Surface and interior measurements of grain boundary sliding during creep

Measurements of grain boundary sliding have been made on polycrystalline specimens of Magnox AL80, a magnesium-0.78 wt% aluminium alloy, at successive strains during creep at 200° C under a stress of 2800 psi. Three independent methods were used to determine the strain due to sliding (_gb) at the surface and two to determine_gbin the interior of the specimens. The one direct method of measuring_gbin the interior used oxide markers introduced by extruding a composite billet. The values of_gbobtained from the offsets in these interior markers were found to agree with those given by the three sets of measurements made on the surface, but not with those from the indirect method for the interior which relies on the measurement of grain strain via grain shape changes.