Another response of materials to irradiation, not discussed up to now, is radiation-induced amorphiza — tion. Amorphization is a structural phase transforma­tion from a crystalline solid to a solid that lacks any long-range order. Typically, the material still main­tains a certain degree of short-range order, but as far as diffraction techniques can discern, any long-range, crystalline order is destroyed following an amorphi — zation transformation.

Amorphization is a metastable process in which material is forced into a glass-like structure, which under thermodynamic equilibrium would be a pro­hibited structure. Amorphization transformations are most prevalent under ambient or low temperature irradiation conditions, such that kinetic recovery mechanisms are not effective at annihilating atomic displacements produced by irradiation. Typically, above a critical temperature, amorphization can be avoided in an otherwise amorphizable material, due to thermal recovery processes.

Amorphization transformations can occur under both ballistic (displacive) and electronic (SHI) radia­tion damage conditions. Under ballistic conditions and depending on the material, amorphization can either occur within a single primary knock-on (PKA) ion track (or other irradiating particle track), or pro­ceed through the accumulation of defects due to overlapping of damage tracks. Amorphization tends to be detrimental to materials employed in radia­tion environments, because the crystal-to-amorphous transformation is usually accompanied by significant volume swelling, mechanical softening, and micro­cracking, to name but a few deleterious effects.

In ceramic materials, tendencies to radiation- induced amorphization are strongly dependent on crystal structure and chemistry, with the vast majority of ceramics exhibiting significant susceptibility to amorphization. One of the key properties that has been correlated quite well to amorphization resistance is ionicity: highly ionic compounds tend to resist amor — phization; highly covalent compounds tend to readily succumb to amorphization at relatively low doses.19

Both spinel and alumina are relatively ionic com­pounds, but interestingly both can be amorphized by both ballistic and electronic damage mechanisms. Single crystal MgAl2O4 spinel was found to amorphize under ballistic ion irradiation conditions at a peak displacement damage level of 25dpa (100 K irradia­tion temperature, 400 keV Xe2+ions)20 A similar result was obtained under in situ ion irradiation con­ditions (30 K irradiation temperature, 1.5 MeV Xe+ ions).21 The critical temperature, Ta for amorphiza — tion of spinel, has yet to be determined, but it is likely to be well below room temperature. (Only below Tc can the material be fully amorphized. Above Ta kinetic recovery dominates and the material is partially to fully crystalline.) Single crystal a-Al2O3 (sapphire) has been observed to amorphize by a ballistic damage dose of about 3.8 dpa (20 K irradiation temperature, 1.5 MeV Xe+ ions, in situ).2 This is a significantly smaller amorphization dose than that for spinel irra­diated under similar conditions. The critical tempera­ture, Tc, for amorphization of a-Al2O3 was estimated to be about 170 K. In both alumina and spinel, the radiation-induced amorphization transformation does not occur by direct, ‘in-cascade’ amorphization but by damage accumulation by overlapping cascades (dam­age tracks). Presumably, neither a-Al2O3 nor MgAl2O4 can be amorphized at ambient temperature or above using displacive radiation damage conditions. However, there is a report of amorphization of a-Al2O3 at a ballistic damage dose of 3-7 keV per atom.23

Under SHI irradiation conditions, where elec­tronic stopping predominates over nuclear stopping, both alumina and spinel undergo amorphization transformations, with significant concomitant volume swelling. In both materials, the transformation does not initiate until ion tracks are overlapped. In poly­crystalline a-Al2O3, the threshold for amorphization was found to be at an accumulated electronic energy deposition of about 1.5 GGy (85 MeV I7+ ions at ambient temperature; amorphization was found to a depth of ^4.5 pm, corresponding to energy deposi­tion cross-sections ranging from ^5 to 20 keV nm-1 per ion.24 In single crystal sapphire irradiated under similar conditions (90.3 MeV 129Xe at room temper­ature), amorphization was found to initiate at the sample surface at an accumulated electronic energy deposition of about 0.3 GGy.25 These authors also observed a correlation between swelling (as measured by surface ‘pop-out’) and amorphization. However, the swelling values obtained from their measure­ments are too large to be realistic (more than 50% volume swelling). Nevertheless, the swelling asso­ciated with SHI radiation-induced amorphization in alumina is substantial. Matzke26 observed ^30% free swelling in Al2O3 irradiated at ^420 K with 72 MeV I+ ions to fluences ranging from 1019 to 1021 ionsm-2 (5-500 GGy at the sample surface).

In MgAl2O4, amorphization and significant surface pop-out were observed in SHI irradiations at 370 K using 72 MeV I+ ions.27 The ion fluences where pop-out was observed were 1 x 1019 and 5 x 1019ionsm~2 (5.3 and 27 GGy, respectively, at the sample surface). The volumetric swelling asso­ciated with this crystal-to-amorphous phase transfor­mation was estimated to be ^35%.28 In summary, huge volume changes appear to be associated with SHI amorphization transformations in model cera­mics such as spinel and alumina.

This concludes the comparison and contrast of radiation damage effects in two model ceramic oxi­des, namely, a-Al2O3 alumina and MgAl2O4 spinel. To make this chapter on radiation effects in nuclear reactor relevant materials as comprehensive as possi­ble, we offer in the following section some notes on additional ceramic materials that are either important currently in nuclear reactor applications or have potential as advanced nuclear reactor materials with respect to future applications. In particular, we con­sider three representative ceramic materials, namely, urania, silicon carbide, and graphite.

Subsequent chapters treat these materials in more detail: uranium dioxide (Chapter 2.02, Ther­modynamic and Thermophysical Properties of the Actinide Oxides; Chapter 2.17, Thermal Proper­ties of Irradiated UO2 and MOX; and Chapter 2.18, Radiation Effects in UO2), SiC (Chapter 2.12, Prop­erties and Characteristics of SiC and SiC/SiC Composites and Chapter 4.07, Radiation Effects in SiC and SiC-SiC), and graphite (Chapter 2.10, Graphite: Properties and Characteristics; Chapter 4.10, Radiation Effects in Graphite; Chapter 4.11, Graphite in Gas-Cooled Reactors; and Chapter 4.18, Carbon as a Fusion Plasma-Facing Material).