Atomistic simulation of the point defects in B2-type MoTa alloy

The formation and migration mechanisms of three different point defects (mono-vacancy, anti-site defect and interstitial atom) in B₂-type MoTa alloy have been investigated by combining molecular dynamics (MD) simulation with modified analytic embedded-atom method (MAEAM). From minimization of the formation energy, we find that the anti-site defects Mo_Ta and Ta_Mo are easier to form than Mo and Ta mono-vacancies, while Mo and Ta interstitial atoms are difficult to form in the alloy. In six migration mechanisms of Mo and Ta mono-vacancies, one nearest-neighbor jump (1NNJ) is the most favorable due to its lowest activation and migration energies, but it will cause a disorder in the alloy. One next-nearest-neighbor jump (1NNNJ) and one third-nearest-neighbor jump (1TNNJ) can maintain the ordered property of the alloy but require higher activation and migration energies, so the 1NNNJ and 1TNNJ should be replaced by straight [1 0 0] six nearest-neighbor cyclic jumps (S[1 0 0]6NNCJ) or bent [1 0 0] six nearest-neighbor cyclic jumps (B[1 0 0]6NNCJ) and [1 1 0] six nearest-neighbor cyclic jumps ([1 1 0]6NNCJ), respectively. Although the migrations of Mo and Ta interstitial atoms need much lower energy than Mo and Ta mono-vacancies, they are not main migration mechanisms due to difficult to form in the alloy.     

Early precipitation stages of aluminum alloys—The role of quenched-in vacancies

We present preliminary results on the structure of nano-sized particles (clusters of solute atoms or Guinier-Preston-Bagaryatsky (GPB) zones) in aluminum alloys. Hindering the motion of dislocations, these GPB zones (precipitates) are responsible for the strength of AlCuMg-alloys - used e.g. as AA2024 - for the outer shell plates in aircraft industry. We will discuss the role of quenched-in vacancies for the formation and growth of the precipitates. Using positron annihilation (Doppler broadening together with the recently developed tool of high-momentum analysis (HMA)) and X-ray absorption spectroscopy measurements, we are able to characterize the local atomic environment in the vicinity of vacancies and selected elements (Cu) forming the precipitates. The interpretation will be based on a comparison to numerical calculations of positron lifetimes and momentum distributions as well as simulated X-ray absorption spectra. Often reliable numerical calculations of experimental quantities are possible only if relaxed atomic positions - calculated employing an ab-initio code like SIESTA - are used as input.     

Interaction of Point Defects with Twin Boundaries in Copper

The interaction between small vacancy clusters and twin boundaries in copper is studied by using many-body potential developed by Ackland et aL for fcc metals. The interaction energies of single-, di- and tri-vacancy clusters with （111） and （112） twin boundaries are computed using well established simulation techniques. For （111） twins the vacancy clusters are highly repelled when they are on the adjacent planes, and are attracted when they are away from the boundary. In the case of （112） twins, vacancy clusters are more attracted to the boundary when they are near the boundary as compared to away from it. Vacancy clusters on both the sides of the boundary are also investigated, and it is observed that the clusters energetically prefer to lie on the off-mirror sites as compared to the mirror position across the twin.