Structural investigations on Nd-doped silica nanocomposites: effect of sintering temperature and dopant concentration

Neodymium-doped silica nanocomposites were prepared from an acid-catalysed sol–gel solution followed by heat treatment. The structural and microstructural properties of the prepared samples as a function of sintering temperature and Nd concentration are reported. Fourier transform infrared spectra show that phase separation occurs during heat treatment. The presence of Nd₂O₃ and α-Nd₂Si₂O₇ phases in the samples was established by X-ray diffraction (XRD), and transmission electron microscopy (TEM) micrographs revealed the microstructure of the nanocomposites. From XRD patterns, the crystallite size was determined using the Debye–Scherrer formula, while the particle size was estimated from TEM micrographs. The results suggest that sintering at high temperature enhances the crystallinity and density of Nd₂O₃–SiO₂ nanocomposites, while the high concentration of neodymium prevents the crystallization of SiO₂.     

Long-term natural ageing of a dissociated dislocation in a Cu-13.43 at.% Al alloy

The nature of impurity-dislocation interactions is one of the key questions governing the strength and plasticity of solid-solution materials. To investigate the influence of impurities on the mechanical properties of intermetallic NiAl, the electronic structure and energy of NiAl with a <100>{010} edge dislocation and transition-metal impurities was calculated using the real-space tight-binding linear muffin-tin orbital method. The localized electronic states, appearing in the core of the dislocation, are found to lead to strong impurity-dislocation interactions via two mechanisms: firstly, chemical locking, due to strong hybridization between impurity electronic states and dislocation localized states; secondly, electrostatic locking, due to long-range charge oscillations caused by the electron localization in the dislocation core. The results obtained explain qualitatively why the solid-solution hardening effect in NiAl correlates with the electronic structure of impurities rather than with size misfit, as expected according to traditional views.     

Formation of ω -Al7Cu2Fe phase during laser processing of quasicrystal-forming Al–Cu–Fe alloy

The formation of an ω-Al₇Cu₂Fe phase during laser cladding of quasicrystal-forming Al₆₅Cu_23.3Fe_11.7 alloy on a pure aluminium substrate is reported. This phase is found to nucleate at the periphery of primary icosahedral-phase particles. A large number of ω-phase particles form an envelope around the icosahedral phase. On the outer side, they form an interface with an α-Al solid solution. Detailed transmission electron microscopic observations show that the ω phase exhibits an orientation relationship with the icosahedral phase. Analysis of experimental results suggests that the ω phase forms by precipitation on an icosahedral phase by heterogeneous nucleation and grows into the aluminium-rich melt until supersaturation is exhausted. The microstructural observations are explained in terms of available models of phase transformations.     

Nanoscale inclusions in the phonon glass thermoelectric material Zn4Sb3

We have investigated the thermoelectric material Zn₄Sb₃ using transmission electron microscopy (TEM). Nanoscale inclusions with a diameter of about 10 nm were observed, constituting on the order of 1% by volume of the material. Studies using energy filtered imaging, electron diffraction, and high-angle annular dark-field STEM indicate that the inclusions consist of Zn. These inclusions are expected to scatter the medium and long-wavelength phonons effectively, thus contributing to phonon glass behavior which results in the exceptionally low thermal conductivity for this thermoelectric material.     

Observation of Cu nanometre scale clusters formed in Fe85Si2B8P4Cu1 nanocrystalline soft magnetic alloy by a spherical aberration-corrected TEM/STEM

Microstructure of a nanocrystalline soft magnetic Fe₈₅Si₂B₈P₄Cu₁ alloy (NANOMET^®) was investigated by the state of the art spherical aberration-corrected TEM/STEM. Observation by TEM shows that the microstructure of NANOMET^® heat treated at 738 K for 600 s which exhibits the optimum soft magnetic properties has homogeneously distributed bcc-Fe nanocrystallites with the average grain size of 30 nm embedded in an amorphous matrix. Elemental mappings indicate that P is excluded from bcc-Fe grains and enriched outside the grains, which causes to retard the grain growth of bcc-Fe crystallites. The aberration-corrected STEM-EDS analysis with the ultrafine electron probe successfully proved that Cu atoms form nanometre scale clusters inside and/or outside the bcc-Fe nanocrystallites.     

TEM observation of the restrained Goss–Brass orientation transformation in a warm-rolled Fe–28Mn–6Si–5Cr shape memory alloy

A dominant Goss texture component instead of a Brass texture component has been observed in a warm-rolled Fe–28Mn–6Si–5Cr shape memory alloy [H. Li, F. Yin, T. Sawaguchi K. Ogawa, X. Zhao and K. Tsuzaki, Mater. Sci. Eng. A 494 (2008) p.217]. In order to clarify the orientation flow mechanism in these grains of Goss orientation, investigations on microtexture and microstructure were carried out by electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM). It is concluded that the Goss–Brass orientation transformation is restrained in the Goss-orientated grains, which were widely observed in both the deformed matrix and shear bands.     

Shape of prismatic dislocation loops in anisotropic α-Fe

Prismatic dislocation loops are the primary manifestation of radiation damage in crystals, and contribute to the phenomenon of radiation embrittlement. This undesirable effect, most serious for materials used in high-dose environments such as next-generation fission and future fusion reactors, results from the strong interaction between gliding dislocations, the carriers of plasticity, with the population of radiation-induced prismatic loops. Ferritic–martensitic steels, the most promising candidate materials for future high-dose applications, are based on iron and are known to become highly elastically-anisotropic at the high temperatures (>500°C) at which they must operate. In this article, we develop a novel modelling approach based on anisotropic elasticity theory to predict the shapes of prismatic loops in anisotropic crystals, paying particular attention to the technologically important case of α-iron. The results are compared with transmission electron microscope observations of the damage structure sustained by ultra-high-purity iron irradiated to a dose of approximately two displacements per atom.     

Observation of phase separation in magnetron sputter-deposited Al–Cu thin films

The microstructures of Al–1.8 to 92.5?at.% Cu thin films prepared by radiofrequency (13.56?MHz) cathodic magnetron sputtering have been investigated by X-ray diffraction (XRD) and transmission electronic microscopy (TEM). A phase separation occurs in films of nominal Al–66.64?at.% Cu composition, consisting of a fcc Al solid solution phase, a fcc Cu solid solution phase and an unexpected sc Cu₃Al ordered phase with a Cu₃Au structure and a lattice parameter of about 0.36?nm.     

Effects of Si addition on the microstructure evolution of Al–Cu–Mg alloys in the α + S + T phase field

The precipitation behaviour and age-hardening response of Al–1.5Cu–4.0Mg (wt.%) alloys microalloyed with Si have been investigated by means of hardness measurement, TEM and HRTEM. Compared to the ternary alloy, the quaternary alloys exhibit a higher hardness. It is found that the underaged microstructure in the Al–1.5Cu–4.0?Mg alloy contains some fine precipitates, which are identified as the T phase by FFT spectra. The peak-aged microstructures of the ternary alloy is dominated by the T phase, while the peak-aged microstructures of the Si-containing alloys are dominated by the S phase. The volume fraction of the S phase is found to increase as more Si is added.     

Enhanced age-hardening response and microstructure study of an Ag-modified Mg–Sn–Zn based alloy

A new Mg–Sn–Zn based alloy modified with a small amount of Ag exhibits a significantly higher aging peak than that of the base alloy and at a considerably shorter aging time. The enhanced aging response of the Ag-modified alloy is ascribed to the precipitation of densely distributed MgZn₂ needles and Mg₂Sn plates stimulated by the Ag. A wide and low plateau behind the hardness peak could be associated with rod-shaped precipitates of the orthorhombic Mg₅₄Ag₁₇ phase, aligned with the hexagonal axis of the Mg matrix.