NONMONOTONIC CREEP IN LIF CRYSTALS CONTAINING MG

Precision measurements of the creep rate using interferometric recordi ng of the process in LiF crystals containing 0.002 to 0.03 wt % Mg usi ng interferometric recording of the process show that nonmonotonic epi sodes in the form of alternating segments with relatively high and low creep rates periodically appear against a background of overall creep attenuation. The accumulation of creep strain takes place mostly on t he segments with increased rates, which causes kinks to appear in the creep curve that are noticeable when shifts are resolved at a level of fractional microns. Measurements were made of the height of steps L b ased on ''rate-strain increment'' curves for stresses smaller and clos er to the yield point. Increasing the stresses causes the value oft to fall off, while increasing the Mg content causes it to increase. It i s proposed that the nonmonotonic behavior of creep is connected with s tructural nonuniformity of the material, which is determined by the im purity content and the nonuniformity of the strain process itself. For small strains in the neighborhood of the yielding area, where strain comes about as a result of a broadening of a pre-existing slip band, t he impurity atoms enhance the nonuniformity of the strain (L increases ). As the strain (stress) increases, L decreases, which corresponds to a smoothing out of the nonuniformities. It is emphasized that the ste p-like character of the strain accumulation is a general properly. (C) 1998 American Institute of Physics.