Strengthening of Alkali Halides by Divalent-Ion Additions

Single crystals of NaCl, NaBr, KCl, and KBr containing divalent additions of Ca²⁺, Sr²⁺, and Ba²⁺ were tested mechanically. In the solution-treated condition, the yield strength, σ _v , as determined from compression testing in a 〈100〉 direction is essentially dependent on the concentration of the dopant only and is independent of the species of either the dopant or of the host material. All crystals soften on aging, with the exception of the NaCl:Ca²⁺, NaBr:Ca²⁺, and NaBr:Sr²⁺ systems. In addition, correlation was good between σ _v , and the Knoop hardness number, H , obtained by indentation with the long axis of the indenter aligned in 〈100〉. The equation is of the form σ _v = C ( H–H ₀), where C ? 0.21 for all four halide families and H₀ is near the hardness value of the pure halides. Furthermore, H ₀?5×10^?3E₁₁₁, the Young's modulus in the 〈111〉 direction. Hence σ _v ?0.21 H –10^?3E₁₁₁.