Papers by Keyword: Dislocation Density

Abstract: The dislocation density of plastically deformed oxygen free copper (OFC) was evaluated by X-ray diffraction profile analysis with synchrotron radiation. The modified Williamson-Hall and modified Warren-Averbach methods were applied to the analysis. The dislocation densities of OFC samples with compressive plastic strains of 1 % and 4 % were 5.1×10¹⁴ m^-2 and 9.2×10¹⁴ m^-2, respectively.

Abstract: Microstructural change during hot compressive deformation at 700 oC followed by isothermal annealing for a Fe-32Ni austenitic alloy was monitored using in situ neutron diffraction. The evolution of deformation texture with 40% compression and its change to recrystallization texture during isothermal annealing were presented by inverse pole figures for the axial and radial directions. The change in dislocation density was tracked using the convolutional multiple whole profile fitting method. To obtain the fitting results with good accuracies, at least 60 s time-interval for slicing the event-mode recorded data was needed. The average dislocation density in 60 s after hot compression was determined to be 2.8 x 10¹⁴ m^-2, and it decreased with increasing of annealing time.

Abstract: To understand the strengthening mechanism of a metallic material with high dislocation density, the plastic deformation behavior of lath martensite was studied by means of in situ neutron diffraction measurements during tensile deformations using a 22SiMn2TiB steel and a Fe-18Ni alloy. The characteristics of dislocation were analyzed and were discussed with the relation of stress-strain curves. The dislocation densities (ρ) induced by martensitic transformation during heat-treatment in both materials were found to be originally as high as 10¹⁵ m^-2 order, and subsequently to increase slightly by the tensile deformation. The parameter M value which displays the dislocation arrangement dropped drastically at the beginning of plastic deformation in both materials, indicating that the random arrangement became more like a dipole arrangement.

Abstract: The distribution of extended defects in silicon carbide (SiC) crystals grown on profiled seeds by the sublimation (physical vapor transport) method has been studied by optical microscopy in combination with chemical etching. It is established that free lateral growth on protruding relief elements (mesas) is accompanied by a sharp decrease in the density of threading dislocations and micropipes. The decreased density of dislocations is retained after growing a thick layer that involves the overgrowth of grooves that separated individual mesas.

Abstract: Three-inch 6H-SiC bulk crystals were grown by the PVT method on the seeds processed by different treatments. The influences of seed surface morphology and subsurface damage on the dislocation density were investigated. The seed surface morphology was characterized by atomic force microscopy (AFM). The extent of the subsurface damage was estimated by electron back-scattered diffraction (EBSD) and Band Contrast (BC) value. The distribution and density of the dislocations were observed by optical microscopy (OM). The results showed that the pit density performed by H₂ 1400°C etching was nearly one order of magnitude lower than that by mechanical polishing (MP) process. So H₂ etching processed at 1400°C for 2h could completely remove the subsurface damage, compared with the MP process with the deep surface damage.

Abstract: Based on the modified couple stress theory the solution to a screw dislocation is obtained, in an isotropic elastic plane, via Fourier integral transform method. The asymptotic analysis of displacement field at the tip of a stationary crack reveals that stress field is not singular. A crack under anti-plane deformation is modeled by the distribution of screw dislocations. The ensuing integral equations are solved numerically to determine the density of dislocation on a crack surface. The dislocation solution is used to study the interaction between two parallel non-collinear micro-cracks.

Abstract: Grain refinement is attracting attention as a strengthening method which does not depend on the alloying elements added to steels. Many reports have described the manufacturing methods and mechanical properties of ultra-fine grained steels. In ultra-fine grained steels, it is well known that yielding stress drastically increases in accordance with the Hall-Petch relationship, while uniform elongation significantly decreases. These tendencies imply that grain size affects not only yielding but also work-hardening behavior. However, the influence of grain size on work-hardening behavior has not been clearly understood. Therefore, in this study, we investigated the work-hardening behavior during tensile deformation of 12Cr stainless steel with various grain sizes. Grain refining was conducted by cold-rolling of annealed and quenched steel specimens, followed by recrystallization annealing. The grain size of the specimens decreased as the cold-rolling reduction rate increased. The minimum grain size obtained by this method was approximately 5 μm. With decreasing grain size, 0.2% proof stress increased and the strain which reached the plastic instability condition decreased. In the session, we report the dislocation accumulation behavior estimated by grain hardness and XRD and the dynamic recovery behavior assessed by the Kocks-Mecking model.

Abstract: The deformation microstructures and their effects on mechanical properties of austenitic stainless steels processed by cold rolling at ambient temperature to various total strains were studied. The cold working was accompanied by the development of strain-induced martensitic transformation because of meta-stable austenite at room temperature. The strain-induced martensitic transformation and deformation twinning promoted the grain refinement during cold rolling, leading to nanocrystalline structures consisting of a mixture of austenite and martensite grains with their transverse grain sizes of 50-150 nm containing high dislocation densities. The rolled samples experienced substantial strengthening resulted from high density of strain induced grain/phase boundaries and dislocations. The yield strength of austenitic stainless steels could be increased to 2000 MPa after rolling to total strains of about 4. The martensite and austenite provided almost the same contribution to overall yield strength. The dislocation strengthening was much higher than the grain boundary strengthening at small to moderate strains of about 2, whereas the latter gradually increased approaching the level of dislocation strengthening with increasing the strain.

Abstract: In this paper, the influence of pre-strain on low-temperature gas carburization of 316L austenitic stainless steel was investigated. A group of flat specimens were uniaxial tensile to several levels of pre-strain including 5%, 10%, 15%, 20% and 25% engineering strain. Then, the pre-strained specimens was treated by low-temperature gas carburization at 470 °C for 30 h. In order to elucidate the effect of pre-strain on low-temperature gas carburization, optical microscopy (OM), X-ray diffractometer (XRD), scanning electron probe micro-analyzer (EPMA), microhardness tester and residual stress analyzer were used. Meanwhile, dislocation density of the pre-strained specimens was semi-quantitatively measured by means of X-ray diffraction analysis and the role of dislocation density on carbon diffusion during low-temperature gas carburization was discussed. The results show as follow: (1) the thicknesses of the carburized layers are independent of the pre-strain degree. (2) dislocation density increases with the increasing pre-strain, but almost has no effect on carbon diffusion at the given carburizing temperature. (3) an outstanding surface with hardness (≈ 1150 HV_0.1) and compressive residual stress (≈1900 MPa) is introduced by low-temperature gas carburization, and the strengthening results of carburization are unaffected by pre-strain.

Abstract: The study of the role of various factors in plastic behavior of materials is carried out using a mathematical model that takes into account fundamental properties of deformation defects in a crystal lattice based on the continuum theory of dislocations. Calculations were performed for copper, nickel, aluminum, and lead using a specialized software system Dislocation Dynamics of Crystallographic Slip. It has been shown that a decrease in the density of dislocations from 10¹² m^-2 to 10¹¹ m^-2 leads to an increase in the dislocation path in 10−16 times, and the maximum velocity in 1.5−2 times in copper and nickel, by nearly 20% in aluminum, and practically remains unchanged in lead. A decrease in the lattice and impurity friction from 2 MPa to 0.1 MPa leads to a linear increase in the path and the maximum velocity of the dislocation by 10−25%.