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Effect of Metalloid Impurities on Grain Boundary Stability in Tantalum.

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Final rept. Oct 94-Sep 95,

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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.

Subject Categories:

  • Inorganic Chemistry
  • Physical Chemistry
  • Properties of Metals and Alloys
  • Crystallography

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