{ }-{ } Double twinning in magnesium

Microstructural investigations have been carried out on a fully lamellar Ti₄₉Al₄₇Cr₂Nb₂     

Ductile fracture mechanism in fine-grained magnesium alloy

The ductile fracture mechanism in a fine-grained magnesium alloy has been investigated by transmission electron microscopy-focused ion beam techniques. In coarse-grained or conventional magnesium alloys, twins form at the very beginning of the deformation process, and crack propagation occurs through the twin boundaries. However, in the alloy used in this study, subgrain structures were found instead of twins at the crack tip. Nanoscale twins formed subsequently owing to large stress in the crack propagation route. The fine-grained alloys showed high fracture toughness resulting from resistance to the twins at the beginning of the deformation.     

Investigation of {111} stacking faults and nanotwins in epitaxial BaTiO3 thin films by high-resolution transmission electron microscopy

{111} stacking faults and nanotwins in epitaxial BaTiO₃ thin films on MgO substrates have been investigated by high-resolution transmission electron microscopy. In many cases, the stacking faults and nanotwins were found to be accompanied by partial dislocations. These partial dislocations can be classified as two different types, analogous to the situation in the fcc structure. One is of the Shockley type with the Burgers vector (a/3)<112>. The other is of the Frank type with the Burgers vector (a/3)<111>. The movements of both types of partial can lead to the {111} stacking faults and the {111} twins observed in these films.     

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.     

The relationship between dynamic strain aging and serrated flow behaviour in magnesium alloy

The relationship between dynamic strain ageing (DSA) and serrated flow has been investigated via alternately switching strain rates at various temperatures in a Mg–3Nd–1Zn alloy. The results reveal that serrated flow is enhanced with decreasing strain rate and increasing temperature and tends to vanish, while the DSA continually intensifies as revealed by a higher flow stress even after the serration flow disappears. A mechanism is proposed, which could explain some abnormal deformation behaviour, such as a negative strain rate sensitivity and thermal hardening.     

Low-energy boron implants into silicon: A molecular dynamics simulation study of implants into a stepped surface

Molecular dynamics simulations have been conducted to study implantation of boron with a low kinetic energy (E _k in the range 5 eV) into the silicon surface. The bombarded surface has a realistic stepped structure and the geometry of the step edge is taken from the theory of Chadi (1987, Phys. Rev. Lett., 59, 1691). It is found that reflection is the more common event. However subsurface implants and adatom formations are also observed. The effect of the step morphology is important and different trajectories are observed if the ions hit steps of different shapes.     

Fracture mechanism of a coarse-grained magnesium alloy during fracture toughness testing

The fracture mechanism during fracture toughness testing has been investigated on a coarse-grained magnesium alloy, with an average grain size of ～50 µm, and a low fracture toughness. The results show that {1012}-type deformation twins are formed at the crack tip and many dislocations pile up on these boundaries. The accumulated strains at these boundaries become the origin of fracture; i.e. cracks propagate along these boundaries between the deformation twins and the matrix.     

Revealing texture architecture in a surface gradient nanostructured Al-Cu-Mg alloy

ABSTRACT

A multiscale crystallographic texture architecture in a surface gradient nanostructured Al-Cu-Mg alloy after surface sliding friction treatment (SSFT) has been revealed by a combination of electron backscatter diffraction and precession electron diffraction (PED)-assisted transmission electron microscopy (TEM) orientation mapping. Accompanying a grain structure variation from lamellar coarse grains to equiaxed nanograins, the major texture components evolve from brass {110}<112> in the coarse-grain matrix, Goss {110}<001> at a depth of ～80?μm, E {111}<011> and F {111}<112> at a depth of ～20?μm, to a mixture of rotated cube {001}<110>, E and F in the topmost surface layer. The through-thickness textural development and evolution are attributed to the cyclic loading of concurrent shear and compression during the SSFT processing. The PED-assisted orientation mapping shows good capability in mapping severe plastic-deformation-induced nanostructures with large residual strains and high defect density.     

Influence of aging pathways on the evolution of heterogeneous solute-rich features in peak-aged Al-Mg-Si-Cu alloy with a high Mg/Si ratio

We report on a study of the influence of aging pathways on the evolution of solute-rich features in peak-aged Al-Mg-Si-Cu alloys. The concentration and partitioning ratios of Mg, Si and Cu and Mg/Si ratios in the heterogeneous solute-rich features all increase with increasing size, with Si exhibiting the highest partitioning ratio, but notably these ratios change dramatically depending on the aging pathway selected. Accordingly, the short-time age hardening response can be enhanced by promoting both homogeneous and heterogeneous precipitate nucleation, while simultaneous improvements in peak-aged strength and elongation can be attained by a vacancy-assisted aging pathway.     

Twinning behaviour of TWIP steel studied by Taylor factor analysis

The twinning behaviour of Twinning-induced plasticity (TWIP) steel has been studied by analysing the grain orientation and the Taylor factor, based on microstructural and electron backscatter diffraction device observations. It is demonstrated that the Taylor factor can give an important guideline for determining the deformation mode of TWIP steel. The higher the Taylor factor, the easier the formation of twins and thus a tendency for the deformation mode to be mechanical twinning, while a low Taylor factor corresponds to a slip deformation mode. When the loading temperature is relatively low, the high Taylor factor regions increase and thus deformation twinning becomes easier while slip becomes more difficult, leading to increased tensile stress and decreased elongation.     

Twinning in bcc metals under shock loading: a challenge to empirical potentials

Using density functional theory (DFT), we found that high pressures intrinsically favor twinning in niobium by reducing the thickness of a stable twin. Five empirical interatomic potentials for niobium were considered in molecular dynamics (MD) shock simulations. The results show that two potentials exhibit the experimentally observed twinning behavior. Comparing with DFT under high pressure, we found that these two potentials are capable of reproducing the generalized stacking fault (GSF) curve, but the others predict several artificial metastable states along the GSF curve resulting in an artificial structural transformation.     

The kinetics of infralow-frequency viscoelastic internal friction induced by irreversible structural relaxation of a metallic glass

This letter presents the results of internal-friction measurements of Co 70 Fe 5 Si 15 B10 metallic glass at temperatures 400K h T h 700K and frequencies 0.01 Hz h f h 0.05Hz. It is shown that Maxwellian viscoelastic damping is predominant in this case. The nature of this damping is determined by irreversible structural relaxation. The kinetic relaxation law is derived.     

Linear instability signals the initiation of motion of a twin plane under load

This letter presents a study of the atomic mechanism of the initiation of motion of a static twin plane under applied mechanical load in a model shape-memory material. By tracking the deformation under load and using linear stability analysis, we find that the eigenvalues of the Hessian matrix provide an indicator of the initiation of motion of the twin plane. The initiation of motion is signaled by a linear instability and a drop in the lowest eigenvalue to zero as well as a sharp drop in higher eigenvalues. Additionally, by comparing with direct molecular dynamics, we see that the eigenmode associated with the zero eigenvalue is found to accurately predict the initial mode of motion. We also find that the initial motion occurs through the formation of a stacking fault just ahead of the existing twin plane and the broadening of the stacking fault drives further transformation.     

Dynamic precipitation at elevated temperatures in a dual-phase AlCoCrFeNi high-entropy alloy: an in situ study

ABSTRACT

The effect of a possible phase transformation or precipitation of the face-centred cubic (FCC) phase on intermediate-temperature deformation of a dual-phase AlCoCrFeNi high-entropy alloy has been studied using in situ tensile testing at 550°C. Electron backscatter diffraction (EBSD) results showed localised precipitation of the FCC phase during the intermediate-temperature deformation. The overall fracture behaviour and crack propagation of the material was not altered much compared to the room-temperature behaviour, namely brittle trans-granular fracture. Deformation at higher temperatures (above 750°C) is suggested as a way to enhance the dynamic FCC phase precipitation, in order to improve the ductility or deformability of the alloy.     

Loss of coherency of the alpha/beta interface boundary in titanium alloys during deformation

The loss of coherency of interphase boundaries in two-phase titanium alloys during deformation was analyzed. The energy of the undeformed interphase boundary was first determined by means of the van der Merwe model for stepped interfaces. The subsequent loss of coherency was ascribed to the increase of interphase energy due to absorption of lattice dislocations and was quantified by a relation similar to the Read–Shockley equation for low-angle boundaries in single-phase alloys. It was found that interphase boundaries lose their coherency by a strain of approximately 0.5 at T = 800°C.     

Nucleation of He bubbles in amorphous FeBSi alloy irradiated with He ions

It is interesting to investigate the formation of He bubbles in amorphous alloys because point defects do not exist in amorphous materials. In the present study, the microstructural evolution of amorphous Fe₇₉B₁₆Si₅ alloy, either irradiated with 5?keV He⁺ ions or implanted with 150?eV He⁺ ions without causing displacement damage, and then annealed at a high temperature, was investigated using transmission electron microscopy (TEM). Vacancy-type defects were formed in the amorphous alloy after irradiation with 5?keV He⁺ ions, and He bubbles formed during annealing the irradiated samples at high temperature. On the other hand, for samples implanted with 150?eV He⁺ ions, although He atoms are also trapped in the free volume, no He bubbles were observed during annealing the samples even up to 873?K. In conclusion, the formation of He bubbles is related to the formation and migration of vacancy-type defects even in amorphous alloys.     

Mechanism of formation of aligned two-phase microstructure in a Fe-0.25wt%C alloy under high magnetic field gradients

The effect of a magnetic force on creating an aligned two-phase microstructure in a Fe-0.25wt%C alloy under magnetic field gradients has been investigated. Through the changes in the heating temperature, both dipolar interactions and magnetic forces work together during the austenite to ferrite transformation. The results showed that the aligned two-phase microstructure is not observed under the influence of the magnetic force alone. The ferrite grains are elongated due to dipole interactions at the early stages of their nucleation and growth and then the magnetic force turns the elongated ferrite grains, whose major axis is not parallel to the direction of magnetic force, to the direction of the field in the presence of magnetic field gradients.