Stacking of hexagonal layers in the structure of β-Mg2Al3

An analysis of cross-sections perpendicular to the main diagonal of a cubic elementary cell of a Samson structure–β-Mg₂Al₃ has been conducted. It has been proved that all skeleton atoms, i.e. which occupy their positions with probability of 100%, form a framework of hexagonal planes. These planes are a part of three domains shifted with respect to each other by one-third of the length of cube's diagonal. Space between domains is filled up by clusters with partially occupied atomic positions. For hexagonal domains, the elementary cell and positions of decorating atoms have been determined.     

Synthesis and characterizations of NiTiO3 nanoparticles prepared by the sol–gel process

Nanocrystalline nickel titanate (NiTiO₃) composite powders were prepared by the sol–gel process combined with a surfactant-assisted template method. The resulting powders were calcined at different temperatures ranging from 150°C to 750°C for 2 h in air. The results revealed that a pure hexagonal phase of NiTiO₃ could be obtained at the low temperature of 750°C. The phase evolution of NiTiO₃ was investigated by X-ray diffraction patterns, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Particle size and morphology were studied by transmission electron microscopy.     

Voids formed from solidifying tin particles in solid aluminium

In this study, voids commonly associated with tin particles in two aluminium alloys containing microalloying additions (0.01 at.%) of tin have been observed by transmission electron microscopy. The voids were generated by quenching the alloys at moderate rates (10²–10³ K s^?1) from a temperature (718 K) in excess of the melting temperature (501 K) of elemental tin in tin–microalloyed aluminium. Estimates of the void volume as a function of the volume of the associated tin particle reveal a linear relationship consistent with the excess volume resulting from the solidification of the tin particle. The formation and stabilisation of shrinkage voids in metallic alloys are suggested to arise from a combination of high vacancy supersaturation, the large volumetric misfit strain of the solidifying tin particle and a reduction in void surface energy associated with segregation of alloying elements.     

Study on the damping behaviour of eutectic high-entropy alloys with lamellar structures

ABSTRACT

Damping is the ability to reduce the energy produced by vibrations in a system. In this research, the eutectic high-entropy alloys (EHEAs), AlCo_xCrFeNi_2.1 (x?=?0.7, 1.0 and 1.5), are firstly studied for their damping behaviour. EHEA is a promising concept of high-entropy alloys. By combining the advantages of eutectic phases and HEAs, the EHEAs show better liquidity, castability and compositional homogeneity than conventional HEAs. Meanwhile, it can strike a great balance in mechanical strength and tensile ductility owing to the lamellar structures. The microstructures and crystal structure of two near-EHEAs are also studied. AlCo_0.7CrFeNi_2.1 is a hypo-eutectic HEA with primary body-centred cubic phases. AlCo_1.5CrFeNi_2.1 is a hyper-eutectic HEA with primary face-centred cubic phases. The results showed that these eutectic and near-eutectic alloys gave rise to greater tan δ values than conventional dual-phase HEAs, which might be attributed to their lamellar structures.     

Thermodynamic analysis of Fe contamination in Cu-Mo alloys processed by mechanical alloying

ABSTRACT

This work aims to verify if Fe atoms coming from balls and jars enter into solid solution of Cu-Mo powders during mechanical alloying. The powders were milled and its structure and composition were characterised by scanning and transmission electron microscopy, and X-ray diffraction. Gibbs free of mixing values for Cu-Mo-Fe system were calculated using the Miedema’s model and showed that Cu-rich corner exhibit smallest energy values (<10?kJ?mol^?1). The observed coherency relation Cu(111) || Mo(101) || Fe(101) confirmed formation of solid solution. Fe atoms have entered in solid solution with Cu and Mo and do not remain as particles of second phase.     

Influence of heat treatment on the microstructural evolution of Al–3 wt.% Cu during high-pressure torsion

Solution-treated, peak-aged and overaged samples of the model alloy Al–3?wt.% Cu, obtained by selective heat treatments of the pre-material, have been subjected to high-pressure torsion at room temperature and at 200?°C. The mechanical behaviour of the samples was investigated with torque measurements during deformation and microhardness measurements after deformation. Irrespective of the initial material condition, in the saturation regime a comparable equilibrium microstructure was found consisting of ultrafine aluminium grains stabilized by precipitates formed at grain boundaries.     

HRTEM observation of misfit dislocations at non-faceted Al–Pb interfaces

Al–Pb samples containing 1?at.%?Pb were produced by high-energy ball milling and investigated by high-resolution transmission electron microscopy. The Pb inclusions embedded in an Al matrix exhibit a non-faceted, curved morphology. Dislocations have been found to accommodate the misfit of about 22% at the Al–Pb interfaces. Burgers circuits, drawn around these misfit dislocations, exhibit closing failures of the type a₀ /4〈211〉.     

Grain growth as a stochastic and curvature-driven process

Grain growth subjected to the interplay of stochastic and curvature-driven mechanisms in a single-phase system has been investigated. Numerical results have shown that when the grains are smaller than several tens of nanometres the dominating mechanism is stochastic diffusion control of boundaries. As the grains grow the influence of the deterministic curvature-driven mechanism increases and finally controls the process. In terms of finite-difference solutions to the Fokker–Planck continuity equation, the predicted grain size approaches a log-normal distribution, which agrees well with experimental observations.     

Anisotropic grain growth kinetics in nanocrystalline nickel

ABSTRACT

Intriguing properties exhibited by nanocrystalline metals, including a high level of mechanical strength, arise from their nanometer-scale grain sizes. It is critical to determine the evolution of grain size of nanocrystalline materials at elevated temperature, as this process can drastically change the mechanical properties. In this work, a nanocrystalline Ni foil with grain size ～ 25?nm was annealed in situ in an X-ray diffractometer. X-ray diffraction peaks were analysed to determine the grain growth kinetics. The grain growth exponents obtained were ～ 2–4 depending upon the crystallographic direction, indicating the anisotropic nature of the grain growth kinetics.