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Warren Oliver Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 2 November 1990; accepted 7 February 1991)

Implantation of Ta into single crystal Ni3Al was conducted to determine the degree of surface hardening in monolithic alloys in relation to its lattice location. Ta was implanted at 400 keV to doses of 0.07, 0.36, and 2.52 x 1016 c m " 2 along the [100] axis of a [100] crystal of Ni3Al at room temperature. Composition versus depth profiles were determined by RBS, and lattice location of Ta was determined by channeling angular yield scans about the [100] axis. The hardness of the surface was measured by ultra-low load indentation. Results show that implantation softens the surface and that the Ta is randomly distributed between Ni and Al sites. Annealing at 1000 °C/1 h significantly reduces the damage and causes preferential occupation of Al sites by Ta, resulting in a slight increase in surface hardness. Further annealing at 1200 °C/0.25 h increases the surface hardness substantially and increases occupation of Al lattice sites to roughly 84%. Results are consistent with a model in which the as-implanted surface is softened by disordering, and -subsequent diffusion of Ta to Al sites during thermal treatment causes hardening of the surface.

I. INTRODUCTION The addition of refractories (Nb, Hf, and Ta) to single crystal Ni3Al has been found to cause substantial strengthening. In particular, the addition of 4.5 at. % Ta has been shown to increase the room temperature axial flow stress of [111] or [001] oriented Ni3Al by a factor of 4.1'2 Studies on the strengthening effect of additions to Ni3Al have suggested that it is caused by solid solution strengthening which depends on the atomic misfit parameter and modulus mismatch.3'4 Others have proposed that the changes in strength more likely result from changes in cross-slip due to the anisotropy of the antiphase boundary (APB) energies on (111) and (010) planes, the main driving force for (111) to (010) crossslip.5"7 In either case, the effect is likely to depend on whether the ternary element substitutes for Ni or Al. Numerous experiments have been conducted to directly determine the lattice location of Hf or Ta on the Ni3Al lattice. Atom probe measurements for Hf, Co, and Fe indicate that while Co favors Ni sites, Hf and Fe preferentially substitute on the Al sublattice.8'9 These results are in good agreement with zone axis electron channeling microanalysis results of Bentley and Miller10'11 and with that predicted by the ternary phase diagram. The results disagree with those from perturbed angular correlation (PAC)12 and ion channeling experiments for Hf,13 both of which show that the Hf preferentially resides on the Ni ^Permanent address: Department of Nuclear Engineering, University of Michigan, Ann Arbor, Michigan 48109.

1200 http://journals.cambridge.org

J. Mater. Res., Vol. 6, No. 6, Jun 1991

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sites. In addition, Lin et al.14 used ion channeling with Rutherford backscattering spectr