Two kind of nickel based superalloys were chosen for the investigation of their phase transformation and solidification behavior in this dissertation. The first alloys were three polycrystalline superalloys DK3, DK4 and DK5, which were derived from the chemical composition of GTD111 alloy and IN738 alloy. The second alloys were directional solidified AM3 alloy and AM3 alloys with different Re additions. The microstructures and tensile mechanical properties of the three DK alloys had been investigated. Quantitive analysis were done to measure the microsegregation and as-cast microstructure of AM3 alloy and Re containing AM3 alloys.
The minor phases in the as-cast microstructures of DK alloys had been indentified. Platelet like η phase existed near eutectics in alloy DK3, while both platelet like η phase and nodular like σ phase existed near eutectics in alloy DK4 and DK5. It was found that big Ti/Al ratio in the interdendritic region would favor the formation of η phase during the casting process. When the (Ti+Al) value in the chemical composition of the alloy was big enough, nodular σ phase would form coupled with η phase near eutectics during casting.
Alloy DK3, DK4 and DK5 had been heat treated with the procedure of 1100℃/2h plus 843℃/6h. During solution treatment, γ′→η phase transformation happened in the eutectics of alloy DK3. η phase nucleated from the Ti rich coarse γ′ in the eutectic periphery, and grew with plate like morphologies towards into the eutectics. In alloy DK4 and DK5, the nodular σ phase transformed into γ phase during solution treatment, the interfaces between the nodular σ phase and γ phase were of serrated like characteristic. When thermo exposed the heat treated alloys at 850℃, γ′→η phase transformation continued in alloy DK3, which accompanied with formation of M23C6 particles at the η phase boundary. Extensive σ phase precipitation happened in the interdendritic region of alloy DK4 and DK5 during thermo exposure. The γ phase which formed from nodular σ phase acted as nucleation sites for the precipitation of plate like σ phase during thermo exposure. Platelet like η phase decreased the tensile properties of alloy DK3 while nodular σ phase didn’t have obvious impact on tensile properties of alloy DK4 and DK5.
The formation mechanism of the nodular σ phase and η phase which existed in the as-cast microstructure of alloy DK5 had been analyzed by SEM and TEM observation. It was found that nodular σ nucleated and grew directly from the liquids, because no orientation relationship existed between nodular σ and γ phase. The σ phase might have grown by trapping of atom clusters from its surrounding liquid, which was similar with the growth mechanism of quasicrystals during rapid solidification. The η phase had regular platelet morphology and definite orientation relationship with γ phase, which indicated that it precipitated from solid during cooling process.
The phase transformation mechanism of γ′→ η phase in alloy DK3 during solution treatment was studied by TEM observation. It was found that η phase nucleated from the stacking faults in γ′ phase, and grew with plate like morphology by slippling of superpartial dislocations in γ′. The η phase had certain orientation relationship with γ′ phase, which was {0001}η//{111}γ′, η//<110>γ′. The phase interface between η phase and γ′ phasse was semicoherent, two phases shared one layer of atoms at their interface. Introducing of alloying elements of big atom sizes such as Ti and Hf decreased the stacking faults energy of γ′, which promoted the phase transformation of γ′→ η.
AM3 alloy and AM3 alloys added with different Re had been directional solidified under different withdrawal rates. Quantitive analysis of partition coefficients of major alloying elements showed that addition of Re aggravated the segregation of alloying elements. The higher content of Re in the chemical composition, the more severe of the alloying elements’ partition will be. The value of primary dendrite arm spacing is determined by the solidification rate, which decreases with the increase of solidification rate. Re addition does not have any effect on the value. As-cast eutectic fraction levels increases with both nominal Re content and withdrawal rate, but Re has more apparent impact on the eutectic fraction than withdrawal rate.
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