A new method for automated reconstruction of pre-transformation microstructures

A new method, referred to as the cluster method, is proposed and tested for the reconstruction of pre-transformation microstructure in Ti–6Al–4V alloy. The cluster method begins by constructing clusters of daughter grains (room-temperature hcp α-phase) that have a high probability of being transformed from the same parent grain (high-temperature bcc β-phase). These clusters are then grown, and back-transformed to the parent grain orientations using the ‘summation of mutual misorientation angle method’ (SMMA method). The cluster method is validated by comparing the obtained results with the results of the SMMA and triplet methods.     

Atomic-scale study of {1 1 2} twin boundary structure in a β-Ti alloy

The atomic structure of {1?1?2} twin boundary in a specialty β-Ti alloy, gum metal, has been studied by means of conventional transmission electron microscopy, bright-field and high-angle annular dark field scanning transmission electron microscopy imaging. The observations provide direct evidence that the {1?1?2} twin boundary has a complex, multiple-layer structure, which is composed of various structural units. These structural units are formed as a result of different degrees of inner atomic-column shifting. The complex structure of the {1?1?2} twin boundary is different to the atomic structure of the previously reported ω phase.     

On the role of Ag in enhanced age hardening kinetics of Mg–Gd–Ag–Zr alloys

The addition of Ag to the age hardenable Mg–Gd–Zr alloy system dramatically enhances early stage age hardening kinetics. Using atom probe tomography (APT), Ag-rich clusters were detected in a Ag-containing Mg–Gd–Zr alloy immediately after solution treatment and water quenching. During subsequent isothermal ageing at 200 °C, a high density of basal precipitates was observed during the early stages of ageing. These basal precipitates were enriched with Ag and Gd, as confirmed by APT. It is posited that Ag-rich clusters in the context of quenched-in vacancies can attract Gd atoms, increasing diffusion kinetics to facilitate the formation of the Ag + Gd-rich basal precipitates. The rapid formation of Ag + Gd-rich precipitates was responsible for accelerated ageing.     

Formation of profuse <c+a> dislocations in deformed calcium-containing magnesium alloys

We report that <c+a> pyramidal slipping could be more easily activated in textured Mg–Ca alloys with increasing Ca contents dissolved in α-Mg matrix under tensile deformation, and it is proposed that the decreased stacking fault energy plays the critical rule. In contrast, only twins and <a> basal dislocations are observed in the compressed samples. The results would provide insight into understanding of the deformation mechanism and designing more ductile Mg alloys.     

Solidification thermal parameters,microstructural evolution and dendritic growth in an AlCuNi alloy

ABSTRACT

Unsteady-state upward solidification experiment was carried out with the Al-3Cu-3Ni (wt.%) alloy. Temperature profiles were obtained through a water-cooled directional solidification device. Analysis on the resulting as-cast microstructure and its adaptation to thermal solidification parameter, such as growth and cooling rates and solidification local time, V_L, T_R and t_SL, respectively, was carried out. The results showed that the resulting microstructure is constituted by an Al-rich primary phase (Al _α ), characterised by a dendritic network, and by second phases that take place within the interdendritic regions, forming a eutectic mixture composed by the β-Al₃Ni?+?θ-Al₂Cu intermetallic compounds (IMCs), whose size and morphology have been affected by V_L and T_R. The length of the microstructural scale of the Al _α phase was measured by the secondary dendritic spacing (λ ₂). The experimental mathematical expression of λ ₂ obtained in this work was compared with mathematical models proposed in the literature that associate t_SL-λ ₂ for non-equilibrium and equilibrium solidification conditions.     

Kinetic study of hydrogen-induced direct phase transformation in Y2Fe17 magnetic alloy on the basis of Kolmogorov’s model

The kinetics of the hydrogen induced direct phase transformation in Y₂Fe₁₇ magnetic alloy has been analysed within the framework of Kolmogorov’s kinetic model. It is established that the transformation can be classified as a diffusion-controlled transformation, which occurs by a mechanism of nucleation and growth with a decreasing nucleation rate of new phases, namely α-Fe and YH₂. A kinetic equation has been obtained that well describes the isothermal kinetic curves of the phase transformation in Y₂Fe₁₇ as a function of the transformation temperature.     

Orientational high-entropy alloys

In high-entropy alloys (HEA), the configurational entropy arising from the presence of multiple elements, stabilizes a disordered solid solution in preference to the possible formation of compounds. In the current work, we identify cluster compounds (of the type AM₄X₈) as orientational analogues of HEA (as a first report on orientational high-entropy systems). In cluster compounds, orientational disorder increases the entropy and plays a role analogous to positional disorder in HEA. In the GaMo₄S₈ compound, at temperatures greater than 50 K, the entropic benefit more than makes up for the strain energy cost and stabilizes the disordered phase in preference to an orientationally ordered compound.     

Elastic and phonon properties of quaternary Heusler alloys CoFeCrZ (Z = Al,Si, Ga and Ge) from density functional theory

The structural, elastic and phonon properties of the quaternary CoFeCrZ (Z = Al, Si, Ga and Ge) Heusler alloys have been investigated using the generalized gradient approximation method within density functional theory. The ground-state properties, including, lattice constant and bulk modulus are in good agreement with the available theoretical and experimental data. The elastic constants C_ij are computed using the stress–strain technique. The calculated results indicate that CoFeCrZ (Z = Al, Si, Ga and Ge) alloys are ductile materials. Debye temperatures are predicted from calculated elastic constants. The phonon dispersion relations of CoFeCrZ (Z = Al, Si, Ga and Ge) alloys are calculated for the first time using the density functional theory and the direct method with 2 × 2 × 2 supercell.     

Analysis of the relation between structure and the superconducting transition in YBa2 Cu3 O∼7

Abstract

It is shown that in the high-T_c YBa₂ Cu₃ O_～7 superconductor the critical temperature is a function of the orthorhombic distortion (b–a)/a of the unit cell. From the extrapolation of the (b - a)/a ratio against T_c , a maximum critical temperature of 66 K for the tetragonal phase of YBa₂ Cu₃ O_～7 was predicted. From the correlation between the transition width δT_c and the orthorhombic distortion there was deduced an upper limit for T_c , in the orthorhombic phase of YBa₂ Cu₃ O_～7 of 94·5K.