Accession Number : ADA310558

Title :   Effect of Metalloid Impurities on Grain Boundary Stability in Tantalum.

Descriptive Note : Final rept. Oct 94-Sep 95,

Corporate Author : ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD

Personal Author(s) : Krasko, Genrich L.

PDF Url : ADA310558

Report Date : MAY 1996

Pagination or Media Count : 27

Abstract : Metalloid impurities have a very low solubility in tantalum and, therefore, prefer to segregate at the grain boundaries (GBs). In order to analyze the energetics of the impurities on the tantalum GB, the first-principles calculations were performed on a simple eight-atom supercell emulating a typical (capped trigonal prism) GB environment. The 'environment-sensitive' embedding energies were calculated for hydrogen, boron, carbon, nitrogen, oxygen, phosphorus, and sulphur, as a function of the electron charge density due to the host atoms at the impurity site. The calculations showed that at the electron density typical of a GB, carbon has the lowest energy (followed by nitrogen and boron) and, thus, would compete with the other impurities for the site on the GB, tending to displace them from the GB. These energies were then used in a modified Finnis-Sinclalr embedded atom approach for calculating the cohesive energies and the equilibrium interplanar distances in the vicinity of an (111) 3sigma tilt GB plane, both for the clean (CL) GB and that with an impurity. These distances were found to oscillate, returning to the value corresponding to the equilibrium spacing between (111) planes in bulk BCC tantalum by the 10th-12th plane off the GB. Carbon, nitrogen, and boron somewhat dampen the deformation wave (making the oscillations less than in the CL GB), while oxygen, phosphorus, and sulphur result in an increase of the oscillations. The cohesive energies follow the same trend, the GB with carbon being the most stable. Thus, carbon, nitrogen, and boron may be thought of as being cohesion enhancers, while oxygen, phosphorus, and sulphur result in decohesion effects.

Descriptors :   *STABILITY, *IMPURITIES, *TANTALUM, *GRAIN BOUNDARIES, ELECTRON DENSITY, DEFORMATION, SITES, ENERGY, AUGMENTATION, EQUILIBRIUM(GENERAL), ATOMS, CARBON, NITROGEN, HYDROGEN, OXYGEN, ELECTRONS, EMBEDDING, SULFUR, SOLUBILITY, BORON, ENERGETIC PROPERTIES, OSCILLATION, PHOSPHORUS, WAVES, COHESION, PRISMS(OPTICS), METALLOIDS.

Subject Categories : Inorganic Chemistry
      Physical Chemistry
      Properties of Metals and Alloys
      Crystallography

Distribution Statement : APPROVED FOR PUBLIC RELEASE