4.07.4.1 Elastic Modulus of Monolithic SiC

Figure 13 summarizes the irradiation temperature dependence of the elastic modulus change. Irradia­tion generally reduces modulus to a greater extent for lower temperature irradiation. The modulus reduction becomes negligible when irradiation temperature reaches or exceeds 1273 K. There seems to be an indistinct stage between 1073 and 1273 K. As expected, the elastic modulus trends with ‘point defect swelling’ of SiC. Point defect swelling is an isotropic volume

expansion that is believed to occur by lattice relaxation due to accumulated isolated point defects and small point defect clusters during irradiation at tempera­tures where vacancies are not readily mobile. In SiC irradiated in the point defect swelling regime, a fairly good agreement between dimensional expansion and lattice spacing has been confirmed by X-ray diffrac- tometry studies. In contrast, the data in the nonsatur­able swelling regime is somewhat limited, although the data suggest that there is little reduction in elastic modulus in spite of the swelling being relatively large. However, in this regime, the defects responsible for swelling are voids and other relatively large defects, which would have less of an effect on elastic modulus as compared to point defects.

An estimation of the influence of lattice relaxa­tion on elastic modulus was attempted using the Tersoff potential.54 The result predicted a linear lattice
swelling of 1% causing approximately 10% reduction in elastic modulus (Figure 14). The predicted elastic modulus change could be varied by more than 10% depending on a selection of interatomic potential, with the Tersoff potential giving a relatively high sensitivity of modulus to the interatomic distance among various empirical interatomic potential functions. Therefore, the measured elastic modulus changes observed in this experiment are generally greater than the theoreti­cal prediction. It is noted that the methods applied for generating the data of Figure 14 are various and of differing quality. Typically, elastic modulus as measured by nanoindentation, which sometimes is the only alternative for miniature specimens, tends to give widely scattered and less reliable data than the mechan­ical or sonic modulus methods. Nonetheless, it is clear that the lattice expansion is a major cause of the irradiation-induced elastic modulus reduction in SiC.