Copper alloys are subjected to severe thermal cycles in high heat flux applications in fusion systems, and so, fatigue as well as creep-fatigue performance is a primary concern. Figure 6 shows the fatigue perfor­mance of OFHC Cu, PH CuCrZr and CuNiBe, and DS CuAl25.53 All three copper alloys show signifi­cantly better fatigue performance than OFHC cop­per. Among the three alloys, CuNiBe has the best

fatigue response. The temperature dependence of fatigue behavior is stronger in CuAl25 and CuNiBe than in CuCrZr at temperatures between 25 and 350°C. Heat treatments have an insignificant effect on fatigue life in CuCrZr.54

The fatigue life of copper and copper alloys can be significantly reduced when a hold time is applied at peak tensile and/or compressive strains during fatigue cycling. The hold time effect is evident even at room temperature and with a hold time as short as a few seconds.53,55,56 As shown in Figure 7, the fatigue life of OFHC copper is reduced significantly by the introduction of a hold time of 10 s at both tensile and compressive peak strains. The reduction in fatigue life is more severe in the high-cycle, long­life regime than in the low-cycle, short-life fatigue regime. A similar effect of the hold time was observed in copper alloys. The hold time effect appears to be more severe in CuAl25 than in CuCrZr. The effect of hold time is stronger in overaged CuCrZr (e. g., HT2 in Figure 7) than in prime-aged CuCrZr. Stress relaxation was observed during the hold periods even at room temperature where thermally activated creep processes are not expected. The reduction in fatigue life is apparently due to a change in the crack initiation mode from transgranular with no hold period to intergranular with a hold period.56,57 The fatigue life reduction under creep-fatigue load­ing could be more severe at high temperatures, particularly in PH copper alloys. Their softening behavior at elevated temperature due to overaging

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and recrystallization could have significant impact on the fatigue life with a very long hold time.

Few studies have been performed to characterize the fatigue propagation rates of copper alloys. The fatigue crack growth rate of CuAl25 was found to be higher than that of CuCrZr at a lower stress intensity range, AK, at room temperature.58 Crack growth rates of CuCrZr and CuAl25 alloys increase with increas-

49,59

ing temperature.