Investigation of the effects of boron on Ni 3 Al grain boundaries by atomistic simulations
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The pronounced effect of grain boundaries on the physical properties of materials has motivated many studies with the aim of understanding the structure, energetics, and properties of grain boundaries. While significant experimental and theoretical progress1'2 has been made in understanding boundaries in pure systems, the understanding in alloy systems is much less developed.3'4 In this paper we present our recent results on atomistic simulations of grain boundaries in the ordered intermetallic compound Ni3Al. The brittle nature of this material and many other Ll 2 compounds arises from the marked propensity toward intergranular fracture, which contrasts with the ductile nature of single crystals of Ni3Al. While Ni3Al precipitate has been widely used as the •y'-strengthener in Ni-based superalloys,5 these brittle characteristics have precluded the use of Ni3Al itself as a structural material, despite the many attractive properties of the single crystal, such as low diffusivity and enhanced yield strength at high temperatures.6 The discovery that microalloying Ni3Al with small amounts of boron (B) significantly reduces the tendency for intergranular fracture7 has stimulated a renewed interest in intermetallic compounds.8'9 Extensive recent studies by Liu and coworkers10 have examined the role of boron in Ni3Al in detail and have established that boron segregates preferentially to the grain boundaries where it has been detected by Auger analysis.11 The influence of boron was also found to depend sensitively on the Ni-Al ratio. With stoichiometric or Al-rich Ni3Al the boron is ineffective in improving
ductility, while in Al-poor samples (between 23 and 25% Al) the boron dramatically improves the ductility of polycrystalline Ni3Al. In this study we examine the role of boron at grain boundaries in Ni3Al and compare the results of the present calculations with our earlier studies of boundaries in Ni, Al, and Ni3Al. Preliminary accounts have been given elsewhere.12"14 The atomistic simulations employ potentials we have developed1516 using an approach based on the embedded atom method.17 The potentials for Ni, Al, and Ni3Al were determined by fitting to known thermodynamic and other physical properties of the bulk materials.3'1516 For B-containing systems, where there is considerably less experimental data, we have used the results of Linear Muffin Tin Orbital (LMTO) density functional calculations18"20 to derive the potentials. The electronic structure calculations have been compared to available experimental results on Ni and Al-containing systems to assess the reliability of the LMTO approach. The paper is organized as follows. The methodology and results of the LMTO calculations are discussed in Sec. II, and the procedure for deriving the interatomic potentials is described in Sec. III. The remainder of the paper is devoted to the atomistic simulations of grain boundaries with and without B atoms present. II. ELECTRONIC STRUCTURE CALCULATIONS
All of the electronic calculations were done using the Linearized Muffin Ti
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