Aluminum R3 grain boundary sliding enhanced by vacancy diffusion Ningning Du a , Yue Qi b, * , Paul E. Krajewski b , Allan F. Bower a a Division of Engineering, Brown University, Providence, RI 02912, USA b Materials and Processes Laboratory, General Motors Technical Center, 30500 Mound Road, Warren, MI 48090-9055, USA Received 18 December 2009; received in revised form 31 March 2010; accepted 9 April 2010 Available online 10 May 2010 Abstract Grain boundary sliding is an important deformation mechanism for elevated temperature forming processes. Molecular dynamics simulations are used to investigate the effect of vacancies in the grain boundary vicinity on the sliding of Al bi-crystals at 750 K. The threshold stress for grain boundary sliding was computed for a variety of grain boundaries with different structures and energies. These structures included one symmetrical tilt grain boundary and five asymmetrical tilt grain boundaries. Without vacancies, low energy R3 grain boundaries exhibited significantly less sliding than other high energy grain boundaries. The addition of vacancies to R3 grain boundaries decreased the threshold stress for grain boundary sliding by increasing the grain boundary diffusivity. A higher concentration of vacancies enhanced this effect. The influence of vacancies on grain boundary diffusivity and grain boundary sliding was negligible for high energy grain boundaries, due to the already high atom mobility in these boundaries. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Molecular dynamics; Grain boundary sliding; Aluminum; Superplasticity; Grain boundary structure 1. Introduction Grain boundary sliding (GBS) involves the rigid transla- tion of one grain over another parallel to the grain bound- ary (GB) interface. GBS mediates plastic flow of polycrystalline materials [1,2], especially when grain sizes drop to the nanometer scale [3] and when the deformation temperature is high [4–12]. At high temperatures, vacancies tend to segregate to GBs for energy minimization [13–15]. Atomistic calculations on many fcc metals and alloys have demonstrated that the vacancy formation energy at most lattice sites in the vicinity of GBs is lower than in the bulk, despite displaying site to site variation [16]. For Al, the equilibrium concentration of vacancies is expected to be higher near a GB [17–20]. This is consistent with experi- mental observations that excess vacancies can be found around GBs in an Al–Mg–Mn alloy during superplastic deformation [21–23] at high temperature. The existence of vacancies at the GB will influence GBS and thus the total deformation behavior of polycrystalline materials. Experimental evidence of the role of vacancies in GBS is rather limited and, most of the time, correlations are only inferred indirectly. Direct experimental observations of the influence of vacancies on GB mobility in magnesium were made by Lambri et al. [24], who found that the mobil- ity of GBs decreased with a reduction in the vacancy con- centration. With the development of atomic simulations many efforts have been made in the last decade to under- stand the role of vacancies on GBS. At the ab initio level, density functional theory (DFT) was used to model GBS quasi-statically, by shifting the top grain rigidly relative to the bottom grain, followed by energy minimization after each shifting [19,25–28]. The details of GBS depend on localization of the metallic bonding and many structural factors, such as the boundary geometry, the local order and the presence of a vacancy near the boundary. In some cases, a vacancy can migrate during GBS [26–28]. Lu and Kioussis [19] concluded that the existence of the vacancy hinders GBS, since it triples the energy barrier for sliding at an Al symmetrical tilt R5(2 1 0)[0 0 1] boundary. The 1359-6454/$36.00 Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2010.04.016 * Corresponding author. E-mail address: yue.qi@gm.com (Y. Qi). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 58 (2010) 4245–4252