A new light optical metallographic technique for revealing grain structures of common 2000, 5000, and 7000 series aluminum alloys

A new metallographic technique has been developed for revealing the grain structures of common 2000, 5000, and 7000 series aluminum alloys. This technique consists of four steps: 1. aluminum alloy is solution heat treated and water quenched so as to keep soluble elements such as Cu, Mg, and Zn in supersaturated solid solution. optimized precipitation heat treatment then produces relatively coarse precipitates at grain boundaries and fine precipitates in the grain interior. These precipitates are θ-CuAl2 + S-Al2CuMg in 2000 series, β-Mg2Al3 in 5000 series, and η-MgZn2 in 7000 series aluminum alloys. allographic samples are prepared from materials heat treated as specified in steps 1 and 2. The samples are then polished and deliberately overetched. The relatively coarse grain boundary precipitates accentuate the chemical attack at the grain boundaries. overetched sample is then lightly polished to remove etch pitting and etched precipitates in the grain interior. Excellent grain boundary contrast is now produced. ain contrast of samples prepared with this technique is far superior to that of any existing etching technique for aluminum alloys. This new technique will make grain-size measurement much easier and more accurate. The concept of this technique may also be applicable to 3000 and 6000 series aluminum alloys, aluminum metal-matrix composites, and other nonaluminum single-phase alloys in which revealing grain boundaries may otherwise be difficult. In this paper, the metallurgical and metallographic principles related to this novel etching technique are presented.