THE CASTING AND HEAT-TREATMENT OF SUPERALLOY 444 UNDER VACUUM

This work describes the casting of a nickel base superalloy '444' and the results of microstructural examination of the as-cast and annealed specimens. Superalloy '444' is used for the production of single crystal turbine blades for aircraft jet engines. The alloy whose composition (wt%) is 0.6% Cr, 1.98% Ti, 5.1% Al, 11.1% W, 0.91% Nb and the remainder nickel was vacuum melted and cast. Microstructural investigation of the alloy was performed using optical and electron microscopy techniques as well as electron-microprobe X-ray diffraction analysis. The results of these investigations show that the as-cast alloy possesses a pronounced dendritic structure with an inhomogeneous distribution of the alloying elements in the dendrites and in the interdendritic spacing. The λ′-phase volume fraction in the as-cast structure is 39% with an average particle size of the cubic-shaped particles being 0.6 μm. It should be pointed out that the particle size varies over a wide range. The alloy contains 2.2% of a non-equilibrium eutectic. Optical microscopy results show that, after annealing, the dendritic structure completely disappears, while microprobe analysis shows that a uniform composition of the alloy is obtained. During annealing λ′-particle size becomes more uniform and the volume fraction of λ′ increases up to 59% while the non-equilibrium eutectic dissolves. © 1990.