Atomistic simulations of interactions between screw dislocation and twin boundaries in zirconium

Molecular statics was employed to simulate interaction between screw dislocation and twin boundaries (TB) in hexagonal close-packed zirconium. In the moving TB model, the interaction of a moving $\left\{10\overline{1}2\right\}$ TB with a static $1/3〈11\overline{2}0〉\left\{10\overline{1}0\right\}$ screw dislocation was investigated. Twinning dislocation (TD) nucleation and movement play an important role in the interaction. The screw dislocation passes through the moving TB and changes to a basal one with a wide core. In the moving dislocation model, a moving $1/3〈11\overline{2}0〉\left\{10\overline{1}0\right\}$ dislocation passes through the TB, converting into a basal one containing two partial dislocations and an extremely short stacking fault. If the TB changes to the $\left\{10\overline{1}1\right\}$ one, the moving $1/3〈11\overline{2}0〉\left\{10\overline{1}0\right\}$ prismatic screw dislocation can be absorbed by the static TB and dissociated into two TDs on the TB. Along with the stress–strain relationship, results reveal the complicated mechanisms of interactions between the dislocation and TBs.     

Atomic scale structural characterization of {101¯2} twin boundaries in zinc

The structure and the migration mechanisms of $\left\{10\overline{1}2\right\}$ twin boundaries (TBs) of pure zinc deformed by rolling were studied using high-resolution transmission electron microscopy (HRTEM) at atomic scale. We found the presence of basal/prismatic (BP/PB) planes serrations on $\left\{10\overline{1}2\right\}$ TBs and the coexistence of two kinds of TBs with different structures in the same $\left\{10\overline{1}2\right\}$ twin: TBs composed of $\left\{10\overline{1}2\right\}$ coherent twin boundaries (CTBs) plus short BP/PB serrations, and TBs composed of successive BP/PB segments without $\left\{10\overline{1}2\right\}$ CTBs. The formation of BP/PB serrations has no relation to the c/a ratio of hexagonal-close-packed (HCP) metals because the BP/PB serrations are energetically preferred and geometrically favored. Based on dislocation theory, we proposed the migration mechanisms of the TBs to be the glide of twinning dislocations (TDs) on the CTBs and the climb of interface dislocations (IDs) on the BP/PB segments.     

Microstructure evolution and recrystallization behavior of cold-rolled Zr-1Sn-0.3Nb-0.3Fe-0.1Cr alloy during annealing

The effects of cold-rolling reduction, annealing temperature, and time on recrystallization behavior and kinetics of cold-rolled Zr-1Sn-0.3Nb-0.3Fe-0.1Cr alloy were investigated using the Vickers hardness test, scanning electronic microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffractometry (EBSD). The results show that the rate of the recrystallization increased with increasing annealing temperature and rolling reduction. Recrystallized grains nucleated preferentially at sites with high density dislocation and deformation stored energy and then grew into integral grains. Recrystallization texture changed from $〈10\overline{1}0〉$//RD to $〈11\overline{2}0〉$//RD. The grain orientation changed from random orientation to the orientation with the maximum misorientation around 30°. Recrystallization kinetics and maps were constructed based on the Johnson–Mehl– Avrami–Kolmogorov (JMAK) equation to derive parameters sensitive to the microstructure. The activation energies for recrystallization of 30%, 50% and 70% cold-rolling reductions were determined to be 240, 249 and 180 kJ/mol, respectively.     

Hydride formation during cathodic polarization of Ti—II. Effect of temperature and pH of solution on hydride growth

The effect of temperature on the kinetics of hydrogen absorption in α-Ti during galvanostatic cathodic polarization in 0·05M H₂SO₄ at c.d. 1·0 mA/cm² has been studied by direct estimation of the hydrogen content and by determination of the thickness of the surface hydride layer. For the temperature range 25°–100°C the rate of hydrogen absorption is parabolic and increases with temperature. The following equation has been determined for the diffusion of hydrogen occurring during the growth of surface hydride layers on α-Ti:

$D_{\alpha }=6\times {10}^{?2}\exp ?(?14,400\pm 800/RT)c{m}^2{s}^{?1}.$

For vacuum annealed Ti the rate of hydrogen absorption decreases as the pH value of the charging solution increases, and for solutions of pH 10·3 and 14·0 no hydrogen absorption was detected. Surface condition affects hydrogen absorption over the entire pH range 0·3–13·0, hydrogen absorption being greatest for abraded specimens and least for pickled and vacuum annealed specimens.     

Grain boundary finite length faceting

We study symmetrical and asymmetrical aluminium grain boundary faceting with molecular dynamics simulations employing two embedded atom method potentials. Facet formation, coarsening, and the reversible phase transition of $\sum 3\{110\}$ boundary into $\sum 3\{112\}$ twin, and vice versa, are demonstrated in the simulations and the results are consistent with earlier experimental studies and theoretical models. The $\sum 11\{002{\}}_1/\{667{\}}_2$ boundary shows faceting into $\{225{\}}_1/\{441{\}}_2$ and $\{667{\}}_1/\{001{\}}_2$ boundaries and coarsens with a slower rate when compared to $\sum 3\{112\}$ facets. However, facets formed by $\{111{\}}_1/\{112{\}}_2$ and $\{001{\}}_1/\{110{\}}_2$ boundaries from a $\{116{\}}_1/\{662{\}}_2$ boundary are stable against finite temperature annealing. In the above faceted boundary, elastic strain energy induced by atomic mismatch across the boundary creates barriers to facet coarsening. Grain boundary tension is too small to stabilize the finite length faceting in both $\sum 3\{112\}$ twin and asymmetrical $\{111{\}}_1/\{112{\}}_2$ and $\{001{\}}_1/\{110{\}}_2$ facets. The observed finite facet sizes are dictated by facet coarsening kinetics which can be strongly retarded by deep local energy minima associated with atomic matching across the boundary.     

Microstructure and textural evolution during cold rolling and annealing of commercially pure titanium sheet

The effects of cold rolling and annealing on the microstructure and textural evolution of a commercially pure titanium (CP-Ti) sheet were investigated. Electron backscatter diffractometry demonstrates that the deformation during rolling is accommodated by twinning and slip. Additionally, twinning is the dominant deformation mechanism when the cold rolling reduction is less than 40%. During rolling, $\left\{11\overline{2}2\right\}〈11\overline{2}\overline{3}〉$ contraction twinning (CT) and $\left\{10\overline{1}2\right\}〈10\overline{1}1〉$ extension twinning (ET) are activated. And, the intensity of the (0002) pole along the ND gradually increases with increasing deformation. During annealing, the fraction of low angle grain boundaries (LAGBs) and the intensity of the (0002) pole along the ND gradually decrease slightly with increasing annealing time, while twinning lamellae disappear rapidly. When the annealing time reaches 60 min, 20% cold-rolled sheet recrystallizes almost completely.     

Nucleation potency prediction of LaB6 with E2EM model and its influence on microstructure and tensile properties of Al-7Si-0.3Mg alloy

The edge-to-edge matching (E2EM) crystallographic model was used to predict the orientation relationships (ORs) between LaB₆ and Al. Three different possible ORs can be predicted between LaB₆ and Al, which are ${\left(100\right)}_{\text{Al}}6{\left(100\right)}_{{\text{LaB}}_{\text{6}}}$, ${\left[001\right]}_{\text{Al}}6{\left[001\right]}_{{\text{LaB}}_{\text{6}}}$; ${\left(110\right)}_{\text{Al}}6{\left(110\right)}_{{\text{LaB}}_{\text{6}}}$, ${\left[001\right]}_{\text{Al}}6{\left[001\right]}_{{\text{LaB}}_{\text{6}}}$; and ${\left(111\right)}_{\text{Al}}6{\left(111\right)}_{{\text{LaB}}_{\text{6}}}$, ${\left[01\overline{1}\right]}_{\text{Al}}6{\left[01\overline{1}\right]}_{{\text{LaB}}_{\text{6}}}$. The prediction results are perfectly confirmed through TEM analysis and prove the nucleation potency of LaB₆. The refining efficacy of Al-2La-1B refiner and its influence on the tensile properties were investigated in the as-cast Al-7Si-0.3Mg alloy. According to the results, LaB₆ has higher nucleation potency than TiB₂, leading to better grain refining efficacy of Al-2La-1B refiner in the as-cast Al-7Si-0.3Mg alloy. Regarding the mechanical performances, tensile properties of the as-cast Al-7Si-0.3Mg casting alloy are prominently improved after addition of Al-2La-1B refiner, due to the refined microstructures.     

Mechanism of the action of weak acids and their salts on the passivation of iron by oxygen

Oxygen containing solutions of salts of weak acids passivate iron only when the pH of the solution exceeds a critical value. The dependence of the critical pH on the nature (pK_a) and concentration of the salt (c_s) as well as dissolved oxygen (c₀₂) can often be represented by an equation of the following form: ${pH}_c=log{c}_B?log{c}_{0_2}+{pK}_a?const.$A quantitative explanation for this relationship is derived in terms of the pH shift nearest the iron surface in course of the diffusion controlled corrosion process.     

Leaching optimization and dissolution behavior of alkaline anions in bauxite residue

Bauxite residue is a highly alkaline waste containing soluble alkaline anions, which can cause environmental concerns. The optimal leaching conditions, distribution of alkaline anions, types of pivotal alkaline anions and their dissolution behaviors were investigated based on the combination of single factors-orthogonal experiments and leaching stage experiment. Using a two-stage leaching, 86% of the soluble alkaline anions $\left({\text{CO}}_3^{2?},{\text{HCO}}_3^{?},\text{Al}{\left(\text{OH}\right)}_4^{?},{\text{OH}}^{?}\right)$ were leached with a L/S ratio of 2 mL/g, at 30 °C, over 23 h. During the first stage of leaching, approximately 88% of alkaline anions were leached from the dissolution of free alkali (NaOH, carbonate, bicarbonate, NaAl(OH)₄) with the rest originating from the dissolution of alkaline minerals (calcite, cancrinite and hydrogarnet). Supernatant alkalinity was 69.78 mmol/L with ${\text{CO}}_3^{2?}$ accounting for 75%. Furthermore, carbonate leaching was controlled by solid film diffusion using the Stumm Model with an apparent activation energy of 10.24 kJ/mol.