278 Prakt. Met. Sonderband 41 (2009) 
Oxide dispersion-strengthened gold alloys can be prepared by mechanical alloying for bulk material 
and by internal oxidation for foils and superfine wires. Mechanical alloying is a solid-state high- 
energy ball milling process consisting of repeated deformation, welding, and fracturing of the 
original components. However, long processing time for obtaining the appropriate size and uniform 
distribution of the second phase particles limits the usability of this process. Another producing 
possibility - the internal oxidation process - is based on the preferential oxidation of the solute 
element in a dilute solid solution with oxygen diffusing in from the surface. The process is 
particularly attractive because it can result in the formation of very fine oxide dispersion throughout 
the matrix. Unfortunately, internal oxidation is not possible in Au-RE alloys where a low solubility 
of oxygen and RE in Au prevents a direct precipitation of RE oxides from the solid solution. In the 
cast microstructure the RE occurs as coarse intermetallic segregates in the interdendritic space and 
because of direct oxidation a high inhomogeneous oxide distribution is obtained after internal 
oxidation. To attain a fine dispersion of the oxide particles in Au—RE alloys the coarse intermetallic 
segregate has to be refined to a much finer scale. For this we could use the extensive deformation of 
the cast samples, or rapid solidification [5, 6]. The microstructures that develop during rapid 
solidification have a great influence on the success of the subsequent internal oxidation process [7] 
and depend on the solidification conditions and the alloy composition. 
In the present paper, the influence of the ribbon thickness (i. e. the average cooling rate) on the 
microstructure and segregation in the melt spun Au-0.5wt%La is reported. The solidification 
conditions developed during the melt spinning experiments, leading to different microstructural 
morphologies, were also analysed. The results of this study were used to determine the primary 
mode of the solidification process at the rapid solidification of the selected Au-La alloy and to 
identify the conditions that can lead to uniformly distributed, fine intermetallic particles in the Au 
matrix. 
2. Experimental details 
Small quantity of the experimental Au-0.5wt.% La alloy was prepared by induction vacuum melting 
using high purity elements as starting materials. A little higher quantity of lanthanum 
(corresponding to the content of 0.6wt.%) was taken for the preparation of the alloy, because of 
expected surface oxidizing of lanthanum pieces in the first stage of heating to the melting 
temperature. The melt was cast into ingots 10mm in diameter and 100 mm in height. The lanthanum 
contents of the ingots were determined by wet chemical analysis to 0.57wt.% La. The rapid 
solidification experiments were performed by Chill-Block Melt-Spinning technique. Smaller 
sections of cast ingots were inductively remelted in the graphite crucible with outpouring orifice of 
1,7 mm diameter. The crucible was closed with a cover to assure an overpressure of Ar in the space 
above the melt (0.2 Mpa) in relation to the pressure of Ar atmosphere in the chamber. The melt was 
heated above the melting temperature and started to flow through the orifice onto a copper- 
beryllium substrate (wheel of 300 mm diameter) at 1230 °C, when the melt viscosity fell below a 
critical level. The peripheral velocity of the wheel, where the melt solidified, was kept constant at 
21 ms”. The height above the copper-beryllium substrate and the angle between a jet of melt and a 
tangent on the wheel surface at the incident point of jet were 0.8 mm and 90°, respectively. 
Continuous ribbons about 2 mm in width and 50 to 90 um in thickness were produced. 
The solidification microstructure and the surface topology of the ribbons were investigated by light 
optical microscopy (Nikon Epiphot 300) and scanning electron microscopy (FEI Sirion NC) 
equipped with EDX analyser. Specimens for optical and scanning electron microscopy were 
obtained as vertical cross-sections, prepared with standard metallographic methods and chemically 
etched in a mixture of 60 vol.% HCI and 40 vol.% HNO (for optical microscopy) and in a solution