4.20.5.3.1 Defect structure in irradiated copper and copper alloys

Copper is among the most extensively studied metals in terms of fundamental radiation damage. Several reviews on the effect of irradiation on the

microstructure of copper and copper alloys are available in the literature.60,96,97 Neutron irradiation of copper at low temperatures produces small defect clusters, dislocation loops, and SFTs. At temperatures above 150—180 °C, the density of defect clusters

starts to decrease with increasing temperature, accompanied by the formation ofvoids. This temper­ature-dependent formation of defect structures is shown in Figure 14.60 Low-temperature neutron irradiation produces a high number density of SFTs and a low number density of dislocation loops in copper. Edwards et a/.64 reported a number density of SFTs, ^2-4 x 1023m~3 and a number density of dislocation loops, 5 x 1021m~3 in OFHC copper neutron irradiated to ~0.01 dpa at 100 °C. Disloca­tion loops are believed to be of interstitial type.

(a) (b)

The size of SFTs is small, ^2-3 nm. As doses increased, the density of SFTs increased to a satura­tion level at ~0.1 dpa, while the size of SFT is inde­pendent of the dose and temperature. In general, the dislocation loop density is low, and a significant dis­location network is not formed in irradiated copper.96

Radiation hardening in copper can be adequately described by Seeger’s dispersed barrier model, and the yield strength increase is due to the formation of defect clusters.98 Singh and Zinkle96 summarized the dose dependence of the TEM-visible defect cluster density in copper irradiated near room temperature with fission neutrons, 14MeV neutrons, spallation neutrons, and 800 MeV protons (Figure 15)96 TEM — visible defect clusters were observed at a very low dose (10-5 dpa). The defect cluster density showed a linear dependence on irradiation dose at low doses. The dose dependence of the defect cluster density shifts to either a linear or a square root relation at intermediate doses (>^0.0002 dpa). The cluster den­sity reaches an apparent saturation (~1 x 1024m~3) at ~0.1 dpa. The dislocation loops range in size from ~1 to 25 nm.9 Differences in the type of irradiation (fission, fusion, spallation, etc.) have no significant effect on the defect cluster accumula­tion behavior in copper. The density of defect clus­ters in irradiated copper shows strong temperature
dependence (Figure 16).100 The defect cluster den­sity is essentially independent of the irradiation temperature between 20 and 180 °C (upper tempera­ture limit is dependent on dose rate). At higher tem­perature, the cluster density decreases rapidly with increasing irradiation temperature. At irradiation temperatures between 182 and 450 °C, the density of defect clusters was reduced by over three orders of magnitude.83,84 The saturation dose of the defect cluster density is similar, ^0.1 dpa, for all irradia­tion temperatures.96 The size distribution of visible defect clusters can be described by an exponential function10: N(d) = N0 exp(—d/d0), where N(d) is the number of defects of diameter d, N0, and d0 are constants, and their values depend on irradia­tion conditions and material purity. As the irradiation temperature decreases, a fraction of small clusters increases relative to large clusters.

Void formation occurs above 180 °C in neutron — irradiated copper.60 The peak void swelling temper­ature in copper is about 320 °C at a dose rate of 2 x 10-7dpas_1. Singh and Zinkle96 summarized the dose dependence of void density measured by TEM in copper irradiated with fission and fusion neutrons at 250-300 °C from several studies. The data showed a large variation (up to two orders of magnitude differences) of void density between

experiments. One possible source could be residual gas atoms in copper that can have a dramatic effect on void swelling in copper. Zinkle and Lee86 discussed in detail the effect of oxygen and helium on the forma­tion of voids in copper. The stacking fault tetrahedron is predicted to be the most stable configuration of vacancy clusters in copper. A small amount of oxygen (~10 appm) or helium (~ 1 appm) in copper is needed to stabilize voids. High-purity copper with low oxy­gen concentration (<5wppm) showed no significant

Irradiation temperature

void formation after 14MeV Cu ion irradiation to 40 dpa at temperatures of 100-500 °C.100

The defect microstructure (SFTs and dislocation loops) in irradiated copper alloys is essentially the same as in irradiated pure copper.22’25’64 Neutron irradiation can affect precipitate microstructure in copper alloys. When irradiated at 100 °C, the precip­itate density in CuCrZr was slightly reduced, and the mean size of the precipitates increased.13’64 Zinkle et a/.25’26 reported that when GlidCop Al25 and MAGT 0.2 were ion irradiated to 30 dpa at 180 °C, a high number density (5 x 1023 m~3) of defect clusters (primarily SFTs) with a mean size of2 nm was produced. The geometry ofoxide particles in GlidCop Al25 was transformed from triangular platelets to nearly circular platelets’ and the particle size was reduced from 10 to 6 nm after irradiation (Figure 17).25’26 The geometry and size of oxide par­ticles in MAGT 0.2 were essentially unchanged by irradiation. In general, DS copper alloys showed supe­rior particle stability under irradiation.

Limited data are available in terms of the effect of solution additions on the irradiated microstructure of copper. A study by Zinkle25 showed that solute additions (e. g., Al, Mn, Ni) to 5 at% in copper do not have significant effect on the total density of small defect clusters at low irradiation temperatures (<130 °C). However, solute additions reduce the formation of SFTs and enhance the formation of dislocation loops. The loop density and mean size in Cu-5% Mn irradiated to 1.6 dpa at 160 °C were 3 x 10[5] [6] m~3 and 23 nm, and 1.8 x 1022m~3 and 18 nm in Cu-5% Ni irradiated to 0.7 dpa at 90 °C

(Figure 18).25’26 These loop densities are more than an order of magnitude larger than the highest loop density observed in pure copper. The effect of the stacking fault energy on void formation in copper alloys was also investigated. Generally speaking, the lower the stacking fault energy, the less favorable for the formation of 3D voids. For example, swelling occurred in Cu—1—2.5% Ge alloys irradiated at 250°C, while no measurable swelling occurred in Cu-3-5% Ge that has lower stacking fault

97

energies.