Nanobeam Electron Diffraction Patterns Research Articles

Local Atomic Structure Analysis of Zr-Ni and Zr-Cu Metallic Glasses Using Electron Diffraction

Local atomic structures in Zr66:7Ni33:3 and Zr66:7Cu33:3 metallic glasses were examined by using nanobeam electron diffraction (NBED), energy-filtered selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). Locally ordered regions of atomic medium range order (MRO) were observed in both of the specimens by NBED, although it was difficult to recognize the regions using HREM. Statistical analyses for NBED patterns revealed such a difference in the extended MRO regions between the specimens that the MRO structure in Zr66:7Ni33:3 is more complex with a large dispersion of interplanar spacings than those in Zr66:7Cu33:3. To understand nearest-neighbor atomic coordination, we performed electron intensity analyses using energy-filtered SAED patterns and constructed structure models including about 5000 atoms with the help of reverse Monte Carlo simulation. The nearest-neighbor atomic environments around Ni atoms in Zr66:7Ni33:3 are also different from those around Cu atoms in Zr66:7Cu33:3, consistent with the NBED study. The local structural difference between the two glasses was discussed in relation to their glass-forming abilities. [doi:10.2320/matertrans.MF200618]

Mechanism of nanocrystalline microstructure formation in amorphousFe−Nb−Balloys

To understand the mechanism of the high number density of $\mathrm{bcc}\text{\ensuremath{-}}\mathrm{Fe}$ nanocrystals in a partially crystallized ${\mathrm{Fe}}_{84}{\mathrm{Nb}}_{7}{\mathrm{B}}_{9}$ alloy, we have investigated detailed local structural and compositional changes on annealing amorphous ribbons using transmission electron microscopy, three-dimensional atom probe, and high-energy x-ray diffraction techniques. Nanobeam electron diffraction patterns from an as-quenched amorphous ribbon indicated a local nanoscale atomic ordering. On annealing, reduced interference functions showed a clear change just below the crystallization temperature $(\ensuremath{\sim}773\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. At this stage, local compositional fluctuations started to appear, and medium-range ordering with a $\mathrm{bcc}\text{\ensuremath{-}}\mathrm{Fe}$ structure as small as $2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ was clearly observed in high-resolution electron micrographs with an extremely high number density. Pair distribution function analyses suggested a structural change at this stage of annealing to increase the chemical bonds in the residual amorphous matrix around the $\mathrm{bcc}\text{\ensuremath{-}}\mathrm{Fe}$ regions. The increase of atomic chemical bonds in the residual amorphous matrix is considered to retard the growth of the $\mathrm{bcc}\text{\ensuremath{-}}\mathrm{Fe}$ nanocrystals after the coalescence of $\mathrm{bcc}\text{\ensuremath{-}}\mathrm{Fe}$ MRO regions in the later stage of annealing. These results suggest that $\mathrm{bcc}\text{\ensuremath{-}}\mathrm{Fe}$ nanocrystallization with the extremely high number density is ascribed to primarily (i) the presence of highly dense $\mathrm{bcc}\text{\ensuremath{-}}\mathrm{Fe}$ MRO regions and (ii) the increase of chemical bonds of matrix atoms on annealing.

Structural characterization of iron silicides nanoparticles grown on Si substrate: Annealing rate dependence

We have synthesized iron silicides on a Si(111) substrate using an electron-beam evaporation technique in combination with thermal annealing. The microstructures, as a function of heating rate on annealing, were characterized by transmission electron microscopy (TEM). Bright-field plan-view TEM observations revealed that a discontinuous thin film is formed in the sample annealed rapidly, while complete separated nanoparticles were obtained in the specimen annealed slowly. Selected-area and nano-beam electron diffraction patterns indicated that the former and latter specimens mainly consist of iron monosilicide (ɛ-FeSi) and iron disilicide (β-FeSi2) phases, respectively.

A direct method to determine the chiral indices of carbon nanotubes

A direct method is described to determine the chiral indices ( u, v) of a carbon nanotube from a nanobeam electron diffraction pattern of the nanotube with high accuracy. This method is a one-step procedure and allows rapid and precise assignment of the chiral indices of carbon nanotubes. Using a contemporary electron microscope, we have been able to achieve an accuracy that enables us to assign the chiral indices unambiguously up to the range of (30, 30), corresponding to a diameter of 4.0 nm. Examples of determination of the chiral indices for two single-walled carbon nanotubes are presented as an illustration of the method.

Amorphous structures of silicon carbonitride formed by high-dose nitrogen ion implantation into silicon carbide

Short-range order in amorphous silicon carbonitride (a-SiC x N y ) has been examined using transmission electron microscopy. Single crystals of 6H-SiC with [0 0 0 1] orientation were implanted with 180 keV nitrogen ions at ambient temperature to a fluence of 5 × 10 17 N +/cm 2, followed by thermally annealing at 1500 °C for 30 min. A fully amorphous layer was formed at the topmost layer in the as-implanted sample. A part of the amorphous phase transformed into crystalline SiC after annealing. Radial distribution functions extracted via nano-beam electron diffraction patterns clearly showed that atomistic structures of the ion-beam-induced amorphous phase are different from those of the remaining amorphous phase in the annealed sample: a-SiC x N y possesses an intermediate bond length between Si–C and Si–N, while Si–N and Si–C bonds become more pronounced in the amorphous layer of the annealed specimen.

Concept of chemical short range order domain and the glass forming ability in multicomponent liquid

The concept of multicomponent chemical short-range order (MCSRO) domain is systematically developed by the experimental investigation of Zr–Ti–Cu–Ni–Al bulk metallic glass (BMG) and thermodynamic modeling and calculation. The existence of MCSRO domains in Zr-based BMG is verified by the observations of high-resolution transmission electron microscopy (HRTEM) images and the analysis of nano-beam electron diffraction patterns. The size of the nano-beam used in this work is 0.5 nm in diameter. Thermodynamic evaluation of the melt composed of multiple-MCSRO domains and glass-forming ability (GFA) based on the concept of MCSRO domains has also been conducted. It is indicated that the thermodynamic calculation of the GFA based on MCSRO model is consistent with the experimental data of crystallization activation energy and glass transition temperature for Ni-Zr and Zr-Cu binary alloys, and with supercooled liquid region (ΔTx) for Zr–Ni–Al ternary alloy. The existence of MCSRO domain lowers the free energy of the melt (ΔGMCSRO), resulting in a large undercooling and a larger energy barrier to the nucleation of a critical crystalline nucleus. Large ΔGMCSRO, low melting point as well as co-existence of multiple MCSRO domains are valid criterion for the valuation of GFA.

Characterization of ion-beam-induced amorphous structures by advanced electron microscopy

Ion-beam-induced amorphous structures of silicon carbide (SiC) have been characterized by cross-sectional transmission electron microscopy. Single crystals of 6H–SiC with [0 0 0 1] orientation were irradiated with 180 keV oxygen ions at elevated temperature to a fluence of 1.4×10 18 O +/cm 2. It was found that a layered amorphous structure is formed inside the irradiated crystal. The intensities of halo patterns obtained by nano-beam electron diffraction techniques were analyzed quantitatively using imaging plates as a recording material. Pair distribution functions extracted by Fourier transforming nano-beam electron diffraction patterns indicated that the layered amorphous structure is due to compositional variations of silicon, carbon, and oxygen.

Microstructure Feature of Bulk Glassy Cu<SUB>60</SUB>Zr<SUB>30</SUB>Ti<SUB>10</SUB> Alloy in As-cast and Annealed States

The microstructures of a bulk glassy Cu 60 Zr 30 Ti 10 alloy in as-cast state and annealed at glass transition temperature (T g ), i.e., 440°C, have been studied using an electron transmission microscope (TEM) and a high-resolution transmission electron microscope (HRTEM). Nanocrystals with a size of 4 nm were observed in the cast rod with a diameter of 4 mm. The nanocrystal was determined to be CuZr, which is primitive cubic structure with a lattice parameter of a = 0.3262 nm by analyzing its nano-beam electron diffraction pattern, suggesting that such nanocrystals have a positive effect on mechanical properties. Obviously distinguished interval dark and bright regions were observed in the glassy Cu 60 Zr 30 Ti 10 alloy annealed for 10 min at T g . Nanocrystalline phase with a size of ∼10 nm was recognized after the annealing treatment. The concentration distribution of Cu, Zr and Ti in the alloy annealed for 10 min at T g was examined with a three-dimensional atom probe field ion microscopy (3DAP-FIM). It was found that the Cu and Zr elements fluctuated along depth direction while the Ti element remained almost unchanged. It is therefore interpreted that the annealing-induced nanocrystalline phase results from nucleation and growth at the sites of the Cu-rich and/or Zr-rich regions as well as from growth of the cast-in nanocrystals.

Direct observation of icosahedral cluster in Zr70Pd30 binary glassy alloy

The medium-range order in the Zr70Pd30 binary glassy alloy, where the nanoicosahedral phase precipitates as a primary phase, was examined using the high-resolution electron microscopic technique. The ordered region in the diameter of ∼2 nm was observed in the as-quenched glassy state. This region grows slightly to the diameter of 3–4 nm by annealing for 120 s at 690 K, where the amorphous structure remains. The nanobeam electron diffraction pattern taken from the medium-range order shows the fivefold symmetry, indicating that this region has the icosahedral structure. This result is recognized as a direct evidence for the existence of the icosahedral cluster in the alloy. For further annealing for 600 s at 690 K, the icosahedral quasicrystalline phase in the diameter of 5–8 nm precipitates by assimilating the icosahedral cluster. The formation of the nanoicosahedral phase originates from the existence of the quenched-in icosahedral clusters in the glassy state followed by their easy growth to the icosahedral particle without significant rearrangement of the constitutional elements.

Precipitation of an icosahedral quasicrystalline phase in Hf59Ni8Cu20Al10Ti3 metallic glass

The crystallization process of a Hf59Ni8Cu20Al10Ti3