314 Prakt. Met. Sonderband 41 (2009) 
the equilibrium pressure for oxidation of base metal (Cu) and causes the formation of the external 
layer of the base metal oxide CuO on the surface of the alloy in front of internal oxidation zone 
(Fig. 2b). Both reactions (formation of the external scale of Cu;O and formation of the internal 
oxidation zone) have influence on the change of the electrical resistivity of the sample during high- 
temperature oxidation (Fig. 1b). Namely, the formation of external oxide scale causes the increase 
of the electrical resistivity due to thinning of the conducting part of the metallic sample. On the 
other hand, the internal oxidation of alloys decreases resistivity due to removal of the solute atoms 
from the metal matrix with precipitation of oxides. From the diagram in Fig. 1b it is evident, that 
during high-temperature oxidation of the Cu-Al alloy the electrical resistance increases almost 
linearly in the beginning of the process. The increase of electrical resistance is also smaller as in the 
case of high-temperature oxidation of pure Cu at the same oxidation temperature due to internal 
oxidation of solute element (Al) [16]. At the terminal stage of oxidation the electrical resistance of 
the alloy increases exponentially, due to the thinning of the remaining cross-section of the sample. 
Finally, at the end of oxidation, the electrical resistance of the sample rises over all limits. due to 
non-conductibility of the totally oxidized sample (point B in Fig. 1b). 
and ul Tor am 
a) b) 
Fig. 2: Microstructure of high-temperature oxidized cylindrical samples of: (a) Ag-Sn (2 at.% Sn) alloy at T=800°C and 
t=5 min and (b) Cu-Al (1.25 at.% Al) alloy at T=800°C and t=240 min 
3.3 Determination of the internal oxidation Kinetics of Ag-Sn alloy 
The in-situ measurements of the electrical resistance during internal oxidation experiments were 
used for determination of the internal oxidation kinetics of Ag-Sn alloy. To accomplish this we have 
to transform measured electrical resistance into an instantaneous internally oxidized microstructure 
which is defined by the depth of the I0Z E&r(z). The change of electrical resistance of such 
composite during the internal oxidation is defined as parallel circuit of two resistors. Measuring the 
electrical resistance during R that gives the instantaneous electrical resistivity p(z) and knowing the 
electrical resistivity of the IOZ pjoz and the electrical resistivity of the unoxidized alloy pp, the 
growth of the IOZ Er(t) can be calculated as: