Materials for High Temperature Applications: Next Generation Superalloys and Beyond: Next Generation Superalloys II
Sponsored by: TMS Structural Materials Division, TMS: High Temperature Alloys Committee, TMS: Refractory Metals Committee
Program Organizers: Akane Suzuki, GE Global Research; Martin Heilmaier, Karlsruhe Institute of Technology (KIT); Pierre Sallot, Safran Tech; Stephen Coryell, Special Metals Corporation; Joseph Licavoli, NETL - Department of Energy; Govindarajan Muralidharan, Oak Ridge National Laboratory

Monday 2:00 PM
February 27, 2017
Room: Pacific 16
Location: Marriott Marquis Hotel

Session Chair: Howard Stone, University of Cambridge; Nathalie Bozzolo, MINES-ParisTech


2:00 PM  Invited
Nickel-based Superalloys Reinforced by Gamma Prime and Gamma Double Prime Precipitates: Howard Stone1; Paul Mignanelli1; Nicholas Jones1; Ed Pickering2; Olivier MessÚ1; Catherine Rae1; Mark Hardy3; 1University of Cambridge; 2University of Manchester; 3Rolls-Royce plc
    A new, high niobium, nickel based superalloy has been identified that can be heat-treated to produce a microstructure comprising appreciable volume fractions of both gamma prime and gamma double prime precipitates. Studies of the microstructural evolution of this alloy during thermal exposure of up to 1000 hours at temperatures between 600 and 900 ˚C have shown that it offers a 100 ˚C increase in thermal stability over existing gamma double prime reinforced alloys. Tensile testing at temperatures between ambient and 800 ˚C also indicate that it possesses exceptional tensile strength, in excess of 200 MPa greater than that of IN718 across this temperature range. The properties and prospects of this new alloy will be discussed. The authors would like to acknowledge the EPSRC/Rolls-Royce Strategic Partnership for funding (EP/M005607/1 and EP/H022309/1).

2:30 PM  
Effect of Alloying on the Microstructure and Properties of Superalloys Containing Gamma Prime and Gamma Double Prime Precipitates: Paul Mignanelli1; Nicholas Jones1; Giles Rought Whitta1; Felicity Dear1; Mark Hardy2; Howard Stone1; 1University of Cambridge; 2Rolls-Royce plc
    The service requirements of nickel-based superalloys demand that they possess considerable strength, thermal stability and environmental resistance at elevated temperatures. Recently, a new nickel-based superalloy has been identified with a microstructure containing both gamma prime and gamma double prime precipitates that offers appreciable performance improvements over IN718. In this study, the effect of various alloying additions on the precipitate morphology, thermal stability and the hardness of this alloy have been assessed. In addition, the effect of microstructural condition on the alloy’s isothermal oxidation behaviour has been investigated and the results compared with existing alloys. Alloys based on this system are believed to have considerable potential for high temperature structural applications and prospects for their future development will be presented. The authors would like to acknowledge the EPSRC/Rolls-Royce Strategic Partnership for funding (EP/M005607/1 and EP/H022309/1).

2:50 PM  
Gamma-Prime Strengthened Superalloys for Heavy Duty Gas Turbine Applications: Andrew Detor1; Reza Sharghi-Moshtaghin1; Ning Zhou1; Shenyan Huang1; Richard DiDomizio1; 1General Electric Global Research
    Increasing combined cycle turbine efficiency beyond 65% requires a new heavy duty rotor material capable of operating at temperatures well in excess of today's H-class levels. Current alloys are limited by the stability of their major strengthening phase, gamma double prime, which coarsens and may convert to delta in service at high temperature. The gamma prime phase is more suited for strengthening in this application; however, gamma prime rapidly overages during the slow cooling rates inherent in processing thick-section gas turbine components. In the present work we are developing two new alloying strategies to control the coarsening of gamma prime during slow cooling. In one approach we use controlled coprecipitation to limit growth, and in the other we modify gamma prime chemistry to reduce coarsening kinetics. Microstructure and mechanical properties will be presented and discussed in the context of enabling a 650˚C+ capable heavy duty gas turbine rotor.

3:10 PM  
ICME Approach to Design γ'/γ'' Composite Precipitate Microstructure for Ni-based IN718 Super Alloys: Rongpei Shi1; Donald McAllister1; Ning Zhou2; Andrew Detor2; Richard DiDomizio2; Sanket Sarkar2; Michael Mills1; Yunzhi Wang1; 1The Ohio State University; 2GE Global Research
    A new, high performance heavy duty gas turbine wheel material capable of operating at 1200℉ and above is desired to increase combined cycle turbine efficiency for the next-generation advanced cycle. As such, a new alloy design strategy is being explored to achieve the goal through microstructure stability control via γ'⁄γ" co-precipitation. By linking directly to CALPHAD thermodynamic and kinetic databases, as well as a generalized nearest-neighbor broken-bond prediction of interfacial energies, a quantitative multi-phase field model is developed to study the co-precipitation behaviors of γ' and γ" phases in the conventional and modified IN718 nickel-based superalloy. The influence of alloy chemistry and heat treatment conditions on the precipitation sequence and thus the co-precipitation behavior of composite γ'⁄γ" particles with different morphologies will be addressed.

3:30 PM Break

3:50 PM  Invited
About the Predictability of Microstructure Evolution upon Thermomechanical Processing of Nickel-based Superalloys: Nathalie Bozzolo1; Charbel Moussa1; Marc Bernacki1; 1MINES ParisTech
    Hot-forging of aerojet turbine disks will be under focus in the presentation, with the aim of discussing the possibility of predicting the microstructure obtained after forging as a function of the applied thermomechanical parameters. In case of classical discontinuous recrystallization, with nucleation along the former grain boundaries and growth driven by stored energy, dynamic and post-dynamic microstructure evolutions can be predicted by using a quite mean field model, both for supersolvus and subsolvus conditions. on the other hand, critical grain growth sometimes occurs, a phenomenon that is clearly driven by local metallurgical conditions and topology. Full field models are then necessary for predicting such topological events. Another example of non-conventional evolution that remains difficult to model and thus to predict, is the heteroepitaxial recrystallization phenomenon recently reported in γ-γ' alloys where nucleation arises from a phase transformation process inside primary precipitates.

4:20 PM  
Solubility Limits and Phase Stability in Advanced Polycrystalline Ni-base Superalloys: Sammy Tin1; 1Illinois Institute of Technology
    Modern polycrystalline Ni-base superalloys for advanced gas turbine engines have been a key component that has contributed to technological advances in modern air transportation and land-based power generation. The superior high temperature mechanical strength of Ni-based superalloys can largely be attributed to the characteristic two phase microstructure, consisting of ordered L12 γ’intermetallic precipitates distributed coherently within a disordered FCC γ matrix . In modern polycrystalline Ni-base superalloys, refractory alloying elements such as W, Mo, Cr and Co provide strength to the γ matrix phase through solid solution strengthening while additions Al, Ti, Nb and Ta are utilized to produce an optimized volume fraction of the γ’ strengthening phase. Alloying studies to investigate and quantify the phase stability and solubility limits of polycrystalline Ni-base superalloys with respect the to formation of δ (Ni3Nb),η (Ni3Ti or Ni6AlNb) and TCP σ phases will be presented and discussed.

4:40 PM  
Comparative Study of High-temperature Grain Boundary Engineering of Two Powder Processed Low Stacking-fault Energy Ni-base Superalloys: Joshua McCarley1; Martin Detrois1; Sammy Tin1; 1Illinois Institute of Technology
    Results of high-temperature grain boundary engineering of an experimental, low stacking-fault energy (LSF) Ni-base superalloy were compared to a commercially available superalloy RR1000. Deformation mechanism maps for thermal-mechanical processing were compared along with the resulting length fractions of Σ3 boundaries following sub-solvus and super-solvus annealing. Compared to the hot deformation processing characteristics of RR1000, lowering the stacking-fault energy reduces dislocation mobility and expands the range of temperatures and strain rates over which dislocation-based plastic flow mechanisms were operative in the LSF alloy. For both alloys, processing conditions conducive to dislocation-based plasticity allowed for the storage of strain energy within the microstructure that was utilized for strain-induced boundary migration (SIBM) and the formation of Σ3 boundaries upon annealing. Based on the results, alloying changes that serve to reduce the stacking-fault energy of Ni-base superalloys also make the alloys more amenable for grain boundary engineering techniques that utilized hot deformation.

5:00 PM  
Benchmarking Multi-scale Models with Microtensile Experiments and 3D Microstructural Characterization of RenÚ 88DT: David Eastman1; Paul Shade2; Michael Uchic2; George Weber1; Somnath Ghosh1; Will Lenthe3; Tresa Pollock3; Kevin Hemker1; 1Johns Hopkins University; 2AFRL; 3University of California, Santa Barbara
    Microstructure-dependent ICME-based deformation models require structural characterization and constitutive properties, in addition to experimentally obtained benchmarks at salient length scales. Traditional methods for calibrating and validating mechanical response models with bulk experiments miss many of the underlying microstructural details. Techniques for machining micro-scale samples, such as high-throughput laser ablation, and corresponding microtensile tests have been developed and are being used to elucidate the underlying microstructure-property relations for polycrystalline RenÚ 88DT. These experiments allow for meso-scale characterization through collection of critical data, such as grain size, grain orientation and local strain accumulation, for a microstructural volume that is tractable in crystal plasticity modeling. It was found that sample strength decreases with size owing to a finite sampling of grain orientations with a biased distribution of Schmid factor values. Experimentally capturing these explicit microstructures and their attendant mechanical behavior provides quantitative metrics for crystal plasticity model development and validation.

5:20 PM  
Development of an Oxide Dispersion Strengthened Ni-Based Superalloy Enabling Heavy Duty Gas Turbine Wheels for Improved Combined Cycle Efficiency: Erica Sampson1; Rich DiDomizio1; Reza Sharghi-Moshtaghin1; Sharon Huang1; Andrew Detor1; 1GE Global Research
    The drive to increase combined cycle turbine efficiency from 62% to 65% for the next-generation advanced cycle requires a new heavy duty gas turbine wheel material capable of operating at 1200░F and above. This work aims to utilize oxide dispersion strengthening (ODS) to increase the strength of a solid solution strengthened Ni-based alloy and a gamma prime strengthened Ni-based alloy. Mechanical milling forces the dissolution of starting oxide powders into the metal matrix allowing for the solid state precipitation of new, nanometer scale oxides (nanofeatures) that are effective at dispersion strengthening. TEM characterization has shown the presence of fine nanofeatures present in each alloy. Altering the nanofeature type, nanofeature distribution, and processing can alter and improve the tensile strength of these ODS Ni-based alloys as compared to their baselines.

5:40 PM  
Room and Elevated Temperature Fatigue Life Improvement of ATI 718Plus Using LSP Treatment: Micheal Kattoura1; Seetha Ramaiah Mannava1; Dong Qian2; Vijay Vasudevan1; 1University of Cincinnati; 2University of Texas at Dallas
    Fatigue experiments on ATI 718Plus were conducted at room and elevated temperatures (650 ˚C) to study the improvement in the fatigue life due to Laser Shock Peening (LSP). Stress controlled uniaxial fatigue tests were conducted on base-material and LSP-treated samples to determine the improvement in fatigue life. LSP created surface residual stresses around -700 MPa and increased the surface hardness by 24%. LSP improved yield strength by 16% and endurance limit by 15% at room temperature. The mechanical, thermal, and thermo-mechanical relaxation of residual stresses have been investigated. Residual stresses retained 66% of its amplitude after 140 hours at 650 ˚C. In addition, changes in the microstructure after LSP-treatment and fatigue testing at both temperatures were characterized using the Scanning Electron Microscopy, Electron Back-Scatter Diffraction and Transmission Electron Microscopy. Near surface compressive residual stresses, microstructure and hardening induced by LSP effects on improvement in fatigue life are discussed.