Phase Stability of Ni 2 Al 3 under Electron Irradiation
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PHASE STABILITY OF Ni 2AI 3 UNDER ELECTRON IRRADIATION E.A. KENIK* AND M.Division, NASTASI** *Metals and Ceramics Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376 **Materials Science and Technology Division, Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, NM 87545 ABSTRACT In-situ electron irradiations at 200, 300, and 1000 keV have been performed on Ni 2Al 3 at 100 and 300 K. At low doses, the Ni2A'3 partially disorders to a B2 structure with a lattice parameter of 0.284 nm, close to that extrapolated for 6i-NiAl at 60 at.% Al, the Ni2 AI 3 composition. The electron dose required to disorder the Ni 2AI3 decreased with increasing accelerating voltage and possibly with decreasing temperature. At high doses, precipitation of aluminum from the B2 matrix occurs, along with radiation-induced sputtering and radiationinduced segregation. Lattice parameter changes for the B2 structure observed during irradiation are explained in terms of the compositional dependence of the lattice parameter of /8-NiAl. The B2 structure remains stable to a dose of 6 x 1028 electrons/m 2 , with no amorphization detected. INTRODUCTION Intermetallic compounds exhibit several types of instability under irradiation, including point defect clustering, chemical disorderingl-3 , dissolution or precipitation of second phases 3 ,4 and amorphization 5- 7. The current electron irradiations of Ni 2Al5 are part of a continuing study of radiation damage of aluminum-nickel intermetallic compounds 8'-. Under Xe or Ne ion irradiation at temperatures from 77 to 400 K, Ni 2AI3 disordered to the B2 structure, with the possibility of some amorphization occurring at the lowest temperatures8 . On the other hand, NiAI3 was amorphized by either ion or electron irradiation near 100 K, while showing little or no indication of prior chemical disordering9 . In the current study, which is a continuation of earlier work10 , the stability of Ni 2 AI3 under electron irradiation is evaluated. From the comparison of the radiation response of these intermetallic compounds, some insight may be gained into the damage mechanisms operative under irradiation and into the factors which determine the stability of intermetallic compounds under irradiation. EXPERIMENTAL Thin (-35 nm) multilayered films of alternating aluminum and nickel were electron beam deposited onto a NaCI substrate. The substrate was dissolved in deionized water and the Ni/Al film was mounted on 3 mm molybdenum grids. The films were vacuum annealed for I to 2 h at 623 K to allow interdiffusion between the layers. Rutherford backscattering was employed to characterize the film composition and homogeneity. Electron diffraction indicated that the films were fine-grained (-20 nim) Ni 2Als, the prototype for the trigonal D5 13 structure, with no appreciable amount of other phases. The specimens were irradiated with 1000 keV electrons in a Hitachi HU-1000B microscope or with 200-300 keV electrons in Philips EM430 and CM30 microscopes. These microscopes are equipped for nominal room temperature or low t
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