Prakt. Met. Sonderband 41 (2009) 279 
for bulk material of CrO, in HCI (for electron microscopy). SEM observations of the surface topology and 
solid-state high- microstructure were performed on the samples cleaned in ultrasound. 
racturing of the 
size and uniform 3. Results and discussion 
other producing 
on of the solute Figure 1b shows transverse cross-sectional microstructure of the melt-spun ribbons. It can be seen 
The process is that the wheel-side of the ribbons has smooth surface and edge, and at the top side of the ribbons the 
rsion throughout surface is wavy as a consequence of streaming of the Ar gas above the melt in the travelling 
a low solubility direction of solidifying ribbons. Contrary to the microstructure of the alloy after vacuum induction 
1 solution. In the melting and casting that consists of Au primary dendrites and coarse intermetallic segregates and 
dritic space and eutectic (probably Aut+Augla) mixture (Fig. la), the rapidly solidified ribbons contain very fine 
ed after internal grains, mostly a few micrometer in size and submicron particles decorating the grain boundaries. 
irse intermetallic The composition of coarser particles has been analysed very carefully by energy-dispersive 
e deformation of spectroscopy and has been found in the region of about 10wt.% La, which is close to the 
op during rapid composition of AucLa intermetallics. 
ation process [7] 
ling rate) on the 
he solidification 
. microstructural 
ine the primary 
La alloy and to 
rticles in the Au 
‘vacuum melting 
of lanthanum Fig. 1: Microstructure of the alloy aft Iti d i d aft It spinning (b 
lloy, because 0 f CL y after vacuum melting and casting (a) and after melt spinning (b). 
; to the melting On the macroscopic scale, both surfaces of all ribbons show the same topographic features. The free 
6 La. The rapid surface (Fig.2b) is always smooth and the contact surface (Fig. 2a) usually exhibits some dimples 
hnique. Smaller and canals, which were formed by some gas picked up at the back edge of the melt puddle on the 
ouring orifice of wheel surface. At higher magnifications very fine structure with grains less than 2pm in size 
£ Ar in the space become visible on the contact surface of ribbons (Fig 3a). On the other hand the microstructure 
or. The melt was obtained at the free surface depends on the ribbon thickness. The surface microstruciure of the 
onto a copper- thickest ribbons (80-90 pm) consists of grains with relative coarse dendritic morphology. Those of 
sity fell below a ribbons with medium thickness contain at the free surface again a structure with dendritic 
kept constant at morphology, but finer in size (Fig. 3b). The thinnest parts of ribbons (50 um) consist of fine grains 
jet of melt and a usually with cellular substructure. Such microstructural changes at the free surface clearly indicate a 
0°, respectively different solidification process at the ribbons parts of different thickness. 
o id ‘ The solidification conditions on the contact surface also determine the microstructure. In the region 
stigate d by light of direct contact of the melt with the wheel surface the microstructure consists of very fine grains 
FEI Sirion NC) mostly of submicron size with fine precipitation of second phase through the volume of the grains 
nicroscopy were (Fig. 4a). We believe that these particles were formed by precipitation from the supersaturated solid 
s and chemically solution. In the region of dimples and canals the contact with the wheel surface was not achieved 
od in a solution and this changed the temperature gradient, growth direction and degree of undercooling.