Effect of the void formation on the explosive fracture of electron irradiated NaCl crystals

Experimental and theoretical results are presented on formation of colloids, halogen bubbles and large vacancy voids in heavily irradiated NaCl crystals leading to their explosive decomposition into small pieces under further irradiation or subsequent heating. The dependence of the radiation stability of material with increasing irradiation dose on the void evolution is analyzed. It is shown that voids can grow very fast as compared to colloids and bubbles. For doses higher than 100 Grad, the void dimensions can exceed the mean distance, first, between bubbles and then between colloids resulting in their collisions with voids. Collisions with bubbles fill the voids with gas, and subsequent collisions with colloids (during further irradiation or heating) bring the halogen gas and metal to a back reaction inside the voids. Such a sudden release of stored energy can be shown to result in a temperature spike (above 10^4 K) and instantaneous gas pressure rise up to 1 GPa within the voids, which may transform voids into penny-shaped cracks along the cleavage planes of the matrix. A subsequent growth of the cracks results in fracture of the material. Dependence of the critical amount of stored energy required for the void-crack transition on the mean size of the voids is estimated and compared with experimental data.