Competing plastic deformation mechanisms in nanophase metals H. Van Swygenhoven * and M. Spaczer Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland A. Caro † Centro Ato ´mico ´ Bariloche, 8400 Bariloche, Argentina D. Farkas ‡ MSE, Virginia Polytechnic Institute, Blacksburg, Virginia 24061 Received 8 March 1999 The mechanisms of plastic deformation in nanocrystalline Ni are studied using three-dimensional molecular- dynamics computer simulations for samples with mean grain sizes ranging from 3 to 12 nm under uniaxial load at ﬁnite temperatures. At the lower limit of this size range, we observe a plastic regime controlled by intergrain sliding; at the upper limit, however, we observe a regime with two competing mechanisms: intergrain sliding and dislocation emission from the grain boundaries GB’s. The latter mechanism constitutes a transition behavior, precursor to the dislocation-dominated regime typical of large grain polycrystals. In samples with mainly low-angle GB’s, the transition occurs at a smaller grain size. S0163-18299900525-1 Grain size is one of the most important variables charac- terizing the microstructure of polycrystalline metals. It has signiﬁcant inﬂuence in the plastic behavior of materials. In going from single crystal to polycrystals, plasticity is af- fected in various ways by the presence of grain boundaries GB’s. Dislocations are the carriers of plastic deformation in crystalline materials and depending on the nature of interface between grains they can travel across them or not. In general terms, the energetics of GB’s is such that for low-angle mis- sorientations between neighboring crystals the GB’s energy is small, while the converse is true for high-angle missorien- tations, with the notable exception of some special coinci- dent site lattice boundaries. High-energy GB’s act as efﬁ- cient obstacles for dislocation motion because they usually do not ﬁnd a plane-matching Burgers vector in the neighbor- ing grain. Dislocations generated by a given source inside a grain repel each other. As a consequence of this mutual in- teraction and the external applied stress, their distribution peaks close to the boundaries in the so-called pileup effect. When the stress ﬁeld resulting from the addition of indi- vidual dislocation contributions reaches some critical value, it activates sources in neighboring grains. This stress ﬁeld can be calculated in the continuum approximation giving a d -1/2 dependence for the yield stress of polycrystals as a function of grain size d. This is the well-known Hall-Petch relation 1,2 veriﬁed in numerous experiments. Nanocrystalline phases have become a focus of attention to material scientists because the Hall-Petch relation, when extrapolated to grain sizes in the nanometer range, predicts extremely hard materials. However, experimental evidence suggests that in this range the Hall-Petch relation apparently fails to describe the observations; a new regime appears, whose quantitative description is still controversial. 3–6 In nanophase solids a large fraction of atoms up to 50% are boundary atoms; thus intercrystalline deformation mechanisms are expected to become relevant, as opposed to intracrystalline mechanisms based on dislocation activity. At the smallest grain sizes, dislocation sources inside grains can hardly exist because size and image force limitations; only dislocation emitted from a boundary can eventually travel across the grain. From these arguments it is then likely to expect a change in regime when decreasing grain size, from dislocation-dominated Hall-Petch to a boundary-dominated new one. In previous works, we studied the properties of nanophase samples in the 3.5–8 nm grain size range. 7–9 Here, by ex- tending the grain size to 12 nm and by deforming textured nanostructures, we present evidence of two competing mechanisms leading to such change of regime, obtained from molecular-dynamics MD computer simulations in nanophase Ni. For some simple materials late transition metals and their compounds, MD computer simulations provide an atomistic view of the deformation process through the mean-ﬁeld ap- proximation for the atomic interactions. 10,11 It is simple enough to deal with about a million atoms in present day computers, allowing computer-generated samples to be in a one-to-one scale with real nanograins. Many physical prop- erties such as the lattice parameter, cohesive energy, elastic constants, phonon-dispersion relations, point defect behav- ior, phase diagrams, stacking fault energies, surface struc- ture, reconstruction and energy, etc., are well reproduced within this model. We performed MD simulations of defor- mation in three-dimensional 3D nanophase Ni samples, in the temperature range 300–500 K, at constant applied uniaxial tensile stress between 0 and 3 GPa, in the grain size range 3.2–12 nm. The nature of the interface structure in real nanophase samples may go from random to quite relaxed or even textured orientations. 12,13 Consequently, we used two procedures to create them: a stochastic method, and a con- strained stochastic method. In the ﬁrst one, the simulation cell volume was ﬁlled with nanograins grown from seeds with random location and crystallographic orientation, ﬁlling the space according to the Voronoi construction. In the PHYSICAL REVIEW B 1 JULY 1999-I VOLUME 60, NUMBER 1 PRB 60 0163-1829/99/601/224/$15.00 22 ©1999 The American Physical Society