Load relaxation studies of four alkali halides

Load relaxation experiments were performed on single crystals of KI, LiF, NaCl and NaF at room temperature (T/T_M = 0.30, 0.26, 0.27 and 0.23 respectively) and on LiF at 200 °C (T/T_M = 0.42). For LiF at 200°C and for KI, the resulting log σ - log ε{lunate} curves, called hardness curves, are concave downward independent of pre-strain history. At 22°C. the log σ - log ε{lunate} curves for well-annealed LiF are concave upward: as LiF is work-hardened, the hardness curves become straight or slightly concave downward. The hardness curves for NaCl and NaF are similar to those for work-hardened LiF. These observations are in qualitative agreement with the work of Hart and Li on metals in that at low homologous temperatures and low hardness levels the hardness curves are concave upward; as the temperature and/or hardness level is increased, the hardness curves become first straight and then concave downward. This behavior suggests the following: at low homologous temperatures, glide controlled processes dominate and a model in which the strain rate ε{lunate} is a function of the effective stress σ_∝ to the power M (ε{lunate} ∞ σ_∝M) is adequate. At higher homologous temperatures, thermally activated processes predominate.