The origin for the stability of the (a) loops in zirco­nium is attributed to the relative packing density of the prismatic plane compared to the basal plane, which depends on the c/a ratio of the hcp lattice. Foll and Wilkens64 have proposed that when the c/a ratio is higher than 3, loops are formed in the basal plane with Burgers vector 1/6(2023), whereas if c/a is lower than 3, then loops are formed in the prismatic plane with Burgers vector (a) = 1/3(1120). For all hcp metals, this means that loops are formed in the prismatic plane except for Zn and Cd. This is not the case for Zr, Ti, and Mg where loops are also formed in the basal planes, depending on the irradiation dose, irradiation temperature, and purity of the metal.56,57

MD computations for a-zirconium have also shown that most of the small interstitial clusters produced in the cascade have the form of a dislocation loop with Burgers vector (a) = 1/3(1120). The small vacancy clusters are also found in the prismatic plane.8,28,65 For larger point-defect clusters,66 it is shown that the point-defect clusters in the prismatic plane always relax to perfect dislocation loops with Burgers vector (a) = 1/3(1120). On the other hand, vacancy clusters in the basal plane form a hexagonal loop enclosing a stacking fault with 1 /2(0001) Burgers vector.

The simultaneous observation of vacancy and interstitial (a) loops in zirconium alloys45,48,50,54,61 is a rather surprising feature.53,57 Indeed, as discussed for usual cubic metals, interstitial loops tend to grow under irradiation and the vacancy loops tend to shrink since the edge dislocations are biased toward SIAs due to the EID.

According to Griffiths,57 the coexistence of these two types of loops in zirconium can be explained by a modified SIA bias in zirconium due to (i) a rela­tively small relaxation volume of SIA relative to vacancy (low bias), (ii) interaction with impurities, and (iii) spatial partitioning of vacancy loops and interstitial loops as a result of elastic interactions or
anisotropic diffusion. Other authors53’68 think that this phenomenon is due to a subtle balance of the bias factors of the neighboring point-defect sinks that lead to an increasing bias as the loop size increases if the loop density is high. Woo44 considers that the coexistence of both types of (a) loops can be explained in the frame of the DAD model’ which induces a strong DAD-induced bias. Indeed’ in this model, the (a) type loops are shown to be relatively neutral and may therefore receive a net flow of either interstitials or vacancies, depending on the sink situ­ation in their neighborhood.

Finally, recent computations,69 using the Monte Carlo method, that take into account the large vacancy and interstitial point-defect clusters created inside the cascade as an input microstructure show that both vacancy and interstitial loops are able to grow simultaneously, the proportion of vacancy loops increasing with increasing irradiation temperature. This last phenomenon can be related to the so-called production bias discussed previously.1