Al Grain Boundary Embrittlement Promoted by Na Impurity: An ab initio Study

  • PDF / 474,802 Bytes
  • 6 Pages / 595 x 842 pts (A4) Page_size
  • 106 Downloads / 245 Views

DOWNLOAD

REPORT


Al Grain Boundary Embrittlement Promoted by Na Impurity: An ab initio Study Guang-Hong Lu, Masanori Kohyama1, Ryoichi Yamamoto Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8558, Japan 1

Department of Materials Physics, Osaka National Research Institute, AIST, 1-8-31 Midorigaoka, Ikedashi,

Osaka 563-8577, Japan

ABSTRACT

We calculate the electronic structure of AlΣ9 tilt grain boundary with substitutional Na impurity atoms by first principles pseudopotential method. Results show that by Na segregation Al grain boundary expands and the valence charge density decreases significantly along the boundary. There is no stronger bond than metallic bond in the boundary even with Na impurity. We therefore conclude that the mechanism of Na-promoted Al grain boundary embrittlement should be one kind of ‘decohesion model’. Introduction It seems that theoretically there are two kinds of mechanism of impurity-promoted embrittlement of metal grain boundaries. One is called ‘decohesion model’. In this model, the metal-impurity or metal-metal cohesion is weakened by impurity segregation in metal grain boundary and thus induces embrittlement. A typical one is presented by Losch[1], which indicates that the metal-metal bonds neighboring to the impurity-metal bonds are weakened by the embrittling element in group IV-VI. Supporting calculations are carried out by Briant and Messmer[2-3] on a nickel cluster with sulfur impurity, and a iron cluster with phosphors impurity, respectively. The other is ‘bond mobility model’ proposed by Haydock[4]. In this model segregated non-metallic impurities at the metal grain boundary bind with the neighboring metal atoms. This increases the covalent character of the bonds and reduces their mobility. Under stress, the bonds between grains cannot slide smoothly and thus stress concentration is generated. Goodwin et al. [5-6] carried out the supercell calculation based on the density functional theory (DFT) on the embrittlement by Ge and As impurities in the host Al[111], supporting ‘bond mobility model’. Until now the embrittling elements seemed to be limited to group III-IV. However, recently it has been experimentally found that elements in group I and II have also the embrittling effect on Al alloys. For example, it was reported that as for Al-Mg alloy, Na atoms tend to segregate on Al boundary thus induces the embrittlement[7]. In this paper we therefore carry out the supercell calculation to investigate the effects of Na segregation on Al grain boundary, Because of the limited computational ability, we don’t consider the lattice vibration and only carry out the calculations at absolute 0 K. Hence the calculation results probably can’t compare directly with the experimental ones. However, our results still will be very helpful to understand the effects of segregated Na impurities.

COMPUTATIONAL METHOD The unit cell is constructed as shown in figure 1. There include 84 atoms in the unit cell. 

Z7.17.1 

Downloaded from https://www.cambridge.org/core

Data Loading...