Papers by Keyword: γ'' Precipitate

Abstract: Laser rapid forming (LRF) is introduced as a novel fabrication process for Ni-based superalloy Rene88DT. The effect of heat treatment parameters of quenching rate and aging time on size and distribution of γ′ precipitation was investigated. The heat treatment parameters were first determined by DSC, and then optimized based on the examination of the microstructure and mechanical properties of heat treated LRF Rene88DT. The experimental results show that, the precipitation of γ′ is inhomogeneously distributed as a result of uneven heat-cycle during LRF in as-deposited Rene88DT, resulting in low mechanical properties. After being heat treated at 1165°C, 2h/AC + 760°C,28h/AC, γ ′ precipitation are homogeneously distributed with the size of 40~60nm, and the tensile strength of heat treated sample shows an increase of 400MPa as compared to that for as-deposited. The yield strength is close to that of the PM+HIP standard.

Abstract: It has been elucidated that the small creep strain must be essential to form the rafted γ/γ' structure using a single crystal nickel-based superalloy, CMSX-4. To confirm the amount of the strain required to make rafting, the single crystals interrupted the creep tests up to a strain of 0.01 at 1273K-250MPa was aged at 1273K without stress. To compare the difference in the morphology of the γ/γ' structure of the as-heat treated and the creep-interrupted single crystal with the simple aging, the as-heat treated single crystal was also aged. Microstructural observations by SEM were conducted for the specimens sectioned parallel to (100). The cuboidal γ' precipitates of the as-heat treated single crystal connected each other with three <100> directions after the simple aging. By employing the aging without stress on the specimen interrupted the creep test at a strain of 0.0074, the drastic morphological change in the γ/γ' structure was confirmed, that is, the γ/γ' structure changed to rafted one perpendicular to the creep-interrupted stress axis. The aspect ratios of the creep-interrupted specimens increased with increasing simple aging time, and attained to the maximum value at the simple aging time of 3.60x106s. The maximum value of the aspect ratios increase with increasing creep-interrupted strain and attained 4 at a strain of 0.0074. The dislocation density at the γ/γ' interface increases with increasing creep strain. Consequently, the formation of the rafted γ/γ' structure requires the dislocation substructure induced by the creep strain.

Abstract: The precipitation behaviors of γ″(Ni3Nb) in four Ni-base alloys were investigated. The four alloys were forged Ni20Cr20Fe5Nb alloy, mechanically alloyed Ni20Cr20Fe5Nb alloy, IN 718 alloy and ECAPed(equal channel angular pressing) IN 718 alloy. Aging treatment was employed at either 600°C or 720°C for 20 hrs. The TEM observation and hardness test were performed to identify the formation of γ″. The precipitation of γ″ was noticed after aging at 600°C for 20 hrs in the mechanically alloyed Ni20Cr20Fe5Nb alloy and ECAPed IN 718 alloy, while it was observed after aging at 720°C for 20 hrs in the forged Ni20Cr20Fe5Nb alloy and IN 718 alloy before ECAP. It seemed that the lower aging temperature for γ″ precipitation in the mechanically alloyed Ni20Cr20Fe5Nb alloy and ECAPed IN 718 alloy than in the forged Ni20Cr20Fe5Nb alloy and IN 718 alloy before ECAP appeared to be due to the severe plastic deformation which occurred during mechanical alloying or ECAP.

Abstract: The formation behavior of γ″(Ni3Nb) precipitates in IN 718 alloy before and after ECAP(equal channel angular pressing) was investigated by microstructural observation and the hardness test. For the alloy examined before ECAP, the γ″ precipitates were formed only after aging treatment at 720
, whereas after ECAP, the γ″ precipitates were formed at the aging temperatures of both 600
and 720
. 600
is normally too low a temperature for γ″ precipitates to be formed in commercial IN 718 alloy, however, they were able to be formed due to severe plastic deformation by ECAP. It was found that the ECAP process changed the formation behavior of the γ″ precipitates in IN 718 alloy.