Anisotropy of Plastic Deformation in Hexagonal Metals

Geometric aspects of the shear processes in hexagonal metals are analysed. They can be divided into three groups: those localized essentially between neighbouring atomic planes, occurring in narrow slabs along particular atomic planes, or covering a large crystal volume. Obviously, dislocation glide and deformation twinning are principal types of such processes. On the geometrical level, the dislocation slip as well as twin propagation are controlled by Schmid factors. Since the sample loaded by external stress can sometimes give way to fracture (cleavage) under tensile stress, it has to be also mentioned. The main aim of this work is to show only on geometrical grounds for which sample orientation which process is more likely to occur. More complex shear processes that take place during double twinning are also briefly considered. In polycrystals, the shear phenomena lead to texture formation when the processes that control the behaviour of materials may be those that act in a similar way in single crystals.

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Dislocation dynamics modelling of the creep behaviour of particle-strengthened materials

Proceedings of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rspa.2021.0083 ◽

2021 ◽

Vol 477 (2250) ◽

pp. 20210083

Author(s):

F. X. liu ◽

A. C. F Cocks ◽

E. Tarleton

Keyword(s):

Elevated Temperatures ◽

Creep Behaviour ◽

Particle Interaction ◽

Three Dimensional ◽

Cross Slip ◽

Dislocation Dynamics ◽

Dislocation Slip ◽

Fundamental Particle ◽

Dislocation Glide ◽

Time Scale Separation

Plastic deformation in crystalline materials occurs through dislocation slip and strengthening is achieved with obstacles that hinder the motion of dislocations. At relatively low temperatures, dislocations bypass the particles by Orowan looping, particle shearing, cross-slip or a combination of these mechanisms. At elevated temperatures, atomic diffusivity becomes appreciable, so that dislocations can bypass the particles by climb processes. Climb plays a crucial role in the long-term durability or creep resistance of many structural materials, particularly under extreme conditions of load, temperature and radiation. Here we systematically examine dislocation-particle interaction mechanisms. The analysis is based on three-dimensional discrete dislocation dynamics simulations incorporating impenetrable particles, elastic interactions, dislocation self-climb, cross-slip and glide. The core diffusion dominated dislocation self-climb process is modelled based on a variational principle for the evolution of microstructures, and is coupled with dislocation glide and cross-slip by an adaptive time-stepping scheme to bridge the time scale separation. The stress field caused by particles is implemented based on the particle–matrix mismatch. This model is helpful for understanding the fundamental particle bypass mechanisms and clarifying the effects of dislocation glide, climb and cross-slip on creep deformation.

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Deformation-induced crystalline-to-amorphous phase transformation in a CrMnFeCoNi high-entropy alloy

Science Advances ◽

10.1126/sciadv.abe3105 ◽

2021 ◽

Vol 7 (14) ◽

pp. eabe3105

Author(s):

Hao Wang ◽

Dengke Chen ◽

Xianghai An ◽

Yin Zhang ◽

Shijie Sun ◽

...

Keyword(s):

Phase Transformation ◽

Amorphous Phase ◽

Structural Evolution ◽

Crack Tip ◽

Dislocation Slip ◽

High Entropy Alloy ◽

Face Centered Cubic ◽

High Entropy ◽

Dislocation Glide ◽

Ultrafine Grained

The Cantor high-entropy alloy (HEA) of CrMnFeCoNi is a solid solution with a face-centered cubic structure. While plastic deformation in this alloy is usually dominated by dislocation slip and deformation twinning, our in situ straining transmission electron microscopy (TEM) experiments reveal a crystalline-to-amorphous phase transformation in an ultrafine-grained Cantor alloy. We find that the crack-tip structural evolution involves a sequence of formation of the crystalline, lamellar, spotted, and amorphous patterns, which represent different proportions and organizations of the crystalline and amorphous phases. Such solid-state amorphization stems from both the high lattice friction and high grain boundary resistance to dislocation glide in ultrafine-grained microstructures. The resulting increase of crack-tip dislocation densities promotes the buildup of high stresses for triggering the crystalline-to-amorphous transformation. We also observe the formation of amorphous nanobridges in the crack wake. These amorphization processes dissipate strain energies, thereby providing effective toughening mechanisms for HEAs.

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High-Stress Compressive Creep Behavior of Ti-6Al-4V ELI Alloys with Different Microstructures

MATEC Web of Conferences ◽

10.1051/matecconf/202032111007 ◽

2020 ◽

Vol 321 ◽

pp. 11007

Author(s):

Zhenhua Dan ◽

Jiafei Lu ◽

Hui Chang ◽

Ping Qu ◽

Aifeng Zhang ◽

...

Keyword(s):

Ambient Temperature ◽

Creep Behavior ◽

Microstructural Analysis ◽

High Stress ◽

Dislocation Slip ◽

Duplex Microstructure ◽

Compression Tests ◽

Compressive Creep ◽

Burgers Vector ◽

Dislocation Glide

Influence of initial microstructure of Ti-6Al-4V ELI alloys on their compressive creep behavior at ambient temperature was investigated with applying compression stresses from 695 to 1092 MPa The experimental results show that the basketweave alloys have better compressive creep resistances than those duplex ones. The constitutive equations in steady-state compressive creeps of duplex or basketweave structure are calculated to be =2.77×10-15(σ-710)2.1 and =2.36×10-14(σ-740)1.7 by fitting the linear regression creep curves after uniaxial compression tests. The noticeable compressive creep strains occur when the applied compression stresses are higher than the threshold stresses, i.e. 710 MPa for duplex Ti-6Al-4V ELI alloys and 740 MPa for basketweave alloys. Microstructural analysis indicates that the creep deformation of Ti-6Al-4V ELI alloys at ambient temperature is mainly controlled by dislocation slip. The creep behavior of Ti-6Al-4V ELI alloy with duplex microstructure is controlled by dislocation slip, like slip dislocations with a-type Burgers vector sliding on the basal or prismatic planes and a few c+a type dislocation sliding on the pyramidal planes. While creep mechanism for basketweave ones is dislocation glide controlled by c+a type Burgers vector sliding on the pyramidal planes and a-type sliding on the basal or prismatic planes.

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Enhancing Protein Crystallization under a Magnetic Field

Crystals ◽

10.3390/cryst10090821 ◽

2020 ◽

Vol 10 (9) ◽

pp. 821

Author(s):

Sun Young Ryu ◽

In Hwan Oh ◽

Sang Jin Cho ◽

Shin Ae Kim ◽

Hyun Kyu Song

Keyword(s):

Magnetic Field ◽

Structural Information ◽

Protein Crystallization ◽

Ion Concentration ◽

Large Crystal ◽

High Quality ◽

Preferred Direction ◽

The Individual ◽

Crystal Cells ◽

Crystal Volume

High-quality crystals are essential to ensure high-resolution structural information. Protein crystals are controlled by many factors, such as pH, temperature, and the ion concentration of crystalline solutions. We previously reported the development of a device dedicated to protein crystallization. In the current study, we have further modified and improved our device. Exposure to external magnetic field leads to alignment of the crystal toward a preferred direction depending on the magnetization energy. Each material has different magnetic susceptibilities depending on the individual direction of their unit crystal cells. One of the strategies to acquire a large crystal entails controlling the nucleation rate. Furthermore, exposure of a crystal to a magnetic field may lead to new morphologies by affecting the crystal volume, shape, and quality.

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Perdeuteration, large crystal growth and neutron data collection ofLeishmania mexicanatriose-phosphate isomerase E65Q variant

Acta Crystallographica Section F Structural Biology Communications ◽

10.1107/s2053230x19001882 ◽

2019 ◽

Vol 75 (4) ◽

pp. 260-269 ◽

Cited By ~ 2

Author(s):

Vinardas Kelpšas ◽

Bénédicte Lafumat ◽

Matthew P. Blakeley ◽

Nicolas Coquelle ◽

Esko Oksanen ◽

...

Keyword(s):

Triose Phosphate Isomerase ◽

Dihydroxyacetone Phosphate ◽

Leishmania Mexicana ◽

Large Crystal ◽

Neutron Data ◽

X Ray ◽

Triose Phosphate ◽

Catalytic Mechanisms ◽

Crystal Volume ◽

Neutron Crystallography

Triose-phosphate isomerase (TIM) catalyses the interconversion of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Two catalytic mechanisms have been proposed based on two reaction-intermediate analogues, 2-phosphoglycolate (2PG) and phosphoglycolohydroxamate (PGH), that have been used as mimics of thecis-enediol(ate) intermediate in several studies of TIM. The protonation states that are critical for the mechanistic interpretation of these structures are generally not visible in the X-ray structures. To resolve these questions, it is necessary to determine the hydrogen positions using neutron crystallography. Neutron crystallography requires large crystals and benefits from replacing all hydrogens with deuterium.Leishmania mexicanatriose-phosphate isomerase was therefore perdeuterated and large crystals with 2PG and PGH were produced. Neutron diffraction data collected from two crystals with different volumes highlighted the importance of crystal volume, as smaller crystals required longer exposures and resulted in overall worse statistics.

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Calculation of the Stress Interaction between Dislocation Pile-Ups in Neighbouring Misoriented Grains

Materials Science Forum ◽

10.4028/www.scientific.net/msf.879.1731 ◽

2016 ◽

Vol 879 ◽

pp. 1731-1736 ◽

Cited By ~ 1

Author(s):

R. Schouwenaars ◽

A. Ortiz ◽

V. H. Jacobo

Keyword(s):

Plastic Deformation ◽

Grain Boundaries ◽

External Stress ◽

Dislocation Slip ◽

Shear Stresses ◽

Slip Systems ◽

Dislocation Loops ◽

Stress Concentrations ◽

Pile Up ◽

Schmid Factor

During the early stages of the plastic deformation of a polycrystal, dislocations can pile-up against grain boundaries. Experimental results on large-grained materials have provided excellent verification of this phenomenon. Such a pile-up may activate dislocation slip in the neighbouring grain. Whether this occurs depends on the misorientation between the grains and the resolved shear stresses in the affected grain. Several approximate criteria have been proposed to predict the occurrence of this mechanism. Here, the problem will be assessed directly by calculating the Peach-Köhler force produced by a single dislocation pile-up in one grain on all the possible slip systems in the neighbouring grain, in combination with the effect of the applied external stress as obtained through calculation of the Schmid factor. It will be seen that the problem is significantly more complex than what is generally assumed in basic explanations of the Hall-Petch effect: highly localised stress concentrations are generated for certain misorientations, which are capable of punching out small dislocation loops which may then propagate into the neighbouring grain.

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Mechanical Properties and Dislocation Structure of Single Crystal Cdte Deformed in Compression at 300°C

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s042482010009292x ◽

1976 ◽

Vol 34 ◽

pp. 592-593

Author(s):

Ernest L. Hall ◽

J. B. Vander Sande

Keyword(s):

Mechanical Properties ◽

Single Crystal ◽

Dislocation Structure ◽

Room Temperature ◽

Elevated Temperatures ◽

Strain Curve ◽

Optical Devices ◽

Semiconductor Compound ◽

Wide Range ◽

Sample Orientation

The present paper describes research on the mechanical properties and related dislocation structure of CdTe, a II-VI semiconductor compound with a wide range of uses in electrical and optical devices. At room temperature CdTe exhibits little plasticity and at the same time relatively low strength and hardness. The mechanical behavior of CdTe was examined at elevated temperatures with the goal of understanding plastic flow in this material and eventually improving the room temperature properties. Several samples of single crystal CdTe of identical size and crystallographic orientation were deformed in compression at 300°C to various levels of total strain. A resolved shear stress vs. compressive glide strain curve (Figure la) was derived from the results of the tests and the knowledge of the sample orientation.

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Structure of stacking faults in pyrite using TEM techniques

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100122198 ◽

1992 ◽

Vol 50 (1) ◽

pp. 358-359

Author(s):

Raja Subramanian ◽

Kenneth S. Vecchio

Keyword(s):

Lattice Structure ◽

Stacking Faults ◽

Covalent Bond ◽

Group Symmetry ◽

Iron Pyrite ◽

Dislocation Glide ◽

Partial Dislocations ◽

Transmission Electron ◽

Contrast Analysis ◽

Chlorine Ions

The structure of stacking faults and partial dislocations in iron pyrite (FeS2) have been studied using transmission electron microscopy. Pyrite has the NaCl structure in which the sodium ions are replaced by iron and chlorine ions by covalently-bonded pairs of sulfur ions. These sulfur pairs are oriented along the <111> direction. This covalent bond between sulfur atoms is the strongest bond in pyrite with Pa3 space group symmetry. These sulfur pairs are believed to move as a whole during dislocation glide. The lattice structure across these stacking faults is of interest as the presence of these stacking faults has been preliminarily linked to a higher sulfur reactivity in pyrite. Conventional TEM contrast analysis and high resolution lattice imaging of the faulted area in the TEM specimen has been carried out.

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