THE MOTION OF NONBASAL DISLOCATIONS IN ICE CRYSTALS

The dislocation structure in ice crystals has been extensively studied using etching techniques. There is a general consensus regarding the motion and structure of basal dislocations. The opposite is true conce rning the motion of non-basal dislocations. This paper presents experi mental evidence of the motion of non-basal dislocations in ice, as rev ealed by etching and replicating. Two distinct etch patterns caused by the motion of dislocations emerged on the basal plane were observed. One is a series of separated flat-bottomed etch pits, corresponding to the jumping motion of the edge dislocations. The other is the uniform zigzag etch channels, corresponding to the cross-slip of the screw di slocation on the prismatic or pyramidal planes. Special attention is p aid to the cross-slip of [11 $($) over bar$$ 23] screw dislocations wh ich can contribute to the plastic deformation along the c axis. An imp ortant consequence of the intersection mechanism proposed in this pape r is that, while the basal deformation of single crystals of ice may b e described in terms of slip of the basal dislocations, the non-basal deformation of ice crystals and the plasticity of polycrystalline ice are controlled by the motion of jogged non-basal dislocations. Given t hat the intersection of basal and non-basal dislocations contributes t o the anisotropy of plasticity of ice crystals, the latter assertion i mplies that the rate of plastic deformation of polycrystalline ice sho uld be climb dependent.