2
It is especially fortunate that considerable
microstructural analysis of successful (and unsuccessful)
alloys has accompanied this standard approach to alloy
design. The property-microstructure relationships
developed as a result of correlations made following this
approach have been instrumental in elucidating many of the
known strengthening mechanisms that are now known to
contribute to both high and low temperature strength in
metals. These mechanisms include, among others,
1) precipitation strengthening, 2) solid solution
strengthening, 3) order strengthening, and 4) dispersion
strengthening. An alphabet of elements may be required to
activate these mechanisms and alloys such as TRWNASAVIA
(composition, at.%: Ni-61.0, Cr-6.1, Co-7.5, Mo-2.0,
W-5.8, Ta-9.0, Cb-.5, Al-0.4, Ti-1.0, C-0.13, B-0.02,
Zr-0.13, Re-0.5, and Hf-0.4) were developed seemingly as
confirmation of the old superalloy adage, "The more stuff
we put in, the better it is."
Though TRWNASAVIA is an extreme example of maximizing
desirable properties based on intentional additions (the
alloy contains 1/7 of all known naturally occurring
elements), many other superalloys also contain a large
number of intentionally added elements. These superalloy
compositions are often based on the Ni-Al system. The
microstructure of this "average" Ni based superalloy
consists of essentially three distinct microstructural
features. The first is the matrix, which is usually