Microstructure of a 12H Mg‐Si‐Al‐O‐N Polytype Alloy: Intergranular Phases and Compositional Variations

The mil‐restructure of a 12H polytype alloy having a composition Mg, 86 A1, 67 Si 2.47 O 3.19 N 3.81 was examined by transmission electron microscopy with the techniques of X‐ray spectroscopy microanalysis and lattice fringe imaging. An intergranular film (2.5 to 10 nm thick) is present at the majority of two‐grain junctions. At several boundaries, the polytype fringe spacing decreases abruptly near the boundary, indicating a change in poiytype structure and composition. The intergranular phase at three‐grain junctions is partially crystallized and the crystalline regions incorporate principally Mg and Si cations, whereas the surrounding amorphous phase contains Si, Mg, Ca, and Al cations. The composition of one of the glassy regions is calculated as about Mg 2.5 Si 0.42 Al 0.13 Ca 0.95 (ON).