Monika Feller-Kniepmeier

Anisotropic creep properties of the nickel-base superalloy CMSX-4

Single crystals of the superalloy CMSX-4 were tested in tensile creep in order to investigate the influence of orientation on the creep behaviour at temperatures of 1123 and 1253 K. The creep response of the CMSX-4 crystals was found to be highly anisotropic at the lower temperature of 1123 K. In particular, the primary creep behaviour of near [001] and [011] oriented crystals is highly sensitive to even small misorientations. Secondary creep strength deteriorates considerably in the order [001]-[011]-[111]. At 1253 K the creep anisotropy is clearly reduced, the creep strength of the [111] orientation, however, remains poor. The evolution of the microstructure during creep was studied as a function of strain and orientation and the relationship between deformation mechanisms and creep properties is discussed.

Modelling thermal misfit stresses in nickel-base superalloys containing high volume fraction of γ′ phase

Abstract Finite element calculations were made in order to simulate coherency stresses in the commercial superalloy SRR 99 containing 70 vol.% of γ′ phase. Stress and strain energy properties are calculated for the relevant morphologies observed in SRR 99. The influence of 2D/3D and isotropic/anisotropic modelling on the results is discussed in detail. Our results are applied to better understand the high temperature evolution of the microstructure. The sphere to cube transition with increasing precipitate size and the appearance of butterfly distorted cubes during cooling will be discussed.

Correlation of microstructure and creep stages in the 〈100〉 oriented superalloy SRR 99 at 1253 K

The microstructure of creep specimens tested at 1253 K is analyzed in the three stages of the creep curve by TEM investigations. Stage I is characterized by the glide of screw dislocations in the matrix, while the transition to stage II is the beginning of γ′ rafting. Throughout stage II, the γ′ phase is not cut by dislocations. From these and other metallographic results, the mechanism of creep in the single-crystal alloy can be deduced. The misfit stress at the γ/γ′ interfaces proved to be of special importance for this type of alloy containing a high volume fraction of γ′.

Temperature dependence of the thermal lattice mismatch in a single crystal nickel-base superalloy measured by neutron diffraction

The constrained lattice parameter distribution in the microstructure of a single crystal nickel-base superalloy caused by thermal mismatch is calculated along with the relation between constrained and unconstrained misfit. The results are applied to interpret the diffraction spectra obtained from a neutron diffraction study of the temperature dependence of the lattice mismatch in SRR 99. The results are compared with other misfit measurements on SRR 99 and on superalloys containing high volume fractions of gamma-prime phase. 11 refs.

Shear mechanisms of the γ′ phase in single-crystal superalloys and their relation to creep

The development of dislocation configurations in a single-crystal superalloy during creep and especially the shear mechanism of theγ′ phase is investigated by a transmission electron microscopy (TEM) study. Detailed analysis shows that at low strains and at coherent interfaces, shearing occurs by dissociation of matrix dislocations at theγ/γ′ interfaces, while at high strains and incoherent interfaces, cutting of theγ′ phase is achieved by antiphase boundary (APB) coupled dislocation pairs. The results are discussed in their connection to creep behavior.

Orientation dependence of dislocation structures and deformation mechanisms in creep deformed CMSX-4 single crystals

Abstract Transmission electron microscopy investigations have been carried out in single crystal creep specimens of the nickel-based superalloy CMSX-4, deformed to different stages of their creep lives. The microstructural background of creep anisotropy was observed at 1123 and 1253 K. At the lower temperature, the pronounced creep anisotropy can be attributed to the superposition of coherency and external stresses, leading to different dislocation configurations. At the higher temperature, the creep anisotropy for [001] and [011] load axes is reduced by γ′ rafting in these orientations.

CALCULATION OF THE INTERNAL STRESSES AND STRAINS IN THE MICROSTRUCTURE OF A SINGLE CRYSTAL NICKEL-BASE SUPERALLOY DURING CREEP

Abstract The evolution of internal stresses and strains in the microstructure of a single crystal nickel-base alloy during annealing and during creep in [001] direction has been calculated using a visco-plastic model. Two limiting conditions are considered: an “overloading” case where the internal stresses reach the critical resolved shear stress of the whole γ′ volume and an “underloading” case where the critical resolved shear stress of the γ′ precipitate is reached only at distinct areas. During creep deformation a triaxial stress state evolves in the microstructure and large pressure gradients are built up. The influence of an initial coherency misfit is shown to be negligible after short times of creep. The calculations allow the prediction of flow patterns in the microstructure, creep-induced lattice parameter changes, type and arrangement of interfacial dislocations and of the dependence of the stationary strain rate on the cube or plate morphology of the γ′ phase.

Transmisson electron microscopy of phase composition and lattice misfit in the Re-containing nickel-base superalloy CMSX-10

Abstract Phase compositions of matrix and γ′ phase in the third-generation superalloy CMSX-10 containing 6 wt.% Re were measured locally by energy dispersive X-ray spectroscopy in a transmission electron microscope. The corresponding lattice misfits between matrix and γ′ phase were determined by convergent beam electron diffraction. All measurements were performed in dendrites and interdendritic regions. Comparing the as-cast condition to the standard heat treated condition, remarkable enrichments of Re in the γ′ phase of dendrites and segregation of W and Ta in the matrix are detected. After standard heat treatment, an enrichment of Re remains in the matrix of dendrites. This results in differences in amount and sign of lattice misfit. Coherency stresses were calculated using the Finite Elements Method.

Dislocation structures in γ-γ′ interfaces of the single-crystal superalloy SRR 99 after annealing and high temperature creep

Abstract Dislocation networks in γ-γ′ interfaces of a single-crystal superalloy containing about 60 vol.% of ordered γ′ phase have been analyzed after annealing and after creep. In both experiments gliding screw dislocations are the cause of dislocation accumulation at the interfaces. During annealing 60° segments left in the interfaces climb into edge orientation under the influence of elastic strain and form flat low energy networks. After creep testing dislocation networks built in stage II at the interfaces are folded and have different dislocation contents for interfaces normal rather than parallel to the external stress. These dislocation networks at γ-γ′ interfaces are highly energetic. They are not misfit dislocations and contribute to the strength of the alloy. Close to the rupture surface additional low energy twist dislocation networks are formed at γ-γ′ interfaces.

Element distribution in the macro- and microstructure of nickel base superalloy CMSX-4

Abstract In the nickel base superalloy CMSX-4 phase compositions were determined with high lateral resolution by energy dispersive X-ray analysis inside a transmission electron microscope. After standard heat treatment, considerable enrichment of elements Re and W was detected in the matrix of dendrites compared to interdendritic regions. Equilibrium compositions of γ ′ and matrix phase and the γ ′ volume fraction depended strongly on temperature. In creep specimens tested at 1123 K, directed diffusion of elements with large atomic radius occurred in the pressure gradient between matrix channels with different orientation to the load axis.