A mechanism for basal vacancy loop formation in zirconium

Two-dimensional vacancy platelets as precursors for basal dislocation loops in hexagonal zirconium

“…Thus, there is the question of where the additional irradiation-induced vacancies went. The fraction of <c> loops is too small to argue that the remaining vacancy <a> loops all coalesced to form <c> loops 29 , or that the interaction between the irradiation cascades and a high density of pre-existing <a> vacancy loops accelerated <c> loop formation 5 . While MD simulations have shown that the small size <c> loop can transform from a 3D pyramidal defect, of which the critical size consists of 400 vacancies 30 , no such 3D pyramidal defects are observed in our experiment and other studies.…”

“…Being no smaller than 10 nm has suggested that the <c> loops require an incubation period in which point defects must cluster to a critical size before a <c> loop can be produced. The possible mechanisms for <c> loop formation have received much attention, and over the years, several hypotheses have been proposed 5 , 29 – 32 . The formation of the <c> loops has been associated with the coalescence and subsequent collapse of <a> vacancy dislocation loops 5 .…”

“…The formation of the <c> loops has been associated with the coalescence and subsequent collapse of <a> vacancy dislocation loops 5 . However, recent studies indicate that a majority of the vacancy <a> loops lying along the basal plane are randomly and widely distributed in different directions, substantially lowering the odds for coalescence 29 . Alternatively, <c> loops have been thought to be induced by the interaction between a cascade and a dense cluster of pre-existing <a> vacancy loops in the basal plane 29 .…”

“…However, recent studies indicate that a majority of the vacancy <a> loops lying along the basal plane are randomly and widely distributed in different directions, substantially lowering the odds for coalescence 29 . Alternatively, <c> loops have been thought to be induced by the interaction between a cascade and a dense cluster of pre-existing <a> vacancy loops in the basal plane 29 . The problem with the two aforementioned mechanisms is that they would require a high density of pre-existing <a> vacancy dislocation loops (~10 22 m −3 ) to match the measured amounts of <c> loops.…”

“…The absence of vacancy <c> loops less than 10 nm on the basal planes and the presence of only interstitial <a> loops on the prismatic plane cannot explain the observed contraction along the c-axis of the irradiation growth phenomenon 1,11 . Being no smaller than 10 nm has suggested that the <c> loops require an incubation period in which point defects must cluster to a critical size before a <c> loop can be produced. The possible mechanisms for <c> loop formation have received much attention, and over the years, several hypotheses have been proposed 5,[29][30][31][32] . The formation of the <c> loops has been associated with the coalescence and subsequent collapse of <a> vacancy dislocation loops 5 .…”