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Mar-M247 is a typical cast Ni-base superalloy for polycrystalline. Due to optimal alloy design and microstructural modification, this alloy exhibits superior strength and excellent creep resistance at elevated temperature. However, Mar-M247 demonstrates a low ductility, particularly under the creep conditions of moderate temperatures and high stresses. Low ductility of Mar-M247 superalloy has significantly affected the safety in aerospace applications. In this thesis, the reasons resulting in brittleness and the mechanisms responsible for ductilization and strengthening of Mar-M247 superalloy are systematically studied. The methods of physical and chemical metallurgy for improving the brittleness problem are employed in the present study.
In the beginning of this thesis, a single crystal of Mar-M247 is prepared via Bridgman technique in order to investigate whether the alloy with single crystal has the brittleness problem or not. The experimental results show that the elongation of single crystal is much greater than those of polycrystalline both at 899℃ and under 760℃/724MPa. The rupture in single crystal is along the ductile γ-γ'' eutectic, while the fracture of polycrystalline occurs primarily along the carbide/matrix interface or at carbides. Apparently, single crystal of Mar-M247 superalloy has no brittleness problem. In further research, two typical grain sizes are prepared using a standard pouring parameter. The test results show that the creep behavior of fine grain alloy (3 mm) is much better that coarse grain (8 mm) under moderate temperature and high stress, in addition, the rupture life and elongation of coarse grain alloy can not meet the requirement of EMS-55447 (Engine Material Specification). However, the alloy with coarse grain has superior creep resistance under 982℃/200MPa.
In third part, the hot isostatic pressing (HIP) is adopted to eliminate the porosity in castings. The results reveal that the improvement of rupture ductility under 760℃ /724MPa is limited, and the fracture still occurs at the sites of GB carbides or the script MC within grain interior. Therefore, the carbide characteristics of Mar-M247 superalloy play a main role for the moderate temperature brittleness during creep.
On this basis, this thesis employs double solution treatment and micro-allying to change the carbide characteristics. For the processing of double solution treatment, a thick γ'' film surrounding the script-like carbide within grain interior and the carbides at GB can be formed. This γ'' film can enhance the toughness at interphases and accommodate the stress state near carbide interphase. Double solution treatment can not only upgrade the high temperature (899℃) tensile elongation but also improve the 760℃/724MPa rupture elongation. In the research of Mg micro-alloying, the contents ranging from 30 to 80 ppm Mg are added in Mar-M247 superalloy. The optical observation and AES analyses demonstrate that Mg segregates to the carbide/matrix interface, changing the primary MC carbide characteristics and inhibiting the script-like carbide formation. The measurement using image analyzer also prove the fact of carbide refinement and spheroidization. The mechanical tests show that the microaddition of Mg can dramatically improve the rupture life and elongation. In particular, the rupture elongation of the alloy containing 30~80 ppm Mg is enhanced up to 3~5 times that of Mg-free alloy during the 760℃/724MPa creep test. The fracture analyses demonstrates that cracks are mainly initiated and propagated at the interface of script-like MC carbides in the Mg-free Mar-M247 superalloy at elevated temperature. Because of the effective refinement and spheroidization of MC carbides, a change in the crack initiation occurred from the carbide/matrix interface to that along the γ-γ'' eutectic, that is, an evidence of ductile of fracture mode. Under the condition of 982℃/200MPa, the rupture life and elongation in an over-addition alloy (80 ppm) clearly decreased owing to the formation of large amount of MC carbides present at GB. Hence, the microaddition of 30-50 ppm Mg is able to obtain the optimal microstructure and excellent creep properties under various creep conditions.
Through the present research and investigation, the brittleness of Mar-M247 superalloy occurred at moderate temperature is attributed to the existence of script-like MC carbides within grain interior and at GB, facilitating the crack initiation and propagation. In this thesis, the appropriate pouring parameters to control optimal grain size, the double solution treatment and Mg microaddition can effectively change the carbide characteristics and improve rupture life and elongation. The applications of present results can upgrade the performance of engine and the key components of system.
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