Effect of post-deformation ageing on the recrystallization of KCl-SrCl2 crystals

Room-temperature recrystallization of KCl single crystals, both pure and do ped with 0.02 and 0.06 wt % Sr, deformed preliminarily at 250 degrees C is studied. It is found that annealing of the original single crystals at 650 degrees C results in the precipitation of the SrCl2 phase from the solid so lution. X-ray diffraction studies reveal that the deformed crystals undergo ageing at room temperature, which is accompanied by a change in the phase composition. In the first place, particles of the KSr2Cl5 phase are formed instead of the phase SrCl2 observed in the original and deformed crystals. These particles, which are located at the boundaries of growing recrystalli zed grains (twins of the original single crystal) occupying not more than 8 % of the volume, impede the motion of grain boundaries and hamper further r ecrystallization. Second, it is shown that post-deformation ageing occurrin g in the remaining 92% of the deformed crystal region that has not undergon e recrystallization lends stability to the strain-hardened state of alkali halide crystals over a period of at least two years at room temperature. (C ) 2000 MAIK "Nauka/Interperiodica".