Irradiation temperature typically invokes a very large influence on the microstructural evolution of irra­diated materials. There are several major tempera­ture regimes delineated by the onset of migration of point defects. Early experimental studies used iso­chronal annealing electrical resistivity measurements on metals irradiated near absolute zero temperature to identify five major defect recovery stages.61-64 Figure 6 shows the five major defect recovery stages for copper irradiated with electrons at 4 K.65 The quantitative magnitude of the defect recovery in each of the stages generally depends on material, purity, PKA spectrum, and dose. Based on the cur­rently accepted one-interstitial model, Stage I corre­sponds to the onset of long-range SIA migration. Stage I often consists of several visible substages that have been associated with close-pair (correlated) recombi­nation of Frenkel defects from the same displacement event and long range uncorrelated recombination of defects from different primary displacement events. Stage II involves migration of small SIA clusters and SIA-impurity complexes. Stage III corresponds to the onset of vacancy motion. Stage IV involves migration of vacancy-impurity clusters, and Stage V corresponds to thermal dissociation of sessile vacancy clusters. It should be noted that the specific recovery stage tem­perature depends on the annealing time (typically 10 or 15 min in the resistivity studies), and therefore needs to be adjusted to lower values when considering the onset temperatures for defect migration in typical

Figure 5 Comparison of the molecular dynamics simulations of 1-50 keV PKA displacement cascades in iron. PKA energies of 1 (red), 10 (green), and 50 (blue) keV for times corresponding to the transient peak number of displaced atoms are shown. The length of the Z (horizontal) dimension of the simulation box is 170 lattice parameters (49 nm). Adapted from Stoller, R. E., Oak Ridge National Lab, Private communication, 2010.

Source: Eyre, B. L. J. Phys. F1973, 3(2), 422-470.

Zinkle, S. J.; Kinoshita, C. J. Nucl. Mater. 1997, 251,200-217.

Schilling, W.; Ehrhart, P.; Sonnenberg, K. In Fundamental Aspects of Radiation Damage in Metals, CONF-751006-P1; Robinson, M. T.; Young, F. W., Jr., Eds. National Tech. Inform. Service: Springfield, VA, 1975; Vol. I, pp 470-492.

Hautojarvi, P.; Pollanen, L.; Vehanen, A.; Yli-Kauppila, J. J. Nucl. Mater. 1983, 114(2-3), 250-259.

Lefevre, J.; Costantini, J. M.; Esnouf, S.; Petite, G. J. Appl. Phys. 2009, 106(8), 083509.

Schultz, H. Mater. Sci. Eng. A 1991, 141, 149-167.

Xu, Q.; Yoshiie, T.; Mori, H. J. Nucl. Mater. 2002, 307-311(2), 886-890.

Young, F. W., Jr. J. Nucl. Mater. 1978, 69/70, 310.

Hoffmann, A.; Willmeroth, A.; Vianden, R. Z. Phys. B 1986 62, 335.

Takamura, S.; Kobiyama, M. Rad. Eff. Def. Sol. 1980, 49(4), 247.

Kobiyama, M.; Takamura, S. Rad. Eff. Def. Sol. 1985, 84(3&4), 161.

neutron irradiation experiments that may occur over time scales of months or years. Table 1 provides a summary of defect recovery stage temperatures for several fcc, bcc, and HCP materials.8,18,63,66-69 Although there is a general correlation of the recovery temperatures with melting temperature, Table 1 shows there are several significant exceptions. For example, Pt has one of the lowest Stage I temperatures among fcc metals despite having a very high melting temperature. Similarly, Cr has a much higher Stage III temperature than V or Nb that have higher melting points. As illustrated later in this chapter, the micro­structures of different materials with the same crystal structure and irradiated within the same recovery stage temperature regime are generally qualitatively similar.

Several analytic kinetic rate theory models have been developed to express the dose dependence of defect cluster accumulation in materials at different temper­ature regimes.6,70-72 In the following, summaries are provided on the experimental microstructural obser­vations for five key irradiation temperature regimes.