Papers by Keyword: Dislocation Density

Abstract: X-ray topography shows that selective KOH etching after CVD growth of n-type epilayers on highly N doped 4H SiC substrates can be used to reliably map pure and mixed Threading Screw Dislocations (TSD). The influence of the mapping grid density and the wafer position in the crystal on the average TSD density are investigated. A reliable mapping of TSD contributed to the development of 100mm SiC wafers with average TSD density down to 200 cm^-2.

Abstract: In order to investigate the effect of austenite grain size on martensite start temperature of Nb-V-Ti micro-alloyed ultra-high strength steel, the phase transformation features of Nb-V-Ti micro-alloyed steel was investigated. It has been found that martensite start temperature increased with the increase of austenite grain size as a consequence of the increase of austenitizing temperature. Based on microstructure observation, two types of MX carbonitrides with different compositions and morphologies have been identified. With the increase of the austenite grain size, both the volume fraction of precipitates and the dislocation density decreased, which may be induced by the strengthening of the austenite matrix directly and increasing the resistance of austenite to plastic deformation. Hence, the increase of martensite start temperature could be attributed to a decrease in volume fraction of precipitates and dislocation density.

Abstract: The paper gives a mathematical model of grain evolution and dislocation density during asymmetric cold rolling of aluminum alloy 7075 in an SPD mode. Correlations between the effect of equivalent and shear strain on Al 7075 structure are obtained. An agreement of simulation results with experimental data is shown.

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 and AFM. 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: In the micro electroforming process, the existence of electroforming layer defects caused by macro internal stress seriously limits the application and development of the micro electroforming technology. Currently, some studies have shown that ultrasonic can reduce the internal stress. But the formation process of the internal stress and the mechanism of ultrasonic stress relief in micro electroforming layer are still unclear now. In this paper, the relationship between dislocation density and internal stress under ultrasonic was studied. The results show that the ultrasonic can make the dislocation density increase and the compressive stress decrease. When the ultrasonic power is 200W, the dislocation density and the compressive stress culminate 3.8×10^-15m^-2 and-144.4MPa, respectively. The ultrasonic can excite the movement of dislocation proliferation, pile-up and opening, which leads to a micro plastic deformation in the crystal, and thereby releases the internal stress.

Abstract: The influence of dislocation density on micromechanical characteristics of Cu₆Sn₅ and Cu₃Sn was studied by nanoindentation. The dislocation density dependent hardness of Cu₆Sn₅ and Cu₃Sn was quantitative analyzed. High dislocation density increased the critical shear stress corresponding to the first pop-in events and induced micro plastic deformation before the first pop-in events. For Cu₆Sn₅, dislocation creep and lattice diffusion dominate the high stress regime (HSR) and the low stress regime (LSR), respectively, regardless of the dislocation density. High dislocation density increases the stress exponent (n) of HSR, but decrease n of LSR. Dislocation motion is the dominate creep mechanism of unstrained Cu₃Sn. After pre strain, the creep behavior of Cu₃Sn was divided into HSR and LSR. But dislocation motion is still the dominate creep mechanism. Potential mechanism of the dislocation density dependent micromechanical characteristics was explained based on our experimental results.

Abstract: As a result of the work of Frank-Read dislocation source the shear zone is formed. It is filled with deformation defects forming as a consequence of the dynamic features of the motion of dislocation loops and due to the interaction of shear forming dislocations with dislocations of non-coplanar slip systems. The accumulation of jogs on screw segments leads to the fact that the edge segments are moving faster than the screw segments so the shear zone is swept out generally by screw segments. The expressions of the intensities of the deformation defects accumulation in shear zones are given in the article. The point defects plays special role in the formation and evolution of misorientation substructures into deformed monocrystals, polycrystals and nanocrystals.

Abstract: Dislocation junctions, formed as a result of dislocation reactions, affect the plastic strain process, at least, for two reasons. First of all, junctions serve as barriers to shear-forming dislocations and restrict their path, therefore, the size of the shear zone. Sizes of the shear zone are determined by the density of reacting dislocations in non-coplanar slip systems, forming long enough barriers in the form of dislocation junctions. Secondly, non-breaking dislocation junctions are accumulated inside the shear zone, which leads to an increase in the intensity of the dislocation density accumulation.The present work is devoted to the study of the influence of dislocation junctions on accumulation of the density of dislocation debris (debris junctions) due to formation of stable junctions. For this purpose, the probability density function of lengths in non-breaking junctions is calculated. The model of dislocation interactions, built by the authors of the paper for FCC single crystals, is used.

Abstract: Effect of a size of closed structural formation on accumulation of dislocation density and its components at plastic deformation is studied. Main attention is given to a role of a division of boundaries of a different type. Structural formation sizes are determined and different parameters of structure defining micro-and mezolevel at development of plastic deformation mechanisms are distinguished. A role of statistically stored dislocations (SSD) and geometrically necessary dislocations (GND) for defect structure formation of a material is examined. It is determined that than a size of closed structural formation is less than that a component of GND is larger and component of SSD is less. The work is based on results of TEM reserches of a structure of deformed materials.

Abstract: In the paper we consider the size effect of the closed structural formations as the function of accumulation of scalar density dislocations and their components under plastic deformation. The focus is on the role of interfaces of different types. The sizes are determined and the various parameters of the structure defining the micro-and mesolevel are identified at the development of the mechanisms of plastic deformation. The role of statistically stored dislocations (SSD) and geometrically necessary dislocations (GND) in the formation of the defect structure of the material is examined. It has been established that the smaller the size of the closed structure, the greater the GND part and the smaller the SSD component. The work is based on the TEM research of the structure of deformed materials.