Twin Lamellar Structure Research Articles

Twinning-dominated microstructure evolution and deformation mechanism during cold rolling in pure rhenium

The extremely high work hardening rate and the high cracking propensity were the main challenges that limit the successful deformation processing of rhenium. Rolling tests to 82% thickness reduction were conducted at room temperature, and three-stage twining evolution behavior was revealed. At 10% rolling reduction, primary {112¯1} tension twins were activated and dislocation accumulation appeared near the twin boundary. As the rolling reduction increases to 45%, twin-twin interactions prevailed and abundant GNDs accumulated around {112¯1} twin boundaries and {112¯1} twin- {112¯1} twin interactions. More importantly, A new twinning sequence behavior in rhenium, secondary {112¯1} twins within primary {112¯1} twins, was investigated. After twinning saturation, shear bands were formed due to the promoted local plastic deformation by the twin lamellar structure and submicron-scale isolated twins, which brought pronounced strengthening of rhenium. The extremely high work hardening rate of rhenium was found to mainly originate from the twin lamellar structure and twin-twin interactions. High temperature annealing released the stored strain energy, achieving a synergistic effect of high tensile strength and ductility. This study provides a systematic investigation into the deformation structure during the large reduction cold rolling in rhenium and provides insights into unique twinning evolution and shear bands.

Effect of cerium alloying on microstructure, texture and mechanical properties of magnesium during cold-rolling process

Hot-extrusion and cold-rolling were conducted on Mg–Ce binary alloys to explore the effect of cerium (Ce) on microstructure, texture and mechanical properties of Mg alloys. The addition of Ce results in significant grain refinement both in as-cast and hot-extruded samples. The basal texture is also weakened after extrusion and an inclined basal texture is formed with Ce addition. The strength and elongation of the Mg–Ce alloys are improved simultaneously due to such grain refinement and texture weakening effect. After cold-rolling, plenty of twins are found in the pure Mg and AZ31 plates while grains in the Mg-0.3Ce plate deform more uniformly without lamellar twin structure. Furthermore, Mg-0.3Ce alloys own strong and continuous strain hardening because Ce atoms and Mg–Ce precipitated phases serve as obstacles for dislocation slip.

Texture and Microstructure Evolution During Single-Point Incremental Forming of Commercially Pure Titanium

In the present investigation, the evolution of texture and microstructure during incremental sheet forming was investigated. Hot-rolled sheets of commercially pure titanium were subjected to single-point incremental forming to obtain truncated pyramid geometries. It was observed from texture measurements that there was splitting of basal poles along the transverse direction. Moreover, a significant number of twins was observed in the deformed microstructure with the fraction of extension twins being highest compared to other variants. The crossing of deformation twins, formation of twin lamellar structure and formation of secondary extension twins within primary contraction twins were additionally observed. The state of deformation in the pyramid walls was analyzed via finite element method which in turn was used as input for carrying out crystallographic texture simulations via visco-plastic self-consistent simulations. It was observed that the state of deformation in the wall regions is plain strain compression plus through-thickness shear.

The influence of lamellar twins on deformation mechanism in nanocrystalline magnesium under uniaxial compression

Molecular dynamics simulation is employed to investigate the deformation mechanism of nanocrystalline (NC) Mg with lamellar $$ \{ 1 0 {\bar{\text{1}}\text{1}}\} $$ twins. Five models including a twin-free model and four nanotwinned models with increasing twin boundary (TB) spacing from 2.5 to 20 nm are simulated under uniaxial compressive strain rate of 1 × 108 s−1 at 300 K. Compared with notwinned NC Mg, the introduction of lamellar twin structure can decrease the strength that behaves as a softening mechanism due to the migration of initial TB and newly formed twins. The strength decreases with the reduction in TB spacing. In addition, the model with TB spacing of 2.5 nm is also simulated under uniaxial compression at 0.01 K and 500 K in order to study the effect of temperature. The result shows that the compressive stress increases as the temperature decrease due to the promotion of grain boundary motion and TB migration.

Optimization of strength, ductility and electrical conductivity of a Cu–Cr–Zr alloy by cold rolling and aging treatment

To balance the paradox of both high mechanical properties and electrical conductivity of copper alloys, a nominal hot-rolled Cu-0.4 wt.%Cr-0.3 wt.%Zr alloy strips were subjected to different thermo-mechanical process. The microstructures of the alloys are characterized by optical microscopy and transmission electron microscopy and the mechanical properties and the electrical conductivity are evaluated. The results show that a desired combination of strength, ductility and conductivity can be obtained by controlling the sequence of cold rolling and aging. It suggests that the treatment route with an order of aging followed by cold rolling (ACR) is better than that with the order of cold rolling with aging (CRA). The ultimate tensile strength 568 MPa and electrical conductivity 75.3%IACS are obtained after aging at 450 °C for 3 h followed by 80% cold rolling at room temperature. The high electrical conductivity and ductility is attributed to twin lamellar structure.

On the Strain-Hardening Behavior and Twin-Induced Grain Refinement of CP-Ti Under Ambient Temperature Compression

In this paper, a systematic investigation on the twin-induced grain fragmentation of commercially pure Titanium was carried out under quasi-static uniaxial compression test at ambient temperature. The compression tests were interrupted at various strain levels in order to examine the progressive evolution of the microstructure and texture. The detailed microstructural analysis was performed using the electron backscattered diffraction technique. The microstructural features indicated the presence of $$ \{ 10\bar{1}2\} $$ extension twins (ET), $$ \{ 11\bar{2}2\} $$ contraction twins (CT) and their interactions. The strain-hardening behavior of the material corroborates with the microstructural evidences. The strain-hardening rate and its derivative clearly distinguish the regions of slip- and twin-dominating zones with the increasing strain. The deformation texture substantiates that along with the twins, dislocation slips were active. The slip-based deformation finally deviates the twin boundaries from its special character at medium-to-high strains. At low strains, ET originate in texturally soft grains. These twin domains are comparatively texturally harder. They interact to form ET–ET with ~ 56.8 deg about $$ \langle 10\bar{1}0 \rangle $$ , with twin lamellar structure and consumes the entire parent grain with further deformation by twin expansion. CT develop on texturally hard grains with subsequent strains and were texturally softer. CT–CT, ET–CT-type twin interactions, and CT–ET double twins with ~ 77, ~ 87, and ~ 44.5 deg about $$\langle 10\bar{1}0 \rangle $$ , $$\langle 4\bar{2}\bar{2}1 \rangle $$ and $$\langle 5\bar{1}\bar{4}0 \rangle $$ axes, respectively, were observed. The microstructural changes corresponding to twin-boundary migration, interaction, and grain refinement are discussed on the basis of grain orientation spread and grain reference orientation deviation.

Molecular dynamics studies on the interface evolution characteristics and deformation mechanisms of Cu/Al multilayers during compression process

The interface evolution characteristics and deformation mechanisms of Cu/Al multilayers are investigated via systematic molecular dynamics simulations. It is found that both the yield strength and ductility increase slightly with increasing strain rate, and the stress-strain curves exhibit two main yield points for all strain rate loadings. The first yield point correlates with the decomposition of perfect misfit dislocations on the interface and the propagation of partial dislocations inside the Al layer, and the second yield point relates with the dislocation transmission from the Al layer into the Cu layer. The lower the loading strain rate, the more severe the fluctuations on the stress-strain curve. However, the strain rates do not change the evolution way of dislocation networks. The calculated evolution curves of dislocation numbers indicate that the dislocation density inside the Cu layer is lower than that inside the Al layer. The interface region displays a serrated structure without voids or cracks, and the higher the loading strain rate, the more serious the interface roughening deformation. The main deformation mechanisms, respectively, are the formation of a lamellar twin structure in the Cu layer and dislocation slip in the Al layer, and the interface roughening is mainly dominated by the formation of a lamellar twin structure. Furthermore, the deformation mechanisms do not depend on the strain rate applied in this paper. In addition, we also discuss the growth curve of interface thickness which is divided into three stages.

Effects of micro-twin lamellar structure on the mechanical properties and fracture morphology of AZ31 Mg alloy

The effects of a micro-twin lamellar structure on the mechanical properties and fracture morphology of AZ31 Mg alloy have been investigated through impact test, tensile test, and SEM observation of the failure surface. A hot-rolled AZ31 Mg alloy sheet was subjected to dynamic plastic deformation with the aim of introducing a {101¯2} twin lamellar structure prior to tee tensile test, because dynamic plastic deformation with a high strain rate (4–6 × 102 s–1) can activate a greater number of deformation twins in AZ31. Compared with the mechanical properties of hot-rolled and annealed Mg alloy before dynamic plastic deformation, it was found that the tensile yield stress and maximum flow stress of the AZ31 Mg alloy increase after dynamic plastic deformation, and ductility improves under tension in the range 0–30°. Based on the analyses of microstructure and fracture morphology, the phenomenon can be ascribed to the fact that the micro-twin lamellar structure can refine the matrix grain and render the microstructure more homogeneous.

Effect of natural homointerfaces on the magnetic properties of pseudomorphic La0.7Sr0.3MnO3 thin film: Phase separation vs split domain structure

We studied the effect of naturally formed homointerfaces on the magnetic and electric transport behavior of a heavily twinned, 40nm thick, pseudomorphic epitaxial film of La0.7Sr0.3MnO3 deposited by molecular beam epitaxy on ferroelastic LaAlO3(001) substrate. As proved by high resolution X-ray diffraction analysis, the lamellar twin structure of the substrate is imprinted in La0.7Sr0.3MnO3. In spite of the pronounced thermomagnetic irreversibility in the DC low field magnetization, spin-glass-like character, possibly related to the structural complexity, was ruled out, on the base of AC susceptibility results. The magnetic characterization indicates anisotropic ferromagnetism, with a saturation magnetization Ms = 3.2μB/Mn, slightly reduced with respect to the fully polarized value of 3.7μB/Mn. The low field DC magnetization vs temperature is non bulklike, with a two step increase in the field cooled MFC(T) branch and a two peak structure in the zero field cooled MZFC(T) one. Correspondingly, two peaks are present in the resistivity vs temperature ρ(T) curve. With reference to the behavior of epitaxial manganites deposited on bicrystal substrates, results are discussed in terms of a two phase model, in which each couple of adjacent ferromagnetic twin cores, with bulklike TC = 370K, is separated by a twin boundary with lower Curie point TC = 150K, acting as barrier for spin polarized transport. The two phase scenario is compared with the alternative one based on a single ferromagnetic phase with the peculiar ferromagnetic domains structure inherent to twinned manganites films, reported to be split into interconnected and spatially separated regions with in-plane and out-of-plane magnetization, coinciding with twin cores and twin boundaries respectively.

Plasticity Induced by Twin Lamellar Structure in Magnesium Alloy

Effect of {10–12} twins on the mechanical properties of magnesium alloy has received considerable research interest. A hot-rolled AZ31 Mg alloy sheet was subjected to dynamic plastic deformation with the aim of introducing {10–12} twin lamellar structure. It has been found that higher strength and better ductility are obtained when tensile loading is perpendicular to the c axis of twin region of the twin lamellar structured sample, indicating that the plasticity improvement caused by twins depends on the special strain path. The fracture morphology of the twin lamellar structured sample shows a dimple fracture mode under tensile loading perpendicular to the c axis, while the cleavage fracture with river pattern has been observed in other fractured samples. Above experimental results indicate that the interaction of dislocations and twin lamellae may play an important role in improving mechanical properties of Mg alloy.

Characteristics of Twin Lamellar Structure in Magnesium Alloy during Room Temperature Dynamic Plastic Deformation

The microstructures of the as-rolled magnesium alloy subjected to dynamic plastic deformation along the rolling direction have been investigated. Mostly one { 10 1 ¯ 2 } twin variant or a twin variant pair is activated in a grain, leading to a parallel { 10 1 ¯ 2 } twin lamellar structure. At the stage of twinning-dominated deformation (e { 10 1 ¯ 2 } twin activity. When plastic strain is greater than ∼8%, the twin lamellar structure disappears because the volume fraction of twins almost saturates at a value of ∼90%.

Orientation and Optical Polarized Spectra (380–900 nm) of Methylene Blue Crystals on a Glass Surface

The crystallographic directions of the crystal toward the vector of polarized light can accurately be positioned, so the information that we gain from polarized spectra can be consistently interpreted according to known crystal structure. The orientation and optical properties of the methylene blue (MB) crystals were analyzed by XRD, XRPD, and polarized VIS-NIR spectroscopy. Cationic dye, MB, was polymerized into crystals on a glass slate. The blue color crystals showed pronounced dichroism, twin lamellar structure and bladed to fibrous habit. According to XRD data, [010] direction lies perpendicular to the crystal surface, so we recognized it as (0k0) face, while [100] and [001] directions coincide with crystal elongation and crystal thickness respectively. In this paper, the polarized spectra of MB crystal are presented, measured with the aim of acquisition of referent values, which could be helpful for the identification of MB molecular aggregation.

Mechanical behavior and microstructural characteristics of magnesium alloy containing {10-12} twin lamellar structure

Abstract

Phase field modeling of twinning in indentation of transparent crystals

Continuum phase field theory is applied to study elastic twinning in calcite and sapphire single crystals subjected to indentation loading by wedge-shaped indenters. An order parameter is associated with the magnitude of stress-free twinning shear. Geometrically linear and nonlinear theories are implemented and compared, the latter incorporating neo-Hookean elasticity. Equilibrium configurations of deformed and twinned crystals are attained numerically via direct energy minimization. Results are in qualitative agreement with experimental observations: a long thin twin forms asymmetrically under one side of the indenter, the tip of the twin is sharp and the length of the twin increases with increasing load. Qualitatively similar results are obtained using isotropic and anisotropic elastic constants, though the difference between isotropic and anisotropic results is greater in sapphire than in calcite. Similar results are also obtained for nanometer-scale specimens and millimeter-scale specimens. Indentation forces are greater in the nonlinear model than the linear model because of the increasing tangent bulk modulus with increasing pressure in the former. Normalized relationships between twin length and indentation force are similar for linear and nonlinear theories at both nanometer and millimeter scales. Twin morphologies are similar for linear and nonlinear theories for indentation with a 90° wedge. However, in the nonlinear model, indentation with a 120° wedge produces a lamellar twin structure between the indenter and the long sharp primary twin. This complex microstructure is not predicted by the linear theory.

Co-Ni-Ga Alloys with Room Temperature Ferromagnetic Martensite Phase

Co-Ni-Ga and Co-Ni-Al alloys are expected to be good ferromagnetic shape memory alloys (FSMAs) owing to their higher ductility resulting from the presence of γ-phase precipitates and higher stability in preparation since they do not contain the highly volatile element Mn. Co-Ni- Ga alloys have a wide range of martensitic transformation and Curie temperatures. In order to explore the possibility of obtaining Co-Ni-Ga alloys with room temperature ferromagnetic martensitic phase, two series of compositions, viz., Co70-xNixGa30 (20 ≤ x ≤ 26) and CoxNi25Ga75-x (43 ≤ x ≤ 50) were taken up for investigation. Polycrystalline ingots of these alloys were prepared by arc melting followed by homogenization and quenching at ice water. Analysis of room temperature X-ray diffraction patterns revealed that most of these alloys had a single-phase (tetragonal) structure typical of a martensitic phase, while some of the alloys exhibited a two-phase (cubic and tetragonal) structure due to the presence of both martensite and austenite phases. All alloys having single martensite phase at room temperature showed martensitic transformation at elevated temperature as well as a linear change of the characteristic martensitic transformation temperatures (As, Af, Ms and Mf) with the number of valence electron to atom ratio (e/a). As, Af, Ms and Mf showed distinctive variations when aged in the martensite phase and austenite phase. All the alloys were ferromagnetic at room temperature and the Curie temperature was determined by high temperature ac magnetic susceptibility and magnetization measurements. The typical twin lamellar structure of martensite phase was observed by optical microscope and the development of the cubic γ-phase along with the parent β′-phase was investigated for different ageing temperatures and annealing temperatures. These studies provide useful information about the potential of these alloys for actuator applications.

Structural Change during Cold Rolling of Electrodeposited Copper

Copper sheet samples composed of nanometer scale lamellar twins was produced by electrodeposition. The coherent lamellar twin boundaries were within 20˚ of being parallel to the sheet plane in more than 60% of the grains. The electrodeposited sample was cold rolled to 30 and 85% reductions in thickness and the structural evolution during cold rolling was examined by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Extensive activity of partial dislocations along twin boundaries and of perfect dislocations within twins (in particular in coarse twins >100nm) were identified. Moreover, it was found that shear banding occurred, which locally destroyed the lamellar twin structure. A dislocation structure developed within the shear bands, and such a structure evolved with strain and gradually replaced the lamellar twin structure. After 85% deformation, a large volume fraction of the lamellar twin structure was replaced by a lamellar dislocation structure characteristic of high strain rolling where the lamellar dislocation boundaries are almost parallel to the rolling plane. It was also found that the structural scales are coarser in the lamellar dislocation structure than in the initial lamellar twin structure.

Structural evolution of Fe33Zr67 and Fe90Zr10 metallic glasses

We report on the structural evolution of melt-spun Fe33Zr67 and Fe90Zr10 glasses. These glasses are subjected to isothermal annealing over a wide temperature range, varying from the onset of crystallization up to near the melting point, for 20–300min. Over 733–1223K, the phase evolution sequence of the Fe33Zr67 glass follows: fcc FeZr2→fcc FeZr2+bct FeZr2→bct FeZr2. In contrast, annealing of the Fe90Zr10 glass over 903–1173K leads to a mixture of α-Fe, Fe3Zr and Fe2Zr phases. Some Fe2Zr crystals are not developed perfectly, showing a special twin lamellar structure. The annealing temperature and alloy composition dependence of grain size in the present Fe–Zr system is discussed.

A high-resolution-electron-microscopy study of ?/??-? directionally solidified eutectic alloy

A master alloy with eutectic compositions of Ni-30.26Mo-6.08Al-1.43 V (wt%) has been directionally solidified (DS) into γ/γ'-α alloy. The microstructural as-ageing treatment was studied by means of high resolution electron microscopy (HREM). A majority of α fibres still display the Bain orientation relationship with the γ'/γ matrix. In a few cases, however, the so-called Nishiyama-Wasserman (NW) orientation relationship is found in specimens aged at 850 o C for 2000 h. Different microdomain structures of the γ phase, corresponding to different ageing temperatures, were revealed. Orthorhombic Ni 3 Mo phase, with a size of tens of nanometres, was found to precipitate inside α fibres after ageing at both 850 and 650 o C. Occasionally, an γ'-Ni 3 Al phase with lamellar twin structure was also found to coexist with Ni 3 Mo precipitate inside the α fibres. The orientation relationships between the precipitates and the α fibres were determined. Energy dispersive X-ray analysis (EDX) showed that the precipitation of Ni 3 Mo and Ni 3 Al is due to solid solution of Ni and Al in the α fibres

Internal stresses and elastic energy in ferroelectric and ferroelastic ceramics: Calculations by the dislocation method

A method is proposed for calculating the internal stresses in ferroelectric and ferroelastic ceramics which is based on the modelling of the mechanical sources of stress by fictitious dislocations continuously distributed on the grain and twin boundaries. For tetragonal and orthorhombic ceramics with a lamellar twin structure, the dislocation models of representative grain boundaries are worked out. On this basis the elastic energy is calculated which is associated with highly inhomogeneous stress fields concentrated near the boundaries of twinned grains. The part of the energy of long-range internal stress fields, which depends on the twin structure, is also estimated with the aid of the dislocation method. The results obtained are used to calculate the equilibrium parameters of the lamellar twin structures in ferroelectric BaTiO3 ceramic and ferroelastic high-Tc superconducting YBa2Cu3O7-δ ceramic.

Influence of annealing conditions on structural properties of YBaCu oxide superconductors

AbstractThe chemical composition, the topography, and the defect structure of high‐temperature superconducting YBaCu oxide samples have been studied in dependence on the annealing conditions by scanning electron microscopy, energy dispersive X‐ray analysis, Rutherford backscattering, nuclear reaction analysis, and transmission electron microscopy. The topography is characterized by smooth and rough grains. The chemical composition of samples varies in dependence on the annealing time and temperature. The microstructure of the superconducting material is dominated by a lamellar twin structure.