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Matrix Transformation in Boron Containing High-Temperature Co–Re–Cr Alloys
Pavel Strunz,Debashis Mukherji,Přemysl Beran,Ralph Gilles,Lukas Karge,Michael Hofmann,Markus Hoelzel,Joachim Rösler,Gergely Farkas 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.5
An addition of boron largely increases the ductility in polycrystalline high-temperature Co–Re alloys. Therefore, the effect ofboron on the alloy structural characteristics is of high importance for the stability of the matrix at operational temperatures. Volume fractions of ε (hexagonal close-packed—hcp), γ (face-centered cubic—fcc) and σ (Cr 2 Re 3 type) phases were measuredat ambient and high temperatures (up to 1500 °C) for a boron-containing Co–17Re–23Cr alloy using neutron diff raction. The matrix phase undergoes an allotropic transformation from ε to γ structure at high temperatures, similar to pure cobaltand to the previously investigated, more complex Co–17Re–23Cr–1.2Ta–2.6C alloy. It was determined in this study that thetransformation temperature depends on the boron content (0–1000 wt. ppm). Nevertheless, the transformation temperaturedid not change monotonically with the increase in the boron content but reached a minimum at approximately 200 ppm ofboron. A probable reason is the interplay between the amount of boron in the matrix and the amount of σ phase, which bindshcp-stabilizing elements (Cr and Re). Moreover, borides were identifi ed in alloys with high boron content.
Effect of Composition on the Matrix Transformation of the Co-Re-Cr-Ta-C Alloys
Přemysl Beran,Debashis Mukherji,Pavel Strunz,Ralph Gilles,Michael Hofmann,Lukas Karge,Oleksandr Dolotko,Joachim Rösler 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.4
Neutron diffraction measurement was performed in-situ at high temperatures on Co-Re-Ta-C alloys with and without Cr addition. This included alloys containing different C content with the C/Ta ratio varying between 0.5 and 1.0. The Co-Re-solid solution matrix of the experimental alloys is polymorphic (like in pure cobalt) and transformed from low temperature hexagonal ε phase to high temperature cubic γ phase on heating. This transformation is reversible and show hysteresis. The main alloying addition, Re, stabilizes the ε Co-phase and increases the transformation temperature to above 1273 K. The onset of the ε γ transformation during heating and cooling was found to differ depending on the alloy composition. In alloys without Cr addition the transformation was not completed on cooling and the high temperature γ phase was partly retained at room temperature in metastable state with the amount depending on the cooling rate from high temperature. The diffraction and microstructural results showed that Cr is ε stabilizer (similar as Re) but the role of Ta is not clear. The C/Ta ratio has no direct effect on the matrix phase transformation. Nevertheless, it influences indirectly by determining the amount of Ta which is freely available in the matrix.
Fangjie Mo,Erdong Wu,Changsheng Zhang,Hong Wang,Zhengye Zhong,Jian Zhang,Bo Chen,Michael Hofmann,Weimin Gan,Guangai Sun 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.5
The present work attempts to reveal the correlation between the microstructural defects and residual stress in the singlecrystal nickel-based superalloy, both of which play the signifi cant role on properties and performance. Neutron diff ractionwas employed to investigate the microstructural defects and residual stresses in a single crystal (SC) nickel-based superalloy,which was subjected to creeping under 220 MPa and 1000 °C for different times. The measured superlattice andfundamental lattice refl ections confi rm that the mismatch and tetragonal distortions with c / a > 1 exist in the SC superalloy. At the initially unstrained state, there exists the angular distortion between γ and γ’ phases with small triaxial compressivestresses, ensuring the structural stability of the superalloy. After creeping, the tetragonal distortion for the γ phase is largerthan that for the γ’ phase. With increasing the creeping time, the mismatch between γ and γ’ phases increases to the maximum,then decreases gradually and fi nally remains unchanged. The macroscopic residual stress shows a similar behaviorwith the mismatch, indicating the correlation between them. Based on the model of shear and dislocations, the evolution ofmicrostructural defects and residual stress are reasonably explained. The effect of shear is dominant at the primary creepstage, which greatly enlarges the mismatch and the residual stress. The dislocations weaken the effect of shear for the furthercreep stage, resulting in the decrease of the mismatch and relaxation of the residual stress. Those fi ndings add some helpfulunderstanding into the microstructure-performance relationship in the SC nickel-based superalloy, which might provide theinsight to materials design and applications.