Atomistic computer study on Mg segregation in the Ni 3 Al grain boundary
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Atomistic computer study on Mg segregation in the Ni3 Al grain boundary Bingyao Jiang, Xianghuai Liu, and Shichang Zou Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, 865 Changning Road, 200050 Shanghai, China
Jian Sun Department of Materials, Shanghai Jiaotong University, 200030 Shanghai, China
Jian Wang Department of Materials, Fudan University, 200433 Shanghai, China (Received 28 March 1997; accepted 29 December 1997)
The embedded atom method (EAM) was applied to calculate the energy on Mg doping in polycrystalline Ni3 Al. The EAM predicted the energy of Mg in Al site in grain boundary is lower than that of Mg in Ni site and much lower than that of Mg in Al or Ni site in bulk and in free surface. It means that Mg would segregate to grain boundary rather than bulk and free surface and Mg will favor to be the substitute of Al rather than of Ni in grain boundary. These results were consistent with the experiments that Mg segregated to grain boundaries with Al depletion and Ni enrichment.
I. INTRODUCTION
It is well known that the intermetallic compound Ni3 Al has high strength at elevated temperature with a positive temperature dependence over a certain temperature range.1 This peculiar property, however, was not sufficient to make it useful for high temperature because of its brittleness. It was found that the addition of a small amount of B, Cr, Zr, or Mg to Ni3 Al remarkably improved its ductility,2–4 but few publications are concerned with its aqueous corrosion behavior.5,6 In recent years, it was found that the corrosion resistance of the intermetallic compound Ni3 Al(0.1B) implanted with Mg ion, measured by multisweep cyclic voltammeter and optical microscopy, was much improved in 1n H2 SO4 .7 The morphology of the unimplanted specimens after treatment in 1n H2 SO4 solution exhibited serious corrosion at grain boundary and pitting corrosion at grain surface, but in Mg-implanted specimens only slight corrosion appeared. The content of Mg measured by electron probe micro analysis (EPMA) demonstrated that the implanted Mg atoms were mostly segregated to grain boundaries. Component profiles across grain boundaries measured by electron dispersive spectrum (EDS) showed that the implanted Mg segregation was present with Al depletion and Ni enrichment. It has been concluded that Mg ion implantation has a beneficial effect of reducing corrosion of polycrystalline Ni3 Al(0.1B) in aqueous solution because of its improvement of the passivation. The embedded atom method (EAM)8,9 was used to calculate the energy of Mg substitution in Al or Ni site in free surface, bulk, and grain boundary. Mg has a very large size difference compared to Ni and Al (aMg J. Mater. Res., Vol. 13, No. 7, Jul 1998
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0.160 nm, aNi 0.125 nm, aAl 0.143 nm) and thus should produce large lattice distortion. Monte Carlo method was used to relax the structure of grain to the minimum energy configuration. Embe
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