Dislocations - 9. Cross-slip of a Screw Dislocation

CROSS-SLIP OF A SCREW DISLOCATION. Electron micrograph and perspective view.

9.0 CROSS-SLIP OF A SCREW DISLOCATION (ELECTRON MICROGRAPH) T17 T18 The zig-zag band on the right electron micrograph gives evidence of cross-slip of a screw dislocation in a foil of aluminum. Cross- slip is the change from one slip plane to another, when the shear stress on the second plane is sufficient to cause the dislocation to glide on it. It is commonly observed in aluminum and in body- centered-cubic metals.
9.l CROSS-SLIP OF A SCREW DISLOCATION. T19 A screw dislocation changes its plane by constricting on the first plane and then bowing out onto the new one. The cross-slip progresses by migration of the constrictions along the dislocation. Gliding up the new plane to the top, it shears the top front quarter relative to the remainder. R The intersection of the cross-slip and primary planes must be parallel to both the dislocation and its Burgers vector; so only screw dislocations cross glide. In addition to stress on the cross glide plane, thermal activation is needed to start the constriction. A more extended dislocation needs greater activation energy. Questions related to: CROSS-SLIP OF SCREW DISLOCATIONS Can a screw dislocation change slip planes? CROSS-SLIP Does cross slip become more frequent at higher temperature? Does the whole new segment need to cross slip at once? What is evidence for such cross slip? WAVY GLIDE What is the result of screw segment of a loop undergoing repeated cross slips? See Answers