Electronegativity difference as a factor for evaluating the thermal stability of Al-rich metallic glasses

This article proposes a factor, the critical electronegativity difference Δx _cri, to correlate alloy composition with thermal stability and glass-forming ability of Al–Ni–RE (RE: Rare Earth element) ternary metallic glasses. The Al-rich metallic glasses with Δx > Δx _cri exhibit glassy behavior, whereas alloys with Δx < Δx _cri are nanocrystalline. Nanoglassy alloys occur when Δx ≈ Δx _cri. The best glass formers are located near Δx _cri. Furthermore, an equation has been deduced to calculate Δx _cri with varying RE covalent atomic radius.     

Efficient atomic packing clusters and glass formation in ternary Al-based metallic glasses

In this article, we propose an efficient atomic packing cluster-based composition protocol to help design Al-based metallic glasses. Its validity is verified by some typical experimental data from the literatures. Furthermore, with this understanding, the Al–Ni–Y alloy system is re-evaluated. As a result, the best glass former Al₈₆Ni₉Y₅ in this system, with the critical thickness of about 500 µm, is successfully fabricated by wedge casting.     

Precipitate characteristics and selected area diffraction patterns of the β′ and Q′ precipitates in Al–Mg–Si–Cu alloys

In this study, the precipitate characteristics and selected area diffraction patterns (SADP) of the β?′ and Q?′ precipitates formed at the over-aged state of Al–Mg–Si–Cu alloys are systematically investigated by means of high-resolution transmission electron microscopy and transition matrix. The β?′ precipitates have two cross-sections, rectangle-shaped and round-shaped aligned with [0?0?1]_Al direction, but only rectangle-shaped cross-section exists for Q?′ precipitates. And, both of them have 12 variants and orientations with Al matrix. However, there are only three different zone axes, [0?0?0?1]_β?′, [1?4?5?0]_β?′, and [5?4?1?0]_β?′ for β?′ precipitates, and [0?0?0?1]_Q?′, [1?4?5?0]_Q?′, and [3?2 1?0]_Q?′ for Q?′ precipitates, parallel to the [0?0?1]_Al direction when they are precipitated from the Al matrix, respectively. Then, a new [0?0?1]_Al SADP model, which superposes diffraction patterns of the β?′ and Q?′ precipitates, is established. Furthermore, some “cross-shaped” diffraction streaks appeared at over-aged state can be explained reasonably by this model, which is good in agreement with the experimental data.     

Solubility criterion for sequential disordering in metal-metal multilayers upon solid-state reaction

An n-body Ni-Ti potential is derived and applied in a molecular dynamics simulation to study the maximum supersaturated solubility of the terminal solid solutions and solid-state reaction in a Ni/Ti bilayer. It reveals that during interfacial reaction of the Ni/Ti bilayer the Ti lattice reaches its maximum solubility by dissolving Ni earlier than Ni does through dissolution of Ti, which results in a sequential disordering of first Ti and then Ni, although Ti has a higher melting point than Ni. In the Ni-Zr, Ni-Mo and Ni-Ta systems, however, the Ni lattice collapses more rapidly because it reaches a maximum solubility earlier than its partners, which have higher melting points than Ni. A solubility criterion is thus relevant for all the above cases; the lower the maximum solid solubility the less stable is the lattice of the metal upon solid-state reaction.     

Surface effect on the screw dislocation mobility over the Peierls barrier

The effect of the image force on the Peierls stress (τ _p ) of a screw dislocation below a free surface is studied via a self-consistent semidiscrete variational Peierls–Nabarro model. The consequence of reduction in elastic energy and increase in stacking fault energy by the presence of the free surface is found to additively increase the Peierls stress (τ _p ). This model gives a physical interpretation of the same tend observed in a recent molecular dynamic study, while previous continuum analysis predicted the opposite.     

Identification of crystal nucleation and growth in an amorphous FeZr2 alloy by means of electrical resistance measurements

A polymorphous crystallization process in an amorphous FeZr₂ alloy has been investigated by means of accurate electrical resistance measurements (ERMs) at elevated temperatures. It was found that, upon crystallization of the amorphous alloy, the electrical resistance increased with increasing temperature, exhibiting three distinct stages. Quantitative microscopy observations revealed that the three stages originated from crystal nucleation, from subsequent growth of crystal nuclei and from coarsening of the crystallites respectively. The activation energies for the crystal nucleation and growth determined from the ERM data agree satisfactorily with the data in the literature. The success in identification of the crystal nucleation and growth processes by means of ERMs may originate from differences in electrical resistance changes due to the crystal formation and the crystalline-amorphous interface formation processes from the amorphous phase.     

The alignment of Bi(Pb)-Sr-Ca-Cu-O near the Ag-(Bi(Pb)-Sr-Ca-Cu-O) interface in silver-sheathed multifilamentary Bi(Pb)2Sr2Ca2Cu3O

The Ag-(Bi(Pb)-Sr-Ca-Cu-O (BSCCO)) interface region has been studied by scanning electron microscopy and transmission electron microscopy to a distance of 2µm away from the silver in the Ag-sheathed multifilamentary Bi(Pb)₂Sr₂Ca₂Cu₃O (Bi(Pb)-2223) tapes. Owing to undulation of the silver surface, silver was found to play an important but not always positive role in the alignment of BSCCO grains. Based on the kink angle variation, the effective thickness of multifilamentary Bi(Pb)-2223 tapes capable of carrying a large current has been defined. We believe that the difference in effective thickness is the key reason leading to different critical current densities J c for these tapes.     

Site preference of refractory elements in γ′-Ni3Al alloyed with Ru

The influence of ruthenium (Ru) on the partitioning behavior of W, Re and Cr in γ′-Ni₃Al has been studied using the Dmol3 method based on the density functional theory. The calculated results show that W, Re and Cr have a preference for the Ni site in γ′-Ni₃Al alloyed with Ru. When Ru substitutes the central Ni atom, the site preference for W, Re and Cr varies. Furthermore, an electronic structure analysis of the alloy in terms of the Mulliken population and partial density of states was performed to elucidate the alloying mechanism of Ru in γ′-Ni₃Al. The results show that the strengthening effect of Ru in the alloy arises from a reduction in the binding energy of Ru as well as p-orbital hybridization between Ru and the host atoms.     

Phase-transformation-induced hardening in Zn–22Al alloys

A phase-transformation-induced hardening effect is reported in Zn–22Al (Al: 22?wt.%) alloys. The Zn–22Al specimens were held at 300?°C for 10?h and then quenched in water. A hardening effect took place in subsequent artificial aging at 100–200?°C, which was accompanied by a phase decomposition of a soft α ₂ phase and a grain coarsening. The phase-transformation-induced hardening affects the hardness more than the grain-coarsening-induced softening, which leads to the age-hardening phenomenon.