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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.
In Germany spent nuclear fuel is stored and transported in casks that possess bolted lid systems equipped with double jacket metal seals of Helicoflex® type in order to enable safe enclosure. A dry interim storage period of 40 years was planned and the casks are licensed for that time. However, due to political reasons and delayed disposal projects this time period is expected to increase significantly. It is therefore necessary to evaluate the long-term sealing behavior of the casks exceeding 40 years of operating time. In this paper, we discuss our approach to investigate the aging behavior of such metal seals, and to predict the long term sealing behavior.
Accelerated aging component tests with seals are performed at temperatures up to 150°C to investigate the course of seal force, useable resilience and leakage rate. Even though these tests have already shown a significant decrease in seal force and useable resilience after aging times of less than 1 year, for most seals the leakage rate stayed better than the specified value. However, observation of the leakage rate has to be continued to determine the effect of the influenced seal properties. As a main influence on the seal long-term behavior the increasing permanent deformation of the outer jacket has been recognized. It is made of aluminum or silver which means that the temperatures that are relevant for the aforementioned application in casks lead to creep deformation.
For further investigation of seal behavior a comprehensive research program concerning the material characteristics and aging behavior of the individual seal components, with an emphasis on the outer jacket material, has been launched. The aim is to get a better understanding of the time and temperature dependent creep mechanisms and deformation. The examinations are separated into tests on seal segments and tests on standardized specimen.
In order to be able to study the aging process, seal segments are compressed in flanges and stored at temperatures ranging from room temperature to 150°C. After defined time intervals the segments are analyzed by using standardized tests including structure investigation and creep tests. The results are compared with the behavior of the basic raw material for the specimen. This is done to ensure the comparability of the thin jacket material and additional material used for standardized tests. Thus, the prediction of component changes after different ageing times at different temperatures can be made. As a result forecasts on the long term seal behavior are intended.
The aluminum alloy 2618A is an Al-Cu-Mg alloy with additions of Fe and Ni, which was designed for long-term operation at elevated temperature in transportation and aerospace industries. Typical applications include aircraft parts and structures (sheet material) or engine components such as turbo charger centrifugal compressor wheels (forged material). Such components are subjected to prolonged aging during service, (e.g. 50 000 h) at temperatures which are close to their age hardening temperature (ca. 190 °C). The microstructural evolution was investigated.
Gas turbines are widely used for a variety of purposes including power generation, compression or as jet engines in aircrafts. The critical components of a gas turbine are the high-pressure turbine blades which operate under severe conditions. These include thermo-mechanical loadings over temperatures ranging from room temperature up to 1100°C.
While a large number of constitutive models for single crystals have been proposed, most applications are restricted to special loading scenarios, temperature range and deformation mechanisms. In particular, a number of models are focused on pure creep. Only a few papers consider application of both creep and fatigue. Applications of the constitutive models to long-term stress relaxation are even scarcer. The new model assumes deformation-induced softening and can properly reproduce the viscous behavior at different time scales.
The model has been calibrated with the uniaxial tests at 800°C and 950°C in [001], [011] and [111] specimens of a nickel-basis superalloy. The predicted creep, short- and long-term relaxation and cyclic tests are in reasonable agreement with the experimental observations.
Data about the creep of metals and their alloys at temperatures close to the melting point are very limited. The reason is that most engineering alloys are used at temperatures below 0.6-0.8 of their melting point, so, investigation of creep at higher temperatures has usually no practical relevance. For some special applications however it is important, in our case hot isostatic pressing (HIP) of single-crystal turbine blades cast from nickel-base superalloys. In order to remove porosity the blades are HIPed at temperatures above GammaP-solvus where superalloy has no strengthening GammaP-phase and therefore is very soft. E.g., the company Howmet Castings hips the superalloy CMSX-4 at 1288°C, which corresponds to a homologous temperature of about 0.97=1561 K/1612 K (solidus temperature). Knowledge about the creep of CMSX-4 at this temperature and understanding of the creep mechanisms are necessary to model the kinetics of pore closure during HIP.
CMSX-4 single-crystals of [001] orientation and few single-crystals of different orientations, [011], [123] and [111], were tested under creep conditions at 1288°C in the stress range between 4 and 16 MPa. At this temperature which is above the GammaP-solvus (for CMSX-4 1280°C) the superalloy has single phase structure representing the Gamma-solid solution of nickel strengthened by solute atoms. On creep curves of CMSX-4 single-crystals of different orientations measured at 1288°C/10 MPa it is seen that despite such a high homological temperature, 0.97, CMSX-4 shows very high anisotropy of creep rate. The average creep rate of [001] single-crystal in the range 0-30% strain is about 11.5 time faster than that for [111], a ratio, which is even higher than at the practically relevant temperatures 750-1100°, see e.g.. Approximation the strain rate – stress dependence by the Norton power law gave a stress exponent n of about 6 which is an indication of dislocation creep. The specimen shape after testing, analysis of traces of plastic deformation by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) investigations indicate on dislocation slip on the octahedral system <011>{11-1}.This result however does not support the current doctrine that at high temperatures creep of metals and alloys are climb controlled. It is concluded from the obtained results that even at such a high homologous temperature, 0.97, dislocation movement by slip is more preferable than by climb if only relatively week obstacles are present like solute atoms and low angle boundaries (LABs).
It is remarkable that under used testing conditions the necking and recrystallization behavior of differently oriented single-crystals is very different. E.g., the [001] single-crystal showed very large local strain during necking, Phi=99.8%, and no recrystallization (see Fig. 3a), while the [111] single-crystal small necking, Phi=62%, accompanied by recrystallization. Such a specific deformation and recrystallization has to be undestood.
The obtained creep data of CMSX-4 was introduced in a finite element model in order to simulate pore closure during commercial HIP at a temperature of 1288°C.
In Germany, for the transport and storage of spent nuclear fuel, casks with double closure lid systems are used, which are equipped with Helicoflex® metal seals. The original interim storage period for these casks was planned to be 40 years. However, recent developments indicate that a storage time of more than 80 years might be necessary. Therefore, the current licensing has to be renewed, which requires extended knowledge of the long-term behaviour of all cask components.
At the Bundesanstalt für Materialforschung und -prüfung (BAM), metal seals have been aged at temperatures of up to 150 °C for up to 7 years. At regular intervals the seals have been tested for leakage rate as well as their mechanical properties. From these tests pronounced influence of ageing could be shown which results in the decrease of the seal force and the useable resilience. The main reason attributed to these performance changes is the increasing permanent deformation of the outer seal component due to creep. So far, an explicit analytical description of the long-term behavior that could be used for predictions exceeding the tested ageing times was not suitable.
In addition to the component tests that were already conducted, a comprehensive research program has been started. The aim is to gain information applicable for the prediction of the seal behaviour for an extended period of time under a certain temperature regime.
The individual seal components are tested regarding different material characteristics including creep, deformation and microstructural properties. The comparability of the material characterization is ensured by comparing the basic raw material of the specimen with the actual component material.
Furthermore, detailed examinations of the time and temperature dependent deformation behaviour are realized by ageing seal segments at multiple temperatures for different periods of time and employing standardized tests.
By analyzing the newly acquired data in combination with the long-term component tests analytical forecasts of the long-term behavior of the seals are intended.