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Reports on the effect of pressure on the crystallization of metallic glass alloys. Liquid regions of iron-aluminum-phosphorus-carbon-boron-gallium alloys; Details on the experiment.

Abstract: We present mechanical properties of monolithic bulk metallic glass (BMG) Zr62Al8Ni13Cu17 under uniaxial compression and tensile tests. It is found that the Zr62Al8Ni13Cu17 BMG exhibits pronounced plasticity of nearly 14% without catastrophic failure upon compression. However, it is destroyed under tension in a brittle manner. High energy X-ray diffraction has been used to detect the structural change in this BMG during both conditions. No deformation-induced nanocrystallization is detected in a 55% strained sample under compression while it only appears in the fracture-affected region upon tension. No excess free volume is obviously found in the fractured samples. We suggest that monolithic BMG alloys with small differences in atomic affinity and atomic sizes among components might have high plasticity under compression. [Copyright &y& Elsevier]

Change in local atomic environment during crystallization of Zr-based glassy alloys was studied by extended x-ray absorption fine structure (EXAFS) spectroscopy. The formation of icosahedral quasicrystalline phase followed by crystallization of tetragonal CuZr[sub 2] has been observed in the Zr[sub 70]Cu[sub 29]Pd[sub 1] glassy alloy during annealing up to 850 K. On the other hand, the binary Zr[sub 70]Cu[sub 30] alloy shows a single glassy to crystalline CuZr[sub 2] phase transformation. The local atomic environment of as-quenched Zr[sub 70]Cu[sub 30] alloy is matched to an icosahedral local atomic configuration, which is similar to that of the as-quenched Zr[sub 70]Cu[sub 29]Pd[sub 1] alloy and the alloy annealed at 593 K containing icosahedral phase. Considering that the supercooled liquid region appears prior to crystallization in the Zr[sub 70]Cu[sub 30] glassy alloy, the observed results support the theory claiming a strong correlation between the existence of local icosahedral short-range order and stability of the supercooled liquid state. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The pressure effect on the crystallization of the Al[sub 89]La[sub 6]Ni[sub 5] amorphous alloy has been investigated by in situ high-pressure and high-temperature x-ray powder diffraction using synchrotron radiation. The amorphous alloy crystallizes in two steps in the pressure range studied (0-4 GPa). The first process, corresponding to simultaneous precipitation of fcc-Al crystals and the metastable bcc-(AlNi)[sub 11]La[sub 3]-like phase, is governed by a eutectic reaction. The second process corresponds to the transformation of a residual amorphous alloy into fcc-Al, Al[sub 11]La[sub 3], Al[sub 3]Ni, and as yet unidentified phase(s). The applied pressure strongly affects the crystallization processes of the amorphous alloy. Both temperatures first decrease with pressure in the pressure range of 0-1 GPa and then increase with pressure up to 4 GPa. The results are discussed with reference to competing processes between the thermodynamic potential barrier and the diffusion activation energy under pressure. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of pressure on the crystallization behavior of the bulk metallic glass-forming Zr[sub 41.2]Ti[sub 13.8]Cu[sub 12.5]Ni[sub 10]Be[sub 22.5] alloy with a wide supercooled liquid region has been investigated by in situ high-pressure and high-temperature x-ray powder diffraction measurements using synchrotron radiation. In the pressure range from 0 to 3 GPa, the crystallization temperature increases with pressure having a slope of 19 K/GPa, which can be explained by the suppression of atomic mobility. This observation is opposite to the results of W.H. Wang, D.W. He, D.Q. Zhao, and Y.S. Yao [Appl. Phys. Lett. 75, 2770 (1999)], reporting a decrease of the crystallization temperature under pressure in a pressure range of 0-6 GPa for the bulk glass Zr[sub 41]Ti[sub 14]Cu[sub 12.5]Ni[sub 9]Be[sub 22.5]C[sub 1] alloy. Compressibility with a volume reduction of approximately 22% at room temperature does not induce crystallization in the Zr[sub 41.2]Ti[sub 13.8]Cu[sub 12.5]Ni[sub 10]Be[sub 22.5] bulk glass alloy. This indicates that the densification effect induced by pressure in the pressure range investigated plays a minor role in the crystallization behavior of this bulk glass alloy. The different crystallization behavior of the carbon-free and the carbon-containing glassy alloys has been critically assessed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure and stored energy of Cu60Zr20Ti20 bulk metallic glass rolled at cryogenic temperature in a wide strain rate range 1.0×10−4 −5.0×10−1 s−1 have been investigated. As the specimen is rolled to be thinner, the stored energy first increases linearly, and then saturates above a critical thickness reduction at lower strain rates, or decreases at high strain rates. At the initial stage of rolling, no phase transformation except shear bands appears in the glass. Phase transformation occurs only when the specimen is severely deformed at strain rates higher than 1.0×10−4 s−1. As strain rate increases, the critical strain for the stored energy to saturate increases, but the critical strain for phase separation to occur decreases, and meanwhile the type of the phase transformation changes from phase separation to nanocrystallization. The stored energy does not change with the occurrence of phase separation, but decreases due to nanocrystallization. It is proposed that coalescence of more free volume in shear bands into nano-voids should be principally responsible for the saturation of the stored energy, which balances the results from the increase in shear band number at higher strains. [Copyright &y& Elsevier]

Clarifying the crystallization path, from the thermodynamic point of view, is of importance for the structure and properties of metallic glasses (MGs). The influence of oxygen on the crystallization pathway of Ni 65 Zr 35 MG was investigated by in-situ heating transmission electron microscopy (TEM). As temperature increases, crystalline Ni 7 Zr 2 and t- ZrO 2 phases are formed first rather than the Ni 10 Zr 7 and Ni 21 Zr 8 phases, which are the main crystallization products following the phase diagram. Oxygen changes the crystallization pathway by varying the Ni/Zr ratio, indicating that the effects of oxygen on the crystallization in nanometer-sized MG cannot be neglected even in high vacuum TEMs. • Oxidation happened in TEM column during in-situ heating. • The primary crystallization products are Ni 7 Zr 2 and t -ZrO 2 phases. • Large amounts of fcc-ZrO phase formed when temperature reached 1073 K. [ABSTRACT FROM AUTHOR]

We study the short-range structural order in liquid gallium prepared by ab initio molecular dynamics simulations. We found that at 400 K, which is close to the melting point (Tm) of Ga, the dominant motif in the liquid phase is identical to that in the stable solid phase Ga-I. Strong directional bonding in this motif prevents it from nucleation in the liquid phase. Meanwhile, a newly identified motif, topologically distinct from yet related to those in β-Ga and Ga-III phases, is also abundant in Ga liquid. This new motif could serve as precursors for β-Ga or Ga-III during crystallization. [ABSTRACT FROM AUTHOR]

The atomic structure evolution of liquid Ca at 3500 K as a function of pressure up to 100 GPa has been studied by ab initio molecular dynamics simulations. The pair correlation function, structure factor and respective peak positions reveal two crossovers at ~ 10 GPa and ~ 50 GPa in liquid Ca while the coordination number shows four pressure regions: about 0.1–5 GPa, about 5–20 GPa, about 20–50 GPa and above ~ 50 GPa and heat capacity shows two different pressure dependences separated at about 30 GPa, i.e., below about 30 GPa a negative one and a positive value above about 30 GPa. It is also observed below 10 GPa the first nearest neighbor Ca Ca distance decreases more sharply as compared to above 10 GPa, this behavior is attributed to the gradual occupation of the interstitial sites. Comparisons of the pressure-dependent bond angle distribution and bond orientation order data of liquid Ca with solid crystalline phases imply that pressure-dependent atomic structural evolution in liquid calcium is, to some extent, similar to that of solid crystalline calcium reported by Teweldeberhan and Bonev [Phys. Rev. B 78 140101(R) (2008)]. These sluggish structural crossovers that have been observed in liquid Ca under pressure will trigger more studies in other disordered systems. [ABSTRACT FROM AUTHOR]

The structural evolution and dynamical properties of Al 2 Ag and Al 2 Cu liquids have been investigated in the temperature range of 943–1153 K by synchrotron X-ray diffraction and ab initio molecular dynamics simulations. It is demonstrated that the local atomic packing of Al 2 Ag liquid has lower ordering than the Al 2 Cu liquid, in which Al and Ag atoms are more randomly distributed while Al and Cu atoms tend to form pairs. With decreasing temperature, the bond-angle distributions of Ag Ag Ag and Ag Al Ag triplets shift to 60°, indicating that the local ordering transforms from the icosahedron-like to the hexagonal close-packed structure, while the Cu Cu Cu and Cu Al Cu triplets have the similar angles to those in the crystalline Al 2 Cu compound. These are consistent with the major Voronoi polyhedrons in both liquids. The dynamic results reveal that Al and Ag atoms diffuse much easier in the Al 2 Ag liquid than Al and Cu in the Al 2 Cu liquid, making Al and Ag atoms more randomly distributed and thus reducing the local ordering in the Al 2 Ag liquid. [ABSTRACT FROM AUTHOR]

Abstract: Materials exhibit a tension–compression asymmetry in terms of plasticity. This phenomenon is usually interpreted by continuum mechanics, which neglects the stress-induced structure change. Here we investigated the structure change of a metallic glass of Zr46.5Cu45Al7Ti1.5 (in at.%) under both tensile and compressive stresses by in situ loading and high-energy X-ray scattering. A relationship between the stress-induced structure change and the extraordinary tension–compression asymmetry of plasticity in the metallic glass is presented. Another interesting phenomenon is that the metallic glass also exhibits tension–compression asymmetry in terms of crystallization. The influence of the stress-induced structure change on the crystallization behavior of metallic glasses is discussed. The results obtained here reveal the structure evolution of a metallic glass under both tensile and compressive stresses, and might provide a new perspective on the tension–compression asymmetry in the structure of amorphous solids. [Copyright &y& Elsevier]

Abstract: The glass forming ability of Zr46Cu37.64−x Ag8.36Al8Be x (x =0, 6 and 10at.%) bulk metallic glasses (BMGs) were significantly improved by Be addition. The critical size of amorphous rods can be over 35mm diameter. The high GFA achieved is mainly due to the decrease of melting point and liquidus temperature, and suppression of the formation of crystalline phases during solidification from liquid state. The high stabilization with supercooled liquid regime of 115K was found for the BMG with x =10at.%. Two independent exothermic events happen in x =0 and 6at.% BMGs, corresponding to the formation of primary crystalline phases Cu10Zr7 and AgZr, then transforming to final stable crystalline phases Zr2Cu and AlCu2Zr. However, in the x =10at.% BMG, the precipitation of primary phases and transformation to final stable phases are within the first exothermic event and the AlCu2Zr phase is totally suppressed. [Copyright &y& Elsevier]

Abstract: Isochronal crystallization kinetics of Cu60Zr20Ti20 bulk metallic glass has been investigated by differential scanning calorimetry. By means of the Kissinger, Ozawa, Kempen, Matusita and Gao methods, average effective activation energies for the first and second crystallization reactions in Cu60Zr20Ti20 are calculated to be about 375±9 and 312±11kJmol−1, respectively, which are smaller than the values deduced from isothermal experiments. Meanwhile, average Avrami exponents, 3.0±0.1 and 3.4±0.2, for two crystallization reactions in isochronal anneals, differ from the value about 2.0 in isothermal anneals. The nonidentity of the Avrami exponents and effective activation energies may be contributed to different crystallization mechanisms and the nature of non-isokinetic between isochronal and isothermal experiments. The values of frequency factor k 0 for the first and second crystallization reactions of Cu60Zr20Ti20 are (1.7±0.3)×1024 and (7.0±0.8)×1018 s−1, respectively, and the large value of k 0 has been discussed in terms of the atomic configuration and interaction. [Copyright &y& Elsevier]

Abstract: Glass forming ability (GFA), mechanical and magnetic properties of Fe72W x Y6−x B22 (x =0, 1, 2 and 3at.%) and Fe72−x W x Y6B22 (x =1, 2, 3, 4, 5 and 6at.%) alloys have been experimentally investigated. It is found that fully amorphous Fe69W3Y6B22 and Fe68W4Y6B22 cylindrical rods with 4mm in diameter can be prepared by using copper mold casting. The newly developed Fe-based quaternary alloys exhibit excellent combination properties: good GFA, high onset crystallization temperature of 970K, high fracture strength of about 4GPa, Vickers hardness of about 1000kg/mm2, and good soft magnetic properties at ambient temperature (saturation magnetization of about 0.8T and coercive force of below 2A/m), which show potential applications in electronic industries. The effect of W addition on GFA in the Fe–Y–B alloy system is discussed. [Copyright &y& Elsevier]

Abstract: Amorphous structure and phase transformations of the rapidly solidified Zr60Fe20Cu20 alloy have been investigated by the DSC, in-situ high temperature XRD and EXAFS techniques. The performed experiments on the amorphous alloy proved significantly shorter Zr–Fe and Zr–Cu distances than those expected from the hard sphere atom model, corroborating strong bonding between Zr and transition elements. The crystallization of Zr60Cu20Fe20 amorphous alloy underwent three-stage process. The formation of stable cubic Zr(Cu, Fe) and tetragonal Zr2(Cu, Fe) phases was confirmed and the formation of a quasicrystalline phase was suggested. [Copyright &y& Elsevier]

Abstract: Microstructure and crystallization of a Cu50Zr45Al5 metallic glass have been investigated by differential scanning calorimetry, X-ray diffraction and extended X-ray absorption fine structure (EXAFS) using synchrotron radiation. The width of the supercooled region is ΔT x = T x − T g =67K, where T x and T g are the crystallization and the glass transition temperature, respectively; the reduced glass transition temperature is T rg = T g/T l =0.65, where T l is the liquid temperature; and γ = T x/(T g + T l)=0.41. After crystallization of the as-prepared Cu50Zr45Al5 metallic glass, crystalline phases are identified as tetragonal CuZr2 and orthorhombic Cu10Zr7. The local atomic structures deduced from EXAFS reveal that Zr atoms in the as-prepared Cu50Zr45Al5 metallic glass are surrounded by about 5 Cu atoms at a distance of 2.70Å and about 5 Zr atoms at 3.14Å. [Copyright &y& Elsevier]

Abstract: A simple method was used to prepare monodispersed ZnO nanoparticles, which can be used as seeds for the synthesis of [0001]-oriented ZnO nanorods by refluxing. Based on the observation of shape and size of ZnO nanocrystals at different refluxing times and surface and attachment energies for various crystallographic planes calculated from the BFDH and HP models, a possible growth mechanism for ZnO nanorods was put forward to explain the experimental observations. The synthesized ZnO nanocrystals show green emission, which are correlated with the degree of oxygen vacancies in the samples. [Copyright &y& Elsevier]

Abstract: Bulk Cu60Zr20Ti20 metallic glass has been rolled at room temperature (RT) and cryogenic temperature (CT) up to 97% in thickness reduction, and the dependences of microstructure on the strain and temperature have been investigated. It is revealed that as the deformation proceeds below a critical thickness reduction, which is 87% at RT and 89% at CT, only the shear band density and the free-volume content increase, whereas the thermal stability of the deformed glass remains unchanged. Deformation above the critical thickness reduction results in phase separation plus nanocrystallization at RT, but only phase separation at CT, indicating that lowering the temperature can effectively retard the deformation-driven crystallization, and that phase separation is the precursor of crystallization. The appearances of phase separation and especially nanocrystallization reduce the thermal stability of the glass since they create advantages in chemical composition and topological structure for the primary crystalline phase to nucleate and grow. [Copyright &y& Elsevier]