Atomistic and first principles studies of lead segregation to aluminum grain boundaries and its influence on thermal stability and mechanical behavior.

摘要

Nanostructured materials have received much interest because they exhibit different properties compared to coarse-grained polycrystals of the same material. Many enhancements in the properties of nanocrystalline materials due to fine grain size are lost if grain growth occurs. Enrichment of the grain boundaries (segregation) with solute atoms with limited solubilities has been shown to diminish or even reverse the free energy available for grain growth by forming metastable structures. Al-Pb is an immiscible alloy system with a positive energy of segregation and therefore is a potential candidate for segregation-induced grain boundary stabilization.;To investigate the segregation-induced stability of grain boundaries in Al-Pb nanoalloys, atomic modeling was used to characterize the structure and energy of substitutional Pb defects in bulk Al, and in an Al bi- and nano-crystal. Monte Carlo simulations using a modified embedded atom method (MEAM) potential fit to first principles results predict the formation of Pb clusters in bulk Al, in agreement with prior experiments. In the case of the bicrystal and nanocrystalline structure, the simulations predict segregation of Pb impurities towards Al grain boundaries prior to cluster formation depending on the Pb content and the number of grain boundaries. Analysis of the relative enthalpies for Pb defects suggests that Pb impurities can help stabilize nanocrystalline Al against grain growth. Subsequent calculations of the energies of Pb clusters embedded in an Al matrix, in a cuboctahedral configuration, using the same potential, predict a cross-over cluster size of approximately 2.8 nm below which Pb prefers to segregate to grain boundaries compared to forming clusters in the Al matrix.;To study the stabilization of Al grain boundaries caused by segregation of Pb impurities, grain boundary energy as a function of Pb content was investigated for two high symmetry Sigma 5 {210} and Sigma 5 {310} Al tilt grain boundaries. Calculations for grain boundary energies were performed using atomistic MEAM and Density Functional Theory calculations. Results from both of these methods showed a reduction in grain boundary energy with an increase in Pb content.;To further explore the boundary stabilization caused by segregated Pb atoms over the entire tilt angle, a dependence of the energies of 100> symmetrical Al tilt boundaries, with and without Pb substitution, on misorientation angles was investigated using a multiscale disclination-structural unit model (DSUM). The DSUM combines an atomistic structural unit model with a mesoscopic disclination based description of grain boundaries. The agreement between grain boundary energy calculations using the multiscale DSUM and the atomistic calculations is reasonably good, with our MEAM+DSUM (model combining MEAM with DSUM) results agreeing with the atomistic MEAM calculation, and our GLUE+DSUM (model combining GLUE interatomic potential with DSUM) results agreeing with the atomistic GLUE calculation at intermediate angles within about 0.08 J/m 2 and 0.06 J/m2, respectively. The predictions given by the MEAM+DSUM and the GLUE+DSUM for an intermediate grain boundary containing Pb at 22.6°, along with matching the full atomistic MEAM and GLUE result also matched the first principles result reasonably well. The multiscale DSUM predicts a strong dependence of the grain stabilization energy on tilt angle.;Although there exist several experimental/theoretical studies on the mechanical behavior of single component nano-crystalline materials, studies on the effect of a second component on the mechanical behavior of nanocrystalline materials are very limited. In this dissertation the effect of Pb on the mechanical behavior of pure Al using molecular dynamics (MD) simulations was investigated. MD simulations were carried out for uniaxial tensile straining of bicrystalline aluminum (Al) and aluminum-lead (Al-Pb) alloys. A softening owing to the presence of Pb on the Al grain boundaries, in agreement with, but less than that found experimentally, was observed. The thickening and disordering of the grain boundaries was believed to be contributing to this softening.