Effects of Grain Boundary Disorder on Yield Strength

It was recently reported that segregation of Zr to grain boundaries (GB) in nanocrystalline Cu can lead to the formation of disordered intergranular films [1,2]. In this study we employ atomistic computer simulations to study how the formation of these films affects the dislocation nucleation from the GBs. We found that full disorder of the grain boundary structure leads to the suppression of dislocation emission and significant increase of the yield stress. Depending on the solute concentration and heat-treatment, however, a partial disorder may also occur and this aids dislocation nucleation rather than suppressing it, resulting in elimination of the strengthening effect

Effects of solutes on migration of incoherent twin boundary in FCC metals

Nanoscale twins form in many metallic materials, especially those with low stacking fault energy. Their presence can significantly enhance the strength of a material. However, Nanotwins are unstable and can be annihilated, e.g., by thermal annealing. We present the results of molecular dynamics (MD) simulations that demonstrate that additions of solutes can significantly stabilize nanotwinned structures. The MD simulations reveal that the mechanism of the solute drag on the twin boundary is associated not with the solute segregation on the incoherent twin boundary (ITB), but rather with changing of the ITB shape to accommodate as many solutes as possible

Asperity contacts at the nanoscale: comparison of Ru and Au

We develop and validate an interatomic potential for ruthenium based on the embedded atom method framework with the Finnis/Sinclair representation. We confirm that the new potential yields a stable hcp lattice with reasonable lattice and elastic constants and surface and stacking fault energies. We employ molecular dynamics simulations to bring two surfaces together; one flat and the other with a single asperity. We compare the process of asperity contact formation and breaking in Au and Ru, two materials currently in use in micro electro mechanical system switches. While Au is very ductile at 150 and 300 K, Ru shows considerably less plasticity at 300 and 600 K (approximately the same homologous temperature). In Au, the asperity necks down to a single atom thick bridge at separation. While similar necking occurs in Ru at 600 K, it is much more limited than in Au. On the other hand, at 300 K, Ru breaks by a much more brittle process of fracture/decohesion with limited plastic deformation.Comment: 10 pages, 13 figure

Molecular Dynamics Study of Self-Diffusion in Zr

We employed a recently developed semi-empirical Zr potential to determine the diffusivities in the hcp and bcc Zr via molecular dynamics simulation. The point defect concentration was determined directly from MD simulation rather than from theoretical methods using T=0 calculations. We found that the diffusion proceeds via the interstitial mechanism in the hcp Zr and both the vacancy and interstitial mechanisms give contribution in diffusivity in the bcc Zr. The agreement with the experimental data is excellent for the hcp Zr and for the bcc Zr it is rather good at high temperatures but there is a considerable disagreement at low temperatures

Molecular dynamics simulation of phase competition in terbium

The competition among multiple solid phases determines the final microstructures of a material. Such competition can originate at the very beginning of the solidification process. We report the results of molecular dynamics simulation of the phase competition between the hexagonal close-packed (hcp), face-centered cubic (fcc), and body-centered cubic (bcc) phases during the solidification of pure Tb. We found that the liquid supercooled below the hcp melting temperature has both bcc and hcp/fcc nuclei, but only the bcc nuclei grow such that the liquid always solidifies into the bcc phase, even at temperatures where the hcp phase is more stable. The hcp phase can only form in the last liquid droplet or at the bcc grain boundaries. Depending on the bcc grain orientations, the hcp phase jammed between the bcc grains either completely disappears or slowly grows via a solid-state massive transformation mechanism. Once the hcp phase becomes large enough, the stresses associated with its appearance can trigger a martensitic transformation. Yet, not the entire bcc phase is consumed by the martensitic transformation and the remaining bcc phase is transformed into the hcp phase via the solid-state massive transformation mechanism. Finally, if the supercooling is too large, the nucleation becomes almost barrier free and the liquid solidifies into a structure consisting of ultra-fine hcp and bcc grains after which the bcc phase quickly disappears