Prakt. Met. Sonderband 38 (2006) 231 
MICROSTRUCTURAL CHANGES DURING INTERNAL 
Titanium OXIDATION OF A Ag-Sn ALLOY 
TIMET M. Bruncko®, I. Anzel*, A. C. KneissI** 
> of the * University of Maribor, Slovenia 
1g 2006, * University of Leoben, Austria 
tructural 
. Mater. 
viour of Abstract 
23 Electrical contact materials are expected to combine improved mechanical and physical 
| properties with good corrosion resistance at increased temperatures. One of the most 
auer, : ; : — : ; . 
hand widely used groups of contact materials are internally oxidized dispersion hardened silver 
alloys. The principle of their production involves selective oxidation of less noble solute 
elements (usually Mg, Al, Sn, Cd and Zn) forming the fine dispersion of oxide particles in 
stic flow the solvent silver matrix. In this study the internal oxidation of dilute silver alloy containing 
9 (2001) 2 at. % of tin was extensively investigated. The morphological evolution of the alloy and 
the internal oxidation kinetics were determined 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. The results of experiments and their analyses 
show that the microstructural changes during internal oxidation of Ag-Sn alloy are strongly 
dependent on the annealing temperature. At the highest temperatures the perfectly 
dispersive distribution of the oxide particles SnO; in the silver matrix was found, while at 
lower temperatures the formation of inner oxide bands and distribution of oxide particles 
along the grain boundaries is predominate. 
1. INTRODUCTION 
Annealing of certain dilute alloys in reactive atmosphere containing species such as 
oxygen, nitrogen, carbon or sulfur results in the selective reaction of one of these elements 
with the less noble solute elements from the alloy and can lead to a fine dispersion of 
precipitated particles in the solvent matrix. This phenomenon is known under generic 
name internal oxidation and is an important method for preparation of oxide dispersion 
strengthened metal matrix composites with improved mechanical properties at elevated 
temperatures [1, 2]. The internal oxidation is well-understood for single phase solid 
solutions. If the oxygen pressure is low enough (the partial pressure of oxygen in the 
atmosphere must be equal or lower than the equilibrium pressure for oxidation of base 
metal — matrix) it is possible to obtain only the subscale without formation of external layer 
of the base metal oxide. In this case, the metal surface remains bright and clean while 
oxidation has occurred in fact within the base metal. The border of the internal oxidation 
zone (I0Z) presents the reaction (oxidation) front. This is the position of the latest oxide 
particles precipitation. In the surface layer dissolved oxygen constantly diffuses inward 
through the 10Z and provides the necessary oxygen concentration for precipitation of the 
oxide particles at the reaction front. The oxygen concentration at the front must be equal 
(or higher) compared to dissociation (decomposition) pressure for the oxide of alloying