Regions Of High Dislocation Density Research Articles

Effects of deformation on the decomposition of austenite: Part I—The ferrite reaction

The influence of plastic deformation at 550°C in the austenite bay on the subsequent isothermal transformation in the range 600–750°C, of a Cr-Mo-Ni steel (En 24) and a 0.2C–5Cr steel, has been quantitatively assessed. Microscopic examination has revealed directly the increased rates of nucleation and growth of the ferrite and the occurrence of intragranular nucleation. The finer ferrite grain sizes resulting from the deformation are shown to give improved mechanical properties. Thin-foil electron microscopy has allowed a close study of the regions giving rise to ferrite nuclei, and also of the frequent irregular growth in regions of high dislocation density. Using an iron-0.2C-1V alloy, it has been shown that small deformations at temperatures as high as 875°C have a pronounced effect on the subsequent reaction at 700°C.

Structure and properties of heavily cold-worked fcc metals and alloys

Al-5%Mg, Ag, Au, Cu-l0%Ni, and Ni in the form of bars have been deformed to large strains by rolling at 20°C, and their microstructures determined by thinfoil transmission electron microscopy. Tensile tests have been carried out on the rolled specimens, and from the results obtained stress/true rolling strain curves have been obtained. It has been found that over a wide range of strains there are linear relationships between stress and true rolling strain; i.e. there is a work-hardening modulus. The workhardening modulus has been found to be related to the homologous temperature of deformation and the stacking fault energy. High moduli are found when plastic deformation gives rise to an increase in the dislocation density. Low moduli are found when shear bands consisting of small highly misorientated grains seem to form as a result of a structural instability within regions of high dislocation density.

The Effects of Dislocations and Crystal Defects on the Energy Resolution and Response Uniformity of Mercuric Iodide Detectors

The energy resolution and response uniformity of HgI2 detectors to nuclear radiation depends strongly on dislocation density and distribution in the HgI2 crystal bulk. Several large planar detectors fabricated from the interiors of large single crystals verified this dependence. Scanning with collimated low-energy (6 keV) and medium-energy (60 keV) gamma-rays confirmed the association of poor resolution and counting efficiency with regions of high dislocation density and crystal imperfection.

Cu-2.5 at%Al合金単結晶の高温定常変形

Copper-2.5 at%Al alloy single crystals oriented for a single slip system were deformed in tension at 600 and 700°C. In the later stage of deformation, the condition of steady state deformation was satisfied; namely the specimens have been deformed at a constant stress, independent of strain. In this steady state deformation stage, the internal stress and the effective stress were determined by the stress dip test. The dislocation substructure formed in this stage were observed by the etch pit technique. The results obtained are as follows:(1) The tensile axis rotated into the direction predicted by the single glide on the (111)[\bar101] slip system. When the tensile axis approached the [001]-[\bar111] symmetry line, the steady state deformation was observed.(2) The strain rate is proportional to the nth power of the steady state stress and the n value is about 5.(3) The ratio of the internal stress to the applied stress is 0.6∼0.9 and depends on the applied stress. The effective stress increases with the applied stress.(4) The parameter m* describing the effective stress sensitivity of dislocation glide velocity was estimated assuming the mobile dislocation density to be proportional to the square of the applied stress and the m* value was found to be 1.1.(5) The subgrains elongating in the [1\bar21] direction are found on the (111) plane. On the (1\bar11) plane the rows of dislocations are found along the direction roughly parallel to the trace of the primary slip plane, and the high dislocation density regions, approximately perpendicular to the slip trace, are observed.

Tempering of martensite: A field ion microscope study

As-quenched martensite in an Fe-0.88% C-0.45% Mn steel was tempered in the field ion microscope. During the first stage of the tempering process, precipitation of two types of ε-carbide is observed—in an ordered fashion along twin planes, and as dispersed random particles in untwinned regions of high dislocation density. Further tempering causes cementite precipitation to occur most rapidly in those regions which contain random ε-carbide dispersions. Continued tempering causes the cementite thus nucleated to grow, and causes the transformation of low-carbon martensite to cementite and ferrite to proceed to completion. This is seen by the steady decrease and ultimate absence of the twin boundaries in the image.

Polygonization of KI surfaces by photon irradiation

Abstract Surface features are reported on KI after irradiation with ultra-violet light. The mechanism proposed to explain these features involves the three different sputtering rates which occur at the pure crystal surface, a dirty surface and a region of high dislocation density. This model differs from an earlier one proposed by Ag[acaron]rbiceanu and Teodorescu following electron irradiation although both types of experiment produced identical features.

Dislocation sources and interactions in silicon

An investigation of dislocation sources in silicon deformed by thermal shock has been made by means of X-ray topography. Deformation occurs by means of surface and internal planar dislocation sources. The dislocations are in 60° or pure screw orientation except in regions of high dislocation density. The sources belong to the three {111} planes inclined to the surface (also a (111) plane). Nearly all Burgers vectors were in the plane of the slice, but a few inclined pure screw dislocations and Lomer dislocations were also observed. Both single and double-ended sources were present, the majority being single-ended. Their configurations (which were usually complex) and some associated dislocation interactions are discussed.

High-Density Saturation Effects of the Dislocations in n-Type Germanium

The dislocation effects on electrical resistivity and Hall coefficient at the liquid-nitrogen temperature were examined in n-type germanium specimens whose dislocation density, distribution, and nature were determined by means of optical and electron microscopy. In particular, the high dislocation density regions were considered, where the space-charge cylinders and the low-mobility rings around the dislocations overlap. Three stages were revealed, the first connected with the bulk conductivity, the second with the conductivity in the low-mobility regions, the third with the prevalence of the minority hole conductivity. The interpretation of the experimental data led to the following determinations: the specific volumes of the space-charge cylinders and the low-mobility rings around the edge dislocations; the ratios between low mobility and bulk mobility of the electrons; the degree of the contamination specific of the deformations; the saturation hole concentrations and mobilities. Further, the availability of differentiated specimens with prevalence of either screw or edge dislocations made it possible to resolve the effects of the two types of dislocations.

The Formation of Cube Texture in 50%Ni–50%Fe Alloy

The recrystallization process in a 50%Ni–50%Fe alloy sheet 96% cold rolled was studied by a transmission electron microscope.In the early stage of recrystallization, a cube grain grew mainly at the band-like structure consisting of (110) [1\bar12] and weak (100) [001] components of cold rolled texture. A high dislocation density region was found in a slightly grown cube grain when annealed at 490°∼520°C for 30 min. Such a region was found only one in each cube grain, and its size was compared to the subgrain size of the band-like structure.It was presumed that the cube grain grew from the region by subgrain boundary migration, without inverse Rowland transformation.

50%Ni-50%Fe合金における立方体集合組織の成因について

In the early stage of the recrystallization process in a cold rolled (96% reduction in thickness) Ni-Fe alloy sheet, it was observed by transmission electron microscopy that the cube-oriented grains were grown mainly in regions adjacent to the band-like structures in the rolling texture which consisted of (110)[1\={1}2] and (100)[001] components. On annealing at 490°∼500°C for 30 min, a high dislocation density region remained in slightly grown cube grains. Such a region was contained at only one site in each cube grain, and its size was comparable with that of the subgrains. It was presumed that the region was the origin of the cube grain, but it was neither the region where any dislocation reactions such as the inverse Rowland transformation had taken place, nor resulted from any impurity effects.It was concluded that the cube grains grew preferentially from the cube components which already existed in the rolling texture.

The effect of surface on the recrystallization of gold single crystals

Gold single crystals were deformed at room temperature to various amounts of deformation at different strain rates. The crystals were cut into several specimens, each of which was etched to remove an amount of the surface layer (0–416 μ,), and “recrystallized.” An effect of the surface on nucleation of recrystallized grains was found. The effect was most pronounced in crystals deformed to the end of stage II of the stress-strain curve. It is concluded that the surface itself has an effect which results in high dislocation density regions in its layers. Several mechanisms, by which the surface layers interfere with dislocations escaping from the surface, are discussed.

Precipitation of Copper in Germanium

The density of dislocations is shown to have a marked effect on the rate of anneal of copper in germanium. At 500\ifmmode^\circ\else\textdegree\fi{}C samples containing high dislocation density (\ensuremath{\sim}${10}^{6}$/${\mathrm{cm}}^{2}$) anneal in about 1 hour in contrast to material of low dislocation density (\ensuremath{\sim}${10}^{4}$/${\mathrm{cm}}^{2}$) which requires about 24 hours. When copper-doped germanium is cooled from a high temperature in regions of high dislocation density, significant precipitation occurs in a cooling cycle of only a few seconds. In this case, in order to prevent precipitation the sample must be quenched from the high temperature in a time of the order of 0.1 second.