Multiscale modeling on bulk nanostructured metal

Bulk Nanostructured Metal whose grain size is smaller than 1µm expresses remarkably peculiar behavior in material and mechanical aspects. In this study, we develop a multiscale crystal plasticity model considering information of dislocation source. In order to express an effect of grain boundary as a role of dislocation source, information of misorientation is introduced into a critical resolved shear stress model of crystal plasticity.

Finite element simulations based on the multiscale crystal plasticity model are carried out for face centered cubic polycrystals with different grain size. It is numerically predicted that yield behavior of fine-grained metals depends on the initial grain size as the following figure.

Nominal stress-nominal strain curves

Whereas deformation is almost uniform when the initial mean grain size is 1.2µm, strain localization can be seen when the initial mean grain size is 0.3µm or 0.6µm. However, the regions where strain is high are deferent. It is predicted that such differences of strain localization affect on softening of nanostructured metal.

Distributions of equivalent plastic strain(0.00 0.01)

Multiphysics simulation on intergranular stress corrosion cracking

Stress corrosion cracking is a critical concern for light water reactors because it can degrade structural components over a long period. It takes the form of intergranular stress corrosion cracking (IGSCC). Many studies on IGSCC have been conducted over several decades in the past. However, the mechanism of IGSCC initiation and propagation is still not fully understood. In this study, a crystal plasticity model expressing IGSCC is proposed by considering information about the oxidation along the grain boundaries and the failure of an oxide film caused by the localization of a deformation.

From a crystal plasticity finite element analysis and an oxygen reaction-diffusion finite difference analysis based on the presented model, the IGSCC is numerically reproduced. The distributions of the equivalent stress and oxygen concentration are depicted in the following figure. Oxygen atoms enter the grain boundaries from the crack tip [see Fig. (a) and Fig. (b)]. The grain boundaries are corroded by oxidation, and the crack propagates further [see Fig. (c)]. Then, crack propagation stops. The same processes are repeated, and another crack propagates further. The stress value around the crack tip increases with the extension of the unloaded area because of crack propagation. Hence, the crack starts to branch off as shown Fig. (c). It is predicted that crack branching such as IGSCC occurs because of an increase in the stress around the cracks originating from crack propagation.

(a) t = 1.473Ms

(b) t = 2.799Ms

(c) t = 4.630Ms

0MPa 500MPa　　0µol/m³ 0.1µol/m³
Distributions of equivalent stress and oxygen concentration

Crystal plasticity modeling on mechanical property of irradiated material

Crystal defects induced by irradiation obstruct dislocation movement. Hence, the critical resolved shear stress of irradiated material increases. While part of the radiation defects are swept by dislocations released from dislocation sources, therefore localization of plastic deformation are caused by decrease of radiation defects at the partial region. In this study, in order to predict increase of flow stress due to irradiation, information of density of radiation defects is introduced into a hardening modulus of crystal plasticity. Moreover, decrease of work-hardening ratio is represented by considering disappearance of radiation defects originating in dislocation movement.

We conduct crystal plasticity simulations for copper single crystal under simple tensile condition. Decrease of work-hardening ratio and reduction of ductility depending on the initial density of radiation defects are numerically reproduced. It is predicted that annihilation of radiation defect affects reduction of ductility.

Shear stress-shear strain curves