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

Abstract

Room-temperature recrystallization of KCl single crystals, both pure and doped with 0.02 and 0.06 wt % Sr, deformed preliminarily at 250°C is studied. It is found that annealing of the original single crystals at 650°C results in the precipitation of the SrCl₂ phase from the solid solution. 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 recrystallized grains (twins of the original single crystal) occupying not more than 8% of the volume, impede the motion of grain boundaries and hamper further recrystallization. Second, it is shown that post-deformation ageing occurring in the remaining 92% of the deformed crystal region that has not undergone recrystallization lends stability to the strain-hardened state of alkali halide crystals over a period of at least two years at room temperature.