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Synopsis :Plasticdeformationanddislocationaccumulationindispersionhardeningalloysarenumericallyanalyzedbyacrystalplasticityfiniteelement technique and work hardening characteristics are discussed. The critical resolved shear stress for slip system is given by the extended expression of the Bailey-Hirsch type model which include the Orowan stress as size effect of microstructure. Work hardening of slip system is estimated by statistically stored dislocation (SSD) density. Increment of the SSD density is evaluated by slip strain and the mean free path of dislocations (the Kocks-Mecking model). The mean free path depends on the average spacing of dispersed particles, which is also used to estimate the Orowan stress. The average spacing of dispersed particles is calculated from the volume fraction and average diameter of dispersed particles.Asaresult,flowstresslevelattheinitialstageofdeformationagreedverywellwithexperimentalresultbutworkhardeningrate was higher than that of experiment. From this fact, it is considered that the mean free path and the average spacing of dispersed particles are different spacing factors. When we assume that the mean free path is two to three times larger than the average spacing of dispersed particles, numerical result of the strain hardening agrees very well with experimental one.

Image-based deformation simulation of microstructures in metals is attracting attention; however, the data conversion processes from the images of microstructures into the geometric models for the deformation simulation are now inconvenient, and there is a possibility that it prevents diffusion of the image-based simulation. In order to solve the problem, we developed an interface geometric models for Crystal Plasticity Finite Element (CPFE) analysis. The interface incorporates several functions for data cleaning and coarse graining of the microstructures: functions to narrow down the limits of Eulerian angles presenting crystal orientations, integrate crystal grains with similar crystal orientations, eliminate small crystal grains, select representative crystal orientation in each crystal grain, and so on. The interface was applied to an orientation map of polycrystal microstructure in pure titanium, and the course-grained geometric models were successfully obtained. Image-based CPFE analysis was conducted using the geometric models with different number of finite elements. The numbers of crystal grains were assumed to be around 50 in any geometric models. A dislocation density dependent constitutive equation was employed and uniaxial tensile loading was applied to the geometric models by the forced displacement. The results showed that spatial distributions of stress, strain, and dislocation density were good agreement among geometric models with different number of elements in both elastic and plastic ranges while values of the strain and dislocation density showed quantitatively dependency of the number of elements on their distributions in the plastic ranges. These features indicate that the qualitatively similar results can be obtained using the developed interface on the condition that coarse graining of the microstructures does not occur even though the number of elements is changed.

The temperature dependence of 0.2% proof stress was investigated in Ti0.49 mass%O with an ¡ single phase. The 0.2% proof stress decreased as the temperature increased from 77 to 573 K. The athermal stress, which was defined as the average of the stress values for which the temperature dependence was absent above 573 K, was found to be 70 MPa. The temperature dependence of the effective stress indicates that the temperature dependence of the effective stress changed at around 400 K. To study the thermally activated process that controls the yielding in Ti, the temperature dependence of the activation volume was also measured. An inverse temperature dependence of the activation volume was found at between 325 and 400 K, suggesting that the thermally activated process of dislocation glide changed at this temperature. Prismatic slip with ©aª dislocations was dominant at low temperatures, whereas other slips were activated at high temperatures. The activation enthalpy for the dislocation glide was also measured between 77 and 573 K.

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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.

Inhomogeneous deformation of a single ¡-¢ colony in a Ti6Al4V alloy under uniaxial tensile conditions was numerically simulated using a crystal plasticity finite element (CPFE) method, and we predicted density changes in geometrically necessary dislocations (GNDs) depending on the vanadium concentration in the ¢ phase (V ¢ ). The geometric model for the CPFE analysis was obtained by converting data from electron back-scatter diffraction patterns into data for the geometric model for CPFE analysis, using a data conversion procedure previously developed by the authors. The results of the image-based crystal plasticity analysis indicated that smaller V ¢ induced greater stress in the ¡ phase and smaller stress in the ¢ phase close to the ¡-¢ interfaces in the initial stages of deformation because of the elastically softer ¢ phase with lower V ¢ . This resulted in greater strain gradients and greater GND density close to the interfaces in the initial stages of deformation within the single ¡-¢ colony when the ¢ phase plastically does not deform. [

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