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.     

Thermal-equilibrium processes and electronic transport in undoped hydrogenated amorphous silicon

Abstract

The temperature and time dependence of the d.c. conductivity of undoped hydrogenated amorphous silicon is presented. Measurements of the electronic transport are reported, with particular emphasis on the effects of annealing and cooling the samples. Two regimes of behaviour are observed. When samples are rapidly cooled from 200°C below a temperature T _E～145°C a non-equilibrium dark conductivity, higher than that corresponding to slow cooling, is observed. The electronic and atomic structure then slowly relax and the time dependence of the excess conductivity is well described by a stretched exponential function. The second regime above T _E corresponds to a relaxation time short compared to experimental times and the conductivity is independent of which order the annealing temperature is chosen. Thus the thermal equilibrium processes observed in undoped samples are qualitatively very similar to those observed in doped samples as recently reported in the literature.     

Phason elastic constants of the icosahedral Al-Pd-Mn phase derived from diffuse scattering measurements

The diffuse scattering in the diffraction pattern of the icosahedral Al-Pd-Mn quasicrystalline phase has been measured on an absolute scale by X-ray and neutron scattering on single-grain samples. Most of the diffuse scattering can be interpreted in the framework of the elasticity theory of icosahedral quasicrystals considering only phason fluctuations. At room temperature the absolute values of the K₁/k_BT and K₂/k_B T phason elastic constants are of the order of 0.06 and 0.031atom^-1. The amount of diffuse scattering intensity is insensitive to the sample annealing treatment.     

Microstructural features of phase transformation in a binary Pd–Si metallic glass

Glassy ribbons of Pd–Si alloys were prepared by a combination of melt spinning and flux treatment. The crystallization behaviour of a Pd₈₁Si₁₉ glassy alloy was studied through isothermal annealing at temperatures ranging lower than the glass-transition temperature T _g to around the onset of crystallization. The evolution of microstructures arising from isothermal annealing was investigated by X-ray diffraction (XRD) and (high-resolution) transmission electron microscopy ((HR)TEM). XRD spectra showed that, after the sample was annealed at a sub-T _g temperature, its first diffraction peak was split into two overlapping broad peaks. TEM analysis revealed the formation of a spherical, particle-like glassy phase embedded in the glassy matrix together with a finely connected network morphology within both. Combining these observations with compositional analysis suggested that phase separation had taken place during sub-T _g annealing. When the glassy alloy was annealed at temperatures higher than T _g, nanocrystalline structures, composed of Pd₃Si and Pd phases plus a Pd₉Si₂ phase with a lamellar structure, was formed.     

Effect of heat treatment on the NO2-sensing properties of sputter-deposited indium tin oxide thin films

Transparent conducting indium tin oxide (ITO) films were deposited onto glass substrates by radio-frequency magnetron sputtering at 648?K, under an oxygen partial pressure of 1?Pa. The effect of annealing on the electrical properties of the films was studied. Characterization of the coatings revealed an electrical resistivity below 6.5?×?10^??3?Ω?cm. The ITO films deposited at 648?K were amorphous, while the crystallinity improved after annealing at 700?K. The surface morphology examined by scanning electron microscopy appears to be uniform over the entire surface area after annealing. The NO₂-sensing properties of the ITO films were investigated and showed sensitivity at concentrations lower than 50?ppm, at a working temperature of 600?K.     

Microstructure evolution of cold-worked Zircaloy-4 under 20 mev Cu ion irradiation

Abstract

Based upon our previously developed microscopic model of intergranular embrittlement in A₃B L1₂ intermetallic compounds, it is shown that a plot of A versus B s-orbital electronegativity leads to a clear discrimination between ductile and brittle materials. A structure-property relationship is uncovered by constructing the associated structure map using phenomenological Mendeleev numbers, which exhibits a topology identical to the ductile-brittle property map.     

Nucleation of He bubbles in amorphous FeBSi alloy irradiated with He ions

It is interesting to investigate the formation of He bubbles in amorphous alloys because point defects do not exist in amorphous materials. In the present study, the microstructural evolution of amorphous Fe₇₉B₁₆Si₅ alloy, either irradiated with 5?keV He⁺ ions or implanted with 150?eV He⁺ ions without causing displacement damage, and then annealed at a high temperature, was investigated using transmission electron microscopy (TEM). Vacancy-type defects were formed in the amorphous alloy after irradiation with 5?keV He⁺ ions, and He bubbles formed during annealing the irradiated samples at high temperature. On the other hand, for samples implanted with 150?eV He⁺ ions, although He atoms are also trapped in the free volume, no He bubbles were observed during annealing the samples even up to 873?K. In conclusion, the formation of He bubbles is related to the formation and migration of vacancy-type defects even in amorphous alloys.     

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.     

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.     

Dependence of the persistent photoconductivity in doping-modulated a-Si: H multilayers on the exposure temperature

Abstract

We find that the photo-induced excess conductivity (persistent photoconductivity, PPC) in doping-modulated hydrogenated amorphous silicon (a-Si: H) is thermally activated for exposure temperatures above 220 K in agreement with Kakalios and with Hundhausen and Ley. However, we find a temperature regime between 80 and 220 Kin which the PPC measured at 300 K is independent of excitation temperature. Whether the PPC is excited in the thermally activated regime above 220 K or in the temperature-independent regime, the annealing occurs at 410 K. The pre-exponential factor and the activation energy of PPC are related by the Meyer-Neldel rule independent of the excitation temperature.     

New insights on tensile plasticity of ultrafine-grained metallic materials

ABSTRACT

The tensile properties of TiNi_43.5Fe_6.5 alloy samples having different grain sizes (0.16, 0.35, 1.7, 2.3, and 3.9?μm) and fabricated by severe plastic deformation and annealing were investigated. It was observed that both the strength and the elongation of the alloy increase with a decrease in the grain size until the average size reaches 1.7?μm. However, for average grain sizes smaller than 1.7?μm, the elongation decreases continuously with further grain refinement. On the other hand, the strain-hardening rate does not decrease with the decrease in plasticity but instead increases slightly. The poor ductility of the ultrafine-grained TiNi_43.5Fe_6.5 alloy is accompanied by a high degree of strain hardening. This newly observed ductility behaviour of the ultrafine-grained TiNi_43.5Fe_6.5 alloy is elucidated by characterising the intragranular and grain boundaries.     

Plastic deformation behaviour of decagonal Al70Ni15Co15 single quasicrystals

High-temperature deformation experiments have been performed on decagonal Al₇₀Ni₁₅Co₁₅ single quasicrystals at a constant strain rate of 10^-5s^-1 in the temperature range between 700 and 860°C. The samples were deformed in compression with the compression axis in different orientations, parallel to, inclined by 45° and perpendicular to the tenfold symmetry axis. Stress relaxation tests and temperature changes were carried out to determine thermodynamic activation parameters. The flow stress and the activation enthalpy were found to depend on the sample orientation whereas dependences of the activation volume and the stress exponent on the orientation were not observed. Additionally, deformation tests were performed on samples of the basic Co-rich modification of the decagonal phase at the temperature of 860°C in the same three orientations. The deformation behaviours of the two different modifications of the decagonal phase are discussed.     

High-resolution electron microscopy observations of microplastic fracture in SiC under ball milling at room temperature

Nanometre-sized kinks and cracks formed in 6H SiC under ball milling (BM) at room temperature have been observed and characterized on the atomic scale using high-resolution electron microscopy (HREM). Observations of the kinks show that numerous positive and negative partials are aligned at either of the kink boundaries, and the stacking sequences in the kink band are considerably different from those in the other areas. It was also observed that the (0001) lattice planes in the kink band are kinked, indicating that microplasticity occurs in the normally brittle material SiC under BM even at room temperature. HREM observations of cracks show that cracks previously observed by transmision electron microscopy are not completely open but are at the initiation stage of fracture. Inside a crack, one residual kink region can be clearly observed, which indicates a correlation between kink and crack, that is a crack evolves from a kink.     

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.     

Comparison of quasicrystalline (T2) and crystalline (R) structures in AlCuLi using high-resolution X-ray diffraction

Abstract

A high-resolution X-ray scattering experiment has been performed on quasicrystalline (T2) and crystalline (R) phases of AlCuLi alloys. Peak widths determined from the diffraction profile of the R phase are found to scale with momentum transfer G, similar to the strain-broadening effect seen in conventional crystals with small unit cells. A narrowing of peak widths on annealing is also observed. On the other hand, diffraction peak widths in I-phase samples obtained by rapid solidification are found to scale with phason momentum G ₁, similar to that observed in large faceted crystals. Thermal annealing of the I phase is found to have no effect on peak widths. Our findings are discussed in the light of models for crystals and quasicrystals.     

Commensurate-incommensurate phase transition in muthmannite,AuAgTe2: first evidence of a modulated structure at low temperature

To study the temperature-dependent structural changes and to analyze the crystal chemical behavior of silver as a function of temperature, a crystal of muthmannite, AuAgTe₂, has been investigated by X-ray single-crystal diffraction methods at 300 K and 110 K. At room temperature, muthmannite was confirmed as belonging to the space group P2/m, while at low temperature (110 K) it undergoes a reversible commensurate–incommensurate phase transition with a modulation wave vector q = 0.215(1)a* + 0.379(2)c*. Muthmannite reconverts to the commensurate type upon returning to room temperature, thus indicating that the phase transition is completely reversible in character. The average structure of the low-temperature muthmannite remains monoclinic, space group P2/m, and shows only normal thermal compression over the entire temperature range investigated. Crystal-chemical characteristics are compared with published data on the other members of the system Au–Ag–Te. Speculations on the possible origin of the modulated structure at low temperature are also given.     

Transmission electron microscopy of dislocations in SrTiO3

Dislocations have been introduced in SrTiO₃ by Vickers indentation at room temperature and analysed by transmission electron microscopy. The slip systems in SrTiO₃ were identified as ?110?-{110}. ?110? dislocations are dissociated into two partial dislocations. The stacking-fault energy γSF was determined to be 136 ± 15 mJm^-2.     

Sol-gel-hydrothermal growth of oxide thin films at low temperatures

Lead zirconate titanate (PZT) (Pb(Zr_0.52Ti_0.48)O₃) and barium titanate (BaTiO₃) thin films have been successfully prepared using a novel sol-gel- hydrothermal (SG-HT) technique at low temperatures, which involves a combination of the conventional sol-gel process and a hydrothermal method. Highly (111)-oriented PZT thin films with a single perovskite phase and polycrystalline BaTiO thin films with well developed crystallites were obtained at a processing temperature as low as 160⁰C. The microstructural characteristics demonstrate that the SG-HT-derived PZT and BaTiO₃ thin films with good crystallinity and surface morphology are converted from the amorphous phase to the desired perovskite phase on platinum-coated and bare silicon substrates at a low processing temperature of 100-200 C. These results suggest that the SGHT technique, which is of great significance because of its low processing temperature, will become a potential and promising process for fabricating PZT, BaTiO₃ and other oxide thin films.     

Chemistry (intrinsic) and inclusion (extrinsic) effects on the toughness and Weibull modulus of Fe-based bulk metallic glasses

Systematic changes in composition were employed to increase the notch toughness of a variety of Fe-based Bulk Metallic Glasses (BMGs). The Fe₅₀Mn₁₀Mo₁₄Cr₄C₁₆B₆ BMG possessed very high hardness (e.g. 12 GPa) but very low notch toughness (e.g. 5.7 MPa m^1/2) at room temperature, consistent with fracture surface observations of brittle features. Many of the other Fe-BMG variants, created to change the Poisson's ratio via systematic changes in alloy chemistry, exhibited higher toughness but more scatter in the data, reflected in a lower Weibull modulus. SEM examination revealed fracture initiation always occurred at inclusions in samples exhibiting lower toughness and/or Weibull modulus for a given chemistry. Implications of these observations on reliability of BMGs are discussed.