1Nagoya Aircraft Works, Mitsubishi Heavy Industries, Ltd., Nagoya
Precipitation behaviors of 18%Ni maraging steels containing Ti were studied by measuring the electrical resistivity at liquid nitrogen temperature.
On isochronal aging, Mo-rich zones which are significant precipitates in lower temperature aging, are formed following the precipitation of Ni3Ti(DO3) ordered phase at lower temperature region, and a metastable Ni3Mo phase is produced after the precipitation of Ni3Ti(DO24)η phase at higher temperature region.
The presence of Co contributes not only to acceleration of the precipitation of Mo-rich zone, but also to promotion of the precipitation of Ni3Ti(DO3) ordered phase.
On isothermal aging in the 18%Ni maraging steels with lower Ti content, decrease of the electrical resistivity is predominantly governed by the precipitation of Ni3Mo at temperatures over 753 K, but the 18%Ni maraging steels with higher Ti content always show the decrease of the electrical resistivity governed by the precipitation of Mo-rich zone at an early stage of the aging, even at 753 K and more, because the precipitation of Ni3Mo is retarded by presence of Ni3Ti(DO24)η phase.
The activation energy for the precipitation reaction of Mo-rich zone increases with Ti content in the 18%Ni maraging steels. The activation energy in the steels without Ti is estimated at 150 kJ/mol by extrapolation from the relationship between the activation energy and Ti content, and the value is consistent with the data reported previously.
It is presumed that the reason why the activation energy in the precipitation reaction of Mo-rich zone increases with Ti content is that precipitation sites of Mo-rich zone are occupied by the Ni3Ti(DO3) ordered phase produced prior to the precipitation of Mo-rich zone, and the Ni3Ti(DO3) precipitates form a potential barrier against the precipitation of Mo-rich zone.
maraging steel, electrical resistivity, isochronal aging, isothermal aging, molybdenum-rich zone, Ni3Ti(DO_3) ordering, Ni3Ti(DO_24)η phase, Ni3Mo metastable phase, activation energy for aging
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