Dislocations In Ice Research Articles

X-Ray Topographic Evidence for Prismatic Dislocations in Ice

X-Ray Topographic Evidence for Prismatic Dislocations in Ice

The effect of hydrogen disorder on dislocation movement and plastic deformation of ice

In the structure of ice at low pressures (ice I-h) the oxygen atoms are crystallographically arranged but the hydrogen atoms are believed to be randomly arranged consistent with the so-called Bernal-Fowler rules. The effect of this randomness is to make it impossible for a dislocation to move through the ice lattice without creating point defects (breaches of the Bernal-Fowler rules). A calculation of the energy of the defects that would have to be created gives a value so large that it would require a stress of about one tenth of the shear modulus of ice to push the dislocation through. It therefore seems likely that dislocations could not move through ice unless the hydrogen atoms are reoriented by thermally activated point defects ahead of the dislocation. This necessity will greatly slow down dislocations in ice and provides an explanation for the observed behaviour of ice single crystals in creep and constant strain-rate tests, and of the softening of ice at low temperatures produced by small concentrations of dissolved fluoride ions.

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.