Nonmonotonic creep in LiF crystals containing Mg

Precision measurements of the creep rate using interferometric recording of the process in LiF crystals containing 0.002 to 0.03 wt % Mg using interferometric recording of the process show that nonmonotonic episodes 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 the 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 based on “rate-strain increment” curves for stresses smaller and closer to the yield point. Increasing the stresses causes the value of L to fall off, while increasing the Mg content causes it to increase. It is proposed that the nonmonotonic behavior of creep is connected with structural nonuniformity of the material, which is determined by the impurity 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, the 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 step-like character of the strain accumulation is a general property. Fiz. Tverd. Tela (St. Petersburg) 40, 690–693 (April 1998)