Swelling data for Ta and its alloys are limited to a few studies.19 Void formation in pure Ta was experimen­tally observed through TEM examination of material irradiated to 2.5 x 1022ncm-2 (E> 0.1 MeV) at tem­peratures between 673 and 1273 K.46 An empirical estimation of the bulk swelling taken from microstruc­tural void size density data of that study is shown in Figure 6. Void concentrations in the material were highest at the peak swelling temperature and decreased with higher irradiation temperature with an associated increase in cavity size. Ordering of the voids at the peak

swelling condition was reported to occur along the {110} planes in the bcc structure. A subject of consid­erable theoretical debate, the mechanisms of void ordering that have appeared in bcc and fcc metals have been examined,4 — 0 since the first reported occurrence in irradiated Mo.51 Disordered void struc­tures in the microstructure of the higher temperature irradiated Ta appear as the size of the voids increase, though some rafting, or grouping, was reported.46

The swelling data of Wiffen46 derived from micro­structural analysis correlate well with the immersion density data of Bates and Pitner47 (Figure 6), from which an empirical equation for percent swelling as a function of temperature, T (K), and fluence, F (in units of 1022ncm-2, E > 0.1 MeV), was developed, which is as follows:

D — = (F)°’4{1.69 exp[—(0.018T — 16.347)2/a]}

where a _ 14.87 + 44.57 exp[0.09(T — 1338.71)]

“_ 1 + exp[0.09(T — 1338.71)] [1]

The broader width of the swelling peak as a function of irradiation temperature for the calculation repre­sented by eqn [1] compared to the microstructural data of Wiffen46 is believed to be associated with errors in the accurate irradiation temperature of these early measurements. Experimental evidence of decreased swelling at higher fluences was reported by Murgatroyd et al. and attributed to the transmu­tation of Ta to W, resulting in a shift in the lattice constant. Similar effects have been more closely examined in Mo and TZM alloys, and attributed to impurity segregation at void surfaces leading to shrink­age of the voids.53

Swelling measurements in Ta-10W and T-111 alloys are limited specifically to work by Wiffen, from which a later summary was given.19 For irradia­tions at 723 and 873 K to a fluence of 1.9 x 1022ncm-2 (E > 0.1 MeV), no swelling in T-111 was observed, though a possible densification of up to 0.36% may have occurred as evidenced in length measurements. In companion irradiations to that of pure Ta already discussed, involving irradiations to 4.4 x 1022ncm — (E > 0.1 MeV) at temperatures between 698 and 1323 K,46 samples of Ta-10W were included with postirradiation examination involving TEM analysis. The microstructure of the irradiated Ta-10W con­tained fewer voids than the companion Ta samples, with a lower swelling assumed in the Ta-10W alloy but with values not accurately quantifiable.19