Lecture Questions for Nickel Superalloys Flashcards
What is the three phases occurring within Nickel Superalloys? How do the phase contribute to its strengthening
properties?
y (gamma). This is a continuous matrix of fcc austenite. It is solid solution strengthened by some of the other alloying elements such as, Cr, Mo, Ti, W, Co, Fe and Al. The slow diffusing elements such as Mo and W are good for enhancing high temperature creep resistance.
y’ (gamma prime). This is the major precipitable phase (Ni3Al). A coherent precipitate (there is only a about 0.1% mismatch between y and y’), coherency is maintained by a tetragonal distortion.
Carbides. There are various types, but mainly “M”23C6 and “M”C. (where M represent the metal). M”C Carbides
(Monocarbides): “M” is usually Ti, Ta, Nb or W. They are very stable and form just below solidification temperature. They
restrict grain growth during solution treatment. Blocky, needle like form, distributed sparsely well at GB. “M”23C6
Carbides “M” is usually Cr but can be Fe, W, Mo or Co. These carbides form at lower temperature heat treatments, and in
service, at temperatures in the range 760 – 980oC. Finer particles at GB and at y channels. “M” 6C Carbides: Form at
temperatures in the range 815 – 980C, they are similar to “M”23C6 and form when Mo and W contents are high. Eg. in
M252 alloy, which contains 6 – 8wt% Mo or W; “M” 6C and “M”23C6 form on grain boundaries. Fine and dispersive “M”
6C particles can pin down dislocation movement, prevent γ’ phase to be sheared and thus strengthen the superalloy.
What is the metallographic structure of Cobalt Superalloys and how does it contribute to its properties?
Wrought Co superalloys contain ~ 40% Co and increased Ni. Cobalt based alloys are not as high strength as Ni based but they
maintain their properties to higher temperatures. There is basically a y matrix and carbide precipitates. The strength
therefore comes from solid solution strengthening and carbide precipitation. The carbides are particularly important as
there is no y’ precipitate, hence the carbon content is relatively high at ~ 0.1 – 1.0wt%. Co have lower strength at intermediate temperatures due to the lack of coherent y’ precipitation which, all the Ni superalloys have. Co based only has carbides and solid solution strengthening, hence it cannot compete in the high stress applications. So it is used for low stress, high temperature, long life applications.
