4.20.4.1 Tensile Properties

The influence of test temperature, strain rate, and thermal-mechanical treatments on the tensile prop­erties of copper and copper alloys has been studied extensively. Figure 3 illustrates the effect of test temperature on the yield strength of pure copper (in the annealed condition), PH CuCrZr and CuNiBe alloys, and DS CuAl25.15-18,28,32-39 The strength of copper alloys decreases with increasing test tempera­ture. The decrease in strength is moderate up to 200 °C. Significant drops in strength occur at higher temperatures, except that the CuNiBe AT alloy shows a relatively small reduction in strength even up to 400 °C. Pure copper has the lowest yield strength. The tensile properties of pure copper strongly depend on the thermal-mechanical treatment and the impurity content.15-1 ,32,33 CuNiBe alloy has the highest strength over the entire temperature range.34 The tensile properties of PH copper alloys are sensi­tive to the thermal-mechanical treatments. CuCrZr in the solution-annealed, cold-worked, and aged con­dition (SA + CW + A) has superior yield strength at low temperatures relative to CuCrZr in the solution- annealed, and aged condition (SAA). However, the strength of CuCrZr SA + CW + A alloy drops more rapidly with increasing temperature.29,34-39 The yield strength of CuNiBe can be quite different, depending on the processing techniques. The tensile ductility of copper alloys also shows strong temperature depen­dence. The uniform elongation of the CuAl25 alloy decreases considerably as the test temperature in­creases, but increases with increasing test temperature above 400 ° C. The CuNiBe AT alloy shows a moder­ate drop of uniform elongation below 200 °C, but a sharp drop in ductility at higher temperature.34 The uniform elongation of the CuCrZr alloy shows the smallest sensitivity to test temperature. Among

the three copper alloys, the CuCrZr alloy has the best ductility over the temperature range, and the ductility of the CuNiBe alloy is the lowest.

Because of the sensitivity of mechanical properties to thermal-mechanical treatments in PH copper

alloys, the strength of large components made of these alloys can be significantly lower. For example, during component manufacturing, CuCrZr often experiences additional thermal cycles, such as braz­ing, welding, or HIPing. While solution annealing
can be conducted during or after a brazing or HIPing process, rapid quenching is not feasible for large com­ponents, and a much slower cooling rate (e. g., furnace cooled or gas cooled) is applied in the manufacturing cycle. Significant reduction in strength due to slow cooling rates has been reported in CuCrZr.30,40-42 A slow cooling rate (50-80 ° Cmin~ ) and overaging at 560 °C/2h significantly reduced the yield stress and the ultimate tensile strength, and tensile elongations of CuCrZr relative to prime-aged CuCrZr.14 Cooling rates >1200 °C min-1 are required to fully quench the Cu-Cr solid solution.43-45

The effect of strain rate on tensile properties for pure copper and PH CuCrZr and CuNiBe alloys as well as DS CuAl25 alloy was studied at temperatures of 20 and 300 °Q14,34,46 All three copper alloys are relatively insensitive to strain rate at room tem­perature. The strain rate sensitivity parameter of m (where ay = Ce’mand C is a constant) is ~0.01 for the CuAl25 alloy at room temperature. The strain rate sensitivity of this alloy increases significantly with increasing temperature as reflected by a strain rate sensitivity parameter of m ~ 0.07 at 300 °C. Stephens et a/.47 reported a strain rate sensitivity parameter of m ~ 0.1 in the temperature range of 400-650 °C for CuAl25. A similar effect of strain rate on ultimate tensile strength was also observed on these materi — als.34,46 Edwards46 investigated the strain rate effect of copper alloys in air and vacuum, and found that
testing in air or vacuum did not appear to change the strain rate dependence of the CuAl25 alloy, but that testing the CuNiBe alloy in air shifted the embrittlement to a lower temperature.