Münster 1999 – scientific programme

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M: Metallphysik

M 17: Diffusion II

M 17.2: Talk

Tuesday, March 23, 1999, 15:30–15:45, S 9

Tracer Self - Diffusion of Ni in NiAl: I. Influence of Temperature and Composition — •St. Frank¹, S. Divinski¹, Chr. Herzig¹, and U. S"odervall² — ¹Institut f"ur Metallforschung, Westf"alische Wilhelms-Universit"at M"unster, Wilhelm-Klemm-Stra"se 10, D–48149 M"unster — ²Department of Microelectronics and Nanoscience, Chalmers University of Technology, S–41296 G"oteborg, SWEDEN

The ordered B2-phase NiAl exhibits the triple defect structure, where excess Ni atoms occupy Al lattice sites in Ni-rich NiAl alloys and constitutional Ni vacancies are formed in Al-rich compounds. If the ordered lattice is to be retained, a random motion of a vacancy is not possible and it is necessary to invoke more complex diffusion mechansims.
In the present work the Ni self-diffusion in well defined NiAl single crystals was investigated experimentally for a set of compositions from 46.8 to 56.6 at.%Ni in a wide temperature range from about 1050 K to 1500 K. A special pre-diffusion treatment was carried out to avoid an effect of ‘frozen‘ vacancies. Two different techniques were applied to determine the diffusion coefficients, namely tracer experiments by conventional serial sectioning using the ⁶³Ni radiotracer and SIMS analysis using highly enriched ⁶⁴Ni. The effect of composition, i.e. constitutional defects, on diffusion behaviour in NiAl alloys was systematically investigated. In contrast to the previous measurements by Hancock and McDonnell (1971) we observed no remarkable increase of the Ni diffusivity for Al-rich NiAl alloys. Ni-rich compositions, on the other hand, show a pronounced increase of the diffusion coefficients with increasing Ni-content. Existing theoretical considerations based on the earlier data are, however, unable to explain the present results. Therefore, a new detailed analysis of suitable self-diffusion mechanisms of Ni in NiAl is required.