Effect of Temperature on the Deformation of KCl-KBr Alloys

At low temperatures, ionic solids of the rock-salt structure deform by planar {110} slip and are brittle; at higher temperatures slip can occur on any plane of the zone and fully ductile behavior is observed. The effect of alloying on this pattern of behavior has been studied for the KCl-KBr system. Pure polycrystalline KCl and a 0.6% KBr alloy are shown to be ductile at temperatures above 250°C, but a 1.3% alloy polycrystal remains brittle up to 350°C. Associated with this brittleness is the persistence of planar slip to much higher temperatures. More concentrated KBr additions suppress wavy glide in single crystals to temperatures approaching the melting point of the alloys. Data are presented to demonstrate the effect of KBr additions on the yield strength, work-hardening rate, and fracture stress of polycrystalline KCl and two dilute alloys. Small solute additions appear to influence work-hardening behavior much more than the yield strength, but more concentrated additions increase both significantly. The effect of KBr additions on glide behavior appears to be associated with changes in the core energies of dislocations of the zone. Although the glide behavior of pure ionic crystals can be related to the polarizability of the constituent ions, a similar correlation is invalid for ionic solid solution alloys.