Prakt. Met. Sonderband 41 (2009) 313 
microstructural during experiments. The curves can be divided into two regions. The first region (left from point A) 
iickness of the presents the stage of heating of the sample in the vacuum up to the desired isothermal annealing 
ontinuously by temperature. The second region (right from point A) comprises the high-temperature oxidation of 
hanges of some selected alloying systems. 
.) by different EN a2 AU 1820 
2) 1,00 315 Te ‚815 
rials have also 0,95 . 
gh-temperature 0,90 310 N Jaro 
resistivity due 0,85 805 2 | 805 
ernal oxidation an I ee ” of Perera rns Tf) rts a0 
etal matrix by g 075 { 9 2 90 
ion of metallic ore 7% res 
arch. The goal wl 790 {70 
tification and ol 8 785 | A LL 785 
rials. Wi IR eel seme Rf enn nee fro Ny 
i" ose 050 ET ee eT 40 90 120 150 180 210 240 270 300 330 360 390 420 + oe 
+ [min] t [min] 
a) v) 
Fig. 1: Change of electrical resistance during high-temperature oxidation of: (a) Ag-Sn (2 at. % Sn) and (b) Cu-Al (1.25 
at. % Al) alloy at 800°C in air atmosphere 
urements were 
tance furnace, 3.1 Monitoring of internal oxidation of Ag-Sn alloy 
| placed in the 
nciple of the The process of internal oxidation of Ag-Sn alloy starts with the dissolution of oxygen into the 
|. Briefly, the surface layer of the alloy (point A in Fig. 1a). Dissolved oxygen diffuses inward through the metal 
during high- matrix and reacts at the advancing reaction front with a less-noble solute element (Sn). After the 
where the four solubility product is exceeded and the critical supersaturation for homogenous precipitation is 
t contacts) are attained, fine oxide particles SnO, precipitate from the solid solution. The process shown in Fig. 2a 
cts) enable the is named internal oxidation and the layer, composed of oxide particles precipitated in a metal matrix 
is called subscale or the internal oxidation zone (I0Z). Consequently, the electrical resistivity of the 
at 800°C and alloy strongly decreases due to removal of the solute Sn atoms from the Ag matrix (F ig. 1a). The 
Pa, (ii) heating decrease of electrical resistance is parabolic and therefore is highest at the start of internal oxidation 
the end of the (point A in Fig. 1a), due to the highest quantity of solute oxides precipitated in the unit of time from 
ie sample was 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 
> examined on solute atoms are oxidized, the internal oxidation of the alloy is completed and the new equilibrium 
Nikon Epiphot value of electrical resistance is established (point B in Fig. 1a). During further annealing of the alloy 
» and software in the reactive atmosphere the electrical resistance remains constant. This confirms that a 
1ding down to thermodynamically stable state was obtained with no further change of the microstructure. 
3.2 Monitoring of the simultaneous external and internal oxidation of Cu-Al alloy 
In the case of high-temperature oxidation of Cu-Al, the oxidation starts with dissolution of oxygen 
al resistance into the surface layer. When the concentration of the oxygen in the surface reaches the equilibrium 
) at 800°C are concentration for the oxidation of the less noble alloying element (AD), the reaction occurs and 
the electrical solute oxides (ALO3) start to precipitate from the solid solution - internal oxidation (Fig. 2b). With 
J temperature continuation of the process, the partial pressure of oxygen dissolved in the metal matrix increases to