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HAMISH FRASER*, NATALIA TYMIAK***, AND WILLIAM GERBERICH *Dept. MSE, The Ohio State University, 2041 College Rd., Columbus, OH 43210-1179, anderson. I @osu.edu. "**Materials B 164 NIST 100 Bureau Dr, Stop 8554 Gaithersburg, MD 20899-8554. ***Dept. of Chemical Eng. and Materials Science, University of Minnesota, Minneapolis. MN.

ABSTRACT This manuscript discusses the morphological instability observed when multilayered samples with alternating layers of y-Ni(AI)/y-Ni 3AI are exposed to 800C for approximately 100 hours. Samples with 20nm/20nm or 120nm/I20nm layer thickness and or crystal orientation to the interface normal were tested. Pinching off of layers is strongly affected by crystal orientation and layer thickness. Corresponding modeling suggests that the stability of this system is sensitive to fluctuations in the volume fraction of the two phases, the aspect ratio of columnar grains in the layers, and whether coherent or semi-coherent interfaces are present. INTRODUCTION Nanolayered materials consisting of alternating A/B type layers of metals exhibit large

values of hardness that increase monotonically as individual layer thickness is decreased [1]. The large resistance to plastic deformation has been explained by considering dislocation motion that is confined to small volumes of material by numerous interfaces [e.g., 2]. However, if such materials are to be used at elevated temperature, a better understanding of the morphological stability of the layered structure at high temperature is needed. The system studied is a multilayered version of y-Ni(Al)/y-Ni 3 Al. These phases form the basis for Ni-base superalloys that are used for turbine blade applications and that derive their strength from a fine dispersion of cuboidal Y' precipitates in a 7 matrix. A morphological transition from a particulate to a lamellar, or rafted, microstructure with 180 nm layer thickness

has been observed when NASAIR 100 superalloy is stressed at 140MPa for 50hrs at 1000C [3] and interfacial misfit dislocations appear to form as part of the process [4]. However, the creep properties of rafted material compared to the particulate morphology appear to be mixed [5,3,6]. This manuscript discusses the morphological stability of y-Ni(AI)/y-Ni 3A1 multilayers that are deposited by sputtering as to control both layer thickness and crystallographic orientation of the layers. The experimental observations from heating the samples to 800C for 101 hours are discussed in light of two models, which suggest regimes of columnar grain aspect ratio, interfacial energy, and volume fraction for which the multilayers are predicted to be stable.

EXPERIMENTAL PROCEDURE Sample fabrication Multilayered y-Ni(AI)/Y-Ni 3 AI thin films were produced using dc magnetron sputtering. Prior to sputtering, a vacuum base pressure of less than 5 x 10-8 torr was produced and during sputtering, Argon pressure was maintained at 2 x 10-3 torr. Prior to introduction into the chamber, the Argon gas was gettered through a titanium sponge at 800C to increase purity. Cl