Annealing twins in nanocrystalline fcc metals: A molecular dynamics simulation

Abstract

We report fully three-dimensional atomistic molecular dynamics studies of grain growth kinetics in nanocrystalline Cu of 5 nm average grain size. We observe the formation of annealing twins as part of the grain growth process. The grain size and energy evolution was monitored as a function of time for various temperatures, yielding an activation energy for the process. The atomistic mechanism of annealing twin formation from the moving boundaries is described. DOI: 10.1103/PhysRevB.75.184111 PACS number͑s͒: 61.72.Mm, 61.82.Rx Annealing twins in face-centered-cubic metals have been observed as early as 1897, 1-3 and are a prominent feature observed in routine metallography of these materials. The formation of these twins is usually associated with the process of grain growth that occurs during annealing at relatively high temperatures. Twin boundaries are usually flat and extend across an entire grain. The resulting density of twins after an annealing treatment is controlled by grainboundary energy, prior deformation, and resulting grain size. [4][5][6][7][8][9][10][11] The formation of annealing twins is a very common experimental observation; the fraction of annealing twins typically increases with annealing time and therefore with the amount of grain growth. Up to two to three twins per grain are typically observed, and it has been suggested that the nucleation and growth of twinning are mechanistically linked to anomalous grain growth. [12][13][14] In addition, it has been shown that imposed shear stresses during annealing of deformed fcc metals with low stacking fault energies tend to result in recrystallized microstructures containing higher fractions of twin boundaries than those annealed without an imposed shear stress. 13 This appears to be true also for materials deformed by severe plastic deformation where high residual shear stresses are present.The mechanisms by which annealing twins are formed are not completely understood. Current theories propose that accidents at growing grains and particularly faults on ͕111͖-type planes are responsible for the formation of annealing twins. [15][16][17][18] The models of twin formation generally require grain-boundary migration. In the model of Mahajan et al., 15 the nucleation of Shockley partial dislocations at growth accidents on ͕111͖ steps is associated with grain-boundary migration.In nanocrystalline materials, there is strong evidence that deformation occurs through emission of Shockley partial dislocations from grain boundaries which are absorbed in the opposing grain boundaries. These mechanisms based on partial dislocations lead to twinning when partial dislocations are emitted in adjacent ͕111͖-type planes. The deformation response of these materials has been studied using molecular dynamics simulation techniques, [19][20][21] and similar theoretical work can help in the detailed understanding of the process of twin formation during annealing treatments.In the present work, we report on relatively long molecular dynamics simulations of grain growt...

Grain-size dependent mechanical behavior of nanocrystalline metals

“…Farkas et al [124] reported three-dimensional simulations of grain growth beginning with 5 nm grain sizes and were able to generate larger grains and possibly more realistic distributions while also obtaining the activation energy for annealing twins and grain boundary motion. Figure 22 shows 26 fivefold twin formation during an annealing simulation of nanocrystalline Cu as directly compared to experimental evidence of such star shaped twins in nanocrystalline NiW [125] as well as earlier studies by Zhu et al [126] that found fivefold twins formed in nanocrystalline Cu by high-pressure torsion.…”

Investigations and computer simulations of the intergrain diffusion in submicro-and nanocrystalline metals

“…The specimen final state that is used in computations by the method of molecular statics at 0 K is attained by annealing at 300 K with the molecular dynamics method which is followed by cooling to 0 K. It is supposed that model NC specimens constructed in such a way represent the actual structure of NC metals produced by compaction. By now this method has been well approved by many authors in studying the properties of the NC state in metals ( [50] and the cited publications).…”

Anneal hardening of a nanostructured Cu–Al alloy processed by high-pressure torsion and rolling

“…In one model the annealing twins form at irregularities on GBs where the presence of a packet of stacking faults leads to twin boundary formation [48] and in another model the annealing twins form due to a growth accident on the {111} propagating steps present on migrating GBs so that, during the recrystallization process, twinning is controlled by the existence of stacking faults and the behavior of the migrating boundaries [49]. An MD simulation showed consistencies with the latter model and it was found that in NS FCC metals the annealing twins nucleate and grow from migrating boundaries through the emission of Shockley partial dislocations where the emission sites are mostly near triple junctions and GB ledges [50].…”