Effect of Sn on microstructure and mechanical properties of (Ti–Cu)-based bulk metallic glasses

A Ti₄₃Zr₇Cu₄₃Ni₇ bulk metallic glass (BMG) exhibits a plastic strain of less than 0.2% under room-temperature compression. However, a Ti₄₅Zr₅Cu₄₀Ni_7.5Sn_2.5 BMG developed by addition of Sn to Ti₄₃Zr₇Cu₄₃Ni₇ shows a significant improvement of ～14.8% to the plastic strain with work hardening. Microstructural comparison of these BMGs reveals that the addition of Sn leads to formation of nano-scale chemical heterogeneities throughout the material. These nano-scale chemical fluctuations play an important role in enhancing the plastic strain of (Ti–Cu)-based BMGs.     

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.     

Effect of aspect ratio on the compressive deformation and fracture behaviour of Zr-based bulk metallic glass

The compressive deformation and fracture features of Zr₅₉Cu₂₀Al₁₀Ni₈Ti₃ bulk metallic glassy samples with aspect ratios in the range of 0.67–2.00 have been investigated. The compressive plastic strain of the glass monotonically increases with decreasing aspect ratio, but the maximum strength almost maintains a constant value of 1.77–1.88?GPa. All the compressive shear-band angles are equal to?～40° if modified by the rotation of the primary shear bands.     

Enhanced plastic deformation in a metallic glass induced by notches

It is shown that the compressive plasticity of a metallic glass, namely Zr_52.5Ni_14.6Al₁₀Cu_17.9Ti₅, can be improved by the introduction of two symmetrical notches. The enhanced plasticity may be ascribed to a blocking effect of the propagation of shear bands caused by large stress gradients around the notches. In contrast to ceramic specimens with similar notches, the plasticity enhancement of metallic glass induced by notches can provide a new approach to understanding its unique mechanism of deformation.     

Compositions of different phases appearing during devitrification of Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass

The devitrification process of Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 metallic glass during annealing in the supercooled liquid region has been studied by conventional, high-resolution and analytical transmission electron microscopy (TEM). Two kinds of particle appear during devitrification: quasicrystals and crystalline precipitates. Nanoanalysis with energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy reveal that quasicrystals are Be free, while the crystallites have the ZrBe₂ hexagonal structure. These experiments confirm the key role of Be during devitrification. They also indicate that quasicrystals can form in the system Zr–Ti–Cu–Ni (without Be), as further evidenced by direct synthesis and an in situ neutron diffraction investigation, according to the chemical composition deduced from the TEM analysis.     

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.     

Effect of crystallization on the surface area of porous Ni-based metallic glass foams

The change of the specific surface area in porous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ metallic glass (MG) upon partial crystallization was investigated. The observed increase in the surface area of the annealed Ni-based MG foams is due to the formation of homogeneously distributed Ni₁₀(Zr,Ti)₇ rod-shape intermetallic phases with nominal diameters around 250?nm and ～800?nm length on the surface of MG struts during the crystallization. For longer annealing, the specific surface area decreases again due to a change of the morphology of the crystals from rod-like to disc-like appearance, thus suggesting an optimum regime for increasing the specific surface area upon isothermal annealing at a given temperature.     

Difference in microstructure of Zr41Ti14Cu12.5Ni10Be22.5 glasses prepared in a 52 m drop tube and by water quenching

A 52 m drop tube has been used to solidify bulk-glass-forming Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 alloy. Glassy balls with different sizes solidified from the droplets whose structural features, glass-transition behaviour and crystallization kinetics have been investigated. The results indicate that the apparent activation energies of the glass transition and main crystallization reaction are significantly different from those of samples prepared by water quenching. The structural difference between the two types of glassy specimen is revealed by compression studies and in situ energy-dispersive X-ray diffraction. The results are important for understanding the structural features of bulk-forming glasses.     

Phase transitions of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass at pressures up to 27 GPa

The phase transitions of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass have been investigated under high pressures and at room temperature. Direct resistance measurements in a diamond anvil cell provide evidence of a reversible transition between amorphous and crystalline phases; crystallization events occurred at 24 and 26.2 GPa on uploading, and crystalline-to-amorphous phase transitions were observed at 16 and 10.6 GPa on downloading. The phase transitions were confirmed by transmission electron microscopy observations.     

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.     

Tensile deformation behaviour of Zr-based glassy alloys

This paper presents a study of the deformation behaviour of a glassy phase in two Zr-based alloys, Zr₆₅Ni₁₀Cu₅Al_7.5Pd_12.5 and Zr₆₅Al_7.5Ni₁₀Pd_17.5, performed in situ in a transmission electron microscope. In contrast to the case of shear localisation and formation of 10–20 nm thick shear bands in deformed bulk glassy samples studied earlier, it is found that in thin (electron-transparent) samples the glassy phase in front of a crack deforms more homogeneously and no nanocrystallisation takes place. The reasons for such behaviour are discussed. According to the observed results, one can conclude that the studied metallic glasses can be intrinsically ductile in submicrometre-sized volumes.     

Highly inhomogeneous compressive plasticity in nanocrystal-toughened Zr–Cu–Ni–Al bulk metallic glass

A Zr₆₂Cu_15.5Al₁₀Ni_12.5 bulk metallic glass with a large supercooled liquid region of 90 K, produced by copper-mould casting, exhibits a high strength of 1730 MPa and superior but highly inhomogeneous plasticity under uniaxial compression at ambient temperature. Micro-X-ray diffraction shows that compressive loading facilitates crystallization in the monolithic glassy alloy, resulting in room-temperature plasticity. The plastic deformation of the Zr₆₂Cu_15.5Al₁₀Ni_12.5 BMG may be attributed to in situ precipitation of nanocrystals during compression in heavily deformed areas.     

Dynamic and quasi-static mechanical properties of fibre-reinforced metallic glass at different temperatures

We have investigated the dynamic and quasi-static mechanical properties of a tungsten fibre-reinforced Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 bulk metallic glass composite (W/BMG) at temperatures ranging from 77 to 473?K. It was found that the yield stress and elastic modulus of the composite increase while strain-to-failure decreases with decreasing temperature. Tests under uniaxial dynamic compression showed that the W/BMG has a higher yield stress than under quasi-static compression. It was shown that yielding of the composite material is dominated by a shear banding process in the glass matrix, which is essentially a viscous flow as a consequence of local adiabatic heating. The interval between the nominal testing temperature and the temperature at which shear bands initiate is a key factor affecting the shear banding process, rendering the properties of the W/BMG very sensitive to temperature.     

Unusual room-temperature compressive plasticity in nanocrystal-toughened bulk copper-zirconium glass

Cast Cu₅₀Zr₅₀ alloy rods with a diameter of 1?mm have been found to consist of a glassy phase containing fine crystalline particles with a size of about 5?nm. They have a glass transition temperature T _g of 675?K, and a large supercooled-liquid region extending 57?K above T _g. The rods exhibit a high yield strength of 1860?MPa and a Young's modulus of 104?GPa. Because they contain a dispersion of embedded nanocrystals, the as-cast bulk metallic glass rods can sustain a compressive plastic strain at room temperature of more than 50%, an exceptional value which is explicable by compensation of any shear softening by nanocrystal coalescence and pinning of shear bands.     

Microstructure and martensitic transformation behaviors of a Ti–Ni–Hf–Cu high-temperature shape memory alloy ribbon

The Ti₃₆Ni₄₁Hf₁₅Cu₈ melt-spun ribbon undergoes a B2 ? B19′ transformation upon cooling and heating. When the Ti₃₆Ni₄₁Hf₁₅Cu₈ melt-spun ribbon is annealed at 873 K for 1 h, the spherical (Ti, Hf)₂Ni particles with a diameter of 20–40 nm precipitate in the grain interior. The fine (Ti, Hf)₂Ni precipitates improve the stability of phase transformation temperatures and cause martensite domains, with (001) compound twins in three orientations dominant instead of (011) type I twins. {111}-, {113}- and (001)//{111}-type boundaries are observed among these martensite domains. When the (Ti,Hf)₂Ni precipitates coarsen, (011) type I twins become main martensite structures in the ribbon annealed at 973 K for 1 h.     

Effect of strain rate on the compressive behaviour of a Zr56Al10.9Ni4.6Cu27.8Nb0.7 bulk metallic glass

We report the study of the effect of strain rate on the compressive behaviour of a Zr₅₆Al_10.9Ni_4.6Cu_27.8Nb_0.7 bulk metallic glass. The results indicated that both the strength and plasticity of the glass increase with increasing the strain rate up to 10^?5 s^?1, above which the strength and plasticity start to decrease. The enhanced mechanical properties under a strain rate of 10^?5 s^?1 are due to the emission/propagation rate of the shear bands being consistent with the strain rate.     

Synthesis of a La-based bulk metallic glass matrix composite

A bulk metallic glass matrix composite, that is a crystalline phase in an amorphous matrix, has been successfully synthesized by chill casting a La-based quaternary La₆₆Al₁₄Cu₁₀Ni₁₀ alloy. A composite rod as large as 12?mm in diameter was obtained. The reinforcing crystalline phase was identified as α-La, which was uniformly distributed and well developed throughout the sample. The critical cooling rate for the formation of the composite was determined by the Bridgman solidification process to be below 15?K?s^?1. Increased thermal stability was observed for the composite compared with the fully glassy rods, the reason for which remains unclear.     

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.     

Effects of crystallization on corrosion resistance and electron work function of Zr65Al7.5Cu17.5Ni10 amorphous alloys

The effects of crystallization on the electron work function and corrosion resistance of Zr₆₅Al_7.5Cu_17.5Ni₁₀ amorphous alloys have been studied. The single-phase amorphous alloy exhibits a better corrosion resistance and has a higher work function than the partially and fully crystallized alloys with the same composition. The close relationship between corrosion resistance and work function indicates that the Kelvin probe technique can be a powerful tool for characterizing the corrosion behaviour of amorphous alloy on an electronic level.