium

Polarization and passivation behavior of three Zr-based BMGs, i.e. Zr_58.3Al_14.6Ni_8.3Cu_18.8, Zr₅₈Al₁₆Ni₁₁Cu₁₅ and Zr_57.5Al_17.5Ni_13.8Cu_11.3 were investigated in 3% NaCl aqueous solution. Electrochemical investigations were carried out by potentiodynamic polarization method at room temperature. The corroded sample surfaces were examined using scanning electron microscope having energy dispersive spectroscopy (EDS) attachment. The results of the present investigation revealed that Zr₅₈Al₁₆Ni₁₁Cu₁₅ and Zr_57.5Al_17.5Ni_13.8Cu_11.3 BMGs having relatively larger supercooled liquid region (ΔT_x) and pitting overpotential (η_pit) values exhibit low corrosion current density (i_corr) and corrosion penetration rate (CPR) values.     

Microstructure and mechanical properties of the Zr66.4Cu10.5Ni8.7Al8Ta6.4 metallic glass-forming alloy

The investigation on Zr_66.4Cu_10.5Ni_8.7Al₈Ta_6.4 alloy shows that Ta promotes the formation of bcc-β-Zr(Ta,Cu,Ni,Al) dendrites and induces the precipitation of Zr₂Cu and Zr₂Ni intermetallics in as-cast cylinders. The ductile dendritic phase counteracts the brittleness of the intermetallics resulting in 1824 MPa compressive fracture strength and 102 GPa Young’s modulus for this alloy.     

Crystallization behavior of Ti61.67Zr17.15Ni14.80Cu6.38 glass-forming alloy

Ti61.57Zr17.15Ni14.80Cu6.38(atom fraction, %) metallic glass has applications in brazing. Using the hammer-and-anvil technique, Ti61.67Zr17.15Ni14.80Cu6.38 metallic glass was prepared. The crystallization behavior for this metallic glass was investigated by differential scanning calorimetry(DSC), X-ray diffractometry (XRD) and transmission electron microscopy(TEM). There are three stages in DSC curves of crystallization. The reduced glass temperature Trg is 0.42. The kinetic parameters of crystallization were calculated by a set of equations of the maximum crystallization rate. The crystalline phase formed in the MSⅠ(Metastable stage D is Zr2Cu, in the MSII is α-Ti and in the MSⅢ is Ti2Ni. This kind of alloy has lower glass forming ability, and the Ti61.67Zr17.15Ni14.80Cu6.38 metallic glass has lower thermal stability.     

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.     

Cyclic compression behavior of a Cu–Zr–Al–Ag bulk metallic glass

The cyclic compression behavior of a Cu₄₅Zr₄₅Al₅Ag₅BMG was investigated in order to elucidate the damage initiation and growth mechanisms. The present Cu₄₅Zr₄₅Al₅Ag₅ BMG was found to have a fatigue-endurance limit of 1418 MPa and fatigue ratio of 0.77. Fracture under cyclic compression occurred in a pure shear mode. The fracture surface forms an angle of 41° with respect to the loading axis. This angle was similar to the monotonic compressive fracture angle for the present BMG. The cyclic compression fracture surface displays a morphology nearly identical to the monotonic compression fracture surface. In addition to many shear bands and cracks, areas of “chipping” were commonly found on the outside surfaces of the fatigue specimens. An attempt was made to measure crack growth rates, and two types of crack growth behavior were found. With the first type, the growth rate decreased with cycles due to the decrease in the driving force for crack propagation. With the second type, the crack growth rate increased with cycles after chipped areas developed. The fatigue deformation process for BMGs under cyclic compression was carefully studied and rationalized.     

Synthesis and characterization of particulate reinforced Zr57Nb5Al10Cu15.4Ni12.6 bulk metallic glass composites

The Zr₅₇Nb₅Al₁₀Cu_15.4Ni_12.6 bulk metallic glass forming liquid is reinforced with WC, SiC, W, or Ta particles. Structure, microstructure and thermal stability of the composites are studied by X-ray diffraction, optical microscopy and differential scanning calorimetry. The metallic glass matrix remains amorphous after adding up to 20 vol.% of particles. The reactions at the interfaces between the matrix and the different reinforcing materials are investigated with scanning electron microscopy, transmission electron microscopy and electron microprobe. The mechanical properties of the composites are studied in compression and tension. The influence of the introduced particles on the thermal stability of the matrix as well as on the mechanical properties is discussed.     

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.     

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.     

Deformation in a Zr57Ti5Cu20Ni8Al10 bulk metallic glass during nanoindentation

Nanoindentation experiments of a Zr₅₇Ti₅Cu₂₀Ni₈Al₁₀ bulk metallic glass were performed with indentation loads ranging from 200 to 2000 μN. Both the indentation hardness and the reduced contact modulus decreased with the increase in the indentation load due to the propagation of shear bands underneath the indenter – the occurrence of a softening effect. The ratio of the indentation hardness to the reduced contact stiffness was a function of the reciprocal of the indentation depth. Based on the concept of diffusion-induced stresses, a one-dimensional constitutive relation between the change of the excessive free volume and the flow stress was proposed. The indentation-size effect as observed in the indentation tests was explained through the consideration of the contribution of the strain gradient in the constitutive relation.     

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.     

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.     

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.     

Electrochemical investigations on the bulk glass forming Zr55Cu30Al10Ni5 alloy

The electrochemical behaviour of the Zr₅₅Cu₃₀Al₁₀Ni₅ amorphous alloy prepared as cylindrical bulk samples with 7 mm diameter has been investigated. Cyclic potentiodynamic polarization measurements carried out on the cross section areas of the samples in air-saturated and nitrogen-purged sulphate and hydroxide solutions, respectively, show that the alloy rapidly forms protective passive layers with low electron conductivity which are stable in a wide pH-value and potential range. Anodic layer formation reactions are characterized. In chloride containing electrolytes a pitting corrosion susceptibility of the bulk material exists. The effect of crystalline defects as active sites for the initiation of localized corrosion processes is critically discussed.