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Aoyagi, Yoshiteru*; Kobayashi, Ryotaro*; Kaji, Yoshiyuki; Shizawa, Kazuyuki*
International Journal of Plasticity, 47, p.13 - 28, 2013/08
Times Cited Count：25 Percentile：71.33(Engineering, Mechanical)In this study, we derive reaction-diffusion equations for dislocation patterning of dislocation cell structures and subgrains. In order to express the generation of dislocation pattern responding to deformation progress, information of slip rate and stress and effect of interactions between slip systems on formation of cell structures are introduced into the reaction rate coefficients of reaction-diffusion equations. Moreover, we propose a multiscale crystal plasticity model based on dislocation patterning. Then we carry out pseudo-three-dimensional FE-FD hybrid simulation for severe compression of FCC polycrystal using the present model. Some processes of ultra fine-graining, i.e., generation of dislocation cell structures, subgrains, dense dislocation walls and lamella subdivisions with high angle boundaries are numerically reproduced, and we investigate the effect of dislocation behavior on the processes of ultrafine-graining.
Aoyagi, Yoshiteru; Shimokawa, Tomotsugu*; Shizawa, Kazuyuki*; Kaji, Yoshiyuki
Materials Science Forum, 706-709, p.1751 - 1756, 2012/01
Times Cited Count：2 Percentile：73.92(Materials Science, Multidisciplinary)In this study, we develop a crystal plasticity model considering an effect of grain boundary. In order to predict increase of local critical resolved shear stress due to existence of grain boundaries, information of grain boundary as a role of dislocation sources is introduced into a hardening law of crystal plasticity. In addition, carrying out FE simulation for plastic deformation of FCC polycrystal, the stress-strain responses such as increase of yield stress due to existence of grain boundary are discussed. We investigate comprehensively the effect of dislocation behavior on the material property of nanostructured metal. The increase of yield stress and the decrease of hardening ratio with the reduction of grain size are caused by local differences on CRSS and dislocation behavior, respectively.
Muramatsu, Mayu*; Aoyagi, Yoshiteru; Shizawa, Kazuyuki*
Nihon Kikai Gakkai Rombunshu, A, 77(780), p.1304 - 1319, 2011/08
A crystal lattice in a metal during recrystallization process is modeled as an elastic bar element subject only to stretch and its kinematics is discussed. The balance laws of mass, momentum, angular momentum and energy of the lattice element are formulated. These laws are summed up over a phase in a representative volume element (RVE) and averaged in the RVE so as to prepare to develop macroscopic balance laws for a continuum mixture consisting of several phases. When the RVE converges on a material point at the final procedure of formulation, the present model can be regarded as a director model whose direction vector expressing the crystal orientation is attached to a material point of simple body. During the averaging process, two useful theorems are proposed for averaging terms associated with mass source and then these theorems are verified. Moreover, defining the representative lengths both in macroscopic and microscopic scales and performing an order-estimation for the balance law of angular momentum, this law can be separated into the bulk and lattice parts. The former results in the usual form, so that the Cauchy stress keeps symmetric even though the spin angular momentum of crystal lattice is taken into account. On the other hand, the latter corresponds to the evolution equation of crystal orientation of KWC type phase-field model.
Aoyagi, Yoshiteru; Kobayashi, Ryotaro*; Shizawa, Kazuyuki*
Nihon Kikai Gakkai Rombunshu, A, 77(775), p.448 - 461, 2011/03
Ultrafine-grained metals whose grain size is less than one micron have attracted interest as high strength materials. However, a mechanism of ultrafine-graining based on evolution of dislocation structures has not been clarified. In this study, we derive reaction-diffusion equations for dislocation patterning of dislocation cell structures and subgrains. In order to express the generation of dislocation pattern responding to deformation progress, information of slip rate and stress and effect of interactions between slip systems on formation of cell structures are introduced into the reaction rate coefficients of reaction-diffusion equations. Moreover, we propose a multiscale crystal plasticity model based on dislocation patterning. Then we carry out two-dimensional FE-FD simulation for severe compression of FCC polycrystal using the present model. Some processes of ultrafine-graining, i.e., generation of dislocation cell structures, subgrains, dense dislocation walls and lamella subdivisions with high angle boundary is numerically reproduced, and we investigate the effect of dislocation behaviors on the processes of ultrafine-graining.
Kurosawa, Eisuke*; Aoyagi, Yoshiteru; Shizawa, Kazuyuki*
Nihon Kikai Gakkai Rombunshu, A, 76(772), p.1547 - 1556, 2010/12
In order to express the increase of critical resolved shear stress, the conventional Bailey-Hirsh's relationship is extended on the basis of physical consideration for grain boundary that plays a role of dislocation source. A triple-scale dislocation-crystal plasticity FE simulation based on the above model, geometrically necessary crystal defects and the homogenization method is carried out for annealed FCC polycrystals with different initial grain size and initial dislocation density. Yield point drop and propagation of L
ders bands observed in macroscopic specimen with annealed FCC fine-grains are numerically reproduced. Moreover, macroscopic yielding of specimen and microscopic grain yielding are investigated in detail so as to clarify the initial yield behavior of annealed ultrafine-grained metals. It is also shown that plastic deformation is easy to be localized and the tensile ductility decreases as the grain size reduces.
Muramatsu, Mayu*; Ajioka, Hideyasu*; Aoyagi, Yoshiteru; Tadano, Yuichi*; Shizawa, Kazuyuki*
Zairyo, 59(11), p.853 - 860, 2010/11
A new recrystallization phase-field model is proposed, in which the three stages of static recrystallization phenomena, i.e., recovery, nucleation and nucleus growth are sequentially taken into account in a computation. From the information of subgrain patterns and crystal orientations in a polycrystal that are obtained by a dislocation-crystal plasticity FE analysis based on a reaction-diffusion model, subgrain groups surrounded by high angle boundary are found out. Next, subgrains in the group are coalesced into a nucleus by rotation of crystal orientation and migration of subgrain boundaries through a phase-field simulation. Then a computation of nucleus growth is performed also using the phase-field model on account of an autonomic incubation period of nucleation, in which stored dislocation energy assumes a role of driving force. It is shown that the present method can numerically reproduce the three stages of static recrystallization as a sequence of computational procedure.
Kurosawa, Eisuke*; Aoyagi, Yoshiteru; Tadano, Yuichi*; Shizawa, Kazuyuki*
Nihon Kikai Gakkai Rombunshu, A, 76(764), p.483 - 492, 2010/04
In this paper, a triple-scale crystal plasticity model bridging three hierarchical material structures, i.e., dislocation structure, grain aggregate and practical macroscopic structure is developed. The homogenization method is introduced into the Geometrically necessary (GN) dislocation-crystal plasticity model for derivation of the governing equation of macroscopic structure with the mathematical and physical consistencies. Using the present model, a triple-scale FE simulation bridging the above three hierarchical structures is carried out for f.c.c. polycrystals with different mean grain size. It is shown that the present model can qualitatively reproduce size effects of macroscopic specimen with ultrafine-grain. Moreover, the relationship between macroscopic yielding of specimen and microscopic grain yielding is discussed and the mechanism of the poor tensile ductility due to fine-graining is clarified.
Aoyagi, Yoshiteru; Shizawa, Kazuyuki*
no journal, ,
Ultrafine-grained metal (UFGM) expresses remarkably peculiar behavior both in material and mechanical aspects, and a computational model predicting the particular behavior of UFGM is expected in the field of materials science and engineering. In this study, we propose a new critical resolved shear stress model. The increase of flow stress at the first stage of deformation due to extremely low dislocation density in an annealed ultrafine-grain, i.e., the exhaustion of dislocation sources, is expressed by the above model. Yield point drop and propagation of L
ders bands observed in annealed FCC-UFGM are numerically reproduced by the triple-scale crystal plasticity simulation based on the above model, geometrically necessary crystal defects and the homogenization method. Moreover, the relationship between macroscopic yielding of specimen and microscopic grain yielding is investigated so as to clarify the initial yield behavior of UFGM from a microscopic viewpoint.
Aoyagi, Yoshiteru; Shizawa, Kazuyuki*
no journal, ,
no abstracts in English
Aoyagi, Yoshiteru; Shizawa, Kazuyuki*
no journal, ,
Ultrafine-grained metals whose grain size is less than one micron have attracted interest as high strength materials. In this study, in order to predict spontaneous dislocation patterning, generation rate of mobile dislocations is newly introduced into reaction-diffusion model. Information of deformation is related with the reference dislocation density through the generation rate depending on stress and activity of other slip system. In addition, carrying out pseudo-three-dimensional FE simulation for severe compression of FCC polycrystal using a multiscale crystal plasticity model based both on dislocation patterning and accumulation of geometrically necessary dislocations, we predict numerically the processes of ultrafine-graining of FCC crystal. We investigate comprehensively the effect of dislocation behaviors on the processes of ultrafine-graining.
