Abstract
The nucleation and growth behaviours of spherical ordered γ' (L12)-phase and disc-shaped ordered γ'' (DO22)-phase precipitates in Inconel 718 have been investigated mainly by micro-Vickers hardness test and transmission electron microscopy.In this study Inconel 718 type alloy (718) and it's modified Ni-base alloy (718M) were compared.The hardness of 718 and 718M aged in various conditions closely related to the size and the quantity of γ', γ'' precipitates and δ-phase.Decreasing the (Al+Ti) concentration in alloy while maintaining the Nb content increased the driving force both to coarsen γ'' precipitates and form δ-phase.It was considered that γ' and γ'' precipitates in 718 nucleated almost simultaneously at the early stage of aging.The growth kinetics of γ' (718) and γ'' precipitates (718, 718M) in the region free of δ-phase followed the predictions of LIFSHITZ-SLYOZOV-WAGNER (LSW) theory of diffusion controlled growth at all aging temperatures. The activation energy for the growth of γ' precipitates in 718 was estimated to be 245 kJ/mol which was nearly similar to those of diffusion of solute atoms; i. e. Al, Ti and so on, in Ni and Ni-base alloys. The activation energies for the growth of γ'' precipitates in 718 and 718M were estimated to be 451 and 464 kJ/mol, respectively.
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