Papers by Keyword: Crystal Plasticity Analysis

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Authors: Yoshiki Kawano, Tsuyoshi Mayama, Ryouji Kondou, Tetsuya Ohashi
Abstract: In this paper, we investigated changes in active slip systems of α-phase of Ti-6Al-4V alloy under a cyclic plastic loading using a crystal plasticity finite element method. In the analyses, a bicrystal model was employed, and the crystallographic orientations were set so as that prismatic <a> or basal slip system was the primary slip system in each grain. The results showed that there was a mechanism where the basal slip systems could reach the stage of activation under the cyclic plastic loading even though the condition was that the prismatic <a> slips initially operate. The reason for the activity changes was due to the changes in the incompatibility between the grains by the work hardening, and the effect of the incompatibility on activities of slip systems appeared even in the perpendicular arrangements of the grains to the loading direction.
Authors: Michihiro Sato, Tetsuya Ohashi, Takuya Maruizumi, Isao Kitagawa
Abstract: Representative length scale of ULSI (Ultra Large Scale Integration) cells is going to be at a nano-meter order, and the atomic level defects, such as uneven oxide films or dislocation accumulation are becoming more and more important. Among these defects, dislocation accumulation is known to be caused by thermo-plastic deformation in silicon during the processes of device fabrication. In this study, we analyse such thermal stress, plastic slip deformation and accumulation of dislocations in STI (Shallow Trench Isolation) type ULSI devices when the temperature drops from the initial at 1000 °C to room temperature. For the analysis, we use a crystal plasticity analysis code CLP, assuming that lattice friction stress for the movement of dislocations is proportional to the hardness of silicon, which is known to have strong dependency on temperature. The results show that dislocations are generated between the temperature range from 880 to 800 °C, and its maximum density is highly dependent on the lattice friction stress in the temperature range above 800 °C. For example, the difference of 16 MPa in the lattice friction stress at 1000 °C caused increase in dislocation density more than ten times. It is concluded that control of lattice friction stress at high temperatures is one of the most promising way for the suppression of dislocation accumulation.
Authors: Tetsuya Ohashi, Yelm Okuyama
Abstract: Macroscopic mechanical responses of alloy steels dispersed with fine vanadium carbide particles [1] were analyzed by crystal plasticity finite element method. Average distance of dispersed particles was used as a microscopic length scale and introduced to the models of the critical resolved shear stress and the mean free path of dislocations. Numerical result of yield stress agreed well with that of experimental result [1], but that of the work hardening rate was slightly higher than that of experimental one. When the dislocation mean free path was set to be 2 to 3 times the average spacing of dispersed particles, the work hardening rate fit better with the experimental one.
Authors: Yohei Yasuda, Tetsuya Ohashi, Takuro Sugiyama
Abstract: Tensile properties of ferrite lamella in pearlite under lattice strain are examined by a strain gradient crystal plasticity analysis. Tensile direction is made to be parallel to the lamella. Obtained results of macroscopic stress-strain relation of the lamella show significant increase of yield stress and strain hardening rate with the reduction of the lamella thickness and further increase of the yield stress with positive normal lattice strain parallel to the tensile direction in the ferrite layer. Whereas normal lattice strain perpendicular to the tensile direction contributes little to the tensile properties.
Authors: Michihiro Sato, Tetsuya Ohashi, Takuya Maruizumi, Isao Kitagawa
Abstract: Thermal stress, plastic slip deformation and accumulation of dislocations in shallow trench isolation (STI) type ULSI devices when the temperature drops from 1000 し to room temperature are analyzed by a crystal plasticity analysis cord. The results show that dislocation accumulation takes place at the temperature range over 800 し, and the difference of 6 MPa in the lattice friction stress at 1000 し!causes increase of dislocation density more than 1.6 times. Dislocations generate and accumulate at the shoulder part of the device area and bottom corners of the trench. Dislocations are categorized into two groups. In one group, dislocation lines are mostly straight and parallel to the trench direction, and in the other group, dislocations make half loop type structure. Possibilities for the suppression of dislocation accumulation through control of lattice friction stress at high temperature region are discussed.
Authors: Ryouji Kondou, Tetsuya Ohashi
Abstract: Slip deformation phenomena in compatible type multi crystal models subjected to tensile load are analyzed by a finite element crystal plasticity analysis code, and accumulation of geometrically-necessary and statistically-stored dislocations (GNDs and SSDs) are evaluated in detail. Crystal orientations for the grains are chosen so that mutual constraint of deformation through grain boundary planes does not take place. We call these models as compatible type multi crystals, because “compatibility requirements” at grain boundaries are automatically maintained by slip deformation only on the primary systems and uniform deformation is expected to occur in each grain. Results of the analysis, however, show non-uniform deformation with high density of GNDs accumulated in a form of band. Growth of such kind of structure of GNDs caused localized accumulation of SSDs at grain boundary triple junctions. Mechanism for the band-shaped accumulation of GNDs in the compatible type multi crystals are discussed from the viewpoint of multi body interactions which arise from shape change of crystal grains after slip deformation.
Authors: Kazuma Shiraishi, Tsuyoshi Mayama, Michiaki Yamasaki, Yoshihito Kawamura
Abstract: Influence of long period stacking ordered (LPSO) phase on non-uniform deformation in polycrystalline as-cast Mg-Zn-Y alloys was investigated by using a crystal plasticity finite element method. Material parameters for α-Mg phase and LPSO phase were identified by fitting to experimental stress-strain curves of Mg99.2Zn0.2Y0.6 and Mg85Zn6Y9 of which volume fractions of LPSO phase were 0% and 100%, respectively. The results of calculations showed that the increase in volume fraction of LPSO lead to the increased strain accumulation in α-Mg phase.
Authors: Yuji Nakasone, Takeshi Yokoi, Yasunao Sato
Abstract: The present paper describes the FEM code the present authors have developed based on the theory of the polycrystal plasticity with dislocation distributions taken into account and the simulations of tensile deformation behavior in FCC polycrystalline materials having bimodal structures by using the developed FEM code. In order to simulate the deformation behavior of materials having bimodal structures, it is necessary for the code to simulate the mesoscopic deformation behavior with the size effect of the initial yield strength, or the 0.2% proof strength. The present study has attempted to simulate the size effect of 0.2% proof strength by modifying the Bailey-Hirsch relation. By using the modified relation, the size effect of the initial plastic yield is successfully reproduced by FE polycrystal plasticity analysis. The results also showed that the 0.2% yield strength is decreased as the volume fraction of coarse grains is increased in the bimodal structure. As the ratio of the average diameter of fine grains to that of coarse grains is increased, the yield strength of the bimodal structure is decreased. The yield strength and work hardening rate of the bimodal structure, however, is not so much decreased as that of fine grain models. It was also revealed that the reason why materials having bimodal structures show higher ductility is that coarse grains yield in earlier stage of deformation and lower the maximum stress in the materials.
Authors: Michihiro Sato, Tetsuya Ohashi, Keisuke Aikawa, Takuya Maruizumi, Isao Kitagawa
Abstract: We numerically evaluate the accumulation of dislocations in periodic structure of the shallow trench isolation (STI) type ULSI cells which has generally been adopted as the latest semiconductor device structure. STI type ULSI cells with gate length less than 62 nm and various trench depths are employed and subjected to a temperature drop from the initial value of 1000 °C. Dislocation accumulation is evaluated by a technique of crystal plasticity analysis. Relations between the geometry of the STI type ULSI cells and dislocation accumulation are discussed.
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