Topic: Atomistic understanding and prediction of mechanical properties of structural metallic materials

Speaker: Professor Shigenobu Ogata

　　Department of Mechanical Science and Bioengineering, Osaka University, Japan
　　Center for Elements Strategy Initiative for Structure Materials (ESISM), Kyoto University, Japan

Time: 15:00-16:30, (Mon.) May 4^th, 2015

Venue: Room 403, Shi Changxu Building, IMR CAS

Keywords: atomistic modeling, mechanical property, Fe alloy, carbon and hydrogen diffusion, bulk nanostructured metals, accelerated molecular dynamics, Mg twin, kinetic Monte Carlo

Atomistic modeling based on electronic and atomic structure calculations is one of the most promising ways to reveal and predict the mechanical properties of structural materials, along with recent atomistic scale experimental observation techniques and multiscale modeling techniques. Here I introduce several our recent atomistic analyses for understanding and predicting the mechanical properties of structural materials. First, I discuss Si solute atom effect on yield stress of Fe-Si alloy [1]. Atomistic details of the interaction between screw dislocation and substitutional solute Si in Fe-Si alloy was investigated by atomistic modeling method. We developed an embedded atom method (EAM) potential for Fe-Si interaction based on the density functional theory calculations and then evaluated the interaction energy between Si and screw dislocation. The interaction energy is found to become larger as the Si atom approaches the screw dislocation. This indicates that attractive driving force acting between the Si atom and screw dislocation can lead to the dislocation drag effect. Using nudged elastic band (NEB) method, we computed the energy barrier for screw dislocation glide associated with double kink formation, and found that the energy barrier is reduced by nearby existing Si atom. From the theoretically estimated activation energies of dislocation glide, we estimated the temperature and strain rate dependencies of the critical resolved shear stress of Fe-Si alloy. The results explain that the solute Si causes both solid-solution hardening and softening of Fe-Si alloy. I also briefly discuss interstitial carbon and hydrogen diffusion behaviors in defective BCC iron and its effects on dislocation motion, using adaptive boost accelerated molecular dynamics method[2] and path integral molecular dynamics method[3][4]. Not only we successfully reproduced experimental temperature dependent diffusivity, but newly found a general diffusion channel in dislocation core[5]. Next, I discuss creep deformation of bulk nanostructured metal[6][7], which is driven by both the atomic diffusion and the dislocation activities, such as, grain boundary (GB) diffusion, GB sliding, and dislocation nucleation from GB. We actually observed deformation mechanism transition from diffusive to displacive in molecular dynamics creep tests with different applying external stress levels, and then drew a creep deformation mechanism map and proposed a constitutive equation for nanocrystalline copper as a function of temperature, external stress, and grain size, based on statistical mechanics. We clearly explained why the deformation mechanism transits with stress by combining the rate equations for the fundamental activation processes. Our molecular dynamics simulations also revealed that activation entropy play a crucial role in the nanocrystalline creep deformation [9]. We also analyzed several key activities necessary to understand the mechanical properties of nanocrystalline metals, such as, GB diffusion, dislocation nucleation from GB, GB sliding and migration at finite temperature and usual stress level using adaptive boost accelerated molecular dynamics method and computed its thermodynamic parameters. Finally, I discuss deformation twining process in Mg. We developed enthalpy maps with respect to degrees of freedom of Mg HCP crystal. Based on the enthalpy maps, a 2D kMC modeling framework was constructed. Using the kMC, we demonstrate temperature, strain rate, size dependences on twining yield stress.

At the end, based on my experiences of atomistic modeling and simulation for understanding and prediction of macroscopic mechanical properties of metallic structural materials, I would emphasize an importance of meso-science which is a promising avenue to connect these atomistic studies to the structural materials engineering.

References:
[1] M.Wakeda, H.Kimizuka, and S.Ogata, J.Japan Inst.Met.Mater., 77-1, 409-414(2013).
[2] A.Ishii, S.Ogata, H.Kimizuka, and J.Li, Phys. Rev. B, 85-6, 064303(2012).
[3] H.Kimizuka, H.Mori, and S.Ogata, Phys. Rev. B, 83-16, 094110(2011).
[4] H.Kimizuka and S.Ogata, Phys. Rev. B, 84-2, 024116(2011).
[5] A.Ishii, J.Li, and S.Ogata, PLoS ONE, 8-4, e60586(2013).
[6] Y.J.Wang, A.Ishii, and S.Ogata, Mater. Trans., 53-1, 156-160(2012).
[7] Y.J.Wang, A.Ishii, and S.Ogata, Phys. Rev. B, 84-22, 224102-1-7(2011).
[8] Y.J.Wang, A. Ishii, and S.Ogata. Acta Mater., 61-10, 3866-3871(2013).