Influence Of Heat Treatment On Microstructure And Creep Behaviours Of GH4169G Superalloy

By means of heat treatment, creep property measurement and microstructureobservation, an investigation has been made into the influence of the heat treatmentregimes on the microstructure and creep behaviours of GH4169G alloy, and the phasestransformation mechanisms during heat treatment, deformation and fracture mechanisms ofalloy during creep are discussed.Results show that, microstructure of GH4169G alloy consist of γ matrix, particle-likeγ′, disc-like γ" phase and δ phase, and the coherence interfaces are kept in between thephases. A large numble particle-like γ′and γ" phase are obtained in the alloy after directlyaged, and significant amount of needle-like δ phase are precipitated along boundaries afterstandard heat treatment and long term aging treatment. Thereinto, the elements Cr, Fe areenriched in the regions near boundaries, and the coherence interfaces are kept in between(220)δ and (200)γplanes, which are thought to be the main reason of the δ phase keepingthe needle-like configuration. During aged, the γ" phase is nucleated and precipitated fromthe γ′phase, so that the γ′-Ni3Al phase with L12structure is transformed into the γ″-Ni3Nbphase with DO22structure. As the aging time prolonges, the γ" phase is gradually grown upinto the congeries of the unit cells and the Nb atoms are diffused into the ones. In thefurther, a new unit cell formed in the congeries with DO22structure occurs the directionalshearing strain and ordering transformtion, which makes the γ″phase transforming into theδ-Ni3Nb phase with DOastructure.In the ranges of the applied temperaturees and stresses, the creep activation energiesfor the directly aging alloy, standed heat treated alloy, solution alloly at high temperatureand long term aging alloy are calculated to be Q1=588.0kJ/mol, Q2=546.8kJ/mol, Q3=480.6kJ mol and Q4=404.3kJ/mol, respectively. And the ITA-DA-GH4169G alloydisplays a better creep resistance. The slipping of dislocations and twinning are determinedto be the deformation mechanism of alloys during creep, and the {111} planes areidentified to be twinning plane. Moreover, the traces P and B may promote the carbide phase precipitated along the boundaries to restrain the boundaries slipping, which isthought to be the main reason of the alloy having a better creep resistance.As the creep goes on, the single oriented slipping of dislocations is hindered in theregions near the boundaries to bring the stress concentration, which may promote theinitiation and propagation of the cracks along the boundaries vertical to the stress axis, upto the occurrence of creep fracture along the intergranular. Comparded to the free-P, Balloy, the fracture of GH4169G alloy during creep displays the non-smooth surface, whichis attributed to the addition of the traces P and B to enhance the bonding strength betweenthe grains for restraining the boundary slipping this is thought to be the main reason forenhancing the creep resistance and causing the non-smooth surface of creep fracture.