Comparison of glass formation by mechanical alloying and solid-state interdiffusion in Ni---Zr composites

The amorphization process in mechanically alloyed Ni---Zr powders has been investigated by optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, differential scanning calorimetry and saturation magnetization measurements. Starting from elemental crystalline Ni and Zr powders, ball milling first produces a characteristically layered microstructure. Further milling leads to an ultrafine composite in which amorphization by solid-state reaction sets in between 4 and 16 h milling time. Longer milling results in fully homogeneous amorphous material. The obtained results corroborate the similarity of the amorphization process during mechanical alloying with the solid-state interdiffusion reaction in artificially modulated multilayer composites. In particular, mechanical alloying prevents intermetallic phase formation during the interdiffusion reaction because of extremely thin starting layers, thus resulting in a wider glass-forming range than obtained by other preparation techniques based on solid-state interdiffusion.     

Amorphization of Co-base alloy by mechanical alloying

Structural transformations and the amorphization of cobalt base alloy particles are possible when a powder mixture of Co and Cr-Mo is subjected to the mechanical alloying (MA) process. Elemental powders mixed in adequate weight ratios were used as precursors. The process was carried out at room temperature in a shaker mixer mill using vials and balls of hardened steel as milling media, with a ball:powder weight ratio of 8:1. The characterization of the crystalline structure of milled powders was carried out by means of X-ray diffraction to analyze the phase transformations as a function of milling time. The aim of this work was to demonstrate that MA can induce the amorphization of Co-Cr-Mo alloy. The evolution of the phase transformation during milling time is reported. The results showed that it is possible to produce a partial solid solution at room temperature of elemental metallic powders Co₇₀Cr₃₀ and Co₉₀Mo₁₀ by mechanical alloying. Using specific experimental conditions, it is also possible to obtain an amorphous phase in a composition Co₆₀Cr₃₀Mo₁₀; first, Cr and Mo were mechanically prealloyed for 9 h to obtain a Cr₈₀Mo₂₀ solid solution, and Co was then added and milled in over 14 h.     

Influence of outphase Cu50Ti50 amorphous alloy addition on microstructural evolution of mechanically alloyed Zr66.7Ni33.3 amorphous alloy

The influence of outphase Cu₅₀Ti₅₀ amorphous alloy addition on microstructural evolution of Zr_66.7Ni_33.3 amorphous alloy has been investigated using a mechanical alloying method. It has been found that the milling induced microstructural evolution is related to the change of peak positions of the first maximum on X-ray diffraction patterns of the as-obtained amorphous alloys. With increasing milling time, the 3 wt.% Cu₅₀Ti₅₀ addition can give rise to the cyclic amorphization transformation of the as-milled alloy. The mechanical stability of the mixing amorphous phase can be greatly enhanced with increasing Cu₅₀Ti₅₀ addition up to 10 wt.%. Moreover, the addition of outphase Cu₅₀Ti₅₀ amorphous alloy not only increases the onset crystallization temperature of Zr_66.7Ni_33.3 amorphous alloy but also alters its crystallization mode. The effect of outphase amorphous addition on the mechanical stability of the Zr_66.7Ni_33.3 amorphous phase has been discussed based upon the bond order theory.     

Amorphization induced by mechanical alloying and cold rolling

Solid state amorphous phase change is studied in the Ni---Ti system. It is found that the crystalline to amorphous transition can be obtained by the mechanical alloying, or by annealing of the multilayers produced by mechanical alloying and/or cold rolling. Different products are found after the annealing of the multilayers. The activation energies and interdiffusion coefficients are obtained for the solid state amorphous reaction.     

Structural evolution in mechanical alloying of Co and B powders

X-ray diffraction and scanning electron microscopy were used to monitor the mechanical alloying of four different mixtures of cobalt and boron with atomic compositions Co₅₀B₅₀, Co₆₇B₃₃, Co₇₅B₂₅ and Co₈₀B₂₀, respectively. The process induces amorphization reactions depending on the boron content; an almost complete amorphization was reached for the Co₆₇B₃₃ and Co₈₀B₂₀ samples, where only minor traces of unreacted cobalt are present. Extended X-ray absorption fine structure data collected on the most amorphous sample confirmed the diffraction results. In all the samples, the formation of t-Co₂B was detected.     

Influence of minor addition of Pd, Ag or Au on the microstructural evolution of mechanically alloyed Zr-Ni alloys

In this work, the influence of minor addition of Pd, Ag or Au (1 at.%) on the microstructural evolution of Zr-Ni alloys during mechanical alloying has been investigated using X-ray diffraction and differential scanning calorimetry. The experimental results show that the minor addition of Pd, Ag or Au has no significant influence on the amorphization process and glass forming ability of Zr-Ni amorphous alloys. In comparison with Ag and Pd, the minor addition of Au can enhance the mechanical and thermal stability of the mechanically alloyed Zr-Ni alloy. Discussion has been made based upon the evolution of the nearest-neighbor distance of atoms with increasing milling time.     

The influence of atmosphere on the amorphization reaction by mechanical alloying for the Ni---Ti system

A study of the amorphous phase reaction by mechanical alloying (MA) in different atmospheres has been carried out. Structural changes during milling were examined by means of X-ray analysis. The amounts of the gaseous elements absorbed by the powder particles during milling were determined by chemical analysis. It was found that there is almost no influence on the MA process either in an argon or a nitrogen atmosphere. During the milling process in air or an oxygen atmosphere, intermetallic compounds and oxides appeared, accompanying the formation of the amorphous phase. The intermetallic compounds appeared due to the presence of oxygen atoms.     

Effect of carbon on the microstructural evolution of Zr66.7−xNi33.3Cx (x = 0, 1, 3) alloys during mechanical alloying

In the present paper, the effect of carbon on the microstructural evolution of Zr_66.7−xNi_33.3C_x (x = 0, 1, 3) alloys during mechanical alloying has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that these three alloys undergo similar amorphization and crystallization processes, and the final milling product is a metastable fcc-Zr_66.7−xNi_33.3C_x phase. The carbon addition can shorten the milling time for the complete amorphization reaction and enhance the stability of the formed amorphous alloy, which can suppress the mechanically induced amorphous-crystalline phase transformation with further increasing milling time.     

EXAFS study of mechanical-milling-induced solid-state amorphization of Se

Complete solid-state amorphization has been realized in elemental selenium by means of mechanical milling of crystalline Se powder. Extended X-ray-absorption fine-structure measurements (EXAFS) indicated that the amorphization process of crystalline Se was accompanied with a decrease of the nearest neighboring coordination number and the bond length, and the increase of the Debye-Waller factor. Combined with the previous study, we conclude that the mechanical milling results in the strengthening of intra-chain and weakening of inter-chain interactions of Se during amorphization. Compared to the structure of the as-quenched amorphous Se, the first nearest neighbor coordination number of the as-milled amorphous Se becomes smaller, while the Debye-Waller factor is larger, which is caused by the milling process.     

Formation of nanocrystalline and amorphous phases in Cu80P20, Ni82P18 and Cu87Ni13 mechanically alloyed systems

Variations in the crystallite size and stored strain caused by mechanical milling of binary alloy systems of copper, nickel and phosphor were compared. During mechanical alloying process, an amorphous phase is partially produced in CuP and NiP alloy systems and only a nanoscale crystalline phase in CuNi alloy system. The CuP alloy system first forms a Cu₃P intermetallic compound and then the partial amorphous phase by the reaction of primary copper phase and Cu₃P intermetallic compound, while the NiP alloy system directly forms a partial amorphous phase by the reaction of primary nickel and phosphor phases. When the crystallite sizes of primary copper and nickel phases become of the order of a few nanometers in the CuP and NiP alloy systems, they are in an almost strain-free state.     

Amorphization from the quenched high-pressure phase of silicon and germanium

The phase transitions from the quenched high-pressure phase, including amorphous state, have been investigated for crystalline silicon and germanium at various pressures. X-ray diffraction patterns have been measured at pressures up to 15 GPa and temperatures down to 90 K by an energy-dispersive method using synchrotron radiation and a diamond anvil cell. The quenched β-Sn phase undergoes an amorphous phase transition when heated at 1.5 GPa for c-Si and 2.0 GPa for c-Ge. On the other hand, the quenched β-Sn phase transforms into a metastable crystalline phase when heated at higher pressures. The phase behavior is discussed in relation to the pressure dependence of the height of potential barrier between the β-Sb and amorphous phases and that between the β-Sn and metastable phases. The coordination number for the pressure-induced amorphous germanium, obtained through amorphization from the quenched high-pressure phase, is estimated to be about 4.     

Smallest (∼10 nm) phase-change marks in amorphous and crystalline Ge2Sb2Te5 films

Electrical nano-scale crystallization and amorphization in amorphous and crystalline Ge₂Sb₂Te₅ films have been studied using scanning probe microscopes. In scanning tunneling microscopes, the phase changes can be induced, not by tunneling currents, but by conducting currents flowing through contacted probes. In an atomic force microscope, metallic cantilevers can produce phase-change marks with minimal sizes of ∼10 nm. The crystallization and amorphization processes show different dependences upon thickness of Ge₂Sb₂Te₅ films. These features are discussed from thermo-dynamical and microscopic structural points-of-view.     

Study of the solid-state amorphization of (GaSb)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> semiconductors by real-time neutron diffraction and electron microscopy

The spontaneous amorphization of high-pressure quenched phases of the GaSb-Ge system has been studied by neutron diffraction while slowly heating the phases at atmospheric pressure. The sequence of changes in the structural parameters of the initial crystalline phase and the final amorphous phase is established. The behavior of the phases and the correlation in the structural features of the phase transitions and anomalous thermal effects exhibit signs of the inhomogeneous model of solid-state amorphization.     

Thermodynamic model for solid-state amorphization of pure elements by mechanical-milling

Mechanical-milling induced solid-state amorphization (SSA) has been observed in some covalent metalloids (Si, Ge, Se), but never in pure metals at room temperature. In this paper, a thermodynamic model was proposed for the SSA of the pure elements. In the model, a large Gibbs free energy of a grain boundary, compared with the free energy of a crystalline–amorphous interface, provides driving force for the amorphization. The critical grain size required for the SSA, therefore, can be determined from the condition when the Gibbs free energy of the nanocrystalline state is equal to that of amorphous state. The predicted critical grain sizes for SSA generally agree with the experimentally observed values for nonmetallic elements, but are much smaller than the minimum grain sizes of the metallic elements obtained by long-time mechanical-milling.     

New structural features of non-crystalline tetrahedrally bonded networks: A modeling approach

A large model of amorphous silicon (2052 atoms) with 0.5% dangling bonds has been built and investigated. The refinement of the coordinates evidenced the presence of small domains with advanced ordering. These domains preclude the formation of crystallization nuclei and play an essential role in the redistribution of the defects in the material with homogenization of the free energy and stabilization against aging. An effect of amorphization of the ordered nuclei due to free energy redistribution is assessed. The glass relaxation of a-Si (a-Ge) occurring during heating below T_g receives a natural explanation as a structural change from local quasi-ordering to homogeneous disordering.     

Amorphous and quasicrystalline Ti–Ni–Zr powders synthesized by mechanical alloying

The formation of amorphous and quasicrystalline phases in the Ti₄₅Zr₃₈Ni₁₇ system both directly by mechanical alloying and after subsequent annealing was studied. The presence of amorphous, icosahedral quasicrystalline and the Ti₂Ni-type with a fcc structure phases together with the initial metallic components was found in as-milled samples by X-ray diffraction. An increase of the milling time results in an increase of the amorphous phase content. Icosahedral quasicrystalline phases of Ti–Ni–Zr system were produced by mechanical alloying and subsequent annealing. Differential scanning calorimetry studies up to 520 °C showed an extended exothermal effect starting from 300 °C, which corresponds to the crystallization of the as-milled samples. The shape and size of the particles of the alloys were investigated by scanning electron microscopy and argon adsorption. The Specific area surface of the as-milled sample was rather small, in agreement with scanning electron microscopy data. The kinetics of the hydrogenation of the amorphous alloy Ti₄₅Zr₃₈Ni₁₇ at different temperatures was studied.     

Short range structure of mechanically alloyed amorphous Ni2Zr investigated by anomalous X-ray scattering

The differential anomalous scattering technique has been used to study the local order in an amorphous Ni₂Zr sample, prepared by mechanical alloying. The resulting structural parameters are compared with previous data obtained for a sample prepared by rapid quenching.     

The role of fuzzy interfaces in the nucleation, growth and agglomeration of aluminum hydroxide in concentrated caustic solutions

Fundamental crystal growth theory relies on classical concepts of monomeric addition at step sites on crystal surfaces. The nucleation and growth of crystalline aluminium hydroxide from concentrated caustic solutions does not follow classical crystal growth mechanistic pathways. Numerous techniques including static and dynamic light scattering, small angle X-ray and neutron scattering, cryovitirification transmission electron microscopy, rheology and atomic force microscopy have been employed in the study of aluminium hydroxide crystallisation from concentrated caustic solutions. The observations from these techniques have been interpreted on the basis polymer crystal growth theory, thermodynamic phase inversions analysis and entropically driven insolubility.

The experimental observations can be interpreted on the basis that aluminium hydroxide nuclei and crystals are surrounded by a diffuse interface which grades in density from the crystalline aluminium hydroxide particle core to the surrounding solution. A mechanism for the nucleation and growth mechanisms of aluminium hydroxide has been proposed: initial solution formation of a loose polymeric network; clustering of this network followed by gradual densification to form amorphous nuclei; further densification of the core of the nuclei to form crystallites and gradual densification but not crystallisation of the still amorphous diffuse interface.

The presence of this diffuse interface enables the slow agglomeration behaviour of aluminium hydroxides particles in concentrated caustic liquors to be explained. In liquors of very high ionic strength (in this case up to 6 M NaOH) particulate agglomeration would be expected to be rapid due to the small double layer thickness as predicted by DLVO theory. During rapid growth the diffuse interface inhibits the sufficiently close approach of the dense part of the particles to the point where attractive inter-particulate van der Waals forces would dominate and agglomeration would take place. As supersaturation is depleted and the growth rate of the diffuse interface decreases but densification is still occurring the particles can approach more closely and agglomeration will occur. Thus it is probable that the observed agglomeration behaviour is supersaturation and growth rate related.     

Phase formation and physical properties of mechanically alloyed amorphous 55Mg35Ni10Si

Amorphous 55Mg35Ni10Si alloy powder has been synthesized by mechanical alloying technique using pure Mg, Ni and Si elemental powders. The transformation of the crystalline powders into an amorphous one has been investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and differential scanning calorimetry. The new material produced has a higher thermal stability than reported results, which is beneficial to the fabrication of Mg–Ni–Si bulk amorphous components through powder metallurgy.     

Zur Struktur mechanisch erzeugter amorpher Quarzschichten

By mechanical treatment of quartz in a vibration mill an amorphization is reached connected with the loss of distance order. The near order of quartz in the range r < 0.65 nm is kept on to 50%. There is a difference in the structure between the near order of mechanical treated quartz and of amorphous silica. The formation of oxygen defect electron centres — they are only formed in presence of O₂ — is discussed in connection with the special structure of quartz, the order of which is mechanically destroyed.