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The creep anisotropy of the single crystal superalloy LEK 94 deformed in tension along [0 0 1] and [1 1 0] directions at 1293 K and 160 MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [1 1 0] as compared to [0 0 1] tensile loading were identified. [1 1 0] tensile creep is associated with a higher number of γ' phase cutting events, where two dislocations with equal Burgers vectors of type <1 1 0> jointly shear the γ' phase. The resulting <2 2 0>-type superdislocation can move by glide. In contrast, during [0 0 1] tensile loading, two dislocations with different <1 1 0>-type Burgers vectors must combine for γ' phase cutting. The resulting <2 0 0>-type superdislocations can only move by a combination of glide and climb. The evolution of dislocation networks during creep determines the nature of the γ' phase cutting events. The higher [1 1 0] creep rates at strains exceeding 2% result from a combination of a higher number of cutting events (density of mobile dislocations in γ') and a higher superdislocation mobility (<2 2 0> glide) in the γ' phase.
Experimental study on M23C6 nucleation and growth mechanisms in Ni-base superalloy single crystals
(2017)
The addition of carbon to Ni-base superalloy single crystals has been increasingly carried out to improve low angle grain boundary (LAGB) resistance and castability. Consequently, the precipitation of carbides is highly probable during long-term application of components subjected to higher temperatures (> 1000 °C). While the view on the role of carbides as strengthening or detrimental is polemical, their inevitable increased presence in carbon-doped alloys must be addressed. In the present work, the evolution of M23C6 carbides forming in the commercial grade Ni-base superalloy LEK 94 during high-temperature and low-stress creep exposure is assessed. Although carbon is not intentionally added to the LEK 94 alloy, it admits up to 0.1 at. %, which together with the high content of M23C6-forming transition metals, leads to their precipitation. The precipitation is induced here during creep experiments at 1020 °C and a nominal applied stress of 160 MPa along [001]. The correlation of precipitation and external load is carried out by evaluating the carbides in the gage section of parallel and circularly notched cylindrical samples, as well as in their heads. Characterization is made by transmission electron microscopy (TEM). Although primary MC carbides form mostly in interdendritic regions during casting, high temperature exposure induces M23C6 carbide nucleation especially in the γ phase of dendritic regions, where a stronger partitioning of refractory elements is present. The carbides have a needle shape with their main axis on 〈100〉 and a cube-on-cube orientation relationship. They present incoherent {100} facets along their elongated region and semi-coherent {111} facets at their ends. Their nucleation and growth mechanisms are discussed based on microstructural observation under different experimental conditions.
The present work describes the shear creep behavior of the superalloy LEK 94 at temperatures between 980 and 1050 °C and shear stresses between 50 and 140 MPa for loading on the macroscopic crystallographic shear system (MCSS) (011)[01-1]. The strain rate versus strain curves show short primary and extended secondary creep regimes. We find an apparent activation energy for creep of Qapp = 466 kJ/mol and a Norton-law stress exponent of n = 6. With scanning transmission electron microscopy, we characterize three material states that differ in temperature, applied stress, and accumulated strain/time. Rafting develops perpendicular to the maximum principal stress direction, gamma channels fill with dislocations, superdislocations cut gamma' particles, and dislocation networks form at gamma/gamma' interfaces. Our findings are in agreement with previous results for high-temperature and low-stress [001] and [110] tensile creep testing, and for shear creep testing of the superalloys CMSX-4 and CMSX-6 on the MCSSs (111)[01-1] and (001)[100]. The parameters that characterize the evolving gamma/gamma' microstructure and the evolving dislocation substructures depend on creep temperature, stress, strain, and time.
The presentation shows how the three-dimensional quantification of dislocations and their characteristic features, e.g. Burgers vector, line direction, dislocation density, is carried out at the transmission electron microscope in scanning mode (STEM) at Division 5.1 at BAM. Exemplarily, the methods are shown for Ni-base superalloy single crystals, for which a short introduction is given using further TEM techniques. Additional examples on low angle grain boundaries, nucleation of oxides at dislocations and interaction of dislocations and carbides are shown.
The content of the presentation was addapted, aiming at scientists who work within the DFG Priority Programme 1713 "Strong coupling of thermo-chemical and thermo-mechanical states in applied materials".
The LEK94 is a Ni-base superalloy single crystal (SX) of the second generation, a materials class which is important for its high temperature creep resistance (>1000°C) in first stage blades of low-to-medium pressure gas turbines. Monocrystalline Ni-base superalloy SXs have a two-phase microstructure consisting of small cubes (' phase with the ordered L12 crystal structure, cube edge length: 500 nm), separated by thin channels (γ phase with fcc solid solution structure, channel width: 20 nm). The microstructural evolution during high temperature and low stress tensile creep has been thoroughly investigated previously, mainly for [001] loading, both in terms of dislocation activity (filling of γ channels, formation of dislocation networks, cutting of the γ’ phase) as well as phase coarsening (rafting, topological inversion). Other loading geometries have received less attention.
The present work studies high temperature and low stress creep deformation of the superalloy LEK 94 at temperatures around 1000°C, where rafting occurs. Differences between loading under different uniaxial, biaxial and triaxial stress states are discussed. Stereo-microscopy and g∙b analysis in the scanning transmission electron microscopy mode (STEM) are combined for microstructural analysis. The focus is set on the role of dislocation interactions with the aging microstructure. Both development in STEM characterization methods, as well as the roles of phase coarsening, γ channel filling, microstructural heterogeneity and γ’ phase cutting are discussed.
Ni-base superalloy single crystals have been used in turbine blades for hot sections of gas turbines for over four decades. In order to increase the efficiency of the turbines, a continuous increase in the inlet temperature of combustion gases into the turbine has driven the design of turbine blades to complicated shapes and the presence of a complex pattern of cooling channels. These three-dimensional shapes, together with the inhomogeneous distribution of stresses along the blade, induce an also complicated triaxial stress state, which does not compare to uniaxial tests that are performed to characterize high temperature properties such as creep. A round notch on a test piece represents a simple configuration that generates a quasi-isostatic stress state across the notch. In the present contribution, the effect of a sharp round notch on the microstructural micromechanisms within the notched region cylindrical bars, loaded along [001] at 1020 °C and 160 MPa net stress, is studied. To this end, a series of interrupted creep tests is conducted on plain and notched bars and the microstructure is compared. Results are discussed in terms of degree microstructural coarsening, and dislocation activity. The effect of notch generation via grinding is also discussed in these terms. The presence of carbides evolving in from residual carbon is also shown and discussed. .Funding by the German Research Association (DFG) [grant number AG 191/1] is acknowledge