Sources of thermally generated vacancies in single-crystal and polycrystalline gold

Sources of vacancies play an important role in phenomena such as sintering, the Kirkendall effect, high temperature creep, polygonization and many other diffusion controlled processes. In spite of their importance very little is known about the fundamental properties of vacancy sources. For example, it is not known if a vacancy can form with the same ease on a surface, a grain boundary, a sub-boundary and a dislocation. Also, the energy to form a jog on a dislocation, and the rate at which the dislocation climbs are at best poorly understood. Of all the potential vacancy sources the dislocation is the one most studied and least understood. The electrical pulsing work of Jackson (1960), Lund (1964) and Koehler (1960, 1964) pioneered the study of vacan??cy generation at high temperatures, when a lattice is subjected to a sudden and large subsaturation of vacancies. Their experiments indicated that vacancies are being produced at dislocations, but the work is hampered by a poor knowledge of the specimen structure (e.g. grain size, subgrain size and dislocation density). Thus their interpretation and conclusions regarding the operative vacancy sources are uncertain. The specimen structure is particularly important if one wishes to observe only the high temperature climb (T/Tm greater than 0.5) of dislocations. [T and Tm are the temperature and melting temperature respectively (in units of ??K).] For example Barnes (1960) has shown that the free surfaces and grain boundaries in copper are excellent sources of thermally generated vacancies, though he found no definitive evidence for dislocation climb. In view of the existing situation an investigation was undertaken to study vacancy generation in specimens with a controlled structure. A new technique is developed for thermally pulsing thick single crystals and distinguishing internally produced vacancies from vacancies generated at the free surfaces, In addition the Lund-Koehler (1964) electrical pulsing technique is used and extended to study generation (formation) kinetics in polycrystalline specimens whose dislocation density, subgrain size and grain size are measured. Analyses are performed, based on the present experimental data, to account for possible dislocation shortcircuiting and distinguishing between vacancy diffusion limited climb and vacancy production limited dislocation climb. Gold is the logical choice for the present study since: (1) the high temperature monovacancy diffusion coefficient and equilibrium concentration are known (Simmons and Balluffi, 1962), (2) the specimens are readily thinned for observation by transmission electron microscopy, (3) previous kinetic studies are available for a comparison with the data obtained, (4) specimens are not contaminated by heating and cooling in air