232 Prakt. Met. Sonderband 38 (2006) 
element according to the activity of alloying element in the solid solution. In the internal sta 
oxidation the oxides of solute elements can be formed as fine discrete particles me 
precipitated in the metal matrix, as coarse particles preferentially precipitated on the of 
crystal defects or as continuous inner oxide films. The morphology and the size of the 
oxidation products, that determine the properties of materials, is strongly dependent on the anc 
proceeding of the reactions [1-9]. However, the method has many shortcomings needed to 
be overcome. For instance, the process of internal oxidation is very complex and therefore 
desired size and morphology of oxide particles throughout the matrix are not easy to 
control. 
In this article the kinetics and the microstructure evolution during internal oxidation of dilute 
silver alloy containing 2 at. % of tin was studied by several metallographic examinations 
and in-situ electrical resistance measurements in the air atmosphere and in the 
temperature range from 600°C to 800°C. 
2. EXPERIMENT 
The experimental alloy Ag-Sn (2 at.% Sn) was prepared by melting the pure Ag (99.99%) 
and Sn (99.99%) metals in an evacuated (10 Pa) quartz ampoule. The obtained cast bar 
was homogenized for 20 hours at 850°C and 10” Pa. After homogenization the bar was 
cold drawn into wire with diameter 0.5 mm. Short pieces of wire (150 mm) were annealed 
and thereby recrystallized (850°C and 102 Pa). Before the internal oxidation experiments 
the surfaces of the samples were polished with diamond paste and cleaned in the 
ultrasonic cleaner. 
The internal oxidation experiments were performed in the laboratory device (Fig. 1a) 
consisting of a three-zone tubular electro-resistance furnace, vacuum system (two-stage 
rotary and diffusion pump) and the retort placed in the furnace. The experiments of internal 
oxidation were performed at different oxidation temperatures and 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 the 10? Pa and the sample was cooled 
down to room temperature. 
The thicknesses of the 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 3. 
DB12 and software program Analysis). The metallographic preparations consisted of 
mechanical wet grinding down to 5 um with SiC and polishing with diamond suspension The 
(1um). Additionally, the characteristic of the precipitated oxide particles (morphology, size OX} 
and distribution as well as the shape of inner oxide bands) and the chemical composition me 
of the reaction products was examined with the scanning electron microscope JEOL 840A Sn. 
and FEI Sirion NC equipped with the EDX analyzer. hor 
The kinetics of internal oxidation was monitored continuously during experiments by in-situ sol 
electrical resistance measurements. The electrical resistance changes of the samples tote 
were acquired with the specially designed measurement cell that enables in-situ electrical 80( 
resistance measurements by the four probe method. It consists of the ceramic cylinder diff 
where the four prismatic contacts made from pure platinum are fixed (Fig. 1b). The outer sol 
contacts (current contacts) are designed for charging the sample with current and the inner (Fi 
contacts (voltage contacts) enable the measurement of the voltage change in the sample. mn 
The contacts are connected to the measurement instrument with Pt wires. The high- Col 
temperature resistant junction between the sample and the contacts is attained with (Fir 
springs and specially designed coupling holders made from high-temperature resistant pre