F. E. H. Müller

Cyclic stress-strain response of the ODS nickel-base, superalloy PM 1000 under variable amplitude loading at high temperatures

Abstract The cyclic stress–strain behaviour of the recently developed oxide dispersion-strengthened nickel-base alloy PM 1000 was studied under constant and variable amplitude loading conditions. Single-step tests with a constant total strain amplitude as well as incremental step tests covering the same amplitude range have been carried out at 1123 and 1273 K. The interaction of the dislocations with the fine, homogeneously distributed oxide dispersoids was found to suppress the formation of dislocation cell structures. Rather, networks with dislocations frequently pinned at the particle/matrix interface have been observed by transmission electron microscopy. However, wavy dislocation slip still contributes to the stress–strain response. Despite the similarity of the resulting microstructures, the cyclic stress–strain curve obtained from constant amplitude tests deviates slightly from the one observed in incremental step tests. While non-Masing behaviour was found for constant amplitude testing, the strong influence of the dispersoids on dislocation mobility in combination with the constancy of dislocation arrangement yields Masing behaviour for the incremental step tests.

The influence of texture on the creep behaviour of the ods nickel-base alloy pm 1000

Oxide dispersion strengthened (ODS) nickel-base alloys are considered to be potential candidate materials for high temperature applications in excess of 1,000 C. Their extraordinary creep resistance results from a beneficial superposition of strengthening by incoherent, finely dispersed oxide particles, which effectively interact with dislocations and of coarse, elongated grains suppressing grain boundary sliding. While these mechanisms have been convincingly discussed in the literature, the influence of texture on creep strength has not been treated equally systematically due to the lack of varying the grain orientation or texture independently on the grain structure during the final recrystallization heat treatment. The availability of appropriate material of the ODS nickel-base alloy PM 1000 (trademark of Plansee AG, Reutte, Austria) enables to investigate creep specimens taken from the same heat with different grain orientations but identical grain and particle structure. Thus, in the present work, the authors will show that texture significantly affects not only the point of maximum deformation resistance (or steady-state flow stress, respectively) but also the homogeneity of deformation.

Contribution of Lattice Distortion to Solid Solution Strengthening in a Series of Refractory High Entropy Alloys

We present an experimental approach for revealing the impact of lattice distortion on solid solution strengthening in a series of body-centered-cubic (bcc) Al-containing, refractory high entropy alloys (HEAs) from the Nb-Mo-Cr-Ti-Al system. By systematically varying the Nb and Cr content, a wide range of atomic size difference as a common measure for the lattice distortion was obtained. Single-phase, bcc solid solutions were achieved by arc melting and homogenization as well as verified by means of scanning electron microscopy and X-ray diffraction. The atomic radii of the alloying elements for determination of atomic size difference were recalculated on the basis of the mean atomic radii in and the chemical compositions of the solid solutions. Microhardness (μH) at room temperature correlates well with the deduced atomic size difference. Nevertheless, the mechanisms of microscopic slip lead to pronounced temperature dependence of mechanical strength. In order to account for this particular feature, we present a combined approach, using μH, nanoindentation, and compression tests. The athermal proportion to the yield stress of the investigated equimolar alloys is revealed. These parameters support the universality of this aforementioned correlation. Hence, the pertinence of lattice distortion for solid solution strengthening in bcc HEAs is proven.

The influence of texture and grain structure on the high temperature low-cycle fatigue behaviour of the ODS nickel-based superalloy PM 1000

Abstract The high temperature low-cycle fatigue (HTLCF) behaviour of the recently developed oxide dispersion strengthened (ODS) alloy PM 1000 is investigated at 1123 K using total strain controlled symmetrical wave shapes ( R = −1). In order to reveal the influence of grain structure and texture (a) a 〈100〉-fibre structured bar material with a grain aspect ratio (GAR) of 10 along the longitudinal direction and (b) a pancake structured sheet material with a GAR of 4 and 〈100〉011-cube on edge texture have been studied. The grain structure determines the fracture path, but cannot be correlated with the cyclic lifetimes. Instead, lifetime can be predicted on the basis of the different Youngs moduli of the grain structures.

Investigation of current breaking capacity of vacuum interrupters with focus on contact material properties with the help of a reference model vacuum circuit breaker

This paper presents the performance of current breaking tests with a model vacuum circuit breaker and copper chromium disc-shaped contacts. The specifications of the model circuit breaker and the characteristics of the test circuit define a reference for the current breaking capability that serves as basis for further optimization. This reference breaking capability is defined with a standard powder-metallurgical contact material with 75 wt% copper and 25 wt% chromium. Information about the statistical reproducibility of the current interruption capability, the permissible number of high current switching events without contact wear, the recovery of breaking capability after current over-stress and the increase of breaking capability of virgin contacts due to high current conditioning could be gathered.

Parameters influencing the electrical conductivity of CuCr alloys

Copper-Chromium alloys are commonly used as contact materials for energy distribution. One of the major requirements is a good electrical conductivity for non-dissipative conduction of the nominal current. Since Cu and Cr are mutually nearly insoluble, CuCr alloys are two-phase composites, usually consisting of chromium particles embedded in a copper matrix. A decrease in electrical conductivity following e.g. a change in process parameters may be caused by (a) the geometry of the Cr phase, (b) phase boundaries between Cr particles and Cu matrix, or (c) diffusion of impurities into the Cu matrix. In the present work the influences of these different factors on the electrical conductivity of powder metallurgically produced CuCr25 material will be investigated. Regarding microstructure, several theoretical (analytical) models are available in literature, which do, however, not explicitly consider measurable features of the microstructure. We tackle this problem by combining quantitative microstructural analyses with an empirical relationship generated from FEM simulations based on simplified microstructures to obtain a prediction of the electrical conductivity of the composite. The electrical conductivities determined show considerable deviation from experiment, which is ascribed to the above mentioned factors (b) and (c) having not been incorporated into the model yet. However, the comparison of longitudinal and transverse conductivity measurements enables the separation and quantification of the effects arising by phase boundaries and impurities.

Investigation of the heat affected volume of CuCr contact material for vacuum interrupters

This work is focused on the microstructural changes during switching of simple geometry, butt-type contacts in a demountable vacuum chamber. The heat affected volume in copper-chromium (CuCr) with different chromium (Cr) content and the particle size in the re-solidified area were investigated. It was found that there is only a negligible difference between copper (Cu) with 25 (CC75) and 43 (CC57) wt% Cr. For CC57 a sudden change in the investigated parameters is observed for transferred charges above 35 As and a peak current (Ip) higher than 6 kA. It is assumed that a transition into the arc mode occurs. The results were assessed against published work, in which the switching process was observed using a camera and compared with the development of arc voltage.

Modelling the tertiary creep behaviour of an oxide dispersion strengthened nickel based alloy

Abstract A modified model of constrained cavity growth controlled by the power law creep of adjacent grains has been presented and exemplified with heats of different grain aspect ratio N of the nickel-base superalloy PM 1000. Tensile creep tests at a constant true stress of σ ≈ 200 MPa have been performed in air at 1123 K. The course of the creep curve in the tertiary stage is calculated by incrementally increasing creep time and subsequent calculation of the damage parameter. The proposed approach accounts for the observed increase of the creep rate g3 in the tertiary stage and correctly reflects the influence of the grain aspect ratio N on g3.

Effect of microalloying with silicon on high temperature oxidation resistance of novel refractory high-entropy alloy Ta-Mo-Cr-Ti-Al

Abstract The effect of 1 at.% Si addition to the refractory high-entropy alloy (HEA) Ta–Mo–Cr–Ti–Al on the high temperature oxidation resistance in air between 900 °C and 1100 °C was studied. Due to the formation of protective chromia-rich and alumina scales, the thermogravimetric curves for Ta–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al–1Si showed small mass changes and low oxidation rates which are on the level of chromia-forming alloys. The oxide scales formed on both alloys at all temperatures are complex and consist of outermost TiO2, intermediate Al2O3, and (Cr, Ta, Ti)-rich oxide at the interface oxide/substrate. The Si addition had a slightly detrimental effect on the oxidation resistance at all temperatures primarily as a result of increased internal corrosion attack observed in the Si-containing HEA. Large Laves phase particles distinctly found in the Si-containing alloy were identified to be responsible for the more rapid internal corrosion.

Interdependency of test environment and current breaking capacity of a model vacuum switch

In contact material development it is elementary to have a fundamental understanding of the effects of the test environment on the switching performance. However, the complexity of those dependencies make it difficult to identify a definite cause-effect relationship between contact material and switching properties, especially if the effect of small variations in the contact material, such as the particle distribution and the morphology, shall be investigated. This paper sensitizes to the strong interdependencies of test conditions and current breaking capacity. To identify these dependencies the test environment was systematically modified. The modifications could reproducibly be correlated with their effect on the test results. In particular, the topology of the high current circuit and the contact bolt design showed strong effects. The circuit topology was optimized for reduction of the magnetic flux density within the contact gap, and it was standardized to ensure long-term reproducibility of future results. The modifications of the test circuit could also be recognized in the characteristic arcing voltage.