Abstract
The pre-precipitation processes of γ' and γ'' intermetallic phases and their effects on age hardening processes in an alloy 718 have been examined in relation to both solution treatment and aging temperature. The results were analyzed mainly in terms of the behaviors of quenched-in excess vacancies as well as constituent atoms of γ' and γ'' precipitates, Ti, Al and/or Nb, around as-grown dislocations. Dislocation dipoles linked to each other formed remarkably during the early stage of aging in the specimen solution-treated at higher temperature. The dislocation dipoles disappeared almost simultaneously with the initiation of γ' and/or γ'' precipitations, and eventually age hardening started. In the specimen solution-treated at the lowest temperature the most rapid age hardening occurred due to the γ' and γ'' intragranular precipitations under the adequate migration of the constituent atoms without the formation of the dislocation dipoles. The initiation of precipitation hardening in the early stage of aging was confirmed to depend greatly on the behavior of the constituent atoms associated with the excess vacancy migration around as-grown dislocations. It is concluded that the formation of dislocation dipoles suppresses the γ' and or γ'' precipitations due to the segregation of the constituent atoms to the dislocation dipoles, and the mechanisms of the segregation and dissipation of the constituent atoms around the dislocation dipoles are discussed.
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