Cyclic hardening rate has been evaluated using the cyclic hardening curves as shown in Fig. 3a. At RT, the cyclic
hardening rate is high at some first cycles, and then the cyclic hardening rate becomes slightly lower. Since the
number of cycles for the maximum stress increases with increase of applied strain, the average cyclic hardening rate
decreases slightly with increase of strain amplitude. On the other hand, cyclic hardening rate increases with
increasing strain and temperature at high temperatures (Fig. 2c and 3a). Another interesting phenomenon is that the
stress increases more near the failure period at a comparatively low temperature such as 600°C than at a higher
temperature. This is probably related to a long fatigue time that can cause the precipitation that increase cyclic
deformation hardening rate. It should be mentioned that the cyclic deformation hardening rate at RT is much higher
than that at high temperaturea. At 650