A NOVEL LIGHT OPTICAL METALLOGRAPHIC TECHNIQUE FOR REVEALING GRAIN STRUCTURES OF ALUMINUM ALLOYS

摘要

A new metallographic technique has been developed for revealing the grain structures of 2000, 5000, and 7000 series aluminum alloys. This technique consists of four steps: (1) The aluminum alloy is solution heat treated and water quenched. (2) An optimized precipitation heat treatment then produces relatively coarse precipitates at grain boundaries and fine precipitates in the grain interior. (3) The samples are then polished and deliberately overetched. The chemical attack at the grain boundaries is accentuated because of increased susceptibility of intergranular corrosion of the alloy after the heat treatment specified in steps 1 and 2. (4) The overetched sample is then lightly polished to remove etch pitting and etched precipitates in the grain interior. The grain contrast of samples prepared with this technique is far superior to that of any existing etching technique for aluminum alloys.