Dislocation dynamics in pulse-loaded NaCl crystals

The mean path l of freshly introduced dislocations in NaCl crystals under t he effect of triangular loading pulses tau (t) is shown to depend only on t he pulse amplitude tau (m) and be insensitive to the rate of stress growth. The replacement of triangular pulses with trapezoidal ones with a constant -load plateau (tau = const) extension of up to 60 min only insignificantly changes the l(tau (m)) dependence. The data obtained are interpreted on the basis of the concept of quasi-static relaxation in a nonequilibrium system of dislocations subject to a combined effect of time-dependent applied str esses tau (t), coordinate-dependent internal stresses tau (i)(x), and "dry friction" tau (p) due to the pinning of dislocations by point defects. In s uch a model, the l(tau (m)) dependence should saturate at tau (m) > 2 tau ( p); this is in fact observed in the range of 0.2 tau (y) < tau (m) < 0.3 ta u (y) (here, tau (y) is the yield stress), which gives an estimate for the pinning stress tau (p) approximate to 0.1 tau (y) . Based on the model sugg ested, a series of experimentally confirmed predictions were obtained, e.g. , a recipe of "switching-off" of anomalies. Thus, a preliminary treatment o f the samples by a series of stress pulses or holding in a magnetic field, which transforms the system of fresh dislocations into a more equilibrium s tate, sharply decreases the density of mobile dislocations quasi-statically responding to a pulsed load. It is shown that the discussed anomalies of d islocation mobility should be observed only in sufficiently pure crystals, where tau (p) < tau (y) , and should be absent in contaminated crystals, wh ere tau (p) similar to tau (y) . (C) 2001 MAIK "Nauka/Interperiodica".