Fracture toughness data for PH copper alloys, CuCrZr and CuNiBe, and DS copper alloys, CuAl15 and CuAl25, are summarized in Figure 4.1, -0 CuCrZr has the highest toughness, and CuNiBe the lowest among these alloys. The large scatter in mea­sured fracture toughness values for CuCrZr in differ­ent studies is likely due to different heat treatments, specimen geometry and dimensions, and testing meth­ods. The temperature dependence of the fracture toughness in CuCrZr, while difficult to accurately define, shows an initial decrease with increasing tem­perature, and then a slight recovery at temperatures above 250 °C. The effect of thermal-mechanical treat­ment on fracture toughness of CuCrZr is insignificant in comparison with its effect on tensile properties.14 The minimum value of the JQ for unirradiated CuCrZr is as high as ^100 kJ m~2.

The fracture toughness of DS CuAl15 and CuAl25 is significantly lower than that of CuCrZr, and shows a strong directional dependence. The toughness is higher in the L-T orientation than in the T-L orientation. The fracture toughness decreases

rapidly with increasing temperature. The JQ value for CuAl25 is only 7 kJ m~2 at 250 °C in the T-L

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orientation.