4.20.5.2.1 Tensile properties

Irradiation causes large changes in tensile properties of copper and copper alloys. Copper and copper alloys can be hardened or softened by irradiation, depending on the irradiation temperature and the amount of the cold work prior to irradiation. Irra­diation hardening of copper and copper alloys due to defect cluster formation is significant at irradia­tion temperatures <300 °C. Irradiation softening oc­curs at irradiation temperatures >300 °C because of radiation-enhanced recrystallization and precipitate coarsening in PH copper alloys.

Low-temperature neutron irradiation of pure copper leads to development of a yield drop and significant hardening. Typical stress-strain behavior of pure copper and copper alloys irradiated to low doses at low temperatures is illustrated in Figure 8. The data of irradiated copper are from the work of Edwards eta/.,64 and the data of irradiated CuCrZr from Li et a/.14 Irradiation significantly changes the work hardening behavior of pure copper. Work hard­ening capability is progressively reduced with increas­ing doses. Appreciable work hardening still exists at the dose of 0.1 dpa. The effect of irradiation on the tensile behavior of copper alloys can be quite different. A complete loss of work hardening capability and

uniform elongation occurs at 0.14 dpa in neutron — irradiated CuCrZr in the prime-aged condition. Irra­diation to 1.5 dpa further reduces the yield strength, and recovers some total elongation in CuCrZr.

The dose dependence of radiation hardening in copper at irradiation temperatures of 30-200 °C is summarized by Zinkle et a/., and shown in Figure 9.65’66 Radiation hardening in copper can be observed at a dose as low as 0.0001 dpa. The yield stress increases dramatically with increasing dose and saturates at ^0.1 dpa. Significant radiation hardening is accompanied by loss of strain hardening capabil­ities, resulting in prompt necking upon yielding.

The temperature dependence of radiation hard­ening ofpure copper at different irradiation tempera­tures was summarized and discussed by Fabritsiev and Pokrovsky.67 The radiation hardening decreases with increasing irradiation temperature in copper. The magnitude of radiation hardening is ^200 MPa at 80 °C, while only ^40 MPa at 300 °C at a dose of 0.1 dpa. Annealing at temperatures higher than 0.4 Tm can effectively reduce the defect cluster den­sity in copper. Annealing at 300 °C for 50 h after irradiation of copper to 0.01-0.3 dpa at 100 °C and annealing at 350 °C for 10 h after irradiation of CuCrZr IG and GlidCop Al25 IG to 0.4 dpa at 150 °C can essentially recover the ductility of the cop­per and copper alloys.68,69 However, postirradiation

Figure 9 Radiation hardening in copper. Reproduced from Zinkle, S. J.; Gibson, L. T. Fusion Materials Semi-annual Progress Report; DOE/ER-0313/27; Oak Ridge National Laboratory, 1999; p 163.

annealing also reduces the critical stress for flow localization in pure copper.70

Irradiation creates a large increase in strength and decrease in ductility in copper alloys for irradiation temperatures below 300 °C. The strengthening effect decreases with increasing temperature. The crossover to radiation softening occurs at approximately 300 °C. The radiation softening effect in CuAl25 alloy is not as strong as for CuCrZr alloy where precipitate stability may be an issue. Neutron-irradiated copper alloys exhibit low uniform elongation after low-dose, low-temperature irradiation. The uniform elongation is recovered to near unirradiated values at 300 °C. Figure 10 compiles the yield strength data for PH CuCrZr and DS copper alloys (CuAl 25, CuAl15, MAGT 0.2) as a function of dose for the irradiation temperature of ^100 °C.14,71 Both alloys show signifi­cant radiation hardening at low doses and an apparent saturation at ~0.1 dpa. Irradiation-induced harden­ing is accompanied by the loss of strain hardening capability and a complete loss of uniform elongation, while the total elongation remains on the level of ~10% for doses up to 2.5 dpa for CuCrZr.

The strain rate dependence of tensile properties in neutron-irradiated CuCrZr was investigated at room temperature by Li eta/.14 The strain rate sensi­tivity is small at room temperature in unirradiated CuCrZr. The measured strain rate sensitivity param­eter, m, is <0.01 for CuCrZr. The strain rate sensitiv­ity parameter increased to ^0.02 in CuCrZr after neutron irradiation to 1.5 dpa. Zinkle eta/.65 observed a small strain rate dependence of tensile strength in GlidCop Al15 and MAGT 0.2 neutron irradiated to ^13 dpa at 200 °C with m ~ 0.02 for GlidCop

Al15 and m < 0.01 for MAGT 0.2. In general, the strain rate and temperature dependence of flow stres­ses is small in fcc metals.