Aging of (nickel-base) alloy N35KT

Conclusions

• 1. The mechanism of decomposition of the solid solution during aging of a prestrained nickel-base alloy N35KT is similar to the mechanism of decomposition of the solid solution and alloys of the Ni-Ti and Ni-Cr-Ti systems. It proceeds in two stages: the first consists in a redistribution of the atoms of Ti, Ni and other elements in the lattice of the solid solution with a consequent precipitation of dispersed particles of a metastable cubic phase with a parameter of 60 A. The second stage consists in a change of the cubic lattice into a hexagonal one.

• 2. The transformation of the cubic lattice into hexagonal is accompanied by a diffusion of iron atoms from the lattice of the secondary phase into that of the solid solution and replacement at the nodes thus freed by nickel atoms. The transformation of the lattices during the early stage of aging (575–625°C) begins from the grain or twin boundaries, while at a higher temperature it develops within the grain also.

• 3. The strength of the alloy depends on the nature of the strengthening phases, their size, distribution and their relative amount in the structure. Precipitation of a finely dispersed cubic phase and the growth of its particles to 450–500 A result in a strengthening of the alloy during aging. The microstructure of a fully hardened alloy consists of a solid solution, a dispersed cubic phase and a small amount of a fine lamellar hexagonal phase.

• 4. Softening (overaging) of the alloy is associated with transition of the cubic phase into a hexagonal one and its coalescence.