236 Prakt. Met. Sonderband 38 (2006) 
resistance changes of the samples, and the temperature curve, T(t), indicates the actual 
temperature during experiment. The curves can be divided into two regions. The first 
region (left from the point A) presents the stage of heating of the sample in the vacuum up 
to the desired isothermal annealing temperature. The second region (right from the point 
A) comprises the internal oxidation process. The decrease of electrical resistance is 
parabolic and it is highest at the start of internal oxidation, due to the highest quantity of 
solute oxides precipitated in the unit of time from the solid solution. Later the change of the 
electrical resistance is smaller, because of larger diffusion paths of oxygen to the reaction 
front and smaller length of the reaction front. Finally, when the last solute atoms are 
oxidized, the internal oxidation of the alloy is completed and the new equilibrium value of 
electrical resistance is re-established (point B on the resistance curve). During further 
annealing of the alloy in the reactive atmosphere the electrical resistance remains 
constant. This confirms that a thermodynamically stable state was obtained at point B with 
no further change in the microstructure. 
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Fig. 4: Microstructure of completely internally oxidized Ag-Sn (2 at.% Sn) alloy at T=800°C 
and t=45min; a) D,=300 nm, b) D,=500 nm, (D; - The mean diameter of rounded particles) 
Tum 
Fig. 5: Microstructure of completely internally oxidized Ag-Sn (2 at.% Sn) alloy at T=700°C 
and t=480min 
al D)