Dislocation Structure Of Single Crystals Research Articles

Effects of annealing on hardness, yield strength and dislocation structure in single crystals of the equiatomic Cr-Mn-Fe-Co-Ni high entropy alloy

We report that in contrast to the anticipation, the macroscopic mechanical strength of single crystals of the face-centered-cubic single-phase equiatomic Cr-Mn-Fe-Co-Ni high entropy alloy is not influenced by thermal annealing during which short-range ordering is expected to develop. Experimental evidence shows that hardness at room temperature, critical resolved shear stress at room temperature and at 77 K and activation volume values of the alloys do not significantly vary with the annealing temperature in the range from 900 to 1200°C. The stacking fault energies in the specimens deformed at 77 K following annealing at 1000 and 1200°C respectively are essentially identical to each other. These results clearly indicate that the impact of the formation of chemical short-range order on the macroscopic yield strength of the equiatomic Cr-Mn-Fe-Co-Ni high entropy alloy is insignificant, even if it exists.

Effects of Annealing on Hardness, Yield Strength and Dislocation Structure in Single Crystals of the Equiatomic Cr-Mn-Fe-Co-Ni High Entropy Alloy

We report that in contrast to the anticipation, the macroscopic mechanical strength of single crystals of the face-centered-cubic (FCC) single-phase equiatomic Cr-Mn-Fe-Co-Ni high entropy alloy (HEA) is not influenced by thermal annealing during which short-range ordering (SRO) is expected to develop. Experimental evidence shows that hardness at room temperature, critical resolved shear stress at room temperature and at 77 K and activation volume values of the alloys do not significantly vary with the annealing temperature in the range from 900 to1200°C. The stacking fault energies (SFEs) in the specimens deformed at 77 K following annealing at 1000 and 1200°C respectively are essentially identical to each other. These results clearly indicate that the impact of the formation of SRO on the macroscopic yield strength of the equiatomic Cr-Mn-Fe-Co-Ni HEA is insignificant, even if it exists.

Study of creep and features of the dislocation structure in single crystals of Ni3Ge alloy

Data from investigating the dislocation structure of Ni3Ge alloy single crystals formed during creep are presented. The creep of the Ni3Ge single crystals with the [001] orientation of the deformation axis was studied. It was found that a fragmented substructure with varying degrees of disorientation occurs in the areas of macrolocalized deformation. A polycrystalline substructure consisting of fragments with a low dislocation density is formed in the local areas.

Anisotropy of transient high-temperature bending creep of single crystals

The orientation dependence of the transient high-temperature creep rate of NaCl and KCl single crystals under nonuniform stresses is experimentally studied. The anisotropy of high-temperature bending creep of single crystals is ascertained by analyzing the distribution of bending stresses, the number of possible dislocations providing deformation, the reduced shear stresses causing sliding of dislocations, and the dislocation structure of single crystals after creep. It is shown that hindered dislocation sliding in the easy sliding system activates a new slip system at a certain temperature.

Quantifying the distributions of dislocation spacings and cell sizes

A new method is proposed to quantify the local dislocation spacings on sections displaying the intersections of dislocation lines. The method was applied to dislocation structures in single crystals of CaF2 introduced by deformation at elevated temperature and made visible by etch pits. The method yields the frequency distributions and the spatial distributions of dislocation spacings. For cellular dislocation structures the method provides a quantitative and objective characterization in terms of frequency distribution of dislocation spacings in cell boundaries and cell interiors and of cell size.

Change of superdislocation splitting with strain in single crystals of the Ni3Fe ordered alloy

The method of transmission electron microscopy is used to study the dislocation structure in single crystals of the Ni 3 Fe ordered alloy for different degrees of strain. The [1812], [011], [111] and [001] single crystal orientations are examined. The superdislocation splitting (r) is measured for different degrees of strain, distributions of r are constructed, and their average values 〈r〉 are estimated. The 〈r〉-e curves are constructed. With the use of the experimental r values, the effective energy of antiphase boundaries ζ* is estimated.

Dislocations in yttrium orthovanadate

Dislocation structures in single crystals of yttrium orthovanadate have been identified by transmission electron microscopy. Electron diffraction was used to predict possible Burgers vectors for the dislocations. Results suggest vectors of the type 1 2 [1 1 1] or 1 4 [0 2 1] . Arguments for the likelihood of each possible vector have been presented.

Quantitative microdiffraction from deformed crystals with unpaired dislocations and dislocation walls

This article describes how unpaired dislocations alter white-beam Laue patterns for either isolated dislocations, dislocation walls, or combinations of dislocation walls and isolated dislocations. The intensity distribution of Laue diffraction is analyzed as a function of local misorientation. We show how to quantitatively determine the dislocation structure of single crystals and polycrystals with plastic deformation. The technique is applied to interpret the complicated plastic–elastic field in an iridium weld sample.

Yield Stress Anomaly for 1/2〈112] {1111} Slip in y-Titanium Aluminide

ABSTRACTTransmission electron microscopy (TEM) studies of dislocation structures in single crystals of TiAl containing 54.5 or 54.7 at% Al deformed at different temperatures revealed the occurrence of slip on ½〈112]{ {111} at room temperature and near the peak of the yield stress anomaly (YSA). Measurements of the corresponding yield stresses revealed the existence of a YSA for this type of slip. Weak-beam TEM showed the presence of locks at room temperature for 30° dislocations and at high temperatures for edge dislocations. Both types of locks involve dissociation on two intersecting { 1111} type planes, driven by reduction in elastic strain energy. The edge dislocation dissociation at high temperatures involves both climb and glide.

Effect of the degree of long-range atomic order on hardening characteristics and dislocation structure of single crystals of the alloy Ni3Fe with the [111] orientation of the strain axis

Transmission electron microscopy has been used to investigate the evolution of the dislocation structure of single crystals of the alloy Ni3Fe with the [111] orientation for different degrees of long-range atomic order. A relation has been established among the staged nature of the hardening curve, the characteristics of strain hardening, and the quantitative parameters of the dislocation structure.

Tem Observations of Dislocation Structures in Single Crystals of NiAl After High Temperature Creep

The evolution of dislocation substructures formed in single crystals of NiAl by tension creep testing at temperatures between 1123 K and 1473 K has been studied. Significant differences in the dislocation structure are evident in samples tested in different orientations. For samples tested in soft orientations, where glide of (001) dislocations is easy, some interacting (001) dislocations can be found. In hard oriented samples, the density of dislocation networks is high and low angle subgrain boundaries are observed. The subgrain size in hard oriented crystals is strongly stress dependent. In hard oriented crystals, (001) dislocations have no resolved shear stress for glide and must move by the slower process of climb; or by glide of (110) dislocations. The (001) dislocations are captured in networks, leading to a high total dislocation density in hard oriented samples compared to soft oriented crystals, whereas the dislocation density within the volume of subgrains is constant for all orientations.

Transmission electron microscopy of dislocations in YBa2Cu3O7−xdeformed plastically at high temperatures

Abstract Dislocation structures in single crystals of YBa2Cu3O7−x with a tetragonal structure, deformed by indentation at 650 and 850°C, have been studied in detail using transmission electron microscopy. The Burgers vectors of the dislocations were determined by standard contrast experiments. At 850°C, perfect dislocations with Burgers vectors (100) were dominant; occasionally 〈110〉 dislocations were observed when two types of 〈100〉 dislocation, that is [100] and [010] dislocations, reacted with each other. At 650°C 〈100〉 dislocations were dissociated into two partial dislocations. Contrast experiments revealed that a 〈100〉 perfect dislocation is dissociated into two 1/2(100) partial dislocations, leaving a stacking fault in between. The stacking-fault energy decreases with decreasing temperature. This is explained by a model in which the stress-induced ordering of oxygen vacancies takes place.

Development of dislocation structure in Kh17N14M3B austenitic steel single crystals during rolling

The evolution of dislocation structure in single crystals of Kh17N14M3B austenitic steel with single and multiple orientations on rolling was studied. A quantitative analysis of the substructures at the stage of developed plastic strain was performed. The development of disoriented substructures in different stressed states of the crystal is discussed. The relation between the structure and texture at the stage of disoriented substructures is analyzed.

Cleavage Fracture and Dislocation Structures of Ductile L12 Type Ni3Ge Single Crystals

ABSTRACTFracture and dislocation structures of single crystals of the L12-type compound Ni3Ge deformed in tension-compression at room temperature are investigated by optical and electron microscopy. After considerable plastic deformation, cracks nucleate along primary (111) planes of [245]-oriented single crystals and grow in zigzag manner by connecting one octahedral plane to another. Fracture occurs by brittle cleavage. Similar cleavage fracture also occurs when thin films (whose plane is (111)) prepared from cyclically deformed specimens are further deformed at room temperature. These observations are different from those reported in a previous study where the cleavage plane is claimed to be {001} for single crystals deformed in tensio at room temperature. A number of piled-up dislocations are emitted due to stress concentration at and ahead of crack tips, evidencing that this compound is intrinsically ductile. Superlattice intrinsic stacking faults are observed in front of crack tips along with a moire structure. Although there is argument that such moire structures are formed in persistent slip bands during cyclic deformation of Ni3Al, this is unlikely in the present case. The moire structure observed is believed to be formed by deformation of the thin (111) film.

Dislocation structures of monocrystalline copper during corrosion fatigue in 0.1 M perchloric acid

We have studied the dislocation structures of single crystals of copper cycled in 0.1 M perchloric acid and under different polarization potentials. TEM samples were cut from representative specimens both after saturation and after fracture. Although much higher strain localization was observed in the crystals cycled at anodic potentials, the dislocation structures observed were very similar to those of specimens tested at cathodic potentials and in air. For a plastic shear strain amplitude of 2×10−3, regular loop patches and dipolar walls were observed. For a higher strain amplitude of 4×10−3, dipolar walls associated with secondary slip were found in addition to regular primary walls. We believe this structure to be associated with breaking the primary persistent slip bands (PSB's) into truncated groups, of which the truncations were lined up along the traces of the secondary slip plane. Dislocation structures observed after final fracture of the crystals were different from those formed by cycling just into saturation; dipolar walls formed first and cell structures developed later. Thus, we confirmed the transition of dipolar walls into cells, reported by other investigators. Moreover, the transition of loop patches into “rungs” structure, the embryo of the PSB, was also observed in the late stages of life.

Dislocation configurations in single crystals of pentaerythritol tetranitrate and cyclotrimethylene trinitramine

A preliminary account of the X-ray topographic characterization of the growth-induced dislocation structure in single crystals of the secondary explosives pentaerythritol tetranitrate and cyclotrimethylene trinitramine is presented. Dislocations in the former material are observed to be straight whereas in the latter a substantial amount of dislocation motion is observed. These contrasting dislocation structures are discussed with reference to the mechanical properties of the materials. The possible contributions of dislocation slip and climb mechanisms to dislocation motion are considered.

Effect of growth rate on the dislocations structure of aluminium single crystals of various orientations

X-ray diffraction topography and the etch figure method were used to investigate the dislocation structure of single crystals of pure aluminium (99.999%) grown from the melt by the Bridgman method with 〈100〉, 〈111〉, 〈210〉 and 〈100〉 (near the pole) orientations of the solid-liquid interface with a growth rate range from 4 to 15 mm/min. It has been shown that the dependence of the dislocation density on the growth rate is determined by the crystallographic orientation of the solid-liquid interface. Thus, for crystals with 〈111〉 orientation the dependence of the dislocation density on the growth rate is weak, for crystals with 〈100〉 orientation it is very strong, while for crystals with 〈210〉 orientation this dependence is extremely large. The results obtained are explained on the basis of existing concepts about the anisotropy of mobility of the solid-liquid interface in the directions of preferred crystal growth.

Growth interface and dislocation structure of single crystals of iron and dilute iron-silicon alloys

Dislocation in as-grown crystals and in the vicinity of the growing interface were investigated in crystals of Fe, Fe-0.5 wt% Si and Fe-0.9 wt% Si prepared by the strain-anneal method. It was found that the dislocation density close to the interface was much lower than in the as-grown crystal, indicating that the main part of dislocations originates in the as-grown crystal, probably due to thermal and other stresses. Groups of dislocations with higher density observed at some places in crystals with low average dislocation density, remain at those places, where grains in the polycrystalline specimen have the same orientation as the growing crystal. The density of dislocations in dilute iron-silicon alloy single crystals was found to be lower than in pure iron crystals. This is supposed to be due to higher resolved shear stress necessary to produce dislocations in Fe-Si alloys compared to that for pure iron.

Polygonization and restoration of long-range order on heating a cold-worked iron ?6.5% silicon alloy

Rearrangements taking place in the dislocation structure of single crystals of cold-worked (rolled) Fe-6.5% Si alloy (initially ordered in accordance with the DO3 system) on subsequent annealing, accompanied by the restoration of the equilibrium degree of long-range order, are considered on the basis of an electron-microscope examination. The microscopic polygonization processes detected are analogous to those which occur in alloys with low stacking-fault energies. The processes underlying the perfection of the defect structure and the restoration of the long-range order disrupted by plastic deformation have a marked influence on one another.

The dislocation structure of single crystals of γ-brass

The dislocation structure of single crystals of γ-brass grown by directional solidification from the melt has been studied by X-ray topography and surface etching. The density of the dislocations observed was 1 to 5 × 104 lines/cm2. The Burgers vectors of most dislocations were not determined but some dislocations have b parallel to [001] and others have b parallel to [112]. The correlation between the dislocation structure seen in X-ray topographs and etched features observed in optical micrographs is discussed. Die Versetzungsstruktur von γ-Messing-Einkristallen, die durch gerichtete Erstarrung aus der Schmelze gezogen wurde, wurden mit Rontgenstrahlentopographie und Oberflachenatzung untersucht. Die Dichte der beobachteten Versetzungen betrug 1 bis 5 × 104 Linien/cm2. Die Burgersvektoren der meisten Versetzungen wurden nicht bestimmt, jedoch hatten einige Versetzungen b parallel zu [001] und andere b parallel zu [112]. Die Korrelation zwischen der bei Rontgenstrahlentopographie beobachteten Struktur und den Atzfiguren, die in optischen Mikrographien beobachtet wurden, wird diskutiert.