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Microhardness measured on cleaved surfaces of nominally pure and lightly calcium‐doped NaCl bicrystals showed a modest increase at the grain boundary that could be attributed to segregation of calcium and a larger increase in both bulk and boundary produced by rapid cooling. Variations observed due to the influence of moisture, to nonuniform quenching rate, and to precipitation at the boundary suggest that the quench hardening is due to residual impurity other than calcium.
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Harris et al. (1982) studied this question.
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