312 Prakt. Met. Sonderband 41 (2009) 
In the first case, monitoring of the process is discontinuous and is performed by microstructural 
examinations of partial or total internally oxidized samples by measuring the thickness of the 
internal and/or external scales. In the second case, the process is monitored continuously by 
controlling the gas consumption in the annealing atmosphere or by monitoring the changes of some 
physical properties of the samples (mass, dimensions, extracted latent heat etc.) by different 
methods of thermal analysis (Thermogravimetry, DTA, DSC, Thermodilatometry etc.). 
The microstructural changes during high-temperature oxidation in metallic materials have also 
significant influences on their electrical resistivity. Consequently, during the high-temperature 
oxidation, 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 by 
precipitation of oxides [9-13]. The in-situ monitoring of high-temperature oxidation of metallic 
materials with electrical resistance measurements was also the main topic of this research. The goal 
was to develop a non-destructive measurement method that enables identification and 
characterization of phenomena during high-temperature oxidation of metallic materials. With the 
novel measurement method we monitored the high-temperature oxidation of Ag-Sn (2 at. % Sn). 
and Cu-Al (1.25 at % Al) alloys at 800°C in air atmosphere. 
2 Experimental 
The high-temperature oxidation experiments with in-situ electrical resistance measurements were 
performed in a laboratory device consisting of a three-zone tubular electro-resistance furnace, 
vacuum system (two-stage rotary and a diffusion pump) and the measurement cell placed in the 
furnace inside the retort. The construction of the laboratory device and principle of the 
measurement method were described in detail in our previous publication (14]. Briefly, the 
measurement method is based on monitoring the electrical resistance changes during high- 
temperature oxidation by the four-probe method. It consists of the measurement cell where the four 
prismatic contacts made from pure platinum are fixed. The outer contacts (current contacts) are 
designed for charging the sample with current, and the inner contacts ( voltage contacts) enable the 
measurement of the voltage change in the sample. 
The experiments of high-temperature oxidation for both alloys were performed at 800°C and 
different oxidation times in the following three steps: (i) vacuuming the retort to 102 Pa, (ii) heating 
the sample to the desired temperature, (iii) annealing in air atmosphere (10° Pa). At the end of the 
internal oxidation experiments the retort was vacuumed again to he 102 Pa and the sample was 
cooled down to room temperature. 
The thickness of the external scale as well as subscale (internal oxidation zone) were examined on 
the transversal cross-section of metallographic samples with an optical microscope, Nikon Epiphot 
300, equipped with a system for digital quantitative image analysis (Olympus DB12 and software 
program Analysis). The metallographic preparations consisted of mechanical wet grinding down to 
5 um with SiC and polishing with diamond suspension (1um). 
3 Results and discussion 
The results of monitoring of high-temperature oxidation by in-situ electrical resistance 
measurements on wire samples of Ag-Sn and Cu-Al alloys in air atmosphere (10° Pa) at 800°C are 
presented in Fig. la and Fig. 1b. The normalized resistance curves, R(t), show the electrical 
resistance changes of the samples, and the temperature curves. T (t), indicate the actual temperature