Glide band formation and broadening in ionic single crystals

Abstract

The many differences in glide band formation and broadening between hard LiF (Oilman and Johnston 1960) and soft NaCl (Mendelson 1962 a) can be understood when the stress fields from neighbouring dislocation arrays are evaluated. The bands form in avalanches in about 1/10 sec. The resulting estimated dislocation velocity is several orders of magnitude greater than that reported for hard LiF. Calculations show that a shear stress of three to four times the applied stress exists on neighbouring planes in a glide band having several like sign closely spaced dislocation arrays. This can account for the high dislocation velocities. It can also account for the observed hardness and stability of the dislocations at the spearhead of the glide band (Mendelson 1962 a). The calculations also explain why the sources in different glide bands tend to lie along a direction normal to the slip planes and account for the tendency for flow to occur on a single system even though several systems are equally favoured for slip. A simple dynamic model is proposed to explain why band broadening occurs during the avalanche and not after it ceases. The model can also explain the effects of strain rate, strength and dimensions of the crystal. The criterion governing whether flow occurs by avalanches or by continuous band broadening appears to be the relative proportion of the total strain associated with each source. If this is large, band broadening can cease; if small, continuous band broadening will be the mode of flow.