Energy Materials 2017: Materials for Gas Turbines: Creep and Failure
Sponsored by: Chinese Society for Metals
Program Organizers: Jeffrey Fergus, Auburn University; Ji Zhang, China Iron and Steel Research Institute Group

Tuesday 8:30 AM
February 28, 2017
Room: 13
Location: San Diego Convention Ctr

Session Chair: Ying Zhang, Tennessee Technological University

8:30 AM  Invited
Alloy Development for Promoting γ/γ' Microstructural Stability and Creep Properties of Multi-component Co-base Superalloys: Wendao Li1; Haijing Zhou1; Song Lu1; Fei Xue2; Qiang Feng1; 1University of Science and Technology Beijing; 2Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
    Novel γ'-strengthened Co-Al-W-base superalloys possess excellent hot corrosion resistance and thermal fatigue properties, and they show potential applications for hot section components in gas turbine industry. Our study suggested that the individual and synergetic effects of Ta and Ti additions improved the γ' solvus temperature, γ/γ' microstructural stability and creep resistance at high temperatures in novel Co-base superalloys. To further improve the balanced properties of the γ' stability and creep resistance as well as oxidation and hot corrosion resistance at high temperature, an ICME approach has been used to develop this class of superalloys. The influence of alloying elements on microstructural stability in Co-Al-W-Ta-Ti-base superalloys were investigated. Meanwhile, thermodynamic calculation was performed to identify the trend for optimizing alloy chemistries. The creep mechanisms of Co-Al-W-Ta-Ti single crystal superalloy will be also discussed. This work is helpful for continuous efforts on alloy development to improve temperature capability of γ'-strengthened Co-base superalloys.

9:00 AM  
Rafting Prediction Criterion and Creep Life for Nickel-based Single Crystal Superalloys under Multiaxial Stress States: Zhixun Wen1; Huan Yang1; Zhufeng Yue1; Chengjiang Zhang1; 1Northwestern Polytechnical University
    The rafting is one of the most important and interesting phenomena which occurs in nickel-based single crystal superalloys during high temperature creep tests. A based on the relief of interfacial energy density by dislocation generation at the γ-γ’ interface, a rafting prediction criterion has been developed for nickel-based single crystal superalloys under multiaxial stress states. The rafting process has been analyzed quantitatively by the relief of interfacial energy to predict the rafting life. The holed plate and notched bar creep specimens have been applied to validate the rafting prediction criterion and creep life models.The SEM photos of the rafting structures in different positions in the shear zone have been obtained and are applied to validate the rafting criterion.The life model results agree with the experimental results.

9:20 AM  
Effect of C Addition on Creep and Microstructure Stability of Lamellar TiAl Alloys: Xiwen Zhang1; Ji Zhang1; Jing Zhu2; 1China Iron and Steel Research Institute Group; 2Tsinghua University
    Compared to the current applications, the use of TiAl alloys for gas turbines needs more sustainable creep performance. It’s proved that the lamellar microstructures are critical for TiAl alloys’ creep resistance, and thus the strain induced lamellar degradation usually results in the acceleration of creep deformation. The benefit of minor carbon addition on creep resistance of the lamellar TiAl alloys has been widely identified and attributed mostly to the carbide precipitation along lamellae, somewhat to the C solution strengthening, but little to microstructure stability. This study recently found that adding minor C in a cast TiAl alloy with monolithic hard lamellar orientation can significantly extend its steady creep stage at 760℃ and meanwhile sustain the lamellar structures for much longer time. The relatively lower strain concentration and pinning effect of C or carbides are believed to be the reason of less lamellar degradation and will be discussed in details.

9:40 AM  
Revisiting the Sources of Creep Dislocations in Ni-base, Single Crystal Superalloys: Farangis Ram1; Zhuangming Li2; Zailing Zhu3; Masood Hafez Haghighat2; Stefan Zaefferer2; Dierk Raabe2; Roger Reed3; 1Carnegie Mellon University; 2Max-Planck Institut für Eisenforschung GmbH; 3University of Oxford
    The origin of creep dislocations in Ni-base, single-crystal superalloys at intermediate stress and temperature regimes is revisited. High angular resolution Electron backscatter diffraction (HR-EBSD), electron channeling contrast imaging (ECCI), and discrete dislocation dynamics modeling (DDD) were employed in this study. We found that grown-in, low-angle boundaries are only a minor source of creep dislocations. They only account for those creep dislocations that are in their close vicinity−not farther that 10 μm. Major sources of creep dislocations are isolated, individual dislocations in the uncrept material. They are homogenously spread in small quantities (1011 m/m3) far from subgrain boundaries before creep begins and are activated after activation of dislocations in subgrain boundaries. Once activated, they emit avalanches of creep dislocation into boundary-free regions and increase the dislocation density by two orders of magnitude.

10:00 AM Break

10:20 AM  
Development Activities for the Manufacture of Rotor Forgings for Turbines in High Efficiency Power Plants: Nikolaus Blaes1; B. Donth2; Andreas Diwo2; D. Bokelmann1; M. Baues2; 1Saarschmiede GmbH Freiformschmiede ; 2Saarschmiede GmbH Freiformschmiede
    Boron containing steel COST FB2 designed for operational steam temperatures up to 620°C has found increasing industrial application in high efficiency turbines. Currently several development programs are going to further increase the steam temperature beyond 700°C. To achieve higher temperatures with steel forgings not only sufficient creep strength has to be achieved, but resistance against steam oxidation has to be improved by increase of Cr content. In the past all attempts to develop stable creep resistant martensitic 11-12% Cr steels for up to 650°C failed due to breakdown in long-term creep strength. Therefore the European project “Z-Ultra” was launched for development and manufacture of a new alloy type replacing in the steel matrix nitride strengthening particles by fine precipitations of the more stable Z-phase. Saarschmiede participates in this project and contributed amongst others by manufacturing a full scale turbine part. Production experience and test results are presented. Steam temperatures of about 700°C can only be achieved by using Ni-base material in the hottest part of the turbine. In previous R&D projects solid solution hardening Alloy 617 proved to be a most promising candidate alloys. For temperatures in the range of 750°C only g‘-precipitation hardened nickel base alloys can be applied. In Europe “NextGenPower” project was funded by the European Commission, where one of the main goals was to develop turbine rotor materials and to demonstrate manufacturability of forgings for full scale turbine rotor parts for that steam temperature range. Contributing to this project, Saarschmiede has produced a large rotor forging from well known Alloy 263 with a diameter of 1000 mm, which is the first worldwide of that size. Experiences in manufacture and test results are presented.

10:40 AM  
Mechanisms of Fracture in Laser Powder Bed Additive Manufactured Superalloys: Håkan Brodin1; Per Sandahl2; 1Siemens Industrial Turbomachinery AB; 2Exova AB
    The current paper reports the outcome of thermomechanical and fatigue testing of Hastelloy X, a well-known gas turbine material. In the current paper the alloy is produced by laser powder bed additive manufacturing. The results strongly indicates that the alloy produced by additive manufacturing suffers when exposed to high temperature. A fine-grained material in combination with weak grain boundaries is put forward as an explanation for the behaviour. The microstructure of additive manufactured alloys is anisotropic and fine-grained. This has an impact on the material response to tensile properties, crack initiation, crack growth and creep. Several different defects are contributing to the damage development. Weak grain boundaries has an effect in how fatigue cracks initiate and develop. Furthermore, the grain boundaries contain small voids that significantly contribute to creep crack formation and coalescence. Fatigue damage also is influenced by spatter particles from the weld process and unmolten powder particles.

11:00 AM  
Wang: High Temperature Oxidation of the New Type γ'-strengthened Cobalt-base Superalloys: Lei Wang1; Yang Liu1; Bo Gao1; Xiu Song1; Shuyu Yang2; 1Northeastern University; 2Shenyang University
    The cyclic oxidation testing was carried out on γ′-strengthened Cobalt-base superalloys with different Ni content in air at a temperature range from 800℃ to 1000℃. The results show that the Co-base superalloys show excellent oxidation resistance at such high temperatures. At 900℃, the oxidation resistance of Cobalt-base superalloys is significantly better than that of Nickel-base superalloy K417G or and Cobalt-base superalloy DZ40M. And, no obvious effect of the Ni addition on the oxidation properties of the Co-base superalloys can be found. At 800℃, the spinel structure oxides riched in Co, Ni and Cr on the surface and the continuous Al2O3 in the inner oxide layer can protect the alloys avoiding the severe oxidation. At 900℃, pyknotic (Co, Ni)O layer with NaCl structure forms on the surface, and the oxide layers are rich in Cr, therefore the oxidation resistance of alloys keeps high.