Theoretical Studies of Ni 3 Al and Nial with Impurities
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THEORETICAL STUDIES OF NI 3 AL AND NIAL WITH IMPURITIES S. P. Chen, A. F. Voter, A. M. Boring, R. C. Albers, and P. J. Hay Los Alamos National Laboratory, Los Alamos, New Mexico 87545 INTRODUCTION Intermetallic compounds have been extensively studied because of their superior strength, low creep rate, and high melting point [1,2]. However, room temperature ductility for the L1 2 and B2 phases are a continuing problem. Both L1 2 Ni 3 Al [3,4] and B2 NiAl [5,6] exhibit an intergranular fracture mode. Understanding grain boundaries in these materials is of particular importance since intergranular fracture limits the applicability of these otherwise promising materials. In an effort to understand the fracture mechanism, we have used embedded atom potentials [7] to study the properties of Ni 3 Al [8,9,10] and NiAl [11]. We also consider the effect of boron, sulfur, and nickel segregation on the strength of grain boundaries in Ni3Al and NiAl. Many of the results presented here appear in literature elsewhere [8,9,10,11]. COMPUTATIONAL METHOD The simulations presented here employed embedded atom [12] descriptions of Ni [7], Al [7], B [13] and S [14]. In order to obtain boron-metal (or sulfur-metal) potentials, the potentials were fitted to data obtained from linearized muffin tin orbital (LMTO) calculations [13] on the hypothetical structures fcc B, B2 NiB, B2 AlB, L1 2 Al B and L1 2 Ni 3 B (similarily for S [14]). Therefore, the interactions of Soron or sulfur with Ni and Al are derived entirely from theory (electronic calculations). The GB simulation results were obtained by first generating ideal symmetric tilt [001] grain boundaries. The resulting cluster of atoms was allowed to relax via a steepest descent, energy-minimization algorithm. The grain boundary (and bulk) cohesive properties were then calculated in two different ways: frozen and slow straining. In the frozen method, the GB structure is first optimized, and then the two grains are pulled apart while keeping all the atomic positions in each grain frozen. The quantity we focus on is max_,the maximum stress required to separate the GB. This approach has the advantage that one can choose where to cleave the system. One result is that values for amax can occur that are greater than the a ax for the surrounding matrix. The disadvantage is that the aMoms near the cleavage plane are not allowed to respond. In the slow straining method, the optimized GB is pulled apart with clamps at four lattice constants away on either side, allowing the system to relax to the lowest possible energy at increments of 2% in the successively increasing strain. The slow straining approach allows the system to respond to the strain and, in principle, the break occurs at the weakest point, possibly away from the boundary. RESULTS OF L1 2 NI 3 AL SIMULATIONS Mat. Res. Soc. Symp. Proc. Vol. 133. ",1989Materials Research Society
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Symmetric [001] tilt boundaries in Ni 3 Al have three unique grain boundary compositions (when the two grains are perfect crystals). The grain boundary com
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