There are many nickel-based superalloys with the most famous
of them often being Inconel 718 which is often used in gas turbine
applications. However, another commonly used precipitation hardenable
nickel-based superalloy is Inconel 706. Inconel 706 is a nickel-iron-chromium
alloy with similar composition to Inconel 718 while being more readily
machined, welded, and fabricated.
Inconel 706 is often used in gas turbine applications for
rotating parts, diffuser cases, and fasteners for its high temperature strength
and creep properties. It is strengthened
by precipitating the gamma prime and gamma prime prime phases which are formed
by the addition of niobium, aluminum, and titanium to the alloy. However,
unlike Inconel 718, the amount of aluminum is reduced with niobium and titanium
being the primary precipitating constituents.
Below are a number of photomicrographs of Inconel 706 at magnifications
of 50X, 100X, and 200X using two different etchants. The first set of photomicrographs
are taken using Beraha’s Tint Etch which works to highlight the grain
orientation while the second set of photomicrographs are taken using Chrome
Regia which works to highlight the grain boundaries. The samples were cut from Inconel
706 round stock which was ground and polished to a sub-micron final polish. The
sample was etched using either Beraha’s Tint Etch or Chrome Regia. The sample
microstructure consists of course equiaxed twinned grains with large MC
carbides.
Beraha’s Tint Etch
100 mL Water, 50 mL Hydrochloric Acid, 1 gram Potassium
Metabisulfite
Chrome Regia:
Chromic Acid Stock Solution (Part A): 250 mL Water, 1 gram
Chromium Trioxide
1 Part Chromic Acid Stock Solution, 1 Part Water, 2 Parts
Hydrochloric Acid
