Hardness of ion-implanted Ni 3 Al and TiAl
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Hardness of ion-implanted Ni3Al and TiAl Gary S. Wasa> Institut fiir Schicht-und Ionentechnik, Kemforschungsanlage Julich GmbH, D-5170, Julich, Germany (Received 26 November 1990; accepted 12 April 1991)
Experiments were conducted to determine the effect of various implanted species on the hardness of single crystal Ni3Al (Ta, Al, B) and polycrystalline TiAl (Ta, N, B). Implantations were conducted to yield about 7 at. % Ta under the peak and ~30% Al, N, or B under the peak in the respective targets. Hardness was measured using the Nanoindenter. Results showed that Ta+-implanted Ni 3 Al softens due to disordering, and subsequent heat treating results in strengthening due to preferential occupation of Al lattice sites. However, B + and Al+ implantations result in increases in the hardness, while heat treating returns the surface hardness to bulk values. Ta+, N + , and B + implantations into TiAl all result in hardening with N+ implantation producing the greatest hardening by a factor of ~1.9. The probable mechanism is solid solution strengthening.
The addition of refractory elements (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 shown1 to increase the room temperature axial flow stress of [111] or [001] oriented Ni3Al by a factor of 4. 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. 23 Others have proposed that the changes in strength more likely result from changes in cross-slip due to the anisotropy of the APB energies on (111) and (010) planes, the main driving force for (111) to (010) cross slip.4"6 Similarly, it is well known that excess Al or Ni in the Ni3Al structure also results in a higher flow stress, and that excess Al has a much more marked effect than does excess Ni.7 The addition of B to polycrystalline Ni3Al increases strength significantly, and the larger intercept,
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