Ernst Affeldt

Influence of Solid Solution Hardening on Creep Properties of Single-Crystal Nickel-Based Superalloys

Improving the creep resistance of the matrix by alloying with refractory elements is a major strengthening effect in nickel-based superalloy with rhenium as one of the most effective elements. In this work, the influence of rhenium on creep properties of single-phase single crystals with varying rhenium content and matrix-near composition is investigated. The use of single-crystalline material leads to very distinct results which are not deteriorated by grain boundary effects. So the strengthening effect can be solely attributed to the alloying element rhenium and is quantified for the first time. By comparing the creep strength of two matrix compositions with the corresponding single-crystal superalloys using the threshold stress concept, the potential of creep strengthening of the matrix in two-phase single-crystal alloys is quantified.

Diffusion Brazing of Single Crystalline Nickel Base Superalloys Using Boron Free Nickel Base Braze Alloys

Brazing is a well established repair technique for high temperature components in both industrial gas turbines and aero engines. Conventional nickel base braze alloys contain boron or silicon as melting point depressing elements. The major benefit of boron and silicon compared to other melting point depressants is its large effect on the melting point and its high diffusion coefficient in nickel base superalloys. However these elements promote precipitation of undesired brittle phases during the brazing process. To avoid these phases, transient liquid phase bonding in combination with boron and silicon free brazing alloys will be examined in this work. The influence of the brazing temperature on solidification and diffusion behaviour during transient liquid phase bonding for a single crystalline first generation and a second generation superalloy will be reported. Our experiments show that isothermal solidification without precipitation of brittle phases in the braze joint or the base material can be achieved. The brazed joint consists of fine γ/γ´ microstructure. EBSD measurements demonstrated that the single crystalline orientation of the base material was maintained throughout the joint. Electron probe micro analysis is used to characterize the diffusion behaviour. Solidification velocity will be compared with the theory of transient liquid phase bonding established by Tuah-Poku [1].

Thermo-mechanical fatigue of a wrought nickel based alloy

Abstract The nickel-based alloy Haynes 230 was investigated under low-cycle fatigue (LCF) and thermo-mechanical fatigue loading at different temperatures and with different phase shift. Whereas the yield strength is low, causing high plastic strains in every cycle, appreciable cyclic hardening up to double the yield strength was observed. The crack propagation rates were evaluated at different temperatures, with different mean stress and a standard waveform. A dwell signal was also tested. The crack propagation was modelled assuming a plasticity controlled fatigue part, which was correlated to the crack tip opening displacement by modified cyclic J-integral. At higher temperatures, an additional thermally activated oxide embrittlement increased the crack growth rate. Both mechanisms were considered separately in the model as they work independently. This model enabled us to describe the experimental growth rate, but cannot account for the threshold effects. As the cyclic life was found to be controlled by crack propagation starting from carbides or oxide spikes, life was predicted based on crack propagation. The results compare well for LCF within a scatterband of factor 2 at the lower cycles to failure. For higher cycle numbers, the measured lives were higher, which was assumed to be caused by the lack of a crack growth threshold in the model.

Effect of B, Zr, and C on Hot Tearing of a Directionally Solidified Nickel-Based Superalloy

The effect of the minor elements B, Zr, and C on the castability of a Nickel-based γ′-strengthened superalloy has been investigated. Tube-like specimens were prepared by directional solidification where the rigid ceramic core leads to hoop stresses and grain boundary cracking. It was found that an important improvement in castability can be achieved by adjusting the minor elemental composition. Too low C (≤0.15 pct) and too high B and Zr contents (≥0.05 pct) lead to material that is very prone to solidification cracking and should be avoided. The results cannot be rationalized on the basis of the current models for solidification cracking. Instead, pronounced hot tearing is observed to occur at high amounts of γ/γ′-eutectic and high Zr contents. The critical film stage where dendrites at the end of solidification do not touch and are separated by thin liquid films must be avoided. How Zr promotes the film stage will be discussed in the paper.

Thermo-mechanical fatigue testing of a rhenium-free single-crystalline super alloy

Abstract Due to high temperatures and mechanical loads, cracks are initiated in aero engine turbine blades which limit the cyclic life of these components. The materials used for components which underlie high thermal and mechanical load are single crystalline (SX) nickel based super alloys that in most cases contain a certain amount of rhenium. Dramatically increasing Re prices lead to the development of Re-free alloys. In this work, low-cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) tests were carried out on the Re-free single crystal M-247LC SX. The test results are shown and a model based on crack propagation was used to predict LCF and TMF life. It was shown, that the modeling results fit properly for out-of-phase TMF and LCF life while for in-phase TMF differences between calculated life and experiments occur due to a different mechanism of fracture.

The effect of temperature gradients in thermo-mechanical fatigue testing

Abstract The focus of this paper is the characterization of the influence of the temperature rates on the TMF lives. Therefore static and the dynamic calibrations of the temperature measurement devices were compared by a custom-built calibration specimen, which included five internal mantle thermocouples. Additionally the temperatures were measured on the specimens’ surface. The experimental setups used were compared by executing TMF pre-test with the identical single-crystalline specimen. Subsequently TMF tests on coated single-crystalline Nickel-based superalloy PWA 1484 were carried out with temperature rates of and . Finally the deviations of the determined TMF lives were evaluated.