Effect of Cascade Remnants on Freely Migrating Defects in Cu -1 % Au Alloys
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ABSTRACT The effects of cascade remnants on Freely Migrating Defects (FMD) were studied by measuring Radiation-Induced Segregation (RIS) in Cu- l%Au at 4000 C during simultaneous irradiation with 1.5-MeV He and (400-800)-keV heavy ions (Ne, Ar or Cu). The large RIS observed during 1.5MeV He-only irradiation was dramatically suppressed under simultaneous heavy ion irradiation. For Cu simultaneous irradiation, the suppression disappeared immediately after the Cu irradiation ceased, while for simultaneous inert gas (Ne or Ar) irradiation, the suppression persisted after the ion beam was turned off. These results demonstrate that the displacement cascades created by heavy ions introduce additional annihilation sites, which reduce the steady-state FMD concentrations. As the cascade remnants produced by Cu ions are thermally unstable at 400 0 C, the RIS suppression occurs only during simultaneous irradiation. On the other hand, the inert gas atoms which accumulate in the specimen apparently stabilize the cascade remnants, allowing the suppression to persist.
INTRODUCTION The concept of freely migrating defects (FMD), i.e., those vacancy and interstitial defects which survive energetic displacement events and become free to migrate, has proved important for understanding the microstructural development of materials irradiated at elevated temperatures. During the past decade, many experimental and computer simulation studies have focused on determining the production rate of FMD. The experimental results show that the production rate of FMD depends strongly upon the energy spectrum of the primary knock-on atoms[I-3). At high recoil energies, the production rate of FMD was found to be only a few per cent of the modified Kinchin-Pease value. The dramatically small production rate of FMD was initially believed to result from intracascade recombination and clustering of defectsf 1]. However, several computer simulation studies have reported values of 10-30% of the modified Kinchin-Pease value for the fraction of defects which survive the intracascade defect annihilation[4,5]. Recently, Wiedersich has presented simple rate-theory calculations which suggest that the cascade remnants, i. e., small interstitial and vacancy clusters generated within a single cascade event may serve as annihilation sites for FMD generated by other cascades[6). He pointed out that such intercacade interactions might explain the apparent discrepancy in FMD production rates between the experimental values and those obtained from computer simulation. In this paper, we report experimental results which indeed demonstrate that intercascade annihilation at cascade remnants reduces the FMD production rate substantially[7].
EXPERIMENTAL PROCEDURE To study the effect of cascade remnants on the FMD production rate, we employed Rutherford Backscattering Spectrometry (RBS) to measure RIS in Cu-1% Au during simultaneous irradiation with 1.5-MeV He and (400-800)-keV Cu, Ne or Ar. The heavier ions generate primarily energetic displacement cascades; the 1.5-MeV He
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