Nanocrystallization in a Zr 57 Ti 5 Cu 20 Al 10 Ni 8 bulk metallic glass

The first stages of thermal relaxation towards equilibrium in a Zr 57 Ti 5 Cu 20 Al 10 Ni 8 bulk metallic glass have been studied by differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and high-resolution electron microscopy. These coupled experiments rule out for this glass the existence of a phase separation on the nanometric scale preceding the onset of crystallization. The first step of crystallization is of the primary type, that is it consists of the nucleation of nanocrystallites in the amorphous matrix with which they coexist in metastable equilibrium. The very high nucleation rate leads to a nanometric composite structure with a number density of about 7 2 10 24 m -3 of 3-4nm crystallites, occupying a volume fraction of about 15%. These features, as well as the crystallization kinetics observed during isochronal or isothermal heating, are discussed.     

Formation of icosahedral quasicrystals in an annealed Zr65Al7.5Ni10Cu12.5Ag5 metallic glass

The formation and thermal stability of an icosahedral quasicrystalline phase in an annealed Zr₆₅Al_7.5Ni₁₀Cu_12.5Ag₅ metallic glass have been investigated by X-ray diffraction and transmission electron microscopy analyses. It was found that the quasicrystalline phase can precipitate from the glassy state and the supercooled liquid of the alloy over a wide range of annealing temperatures. After optimizing the heat-treatment conditions, the volume fraction of the quasicrystalline phase in the alloy can reach as high as about 80%. Investigation of the thermal stability of the quasicrystalline phase demonstrates that it is very stable when the annealing temperature is below the glass transformation temperature T _g of the alloy.     

Low-temperature mechanical properties of Ce68Al10Cu20Co2 bulk metallic glass

Although it is well known that traditional metals and alloys will become brittle at low temperatures, the effect of low temperature on the mechanical properties of bulk metallic glasses (BMGs) is yet to be fully understood. In this research, the mechanical properties of Ce₆₈Al₁₀Cu₂₀Co₂ BMG are investigated at low temperatures. It is found that the yield strength of the Ce-based BMG increases significantly with the decrease of temperature. The elastic moduli of the BMG also increase monotonically with the drop of temperature, indicating the continuous stiffening of the BMG, while both Poisson's ratio and global plasticity decline at low temperatures. It is considered that the stiffer atomic bonds of the Ce-based BMG at low temperatures result in the increase of strength, and the higher energy required for nucleation of shear bands also leads to the increase in yield strength.     

Comparative study of local structure in Zr70Al10Ni20 and quasi-crystal-forming Zr70Al9Ni20Pd1 metallic glasses

The local structure of Zr₇₀Al₉Ni₂₀Pd₁ metallic glass, in which a nano-icosahedral quasi-crystalline phase (I-phase) is formed in the primary stage of crystallization, has been examined and compared with that of Zr₇₀Al₁₀Ni₂₀, the supercooled liquid state of which has a high stability. Since the local environments around the Zr and Ni atoms do not change drastically by the addition of 1 at.% Pd to Zr₇₀Al₁₀Ni₂₀, as evidenced by radial distribution function (RDF) and extended x-ray absorption fine structure (EXAFS) studies, we deduce that the icosahedral phase formed in the Zr₇₀Al₉Ni₂₀Pd₁ metallic glass has a local structure similar to that in Zr₇₀Al₁₀Ni₂₀. Although a very slight rearrangement of Zr–Zr atomic pairs occurs during quasi-crystallization, the I-phase formation is achieved without disturbing the dominant local structure in the glassy state of the Zr₇₀Al₉Ni₂₀Pd₁. An icosahedral local structure is proposed for Zr–Al–Ni metallic glass system as well as for primary quasi-crystal (QC)-forming Zr-based metallic glasses.     

Changes of hardness and electronic work function of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass on annealing

The hardness and electronic work function (EWF) of a bulk metallic glass, namely Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5, have been studied experimentally, with an emphasis on the effect of heat treatments. The glass was annealed at different time and temperatures, and its hardness and EWF measured using the Rockwell indentation technique and a scanning Kelvin probe system, respectively. It is found that the EWF decreases with annealing time and temperature, whereas the hardness increases. This study shows a close relationship between hardness and EWF, indicating that the EWF could be a sensitive parameter for characterising and investigating the mechanical behaviour of BMG at the electronic level.     

Effects of Ag and Pd on the nucleation and growth of the nano-icosahedral phase in Zr65Al7.5Ni10Cu7.5M10 (M = Ag or Pd) metallic glasses

The nucleation and growth of a nano-icosahedral phase from a supercooled liquid region of Zr₆₅Al_7.5Ni₁₀Cu_7.5M₁₀ (M = Ag or Pd) glasses have been examined by differential scanning calorimetry and transmission electron microscopy. The growth rate of the icosahedral phase is nearly constant at the initial stage and much slower than that of the Zr₂Ni phase in the Zr₆₅Al_7.5Ni₁₀Cu_17.5 metallic glass. The homogeneous nucleation rate has a maximum value of 4.4 x 10²⁰ m^-3 s^-1 at 695 K in the Zr₆₅Al_7.5Ni₁₀Cu_7.5Ag₁₀ glass, which is approximately 10² times higher than that for the formation of quasicrystalline phase in the Zr_69.5Al_7.5Ni₁₁Cu₁₂ glass and 10⁴ times higher than that of the Zr₂Ni phase in the Zr₆₅Al_7.5Ni₁₀Cu_17.5 glass. With increasing Pd content, the nucleation rate of the primary phase increases significantly and the growth rate decreases at the crystallization temperature. Thus, the addition of Ag and Pd is effective for an increase in the number of nucleation sites and the suppression of grain growth, which is the main reason for the formation of icosahedral nanoparticles. The significant increase in the nucleation rate is due to an increase in the number of nucleation sites resulting from the short-range ordering consisting of Zr-(Ag or Pd) strong pairs. It is implied that the strong pair Zr-(Ag or Pd) also contributes to the restraint of the long-range rearrangements of the constitutional elements. The formation of the nanoicosahedral phase suggests that icosahedral short-range order exists in the glassy state in the present alloys.