Molecular dynamics simulations of critically percolated,cluster-packed structure in Zr–Al–Ni bulk metallic glass

We have studied the local atomic arrangements of a Zr_0.60Al_0.15Ni_0.25 bulk metallic glass (BMG) with molecular dynamics (MD) simulations based on a plastic crystal model (PCM). We have utilized features of orientationally disordered state of a molecule in plastic crystals. A Zr_0.618Al_0.146Ni_0.236 alloy with an approximated composition to the Zr_0.60Al_0.15Ni_0.25 has been created using MD–PCM from a Zr_0.73Ni_0.27 glassy alloy that possesses critically percolated Ni atoms. The MD–PCM dealt with icosahedral and tetrahedral clusters with 13 and five atoms, respectively, with a Ni, Al, or Zr atom at each center site of the clusters. After the Zr_0.73Ni_0.27 glassy alloy had been created with monatomic MD simulation by quenching from a liquid, the Zr and Ni atoms in the Zr_0.73Ni_0.27 glassy alloy were replaced with randomly oriented icosahedral and tetrahedral clusters, respectively. Subsequently, structural relaxation was performed after adjusting the density to that of the Zr_0.618Al_0.146Ni_0.236 alloy. Total pair-distribution and interference functions revealed that the Zr_0.618Al_0.146Ni_0.236 alloys created with MD–PCM exhibit the characteristics of a non-crystalline phase. Further, Voronoi polyhedra analysis revealed that the Ni-centered polyhedral clusters used as initial atomic arrangements for MD–PCM tend to reproduce the features of the conventional MD results. The origin of the excellent glass-forming ability of the Zr_0.618Al_0.146Ni_0.236 alloy is attributed to the critically percolated cluster-packed structure.     

High-temperature deformation and crystallization behavior of a Cu<Subscript>36</Subscript>Zr<Subscript>48</Subscript>Al<Subscript>8</Subscript>Ag<Subscript>8</Subscript> bulk metallic glass

The influence of annealing on the crystallization behavior of a Cu₃₆Zr₄₈Al₈Ag₈ (at.%) bulk metallic glass (BMG) was investigated. In both isochronal and isothermal annealing processes, the effective activation energies of the primary crystallizations were obtained as 295.8 ± 13.4 and 302.7 ± 14.5 kJ/mol by applying the Kissinger and Ozawa methods, respectively. Using the isothermal transformation kinetics described by the Johnson–Mehl–Avrami model, the Avrami exponent n was found to range between 2.56 and 3.25, which indicates that the primary crystallization behavior was three-dimensional diffusion-controlled growth with an increasing nucleation rate. The high-temperature deformation behavior of a Cu₃₆Zr₄₈Al₈Ag₈ BMG was then investigated by performing a series of compression tests after rapid heating within a supercooled liquid region. It was found that at least 14–17 dense randomly packed atoms are necessary to produce a unit local flow when the present BMG is subjected to non-Newtonian homogeneous deformation, as described by the transition state equation. Deformation and processing maps were also constructed based on the dynamic materials model to predict optimum bulk formability in a Cu₃₆Zr₄₈Al₈Ag₈ BMG taking warm deformation-induced crystallization within a supercooled liquid into account.     

Crystallization kinetics of an amorphous Al<Subscript>75</Subscript>Ni<Subscript>10</Subscript>Ti<Subscript>10</Subscript>Zr<Subscript>5</Subscript> alloy

The formation and crystallization behaviors of a mechanically alloyed Al₇₅Ni₁₀Ti₁₀Zr₅ amorphous alloy were studied by X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry in the present study. The effective activation energy of the crystallization was determined by the Kissinger and Ozawa equations, respectively. The two equations yield close results and the average activation energy is 252 ± 13 kJ/mol. The resultant crystalline products were Al and Al₃Ni, and the crystallization mechanism is two- or three-dimensional nucleation and growth controlled by the diffusion of atoms. The thermal stability of the alloy was evaluated by a continuous transformation diagram obtained by the extended Kissinger equation.     

Characterization of strengthened rapidly quenched Zr-based alloys

The nominal composition components of alloy Zr_66.4Nb_6.4Ni_8.7Cu_10.5Al₈ (Alloy A) were fabricated and characterized. The strengthening of in-situ alloys depends on the role of both the glassy matrix and the second phases. The glass transition and the crystallization kinetics were studied using DSC and X-ray diffraction as a function of element distribution. The amorphous and semi-crystalline structures were identified with the existence of nano crystals in the alloy nominal compositions. The Elastic compression modulus were found to increase with transition to crystallite phase. Where as, the microhardness decreases dramatically with the change from crystalline to amorphous phase. The compression fracture surface shows classic veins behavior. In mode of continuous heating and adiabatic annealing the glass transition, T _g , and the crystalline peak, T _p , temperatures display a strong dependency on heating rate. The activation energy for glass transition and crystallization were determined as E _g  = 226 KJ/mol based on Kissinger method, but during the isothermal process E _g  = 121 KJ/mol.     

Development of nanocrystals in an amorphous alloy Zr47Ni30Ti23

Development of nanocrystals during crystallization of an amorphous alloy Zr₄₇Ni₃₀Ti₂₃ is studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Up on heating the amorphous ribbon in DSC, three exothermic peaks including a broad peak are observed. Stable nanocrystals embedded in the amorphous matrix form through primary crystallization. The nanocrystals have uniform sizes after prolonged annealing at 400°C for 180 minutes. Ni plays an important role for the stability of the nanocrystals. Due to the Ni partitioning between the nanocrystals and the residual matrix, the crystallization temperature (T _xI) of the residual amorphous matrix increases as crystallization proceeds. The formation of nanocrystal-amorphous composites that have high microstructural stability is possible through the controlled crystallization of the amorphous alloy Zr₄₇Ni₃₀Ti₂₃.     

Crystallization kinetics of magnetron-sputtered amorphous CoNbZr thin films

For non-isothermal and isothermal annealing, the crystallization kinetics of magnetron sputtered Co_85.5Nb_8.9Zr_5.6 amorphous alloy thin films have been investigated by differential scanning calorimetry measurements. As a result, in the case of non-isothermal crystallization, one distinct exothermic peak is observed at 470 °C, which is due to the crystallization of hcp α-Co. With the Kissinger method, the apparent activation energy was obtained to be 99.82 kJ/mol. By using the Deloy-Ozawa method, the local activation energy of non-isothermal crystallization was calculated. For isothermal crystallization, the Avrami exponents were determined by means of the Johson-Mehl-Avrami equation, which is in the range of 1.19-1.37. Based on an Arrhenius relationship, the local activation energy was analyzed, which yields an average value E_c=88.51 kJ/mol. Finally, the local Avrami exponent was used for discussing the details of the nucleation and growth behaviour during the isothermal crystallization.     

Structures and properties of a Zr-based bulk glass alloy after annealing

A bulk glass Zr_52.5Ni_14.6Al₁₀Cu_17.9Ti₅ alloy with 6 mm diameter is prepared by pre-melting sponge zirconium together with other pure metal elements and followed by injecting cast. In the samples, the content of oxygen is chemically analyzed in the level of 706 ppm (atomic concentration), which significantly affects the crystallization and the microstructure. When the bulk glass samples are annealed at the temperature far below the crystallization temperature(T_x), the predominant phases of Zr₂Ni_0.67O_0.33 and Zr₂Ni compounds crystallize and uniformly distribute on glass matrix. These predominant phases will grow and join together to form net-shape phase when the annealed temperature is in the range of T_x to above T_x. The glass matrix phase separated by the net-shape phase into the size of about 25 μm at 703 K to 15 μm at 823 K almost fully transforms into Zr₂Ni and a small amount of Zr₂Cu and Zr₄Al₃. At annealing temperatures far above T_x, Zr₂Cu and Zr₄Al₃ compounds crystallize by phase separation to form nanostructure with nano-scale phases of Zr₂Cu and Zr₄Al₃ compounds distributed on the matrix of Zr₂Ni. The micro-compressive tests by Nanoindenter II reveal that the bulk glass phase has a lower elastic modulus and lower microhardness. Increasing the annealing temperature, the modulus and microhardness for the crystallized microstructure increase. With the phase separation taking place, the modulus and microhardness for the nanostructure are improved slightly. But the different deformational mechanism between micro-scale and bulk specimens is unknown.     

Synthesis and thermal stability of ball-milled and melt-quenched amorphous and nanostructured Al-Ni-Nd-Co alloys

In this work the Al₈₅Ni₉Nd₄Co₂ alloy was used as a starting point for examining the possibility of forming bulk glassy Al-based materials by combining rapid quenching and ball milling techniques. Fine glassy powders were obtained by ball milling melt-spun amorphous ribbons using a severe cryogenic processing regime. The thermal stability data of the powders as obtained by constant-rate heating and isothermal DSC experiments together with viscosity measurements are discussed with respect to feasible consolidation conditions. The powder compaction was done by two methods (uni-axial hot pressing and extrusion) at 513 K for up to 15 min. Only the uni-axial hot pressing led to bulk Al₈₅Ni₉Nd₄Co₂ samples with similar glassy structure and Vickers microhardness values comparable to those of the initial melt-spun ribbons.     

Crystallization kinetics and martensitic transformation of Ti49Ni46.5Ce4.5 alloy thin film

The Ti₄₉Ni_46.5Ce_4.5 alloy thin film was prepared by direct current (DC) magnetron sputtering system for the first time. Crystallization kinetics, phase composition and the behaviors of martensitic transformation were studied. The results by X-ray diffraction (XRD) and differential scanning calorimeter (DSC) demonstrated that the primary second phase of TiNiCe alloy thin films was Ce₂Ni₇ phase, apparent activation energy was determined to be 510 kJ/mol at the continuous heating process, Avrami exponents for different isothermal temperature were in the range of 1.1-1.88 between 713 and 730 K, one-step martensitic transformation was observed in the crystallized Ti₄₉Ni_46.5Ce_4.5 alloy thin films. The influence of thermal process on martensitic transformation temperature was investigated with non-isothermal and isothermal crystallization. The reason behind the transformation temperature change was also discussed.     

Thermal Relaxation and High Temperature Creep of Zr55Cu30Al10Ni5 Bulk Metallic Glass

The free volume model is applied to isothermal relaxation and hightemperature creep. For this analysis, the time dependent flow behaviourof Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glass (BMG) near the glass transition temperature (T _g ) is expressed as a trade off between stress inducedgeneration and diffusion controlled annihilation of free volume. Thestrain rate-stress relation over a wide strain rate-range (10^–7to 10^–2 s^–1) was established for three different temperatures near T _g . It was found that the thermal relaxation behaviour and creep kinetics arecontrolled by the mobility of atoms with an activation energy of 161 kJ/mol.     

Micro and nano indentation studies on Zr60Cu10Al15Ni15 bulk metallic glass

Partially vitrified Zr₆₀Cu₁₀Al₁₅Ni₁₅ bulk metallic glass has been synthesized using water cooled copper mold drop casting technique. Kinetically favorable microstructures having different morphologies are observed throughout the volume of the bulk metallic glass sample. X-ray diffraction studies indicate formation of hard intermetallic compounds such as Zr₃Al₂ and Zr₂Ni in certain regions along with amorphous structures. Microindentation studies carried out in different regions of the sample reveal microstructure dependent deformation behavior. Highest hardness is observed in the fully crystallized regions compared to pure glassy regions in the same sample. Further nanoindentation in the same sample is used to understand dynamic mechanical properties of microstructures in different regions. The pile-up morphologies around the indent and differences in load–displacement curves provide vital information on deformation behavior of sample in different microstructure sensitive regions.     

Grain-growth in nanocrystalline zirconium-based alloys

Crystallization and subsequent grain growth in nanocrystalline Fe₃₃Zr₆₇ and (Fe, Co)₃₃Zr₆₇ alloys were studied by TEM, differential scanning calorimetry and X-ray diffraction. The grain-growth data for both alloys obtained over a wide temperature range (about 200 °C) were fitted to different kinetic equations (with different grain-growth exponent, n). The model with n=3 (Equations 4 and 5) was found to predict in the best way the isothermal experimental data. This result gives strong evidence that crystallization (in our case by a polymorphic reaction) is indeed observed of a glass into a nanocrystalline material prior to the coarsening, rather than grain growth in an extremely fine-grained material which was never glassy at all. The activation energies for grain-growth, — 260±25 kJ mol^–1, were found to be practically the same for both systems. Additional information about the crystal growth kinetics of the nanocrystals in the amorphous matrix was obtained for (Fe, Co)₃₃Zr₆₇ glass.     

Experimental investigation of phase equilibria in the Cu–Ni–Zr system

Phase equilibria in the Cu–Ni–Zr ternary system have been measured through alloy sampling combined with diffusion couple approach. According to the phase relations identified with electron probe microanalysis and X-ray diffraction techniques, isothermal sections at both 1073 and 1293 K were constructed. It is evident that remarkable ternary solubility occurs in almost all binary intermetallic phases at both temperatures. The formerly reported ternary compounds T1 (Cu_20–40Ni_40–60Zr₂₀) and T2 (Cu_20–25Ni_60–65Zr₁₅) were not verified in this work. No other ternary compound was detected. In addition, continuous dissolution between Cu₁₀Zr₇ and Ni₁₀Zr₇ at 1073 K was observed.     

Crystallization study of amorphous Al87.5Ni7Mm5Fe0.5 alloy by electrical resistivity measurement

Electrical resistivity measurement has been used to study the crystallization in amorphous Al_87.5Ni₇Mm₅Fe_0.5 alloy. Differential scanning calorimetry (DSC) and resistivity measurement showed that the alloy undergoes a three-stage crystallization process compared to the two-stage crystallization behaviour in many metallic glasses. The first and second peaks around 425 and 609 K, respectively, correspond to the precipitation of fcc-Al and Al₁₁(La,Ce)₃ phases. The activation energy for the formation of Al₁₁(La,Ce)₃ phase has been found to be 153±5.3 kJ/mol. The Avrami exponent is in the range of 1.50–1.76 indicating a three-dimensional growth mechanism with a decreasing nucleation rate.     

Thermodynamic Properties of the Ni0.333Zr0.667 Alloy in Amorphous and Crystalline States

The heat capacity of the Ni_0.333Zr_0.667 alloy in amorphous and crystalline states is measured by adiabatic calorimetry from 13 to 326 K. The thermal behavior of the amorphous alloy is studied by differential scanning calorimetry between room temperature and 800 K. Amorphous Ni_0.333Zr_0.667 is found to crystallize in the range 628–686 K, with a heat evolution maximum at 655 K and an enthalpy increment _cr H = 2.91 kJ/mol. The heat capacity data are used to evaluate the thermodynamic properties of the Ni_0.333Zr_0.667 alloy in amorphous and crystalline states in the temperature range 15–320 K.     

Atomic configuration evaluation of Zr60Ni21Al19 bulk metallic glass under high pressure

The atomic configuration evaluation in Zr₆₀Ni₂₁Al₁₉ bulk metallic glass at high pressures has been revealed by using in situ synchrotron X-ray diffraction. The radial distribution function is gained by Fourier transformation. The investigation shows that the amorphous structure is retained and the coordination number keeps 12.0 within the experimental pressures (0–24.5 GPa). The quantitative determination of the neighbor atomic distance suggests that high pressure alters topological but not chemical short range ordering through shortening the second nearest neighbor atomic distance. The atomic coordination is analyzed by the inherent chemical parameters of the ternary Zr₆₀Ni₂₁Al₁₉ amorphous alloy.     

High formability of glass plus fcc-Al phases in rapidly solidified Al-based multicomponent alloy

A multicomponent Al₈₄Y₉Ni₄Co_1.5Fe_0.5Pd₁ alloy was found to keep a mixed glassy + Al phases in the relatively large ribbon thickness range up to about 200 μm for the melt-spun ribbon and in the diameter range up to about 1100 μm for the wedge-shaped cone rod prepared by injection copper mold casting. The glassy phase in the Al-based alloy has a unique crystallization process of glass transition, followed by supercooled liquid region, fcc-Al + glass, and then Al + Al₃Y + Al₉ (Co, Fe)₂ + unknown phase. It is also noticed that the primary precipitation phase from supercooled liquid is composed of an Al phase instead of coexistent Al + compound phases, being different from the crystallization mode from supercooled liquid for ordinary Al-based glassy alloys. In addition, it is noticed that the mixed Al and glassy phases are extended in a wide heating temperature range of 588–703 K, which is favorable for the development of high-strength nanostructure Al-based bulk alloys obtained by warm extrusion of mixed Al + amorphous phases. The Vickers hardness is about 415 for the glassy phase and increases significantly to about 580 for the mixed Al and glassy phases. The knowledge of forming Al + glassy phases with high hardness in the wide solidification and annealing conditions through high stability up to complete crystallization for the multicomponent alloy is promising for future development of a high-strength Al-based bulk alloy.     

Synthesis and characterization of bulk amorphous/amorphous composite alloys through powder metallurgy route

In the present study, amorphous Ni₆₀Nb₂₀Zr₂₀ and Ti₅₀Cu₂₈Ni₁₅Sn₇ alloy powders were synthesized separately using a mechanical alloying (MA) technique. The dual-amorphous-phased (Ti₅₀Cu₂₈Ni₁₅Sn₇)_100−x (Ni₆₀Nb₂₀Zr₂₀) _x (x = 0, 10, 20, and 30 vol%) powders were prepared by mixing the corresponding amorphous powders. The dual-amorphous-phased powders were then consolidated into bulk amorphous/amorphous composite (BA/AC) alloy discs. The amorphization status of as-prepared powders and bulk BA/AC composite discs was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructure of the BA/AC discs showed that the Ni₆₀Nb₂₀Zr₂₀ phase is distributed homogeneously within the Ti₅₀Cu₂₈Ni₁₅Sn₇ matrix. The (Ti₅₀Cu₂₈Ni₁₅Sn₇)₇₀(Ni₆₀Nb₂₀Zr₂₀)₃₀ BA/AC disc exhibited a relative density of 96.6% and its Vickers microhardness was 726 kg/mm².     

Effect of thermo-mechanical histories on the microstructure and properties of Zr<Subscript>65</Subscript>Al<Subscript>10</Subscript>Ni<Subscript>10</Subscript>Cu<Subscript>15</Subscript> metallic glassy plates

Effect of thermo-mechanical histories during hot rolling in the supercooled liquid region on the microstructure and properties of Zr₆₅Al₁₀Ni₁₀Cu₁₅ metallic glassy plates was investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), differential scanning calorimetry (DSC), microhardness and electrical resistivity measurements. It was found that some nano-scale clusters and a few crystalline phases were dispersed in the amorphous matrix, which may depress the crystallization onset temperature (T_x). The microhardness increased while the electrical resistivity first increased and then decreased with hot rolling times. So, it is important for the working and forming of bulk metallic glasses in the supercooled liquid region to take the thermo-mechanical histories into account.     

Effect of cold-rolling on the crystallization behavior of a CuZr-based bulk metallic glass

The effect of pre-existing shear bands induced by cold-rolling on the crystallization behavior was investigated in the Cu₄₆Zr₄₆Al₈ bulk metallic glass. It was found that with increasing degree of pre-deformation, more shear bands are created resulting in the decrease of the crystallization activation energy for the alloy. Our experimental results demonstrate that pre-existing shear bands can promote the nucleation of crystals, which is discussed in terms of nucleation thermodynamics and kinetics aspects.