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- Kontis, Paraskevas, et al.
(författare)
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The Role of Oxidized Carbides on Thermal-Mechanical Performance of Polycrystalline Superalloys
- 2018
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Ingår i: Metallurgical and Materials Transactions. A. - : SPRINGER. - 1073-5623 .- 1543-1940. ; 49A:9, s. 4236-4245
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Tidskriftsartikel (refereegranskat)abstract
- Oxidized MC carbides which act as main crack initiation sites in a polycrystalline superalloy under thermal-mechanical fatigue (TMF) conditions at 850 degrees C were studied. Microstructural observations in the TMF tested specimens were compared to findings from bulk samples exposed isothermally in air at 850 degrees C for 30 hours in the absence of any external applied load. Carbides were found to oxidize rapidly after exposure at 850 degrees C for 30 hours resulting in surface eruptions corresponding to oxidation products, from where micro-cracks initiated. Plastic deformation due to volume expansion of the often porous oxidized carbides led to high dislocation densities in the adjacent matrix as revealed by controlled electron channeling contrast imaging. The high dislocation density facilitated the dissolution kinetics of gamma precipitates by segregation and diffusion of chromium and cobalt along the dislocations via pipe diffusion, resulting in the formation of soft recrystallized grains. Atom probe tomography revealed substantial compositional differences between the recrystallized grains and the adjacent undeformed gamma matrix. Similar observations were made for the TMF tested alloy. Our observations provide new insights into the true detrimental role of oxidized MC carbides on the crack initiation performance of polycrystalline superalloys under TMF.
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| 4. |
- Moverare, Johan, et al.
(författare)
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Thermomechanical fatigue crack growth in a cast polycrystalline superalloy
- 2014
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Ingår i: EUROSUPERALLOYS 2014 - 2ND EUROPEAN SYMPOSIUM ON SUPERALLOYS AND THEIR APPLICATIONS. - : EDP Sciences.
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Konferensbidrag (refereegranskat)abstract
- Thermomechanical fatigue (TMF) crack growth testing has been performed on the polycrystalline superalloy IN792. All tests were conducted in mechanical strain control in the temperature range between 100 and 750 degrees C. The influence of in-phase (IP) and out-of-phase (OP) TMF cycles was investigated as well as the influence of applying extended dwell times (up to 6 hours) at the maximum temperature. The crack growth rates were also evaluated based on linear elastic fracture mechanics and described as a function of the stress intensity factor K-I. Without dwell time at the maximum temperature, the crack growth rates are generally higher for the OP-TMF cycle compared to the IP-TMF cycle, when equivalent nominal strain ranges are compared. However, due to the fact that the tests were conducted in mechanical strain control, the stress response is very different for the IP and OP cycles. Also the crack closure level differs significantly between the cycle types. By taking the stress response into account and comparing the crack growth rates for equivalent effective stress intensity factor rages Delta K-eff defined as K-max - K-closure, very similar crack growth rates were actually noticed independent of whether an IP or OP cycle were used. While the introduction of a 6 hour dwell time significantly increased the crack growth rates for the IP-TMF cycle, a decrease in crack growth rates versus Delta K-eff were actually seen for the OP-TMF cycle. The fracture behaviour during the different test conditions has been investigated using scanning electron microscopy.
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- Segersäll, Mikael, et al.
(författare)
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Thermal-Mechanical Fatigue Behaviour of a New Single Crystal Superalloy : Effects of Si and Re Alloying
- 2015
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 95, s. 456-467
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Tidskriftsartikel (refereegranskat)abstract
- The mechanical behaviour of a new single crystal superalloy suitable for power generation applications is considered. Effects of alloying with either Si or Re are elucidated. Out-of-phase thermal-mechanical fatigue is emphasised, although to clarify the effects arising some static creep deformation tests are also carried out. A significant Si-effect is found: a modest addition of 0.25 wt. % Si increases the TMF life by a factor of 2. Thinner deformation bands which traverse the γ'-phase are promoted by Si alloying, with a concomitant greater resistance to recrystallization and cracking along them. Alloying with Re, whilst improving the creep behaviour more markedly than Si, does not have such a strong effect on TMF life. The results provide insights into the composition/performance relationships relevant to the TMF performance of single crystal superalloys.
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| 6. |
- Xu, Jinghao, et al.
(författare)
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Modelling of additive manufacturability of nickel-based superalloys for laser powder bed fusion
- 2022
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Ingår i: Acta Materialia. - : Pergamon-Elsevier Science Ltd. - 1359-6454 .- 1873-2453. ; 240
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Tidskriftsartikel (refereegranskat)abstract
- The additive manufacturability of nickel-based superalloys for laser powder bed fusion (LPBF) technologies is studied by considering the in-process cracking mechanisms. The additive manufacturability of nickel-based superalloys largely depends on the resistance to the liquid and solid-state cracking. Herein, we propose a two-parameter-based, heat resistance and deformation resistance (HR-DR) model, accounting for the relation between chemical composition (both major and minor elements) and cracking susceptibility, which is generalized from the elemental microsegregation behavior and mechanisms of LPBF process induced cracking. The proposed model is validated by the LPBF experiments in this study and by the hitherto reported data in LPBF superalloys community. The HR-DR-model is found to be a theoretically acceptable and easy-to-use approach for the prediction of in-process cracking of nickel-based superalloys during LPBF. The influence of alloying elements and the gamma precipitates on the additive manufacturability is discussed. The model provides a path for designing not only new solid solutioning, but also and more importantly gamma strengthened nickel-based superalloys for LPBF applications. (c) 2022 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc.
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