Papers by Keyword: Slip System

Abstract: Aggregate of bainite laths with almost parallel slip systems between neighboring bainite laths, hereafter referred to as ALPS, has great effect on the improvement of mechanical properties in steels. Elongation increases remarkably with increasing the number of bainite laths within an ALPS. When a bainite lath begins to deform, the neighboring bainite lath also easily deforms to relax the deformation strain, because of good parallelism between their slip systems. Under the cooperative deformation of bainite laths, the area of interface between neighboring bainite laths increases during deformation. The increase in the area of interface between neighboring bainite laths suppresses localization of dislocations at the interface, that is, dislocation density per unit area of the interface between neighboring bainite laths hardly increases, resulting in the suppression of nucleation of cracks at the interface between neighboring bainite laths. Ductile fracture would occur along the boundary between ALPSs. It could be suggested that larger ALPS and/or ALPS consisting of large number of bainite laths induce larger elongation in steels.On the other hand, it has been reported that tensile strength increases in proportion to inverse of square root of d, the d being the average size of bainite laths [1]. In order to form fine bainite lath, dislocation network instead of inclusion in austenite was utilized as nucleation site for bainite lath. Great barriers to be overcome exist for the improvement of both strength and toughness. An idea for the improvement of both strength and toughness is shown in this study.

Abstract: In this study an advanced method for investigation of the microstructure such as electron backscattering diffraction (EBSD) together with in-situ tensile test in a scanning electron microscope (SEM) has been used at room temperature and 300°C. EBSD analyses provide information about crystallographic orientation in the microstructure and dislocation structures caused by deformation. The in-situ tensile tests enabled the same area at different strain levels to be investigated. For the same macroscopic strain values lower mean misorientation angles of individual grains at the elevated temperature indicated that less residual strains at grain levels are gained compared with room temperature. The influence of grain size on plastic deformation on microscopic level, where larger grains tend to accumulate more strains, is discussed.

Abstract: In this paper, we report on the synchrotron white beam topographic (SWBXT) observation of “hopping” Frank-Read sources in 4H-SiC. A detailed mechanism for this process is presented which involves threading edge dislocations experiencing a double deflection process involving overgrowth by a macrostep (MP) followed by impingement of that macrostep against a step moving in the opposite direction. These processes enable the single-ended Frank-Read sources created by the pinning of the deflected basal plane dislocation segments at the less mobile threading edge dislocation segments to “hop” from one slip plane to other parallel slip planes. We also report on the nucleation of 1/3< >{ } prismatic dislocation half-loops at the hollow cores of micropipes and their glide under thermal shear stress.

Abstract: Single crystals of the alloy Ti-48%Ni-2%Fe, consisting of the phase B2, were rolled at 350°C with deformation degrees up to ~80% in 11 different initial orientations. Texture development in single crystals under rolling was analyzed to determine the active mechanisms of their plastic deformation. There are three stable initial orientations, which remain unchanged during rolling of Ti-Ni single crystals: {011}<011>, {111}<011> and {111}<112>. It was shown, that Ti-Ni single crystals deform by means of combined action of slip in systems {011}<001> and twinning in systems {114}<221> and {118}<441>. Variants of rolling texture formation in single crystals are very diverse and depend on their initial orientations and deformation degrees.

Abstract: Formation process of textures in AZ80 is investigated on polycrystal specimens with different initial textures by plane strain compression deformation at 673 K and 723 K with strain rates between 5.0×10^-4s^-1 and 5.0×10^-2s^-1. Three kinds of specimens were taken out from the extruded bars with (direction of the extrusion) fiber texture by changing the geometrical relationship with the extrusion direction. In the case of the specimen having parallel to the compression direction before deformation, the initial fiber texture gradually transformed into several orientations with increasing strain at 723K with a strain rate of 5.0×10^-2s^-1. oriented grains, which was not seen in the pole figures before deformation appeared after the deformation up to -1.0 in true strain. The other two kinds of specimens have compression directions perpendicular to direction. The textures after deformation of these two kinds of specimens also consisted of several components. Some of them are common among the three kinds of specimens and the others are retained components of the initial texture.

Abstract: Single crystal nickel base superalloys, such as Chinese material DD6 have been used in gas turbine blade in China more and more widely. In order to make better use of single crystal superalloys with many excellencies, constitutive models have been developed. In this paper, general method of crystallographic constitutive modeling was summarizes and a new constitutive model, based on crystallographic theory was proposed with phenomenological models' advantages. Based on crystallographic slip system principle, the basic slip-based viscoplasticity theory equations were set up on 12 octahedral slip systems and 6 cubic slip systems, total 18 slip systems. In micro-level slip system, the general unified constitutive formulations were used as the flow equations and hardening law. In the model, scalar forms were applied for constitutive equations on slip systems and the number and types of active slip systems were used to describe the material anisotropy, which was satisfied automatically by slip systems not anisotropic tensors and. The experimental and calculation results of two kind single crystal superalloys PWA1480 and DD6 were compared. The model had the capability to predict many mechanical response and analyze structure of single crystal superalloys. The modeling procedures and results showed that this crystallographic model had more clear physical meaning and was exact.

Abstract: Founded on the energy storing characteristics of microstructure during irreversible deformation, a viscoplastic constitutive model with no yielding surface introduced was developed for single crystals by adopting a spring-dashpot mechanical system. Both plastic dashpots reflecting the material time-independent responses and Newtonian dashpots mirroring the material time-dependent viscous responses were introduced to describe the viscoplasticity of slip systems. The single crystal constitutive model was established based on the thermodynamics of internal variables and the theory of absolute reaction rate. By implementing the KBW self-consistent theory, a polycrystal viscoplastic constitutive model was formed. The numerical analysis in corresponding algorithm was significantly simplified as no searching process for the activation of the slip systems and slip directions was required. The numerical simulation of creep-plasticity behaviors demonstrated excellent agreement with the corresponding experimental data.

Abstract: The evolution of texture and deformation in the grains during one pass of equal-channel angular pressing (ECAP) was examined for fine grained high strength and low strength Al alloys and a coarse grained low strength Al alloy. The materials were analysed using electron back-scatter diffraction (EBSD). The results are consistent with the materials responding to the intense macroscopic shear stress by deformation of individual grains through movement of dislocations on one or more of the slip crystallographic slip planes {hkl} that are favourably oriented, combined with the rotation of grains to directions that bring main crystallographic slip planes parallel to the macroscopic shear direction and crystallographic slip directions parallel to two main shear directions. Contrary to reports claiming up to 4 slip systems are activated, it was observed that only the {111}<110> and {001}<110> shear systems are activated. Macroscopic shear deformation occurs on two shear planes: the main shear plane (MSP), equivalent to the simple shear plane, and a secondary shear plane which is perpendicular to the MSP.

Abstract: In this paper, we present a fundamental model of FCC single crystal behaviour at room temperature: this model includes kinematic work hardening derived from the elementary description of the collective dislocations density evolution during cyclic loading. This kinematic work hardening is then coupled with the isotropic work hardening mechanism. Using this original model, a simulation of a tensile test on a single crystal sample is carried out in the case of an initial crystal orientation that promotes single glide even at rather large strains. The evolution of resolved shear stresses on the primary and secondary slip systems are interpreted by means of the interaction between the evolution of isotropic and kinematic work hardening variables. The evolution of the model state-variables including applied resolved shear strain, dislocation densities, and critical shear stresses are represented as functions of the evolution of crystalline orientation during plastic deformation.

Abstract: The yield strength was investigated in a prestrained and baked bake-hardenable sheet steel, which can increase the dent-resistance of exterior automotive body panels. Instead of a standard bake-hardenability test in which the final tensile test is in the same direction as prestrain, the strain-path effect in denting of stamped and paint-baked panels was examined by changing the direction of final tensile tests in uniaxially prestrained and baked samples. The prestrained samples showed the maximum of 0.2% proof stress at 45° to the prestrain axis and the minimum at 90°. Baking raised the minimums at 0° and 90° so that the anisotropy decreases. Furthermore, the strain path changes were characterized by a scalar product of previous and current strain-rate mode tensors. By regarding this parameter as the degree of reactivation of slip systems, the anisotropy observed after prestraining and baking was microscopically explained from the viewpoints of residual stress, crystal plasticity, and dislocation pinning.