Dislocations in Ice.

The dislocation structures of dendritic ice crystals have been directly observed by x-ray diffraction topography. Growth occurs without the intervention of dislocations and very low dislocation dencities may initially prevail. Straining expands dislocations having 11[unknown]20 direction Burgers vectors from sources at grown-in inclusions. Slip of these dislocations in the basal plane (0001) and on pyramidal planes, probably (1 101), with frequent occurrence of primatic punching, is observed. Dislocation reactions of the type (a(0)/ 3) [11 20] + (a(0)/ 3) [1 210] = (a(0)/ 3) [2 110] and a strong preference for pure screw orientation are characteristic. These dislocations generally account for the known anisotropy of plastic flow in ice.