Aluminum is a material that is often used for its strength to
weight ratio, good machinability, corrosion resistance, and cost. Aluminum 6061
is the most commonly used aluminum alloy. It contains magnesium and silicon as
its primary alloying elements and is precipitation hardenable.
While 6061 is precipitation hardenable, grain
size does impact the strength of the alloy due to grain boundary strengthening.
Typically, fine equiaxed
grains are desired to maximize strength and ductility.
For many
aluminum alloys, it is very difficult to reveal the grain boundaries of the
alloy. The below photomicrographs show a 6061 alloy with elongated grains. The
grain boundaries were revealed using a two-step etch. First, Graff-Sargent
reagent was used. Then, Weck’s TInt etch was used on the sample. This two-step
etch allowed crisp grain boundaries to be resolved as well as many of the
intermetallic precipitates within the grains and along the grain
boundaries.
Graff-Sargent
Reagent:
84 mL water,
15.5 mL nitric acid, 0.5 mL hydrofluoric acid, 3 grams chromium trioxide
Weck’s Tint Etch:
100 mL water, 4 grams potassium permanganate,
1 gram sodium hydroxide
