124 Prakt. Met. Sonderband 41 (2009) 
excessive density of defects and plastic deformation reduce the TWSME. The two-way effects were 
not very high (&2w= 0.98 % after 5 constrained training cycles); however, after several hundred free 
thermal cycles (not constrained) the two-way effect became larger (&2w = 1.82 % after 400 thermal 
cycles) with higher shape recovery and finally stabilized. It is not yet known exactly why this 
interesting functional behaviour occurs but it seems that shape memory samples cannot only be 
trained under the usual constrained conditions but also to a certain extent by free thermal cycles. 
3.2 Effect of training on precipitation 
TEM examinations were performed on as-spun and trained ribbons. Figure 2a displays a fine 
martensitic structure (B19’ phase) with nano-sized twin boundaries in the as-spun NiTi ribbons. 
After 500 thermal cycles, many fine spherical particles were observed by TEM as shown in Figures Fig. 3: 
2b and 2c. By selected area diffraction and by EDX analysis these particles (indicated by arrows) 
could be identified as NiTi,. The size of the precipitates was estimated to be in the order of several 3.4 
hundred nanometres. 
Tensil 
platea 
strain 
of the 
tensile 
cause 
Fig. 2: TEM micrograph of the B19” martensite microstructure in a melt-spun NiTi ribbon: (a) before training, (b) and 
(¢) after training, NiTi, particles. 
3.3 DSC results 
The transformation behaviour with increasing number of free thermal cycles after one constrained 
training cycle is shown in Fig. 3. The height of the peak of austenitic phase transformation . 
(endothermic behaviour) increases and the transformation temperatures decrease slightly. The Fig. 4 
decrease in austenitic transformation temperatures and the increase in the height of the austenitic 
peak indicate an easier transformation from martensite to austenite which may be related to the 
presence of a dislocation structure that serves as a network of nucleation sites for the austenite 
phase [16]. The exothermic behaviour (during cooling) of the transformation shows that an 
intermediate phase (which is NiTi,) appears during thermal cycles. The peaks shift gradually to the 
lower temperature side, and the temperature region of the intermediate phase is expanded. It is 
thought that the accumulation of lattice defects introduced by thermomechanical training and 
thermal cycles leads to the stabilization of the intermediate phase.