Uwe Glatzel

Temperature dependence of the elastic moduli of the nickel-base superalloy CMSX-4 and its isolated phases

Abstract Elastic constants of anisotropic cubic materials were determined by acoustic resonance measurements at temperatures ranging from RT to 1300 K. The temperature dependence of Youngs modulus E , shear modulus G , and Poissons ratio ν of superalloy CMSX-4 and its isolated γ ′ and matrix phases were measured and compared with room temperature ultrasonic speed measurements. The orientation dependence of the engineering constants E , G and ν was determined. A comparison to models in literature predicting γ ′ morphology changes during creep is presented and agreement is found with the experimental data measured in this work.

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.

Phase separation in equiatomic AlCoCrFeNi high-entropy alloy

The microstructure of the as-cast AlCoCrFeNi high entropy alloy has been investigated by transmission electron microscopy and atom probe tomography. The alloy shows a very pronounced microstructure with clearly distinguishable dendrites and interdendrites. In both regions a separation into an Al-Ni rich matrix and Cr-Fe-rich precipitates can be observed. Moreover, fluctuations of single elements within the Cr-Fe rich phase have been singled out by three dimensional atom probe measurements. The results of investigations are discussed in terms of spinodal decomposition of the alloying elements inside the Cr-Fe-rich precipitates.

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.

Chemical composition measurements of a nickel-base superalloy by atom probe field ion microscopy

Abstract Single crystals of the rhenium-containing nickel-base superalloy CMSX-4 were investigated using a time of flight atom probe. The distribution of the alloying elements in the two phases, matrix (γ phase) and precipitate (γ′ phase), was studied. The high spatial resolution of the atom probe allows to analyse the γ−γ′ interface. The transition from matrix to γ′ phase takes place within one atomic layer. The boundary is enriched with titanium. Layer-by-layer analysis of (001) planes of the L 1 2 ordered γ′ phase reveals alternating Ni- and Al-rich planes. The probability of site occupation by different elements could be evaluated. An approximate formula of the γ′ phase is given by (Ni 81 Ti 3 Ta 2 Cr 5 Co 7 W 2 ) 3 (Ni 20 Al 70 Ta 6 Co 2 W 2 ). Rhenium is found mainly in the matrix and has a tendency to build clusters.

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.

Fluxless laser beam joining of aluminium with zinc coated steel

Abstract A laser welding–brazing (LWB) process to join zinc coated steel and aluminium sheets in two different flange geometries is reported. The deep drawing steel sheets are covered by a zinc layer of maximum thickness 10 μm, and a zinc based filler wire was used in the welding experiments with a Nd–YAG laser. Because of the differences in melting temperatures between iron (1808 K), aluminium (933 K), and zinc (693 K), it is possible to weld the aluminium alloy only. Owing to the zinc coating on the steel side, a Zn–Al alloy can be brazed onto the steel without any flux agent. The inevitable formation of a Fe–Al intermetallic phase at the bondline of the weld seam and the steel can be limited to a thickness of less than 5 μm and to a proportion of the contact area only. Mechanical as well as dynamic tests show results comparable to those obtained via other joining techniques. Salt chamber corrosion tests of varnished specimens display minor damage and no decline in tensile strength.

Characterization of phases of aluminized nickel base superalloys

Aluminum rich coatings, built up by a diffusion zone and a NiAl-cover layer, can protect the surface of turbine blades against oxidation. Within the single crystalline substrate and the adjacent layer, phases in the range of several tens of nanometers up to a few micrometers develop during production and operation of the turbine blade, were characterized. Investigations with transmission electron microscopy, nanoindentation and local crystal orientation mapping with a scanning electron microscope have been carried out in order to determine composition, morphology and distribution of the different phases. The diffusion zone has in general a defined orientation relative to the superalloy substrate and is built up by at least three phases embedded in a softer matrix, with significant differences in nanohardness. Local internal stress states in the diffusion zone are estimated. The NiAl-cover layer is a coarse columnar grained, non-textured B2 ordered intermetallic NiAl-phase.

Carbon onions produced by laser irradiation of amorphous silicon carbide

Abstract Onion-like carbon clusters embedded in crystalline SiC were observed after laser-induced crystallization of amorphous SiC and a process of their formation has been suggested. The carbon onions have a size in the range of 5–30 nm. The shells are defective with interplanar distances about 0.37 nm.

Change of phase morpholigies during creep of CMSX-4 at 1253K

Morphology changes of {gamma}{prime} precipitates during uniaxial stress of nickel base superalloy single crystals containing high volume fractions of {gamma}{prime} phase are well known. For negative misfit material and tensile stress in [001] direction, the initially cuboidal {gamma}{prime} particles transform into rafts oriented perpendicular to the load axis. In [011] orientation {gamma}{prime} rods, aligned perpendicular to the tensile external load direction, are generated. For the [111] load axis no shape changes were observed neither for tensile nor for compressive stresses. During investigations of creep anisotropy of CMSX-4 and microstructural evolution connected with creep in different orientations, the changes in matrix plate thickness occurring during creep at elevated temperatures are of interest. In [001] as well as in [110] orientation, bowing of dislocation dipoles in matrix channels is a frequently observed process which is controlled by the local resolved shear stress and the thickness of matrix plates. The rhenium containing alloy CMSX-4 has a volume fraction of about 70% of ordered {gamma}{prime} phase after standard heat treatment (solutioning: 1h/1,553 K, 2h/1,563 K, 6h/1,573 K and annealing 6h/1,413 K, 16h/1,143 K).