H. Biermann

An X-ray study of creep-deformation induced changes of the lattice mismatch in the γ′-hardened monocrystalline nickel-base superalloy SRR 99

Abstract X-ray line profile measurements were performed on monocrystalline specimens of the γ′-hardened nickel-base superalloy SSR 99 with axes close to the [001] direction. The aim was to measure the local lattice parameters in the γ-matrix and in the γ′-particles and to obtain information on the lattice mismatch and internal stresses. For this purpose, a special high-resolution double crystal diffractomoter with negligible instrumental line broadening was used. Measurements were performed on specimens in the initial state with cuboidal γ′-particles and on creep-deformed specimens containing the so-called γ/γ′-raft structure. In several cases the line profiles were measured as a function of the rocking angle, and the intensity distributions were mapped in reciprocal space around the (002) and (020) Bragg reflections. In the undeformed state these intensity distributionsindicate that the local lattice parameter varies spatially in the γ-phase. The line profiles of specimens in the initial state were asymmetric. A remarkable result obtained on creep-deformed specimes was that, whereas the asymmetry of the (020) line profiles was enhanced to the extent that a hump or a second peak appeared, the asymmetry of the (002) line profiles was reversed in sign. A quantitative evaluation yielded mean values of the constrained lattice mismatch which, in the case of creep-deformed specimens, differ significantly for the (001) and (010) lattice plane spacings. It is concluded that the orientation-dependent lattice spacings represent a triaxial state of residual stress which has its origin in the superposition of the originally present coherency stresses and the deformation-induced internal stresses. All observed features could be explained in detail in terms of a composite model of plastic deformation, which takes into account the dislocation networks deposited at the γ/γ′-interfaces during deformation.

Local variations of lattice parameter and long-range internal stresses during cyclic deformation of polycrystalline copper

Abstract X-ray line profiles and line positions were measured on single grains of cyclically deformed polycrystalline copper specimens unloaded from characteristic points of the stress-strain hysteresis loop. A high-resolution double crystal diffractometer was used to obtain local lattice parameter changes, from which long-range internal stresses and mean stresses were evaluated. In the case of samples unloaded from the load-reversal points the asymmetry of the characteristically broadened X-ray line profiles indicated forward and backward internal stresses prevailing in the dislocation cell wals and cell interiors of the dislocation cell structure, respectively. The changes of the long-range internal stresses after unloading from different points of the stress-strain hysteresis loop can be explained in terms of the composite model. In addition, mean strains and stresses, varying from grain, could be determined from the shifts of the Bragg reflections. A detailed microstructure-based discussion of the elastic stored energy, evaluated from the analysis of the stress-strain hysteresis loop, is given.

Measurement of service-induced internal elastic strains in a single-crystal nickel-based turbine blade with convergent-beam electron diffraction

Abstract During tensile creep deformation, single-crystal nickel-based superalloys develop a characteristic internal stress state, which leads to counteracting tetragonal lattice distortions of the two phases γ and γ. The aim of this work was the evaluation of the related elastic lattice distortions in the microstruture of a turbine blade after service in an accelerated mission test with high lateral resolution by convergent-beam electron diffraction. For the determination of the local lattice parameters, the higher-order Laue zone (HOLZ) lines of the central diffraction disc were evaluated under the assumption of a tetragonal distorted crystal lattice. The zone-axis patterns were simulated considering dynamical effects of the diffraction process. In order to reduce the computation time, a set of HOLZ line patterns was calculated in advance. Experimental patterns were compared with dynamically simulated patterns by the analysis of characteristic areas between HOLZ lines. The dependence of two area fractions on the lattice parameters were fitted by an interpolation function. This function was used for the evaluation of the experimental patterns. With this new approach a relatively fast evaluation of lattice strains was possible.

Determination of local strains in a monocrystalline turbine blade by microbeam X-ray diffraction with synchrotron radiation

Abstract X-ray peak profiles of the types {400} and {300} of monocrystalline nickel-based superalloy turbine blades were measured with a new microbeam synchrotron radiation X-ray diffraction technique. Under the coating of a turbine blade in the virgin state, a plastically deformed layer was found which is responsible for anomalous rafting. In a turbine blade subjected to an “accelerated mission test”, the phases γ and γ ′ were found to be under elastic distortions arising from the superposition of counteracting internal stresses. It is concluded that the investigated section was under tensile creep load during service. In the regions near the surfaces, the lattice parameters of both γ and γ ′ increase. The reason for this increase is the diffusion of Al from the NiAl coating into the bulk of the blade. Finally, the differences between the peak profiles measured with the microfocus technique and with a conventional diffractometer are discussed.

Microbeam synchrotron radiation diffraction study of a monocrystalline nickel-base turbine blade after service

In turbine blades subjected to service, the hot regions near the leading and trailing edges are subjected to temperatures up to 1,100 C, whereas the regions near the cooling channels are subjected to temperatures of about 800 C. These temperature gradients cause strong inhomogeneities in the local thermal and mechanical loads. In the present paper a monocrystalline turbine blade of the nickel-base superalloy CMSX-6 with an orientation near [001] from a developmental turbine was investigated with high lateral resolution using a Bragg-Fresnel focusing optics in combination with synchrotron radiation at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The blade had been exposed to service in so-called accelerated mission tests for several hundred hours in two test turbines. In the bulk of the material, a {gamma}/{gamma}{prime} raft structure perpendicular to the [001]-direction is observed. At the surface, the protective aluminide coating is visible. Between these two regions, a {gamma}{prime}-enriched zone exists.

Classification of γ-γ and γ-α2 lamellar boundaries on the basis of continuity of strains and slip-twinning planes in fatigued TiAl polysynthetically twinned crystals

Abstract Effects of lamellar boundaries on deformation in two-phase TiAl(γ)-Ti3AI(α2) alloy were geometrically and experimentally examined and the types of the boundaries were classified focusing on the continuity of macroscopic strain components and slip-twinning planes. TiAl polysynthetically twinned crystals were fatigued at 420 MPa with the loading axis parallel to the lamellar boundaries and (112) in γ domains. The lamellar boundaries were classified into 13 groups composed of three γ-γ true-twin, three γ-γ pseudotwin, three γ-γ 120° rotation boundaries and four γ-α2 interfaces. They were further subdivided into two groups with continuous and discontinuous geometry. Transmission electron microscopy observations showed that deformation modes unexpected from Schmid factor consideration were activated in some γ domains to maintain the macroscopic strain continuity through the lamellar boundary and that the boundary acted as an effective barrier to the transfer of deformation.

Synchrotron Microbeam Diffraction Study of the Microstructure and the Chemical Composition in a Monocrystalline Ni-Base Turbine Blade After a Thermomechanical Mission Test

High-performance turbine blades composed of monocrystalline Ni-base γ/γ′ superalloys are often protected by coatings to resist the high-temperature corrosive atmosphere environment applications. During applications severe thermal and mechanical loads cause drastic and complicated changes in the microstructure and the chemical composition, also combined with strong lateral gradients of both temperature and stress. Microbeam X-ray diffraction experiments with a spatial resolution of about 5 μm enabled the determination of both the microstructural and the chemical composition gradients within material volumes of the sizes of the order of about 1 mm3 or smaller. The high angular and spatial resolution was achieved by a special focusing technique using a Bragg–Fresnel focusing monochromator at the BM5 beamline of the ESRF synchrotron at Grenoble. The X-ray diffraction experiments were complemented and supported by electron microscopy investigations of the microstructure and the chemical composition.