Irradiation-Enhanced Second-Phase Precipitation in Zr-Fe Nanocrystalline Thin Films
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0908-OO04-04.1
Irradiation-Enhanced Second-Phase Precipitation in Zr-Fe Nanocrystalline Thin Films D. Kaoumi1, A. T. Motta1, R. C. Birtcher2 Department of Mechanical and Nuclear Engineering, Pennsylvania State University, PA, USA 2 Materials Science Division, Argonne National Laboratory, Argonne, IL, USA
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ABSTRACT In situ observations in a transmission electron microscope (TEM) were used to study ion-beam enhancement of second-phase precipitation in Zr-Fe nanocrystalline thin films. The freestanding films were prepared by co-sputter deposition with an Fe content of 1.2 at%. TEM diffraction analysis showed that only the hcp Zr crystal structure was present in the as-deposited films. No second phases were detected, although Rutherford Backscattering Spectroscopy (RBS) confirmed a Fe content beyond the solubility limit of Fe in Zr (of the order of ppm). This means the thin films were Zr solid solutions supersaturated with Fe. Heat treatment in the absence of irradiation was observed to cause precipitation of the Zr2Fe intermetallic phase, but only above 673 K. The same second-phase precipitation can occur at lower temperatures in the presence of ion irradiation. Samples were irradiated in-situ at the Intermediate Voltage Electron Microscope (IVEM) at Argonne National Laboratory with Kr ions to fluences in excess of 1016 ion/cm2, at temperatures ranging from 50 to 573 K. Second phase precipitation was detected by electron diffraction patterns and by dark field imaging comparing regions exposed to the beam with regions protected from the beam by the TEM support grid. Precipitation of Zr2Fe intermetallic phase was observed at all irradiating temperatures above room temperature. In the bulk, this phase is thermodynamically metastable in the range of temperatures investigated (relative to the orthorhombic Zr3Fe intermetallic phase). The kinetics of the irradiation-enhanced second-phase precipitation was followed by recording the diffraction patterns at regular intervals. The dose to precipitation was found to decrease with increasing irradiation temperature.
INTRODUCTION The binary Zr-Fe alloy system has been the focus of a number of studies because of its technological importance especially in the nuclear industry where Fe-containing Zr alloys are used as fuel cladding material. The system has also received attention because the Zr-Fe binary alloys are excellent glass formers over a wide range of compositions. Studies of the crystallization of Zr-Fe glasses showed that the immediate products of crystallization are often metastable phases which would not have been predicted from the Zr-Fe phase diagram [1]. In general, the problem of irradiation induced or irradiation enhanced precipitation, and more generally phase formation, has received both experimental and theoretical attention [2-5]. One of the problems with studying irradiation induced phase transformations is that the kinetics are not usually accessible to the experimentalist and thus the mechanisms of phase transformation are difficult to determine. In this st
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