Direct evidence of reversed plastic flow around indentations in magnesium oxide

An etch-pitting technique has been used to compare the dislocation distribution around unrecovered, i.e. with a load still applied to the indenter, and recovered impressions in single crystal MgO. Thus it is shown that, immediately upon removing normal loads greater than 100 g (0.98 N), additional slip lines are developed on the surface surrounding the actual indentation. Reversed plastic flow, in previously undeformed material adjacent to the region which has become work-hardened during the formation of the impression, is considered to be responsible for this phenomenon. It is suggested that this effect is caused by elastic recovery in the work-hardened region as the load is removed. Whilst this appears to have a negligible effect on the accuracy of hardness measurements, it could be an important contribution to cumulative processes of deformation and fracture as in comminution and wear. (author) 23 refs., 3 figs