MOLECULAR-DYNAMICS SIMULATION OF TWEED STRUCTURE AND THE OMEGA-PHASE IN NI-AL

Using computer molecular dynamics (CMD) we have studied the occurrence of tweed structure and of the omega phase in Ni62.5Al37.5. Tweed is a mottled structure that is observed prior to transformations such as martensitic transformations. In some cases, it has been found to accompany omega-phase formation. The characteristic features of tweed have been simulated. To achieve the 62.5-37.5 composition we started with a 50%-50% stoichiometry in which some aluminum atoms were replaced by nickel. Two different lattices were used: one where the nickel atoms were distributed at random on the aluminum sublattice (referred to as the disordered B2 array) and one where they were arranged in an ordered manner (ordered supercell bcc). To describe the forces that interact between the atoms an embedded-atom method was chosen with an interatomic potential derived by Voter and Chen [1] [in Characterization of Defects in Materials, edited by R. W. Siegal, J. R. Weertman, and R. Sinclair, MRS Symposia Proceedings No. 82 (Materials Research Society, Pittsburgh, 1987)], which has been designed to fit some properties of Ni3Al, and some of NiAl, while maintaining good behavior for compositions in between. In the case of the disordered B2 array, diffuse streakings were observed in the diffraction patterns of the structures obtained from the CMD simulations, mainly along the [110]* directions but some also along the [112]* direction. The [112]* streaking is usually associated with the formation of the omega phase. However, with the ordered array, virtually no streaking appeared, strongly suggesting that heterogeneous defects (such as compositional disorder) are necessary to induce both tweed structure and the omega phase. The main type of streaking is typically seen on diffraction patterns of materials displaying tweed. By plotting the positions of the atoms and their displacements after the CMD runs we also noticed [110] {100} shears, again in the disordered lattice. The influence of temperature on tweed, and its evolution versus time was then studied. It was found that tweed decreases in intensity when the temperature increases as is also observed experimentally. It was also found that tweed has two different regimes: a static regime at low temperature and a dynamic regime at high temperature, in agreement with inelastic-scattering experiments.