N Metallographic examination of the ribbons after partial IO in the solid state at temperatures below
a Prepared by the eutectic temperature (Tio<Tg) revealed the microstructure consisting of internal oxidation zone
nn A bars (200 (I0Z) and unoxidiezd two phase zone (UTPZ) (Fig. 2). Closer examinations of I0Z by SEM and
"Tg Technique STEM revealed two types of oxide particles in this zone: very fine oxide particles (10 to 50 nm in
! ak With an size) that are homogeneously distributed through the volume of the matrix and coarser oxide
4-13 bars onto particles with the size which is close to the size of the intermetallic particles in the UTPZ.
US Mbbons about Additionally, as it was identified, not only the size but also the number of coarse oxide particles in
200 experiments the unit volume and their position in the microstructure correspond to those of the secondary phase
re equal to the particles. Therefore, the coarse oxide particles had to be formed by direct oxidation of the Cu-RE
ICked In a mixture intermetallics. On the other hand, the fine oxide particles can arise whether by precipitation of the
impoule oxides from a supersaturated solid solution or by dissolution of intermetallic particles ahead of the
sed in detail using IO front and oxidation of the alloying element from the solid solution. According to the facts, that
40 A), scanning the fine oxide particles were found throughout the entire volume of the ribbons and that the
croanaivsis (Link oxidation from the supersaturated solution depends on the ratio between the velocity of the 10 front
13 mire of 60 and the velocity of decomposition of the solution, most of these particles were probably formed by
din NELOH-HLO, the second mechanism. Because of the Ostwald ripening ahead of the IO front the oxidized
wre prepared. by intermetallic particles are smaller closer to the surface than in the middle of the ribbons. At all 10
experiments in the solid state the oxide particles in the middle of the ribbons became too coarse for
the effective dispersion strengthening.
t features as the
ons of both alloys
ar grains and zone
e ribbons (Fig. 1).
olid solution (tg
2 solid solution
mnar zone. [nthe
top of the coarse
valloy A and 250
was reduced to 3
quent 10
Fig. 2:Optical micrograph of the alloy B after partial IO at 873 K; region A: IOZ; region B: UTPZ.
The size of the oxide particles in the microstructure of internally oxidized ribbons depends also on
the alloy composition and on the IO temperature. At lower IO temperatures (873 and 973 K) the
particles are smaller in the alloy B while at 1073 K in the alloy A. Such behavior can be explained
with the higher diffusivity of Er in Cu (atomic radius of erbium is lower than that of ytterbium -
Re:=0.175 nm, Ryp=0.193 nm) and with the fact that the velocity of coarsening of the Cu-RE
intermetallic particles increases much faster with temperature than the velocity of internal
oxidation. Higher diffusivity of Er enables at lower IO temperatures higher portion of dissolving
intermetallic particles, while at higher IO temperature it causes their faster coarsening ahead of the
10 front. Therefore, at the 10 process in the solid state at the highest IO temperature (1073 K), the
intermetallic particles coarsen before the oxidation front so rapidly, that the oxidized particles in the
middle of the ribbons reach the size of approximately 1um (Fig. 3).
At the heat treatments above the eutectic temperature of both alloys the intermetallic particles in the
| UTPZ melt together with the nearest, surrounding Acu into a liquid with a composition
247
