Assessment of rheological and thermodynamic properties of the Pd40Ni40P20 bulk metallic glass around glass transition using an indentation creep technique

The thermodynamic and rheological properties of the Pd₄₀Ni₄₀P₂₀ bulk metallic glass are explored by means of an indentation creep technique around the glass transition. We have developed a dedicated instrumented indentation apparatus allowing to assess the mechanical properties at elevated temperatures. The analysis of results is made possible by using the viscoelastic solutions of contact mechanics. We also analyse the thermodynamics of creep around glass transition to estimate the activation free energy changes from the activation free enthalpy changes via the shear modulus – temperature data. The shear viscosity values extracted using this technique allow for the derivation of activation energies (free enthalpy 210 kJ/mol, enthalpy 456 kJ/mol, entropy 410 J/mol/K) for the flow process. All these properties were found to closely match with those obtained using conventional techniques for viscosity measurements. Compared to the latter, the indentation creep technique requires small volumes and samples are easy to prepare. It is therefore expected that such a technique might be employed for the study of glass transition in metallic glasses.     

The isothermal and isochronal kinetics of the crystallisation of bulk amorphous Pd40Cu30P20Ni10

The amorphous alloy Pd₄₀Cu₃₀P₂₀Ni₁₀ has been produced by water quenching the molten alloy. The kinetics of crystallisation has been measured by means of isothermal and isochronal differential scanning calorimetry. The associated microstructural changes have been analysed using scanning electron microscopy. Special interest has been devoted to the influence of isothermal pre-annealing on subsequent isochronal crystallisation kinetics. By applying appropriate isothermal pre-annealing temperatures, nucleation of the crystalline phases takes place to different extents. Accordingly, the initial state for the subsequent isochronal crystallisation could be changed gradually with respect to the number of pre-existing nuclei. This leads to a gradual change of the mode of nucleation from site saturation to continuous nucleation during the crystallisation. The corresponding change of the kinetic parameters has been used to determine the activation energies of nucleation and growth separately.     

Micromechanisms of serrated flow in a Ni50Pd30P20 bulk metallic glass with a large compression plasticity

Serrated flow is a characteristic feature of plastic deformation of bulk metallic glasses (BMGs) with a large compression strain. However, the underlying mechanisms of the discrete plasticity in the disordered solids have been debated for many years. Here, we report mechanical behavior and microstructural evolution of a Ni₅₀Pd₃₀P₂₀ BMG subjected to uniaxial compression testing. Extensive nanocrystallization within shear bands and in the vicinity of fracture surfaces was observed and various crystal defects, including dislocations, twins and kink bands, were detected in the resultant nanocrystals. These observations suggest a microscopic mechanism of the serrated flow of the BMG, i.e. the stress drop is caused by local strain-softening and the arrest of shear bands is associated with in situ nanocrystallization.     

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.     

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.     

Local atomic structure of Pd–Ni–P bulk metallic glass examined by high-resolution electron microscopy and electron diffraction

Structural fluctuation in a Pd₄₀Ni₄₀P₂₀ bulk metallic glass is investigated by transmission electron microscopy and electron diffraction. Local atomic ordered regions with a fcc-(Pd,Ni) type structure was sharply imaged by a high-resolution electron microscopy (HREM) attached with a Cs-corrector. Interference function for the glassy state was obtained from electron-diffraction intensity profiles using energy-filter and imaging-plate techniques. We used a reverse Monte Carlo (RMC) simulation method to develop a realistic structure model. The model consists of a dense-random-packing structure, in which an fcc ordered region with Pd, Ni, and P atoms was embedded. The structure model is consistent with the diffraction and HREM results. In Voronoi polyhedral analysis of the RMC simulated structure, P-centered (Pd,Ni)-P trigonal prisms are found primarily in the matrix structure embedding the fcc-cluster. Around Pd and Ni atoms deformed-fcc type polyhedra were frequently observed. From these local structural features, nanoscale phase separation was revealed to occur during the glass formation.     

Thermodynamics,kinetics, and crystallization of Pt57.3Cu14.6Ni5.3P22.8 bulk metallic glass

We report on differential scanning calorimetry (DSC) studies to characterize the thermodynamics, kinetics and crystallization processes of the bulk metallic glass-forming alloy Pt_57.3Cu_14.6Ni_5.3P_22.8. The heat capacity of the alloy is measured for the crystalline, glassy and supercooled liquid phases. The heating rate dependence of the glass transition is used to calculate the kinetic fragility. Crystallization kinetics are determined under isothermal conditions and used to construct a time–temperature-transformation (TTT) diagram. The experimentally determined crystallization kinetics are fit to calculate the activation energy for crystallization. Our results suggest that Pt_57.3Cu_14.6Ni_5.3P_22.8 is neither a thermodynamically nor a kinetically stabilized glass former. Other contributions, including the activation energy for crystallization and the use of a flux are considered in the discussion to explain the good glass formability of this alloy.     

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.     

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     

Deformation behavior of Zr55.9Cu18.6Ta8Al7.5Ni10 bulk metallic glass matrix composite in the supercooled liquid region

A Zr_55.9Cu_18.6Ta₈Al_7.5Ni₁₀ bulk metallic glass (BMG) composite with an amorphous matrix reinforced by micro-scale particles of Ta-rich solid solution was prepared by copper-mold casting. Isothermal compression tests of the BMG composite were carried out in the range from glass transition temperature (∼673 K) to onset crystallization temperature (∼769 K) determined by differential scanning calorimetry (DSC). The compressive deformation behavior of the BMG composite in the supercooled region was investigated at strain rates ranging from 1 × 10⁻³ s⁻¹ to 8 × 10⁻² s⁻¹. It was found that both the strain rate and test temperature significantly affect the stress–strain behavior of the BMG composite in the supercooled liquid region. The alloy exhibited Newtonian behavior at low strain rates but became non-Newtonian at high strain rates. The largest compressive strain of 0.8 was achieved at a strain rate of 1 × 10⁻³ s⁻¹ at 713 K. The strain rate change method was employed to obtain the strain rate sensitivity (m). The deformation mechanism was discussed in terms of the transition state theory based on the free volume.     

块体Ti40Zr25Ni8Cu9Be18非晶合金的低温压缩行为

研究块体Ti40Zr25Ni8Cu9Be18非晶合金在液氮温度下的准静态压缩力学行为,并对不同应变速率时的低温力学性能和室温性能进行对比.结果表明:块体Ti40Zr25Ni8Cu9Be18非晶合金在低温时的压缩强度明显高于室温时的压缩强度;且随着应变速率的增大,低温压缩强度增加的幅度较大,即正应变速率敏感因子增大;在液氮温度和低应变速率条件下,光滑断面的出现说明低温对降低粘度起着阻碍作用,粘度降低的量级没有达到形成脉状花样所需要的要求;随着应变速率的增加,剪切断口脉状花样的出现说明高应变速率能够明显降低剪切面粘度.     

Thermodynamic approach to free volume kinetics during isothermal relaxation in bulk Pd–Cu–Ni–P20 glasses

Isothermal relaxation behavior just under glass transition region was investigated for bulk Pd₄₀Ni₄₀P₂₀ and Pd_42.5Cu₃₀Ni_7.5P₂₀ metallic glasses. The densification of the bulk sample during relaxation was examined directly by density measurement using conventional Archimedean technique. The density of as-quenched Pd₄₀Ni₄₀P₂₀ increased monotonously with time. From the density data of Pd₄₀Ni₄₀P₂₀ glass, the free volume relaxation was examined and the kinetics was well described by a stretched exponential function with Kohlrausch exponent less than unity. These glasses also showed a clear two-step relaxation that may be a feature peculiar to bulk metallic glasses.     

Homogeneous deformation of Ti41.5Cu37.5Ni7.5Zr2.5Hf5Sn5Si1 bulk metallic glass in the supercooled liquid region

The homogeneous deformation behavior of Ti_41.5Cu_37.5Ni_7.5Zr_2.5Hf₅Sn₅Si₁ BMG has been investigated by compression tests. The results show that its high-temperature deformation behavior is strongly dependent on strain rate and temperature, and there exists a transition from non-Newtonian flow to Newtonian flow with decrease in strain rate, which can be explained based on the transition state theory. In addition, this alloy can reach a large compressive strain than 0.8 at high strain rate; however the much higher flow stress and lower value of S parameter compared with typical BMGs indicates its worse formability in the SLR. A beneficial domain (temperature and strain rate) for optimum hot workability of this alloy has been roughly located by constructing the power dissipation efficiency map, where the power dissipation efficiency is larger than 0.8.     

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.     

In situ observation of pitting corrosion of the Zr50Cu40Al10 bulk metallic glass

The polarization behavior of a Zr₅₀Cu₄₀Al₁₀ (atomic percent) bulk metallic glass was investigated in a 0.6 M NaCl electrolyte while being observed using optical microscopy (OM). In situ anodic-polarization experiments revealed a tendency towards a diffusion-controlled mechanism other than passivity. In situ OM images illustrated pits initiated at the corrosion potential (that is, open-circuit conditions). Scanning electron microscopy (SEM) was utilized to examine the region where the corrosion pits initiated. SEM images showed that the area of pit initiation was surrounded by an oxide that formed on the surface due to the exposure to the NaCl electrolyte.     

Interface structure and properties of a brass-reinforced Ni59Zr20Ti16Si2Sn3 bulk metallic glass composite

Interfaces between a Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ bulk metallic glass (BMG) and crystalline brass reinforcements were characterized using transmission electron microscopy and nanoindentation. An interfacial layer with a thickness of ∼50–100 nm was observed in the composite prepared by warm extrusion of gas atomized powders. Microstructural characterization and chemical analysis suggest that the formation of interfacial layer was caused by interdiffusion between the BMG and brass during the warm extrusion. Nanoindentation in the vicinity of BMG–brass interfaces does not cause interface decohesion or crack formation, suggesting a strong interface bonding. Apparently, the resultant interfacial layer not only enhances interfacial bonding but also provides a buffer zone to prevent the catastrophic shear band propagation in the BMG matrix.     

Thermal behaviour of Pd40Cu30Ni10P20 bulk metallic glass

The thermal behaviour of differently milled Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glass through the glass transition has been investigated by in situ high-energy synchrotron X-ray diffraction. Repeated heating and cooling were performed between the glassy and the supercooled liquid state. The changes in positions and intensities of the first and second diffraction maxima of the as-milled powder indicate irreversible changes during first heating up to the glass transition temperature T_g due to structural relaxation. After annealing, reversible structural changes with temperature are observed upon heating and cooling in the glassy phase, and in the supercooled liquid state respectively. The shift in the position of the first maximum scales approximately with the linear thermal expansion for the glassy state; however, this relation does not hold for the supercooled liquid. The structural transition from the glass to the supercooled liquid at the glass transition temperature is reflected by the intensity of the diffraction maxima and by a reversed temperature dependence of the position of the second diffuse maximum below and above T_g. The changes of the glass structure for the decrease of free volume by annealing are found to be different from those observed for the reversible volume expansion or shrinkage by varying the temperature. Therefore, the shift of the first diffuse maximum position of bulk metallic glasses cannot be used as a measure of the change in free volume.     

Influence of minor Si addition on the glass-forming ability and mechanical properties of Pd40Ni40P20 alloy

The influence of a minor Si addition on the glass-forming ability and mechanical properties of Pd₄₀Ni₄₀P₂₀ alloy was investigated. It is suggested that the minor Si addition can adjust the composition to be closer to eutectic, which favors a large undercooling. Furthermore, Si addition led to an enhancement in the ductility of Pd–Ni–P bulk metallic glasses. Simultaneously it is of great interest that there are nanoscale and microscale wavy steps observed on the fracture surface of Pd₄₀Ni₄₀Si₄P₁₆ metallic glass. It is suggested that a quickly running crack dissipates excess energy in terms of stress wave, leaving such wavy traces on the fracture surface of the deformed Pd-based metallic glass.