关键词: 分子动力学/
晶粒尺寸/
力学性质/
变形孪晶
English Abstract
Molecular dynamics simulation of effect of grain on mechanical properties of nano-polycrystal -Fe
Wang Peng1,2,Xu Jian-Gang1,
Zhang Yun-Guang1,
Song Hai-Yang2
1.School of Science, Xi'an University of Posts and Telecommunications, Xi'an 710121, China;
2.College of Material Science and Engineering, Xi'an Shiyou University, Xi'an 710065, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11572259), the Program for New Century Excellent Talent in University of the Ministry of Education of China (Grant No. NCET-12-1046), the New Scientific and Technological Star of Shaanxi Province (Grant No. 2012KJXX-39), and the International Cooperation and Exchanges of Shaanxi Province (Grant No. 2016KW-049).Received Date:01 June 2016
Accepted Date:03 September 2016
Published Online:05 December 2016
Abstract:The nanocrystalline metals are widely investigated due to their unique mechanical properties. Currently, the available studies about deformation mechanisms of metals mainly focus on face-centered cubic metals such as Ni, Cu and Au. However, the body-centered cubic metals are still very limited, despite their industrial importance. Here, we investigate the effects of grain size and temperature on the mechanical behavior of nano-polycrystal -Fe under uniaxial tensile loading by using molecular dynamics (MD) simulation. The models of nanocrystalline -Fe with the grain sizes of 3.95, 6.80, 9.70, 12.50, 15.50, 17.50, 20.70 and 26.00 nm are geometrically created in three dimensions by using Voronoi construction, and these models are relaxed to reach an equilibrium state. Then, each of them has a strain of 0.001 along the Z-direction in each step, keeping zero pressure in the X- and Y-directions until the strain increases up to 0.2. A 1.0 fs time step is used in all of the MD simulations. Based on the data output, the stress-strain curves at different grain sizes are obtained. The results indicate that the peak stresses of nano-polycrystal -Fe decrease with the decrease of grain size, exhibiting a breakdown in the Hall-Petch relation when the grain size is smaller than a critical size. The major deformation mechanism is found to change from dislocation slips and twinning-mediated plasticity in a model with a larger grain size to grain boundary sliding in a model with a smaller grain size. It should be noted that twinning is formed by the emission of 1/6111 partial dislocations along the {112} slip plane. The results show that crack formation during tension is a cause of reducing the flow stress of nano-polycrystal -Fe with a large grain size and that the Young's modulus of nano-polycrystal -Fe decreases with the grain size decreasing. The main reason for the crack nucleation is here that grain boundaries perpendicular to the loading direction bear higher stress and the twin band interacts with grain boundaries at a larger grain size, causing the stress to concentrate at the intersections of grain boundaries. The results also show the detwinning behavior and migration of deformed twins in nano-polycrystal -Fe. The detwinning behavior occurs via the migration of the intersection of grain boundary and twin, and this intersection is incoherent boundary. The migration of deformed twins proceeds by repeating initiation and glide of 1/6111 partial dislocations on adjacent {112} planes. In addition, we find that the nucleation and propagation of dislocation become easier at higher temperature than at lower temperature.
Keywords: molecular dynamics/
grain size/
mechanical properties/
deformed twins
