Prakt. Met. Sonderband 41 (2009) 123 
‚ocess, the with gauge size of 25-30 mm in length, 2 mm in width, and 80 um in thickness were cut from the 
te variants melt-spun ribbons and tested at room temperature on a universal mechanical testing machine, 
designed for small samples. 
earance of 
ging at an 
termediate 3 Results and discussion 
Therefore, 
Some researchers (e.g. [12—14]) consider it necessary that after rapid cooling a heat treatment has to 
refore, the be performed in order to achieve shape memory effects. The reason for this is that in their work 
efficiency, higher cooling rates and, therefore, partially amorphous structures were obtained which have to be 
> annealed for crystallization. We have performed our experiments on three different melt-spinning 
devices and on a splat-cooling device and always obtained martensitic samples which exhibited a 
fully crystallized microstructure and shape memory effects immediately after processing. It was 
observed that the increase in the wheel speed from 5 to 30 ms results in a decrease in the ribbon 
thickness from ~100 to ~30 um. As the increase in the wheel speed leads to a reduced ribbon 
hearth in a thickness, the cooling rate increases and therefore the martensitic substructure gets finer. Figure 1 
»s for each shows the martensitic microstructure of a ribbon produced by melt-spinning at a medium wheel 
speed (15 ms™). In previous studies, the samples have been etched in a conventional solution 
ss crucibles containing HF:HNO;:H,O = 1:4:5 and by this it was possible to reveal partially the martensitic 
pressure of structure but not detailed and not very clear (Fig. 1a). In this study, by using CH3COOH instead of 
wheel (200 water and optimizing the composition of the etchant a fine reproducible microstructure could be 
etween the obtained. 
, based on 
a TWSME 
inear shape 
; of several 
he value of 
During the 
from room 
the changes 
ons of melt- iy) 
lished. The 
ching times Fig. 1: Optical microstructures of a melt-spun ribbon: (a) HF:HNO;:H,0 = 1:4:5, for 10 s; (b) HF:HNO;:CH,COOH = 
Dg 2:4:5, for 5s 
ts, thin discs 
ished nan 3.1 TWSME induced by thermomechanical training 
g a twin jet 
30 minutes. In this method, pieces of NiTi ribbons (~50 mm long and 2 mm wide) were bent to a small radius 
44°, and the (about 0.9 mm) with a surface strain of 8 % and cycled through the transformation range several 
studies were times (3-10 times) in a constrained condition. The resulting two-way effect increases with 
increasing number of constrained thermal cycles in the early stage and then decreases after reaching 
or DSC 821e a maximum point, while with increasing number of constrained thermal cycles, the plastic 
der static air deformation increases strongly and therefore the shape recovery ratio reduces. This fact suggests 
1s cut with a that an appropriate density of slip defects is effective for the improvement of the TWSME while an 
e specimens