Materials for High Temperature Applications: Next Generation Superalloys and Beyond: Coatings and Environmental Resistance
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

Wednesday 2:00 PM
March 1, 2017
Room: Pacific 16
Location: Marriott Marquis Hotel

Session Chair: Carlos Levi, University of California, Santa Barbara; Daniel Monceau, CNRS, CIRIMAT laboratory

2:00 PM  Invited
Design of Next Generation Intermetallic Bond Coatings: David Jorgensen1; Wesley Jackson1; Akane Suzuki2; Tresa Pollock1; 1University of California, Santa Barbara; 2General Electric Global Research
    Bond coat interlayers in thermal barrier coating (TBC) systems must perform reliably in the complex thermal, chemical and mechanical environment of the turbine engine in order to increase temperature capability. To achieve higher temperatures and /or longer cyclic lives, a spectrum of intrinisic failure modes must be suppressed. Failure may occur at the bond coat – thermally grown oxide (TGO) interface, at the TGO-TBC interface or by oxidation-enhanced propagation of cracks from the coating into the substrate. New tools that guide the design of new intermetallic bond coatings will be discussed. Models for the failure processes suggest that bond coatings should possess improved high temperature strength (creep resistance), low oxide growth stresses and high interfacial toughnesses. Combinatorial experiments on B2-base and L12 intermetallic bond coatings reveal new failure resistant compositional domains for further development.

2:30 PM  Invited
Modelling of Kirkendall Pores Formation during the Fabrication and the Ageing of Pt-based Diffusion Coatings on Nickel Base Superalloys: Daniel Monceau1; Pauline Audigié2; Clara Desgranges3; Aurélie Rouaix Vande-Put2; 1CNRS, CIRIMAT Laboratory; 2CIRIMAT Laboratory; 3CEA
    Pt-rich gamma-Ni+gamma prime Ni3Al coating (GGP coatings) can be excellent coatings on nickel base superalloys by improving both oxidation and hot corrosion resistance, with limited rumpling during thermal cycling. Therefore, they are good bond-coatings for TBC systems. Such coatings can be obtained by interdiffusion of a deposited Pt layer with a superalloy. It can be also combined with an aluminization step. Unfortunately, these Pt-rich GGP coatings often include many pores. This paper presents a numerical modelling of the interdiffusion process under chemical potential gradients, including the transport of vacancies. This analysis allows the prediction of the concentration profiles of Ni, Al, Pt, Cr and vacancies V. The composition of the coating and the location of Kirkendall voids can be predicted.

3:00 PM  
The Influence of Bond Coats on Crack Progression during Sustained Peak Low-Cycle Fatigue: Marissa Lafata1; Tresa Pollock1; 1University of California, Santa Barbara
    Bond coats for single crystal turbine blades have been designed primarily for oxidation protection with minimal consideration of mechanical and microstructural optimization. Sustained peak low-cycle fatigue (SPLCF) cracks initiate at and subsequently propagate through the bond coating before entering the single crystal. Fatigue specimens of René N5 with several different bond coatings were compressively loaded in isothermal strain controlled tests. Initial results show that bond coatings can extend or reduce the lifetime of a specimen as compared to uncoated single crystals. Analysis using scanning electron microscopy, EBSD and EDS, provide insights into the deformation occurring as oxide-filled cracks propagate through the layers. Finite element modeling shows the oxide growth stresses on the crack faces significantly influence the crack propagation rate. Changes in composition, oxide growth stresses, and microstructure occur through the coating, IDZ, and superalloy. The role of microstructure and oxidation properties in minimizing crack propagation rates will be discussed.

3:20 PM  
Design of Nickel-base Superalloys with High Creep and Oxidation Resistance: Franck Tancret1; Edern Menou1; Daniel Monceau2; Gérard Ramstein1; Pedro Rivera-Díaz-del-Castillo3; 1Université de Nantes; 2CNRS; 3University of Cambridge
    The resistance of nickel-base chromia-former superalloys to high temperature oxidation is critical in applications like gas turbines, power plants or chemical engineering. Usually, the nominal chromium content or the free Cr concentration in austenite is chosen to trigger the formation of a protective chromia layer. However, this is not sufficient since scale spallation and internal oxidation can occur. Exploiting both computational thermodynamics (Thermo-Calc CALPHAD software and related mobility data) and Gaussian processes models for the coefficient of thermal expansion (CTE) and for creep resistance, genetic algorithm multi-objective optimisation is used to (i) maximise Wagner’s criterion against internal oxidation by ensuring that chromium diffuses faster towards the surface to form the chromia scale than oxygen diffuses inwards, (ii) minimise CTE to lower thermal stresses between alloy and oxide scale, and (iii) maximise creep resistance. New superalloys with good creep and oxidation resistance are then designed for high temperature applications.

3:40 PM Break

4:00 PM  
Kinetic and Structural Processes Affecting Alumina-scale Establishment during Early-stage Oxidation of Ni-base Alloys: Yihong Kang1; Juan Alvarado-Orozco2; Judth Yang1; Brian Gleeson1; 1University of Pittsburgh; 2CiDESi
    The oxidation behavior of Ni-Cr-Al model alloys with a fixed Al contents, variable Cr contents and different surface preparations was studied in air at temperatures up to 1150°C and compared with comparable Ni-Mn-Al and Ni-Al alloys (same amount of solute, e.g., Ni-6Cr-8Al vs. Ni-6Mn-8Al vs. Ni-14Al). Short-term oxidation experiments revealed that a surface recrystallization region, which forms during the early-stage thermal exposure of abraded alloys, can serve to facilitate the alumina-scale formation process. This is because subsurface diffusion is enhanced due to the recrystallization region providing multiple short-circuit paths. However, the extent of this enhancement of diffusion varies with the elements, namely Cr, Mn and Al in present study, at different temperature. A modified interpretation of the beneficial “third-element effect” observed with Cr is presented based on these observations

4:20 PM  Invited
A Perspective on the Challenges to Thermal Barrier Coatings: Carlos Levi1; 1University of California, Santa Barbara
    Gas turbine technology is undergoing some of the most transformational changes in recent history, demanding materials with improved capabilities over current Ni based superalloys. Candidate materials range from Co alloys to refractory metals and CMCs, all of which require coatings to survive the increasingly extreme environment of the engines. TBC systems based on ZrO2-7±1wt%Y2O3 and current bond coats have intrinsic limitations to their use at substantially higher temperatures. Candidate oxides retain ZrO2 as the base oxide and can be conceptually classified into two groups based on the tetragonal or the cubic form of zirconia, of which rare earth zirconates are prototypical, but no candidate in either group meets all the requirements for the more advanced applications. This presentation will provide a perspective on the challenges faced in developing advanced thermal barrier coating systems (TBCs) for metallic components and discuss the scientific foundation of the design strategies for these materials.

4:50 PM  Invited
The Effect of Borosilica Pack-Cementation Coatings on the Oxidation Resistance of Mo-Si-B Based Alloys: John Perepezko1; Daniel Schliephake2; Camelia Gombola2; Martin Heilmaier2; 1University of Wisconsin-Madison; 2Karlsruhe Institute of Technology
    High-temperature materials like Mo-Si-B alloys can increase the efficiency of engines by higher combustion temperatures. They show good creep and oxidation resistance at high temperatures but suffer from low ductility at ambient temperatures. Recently, novel Ti-rich Mo-Si-B alloys have shown an increased creep resistance compared to ternary reference alloys by the formation of Ti-silicide precipitates. However, Ti additions deteriorate the oxidation resistance. Hence, coatings are required to provide sufficient oxidation resistance. The coating was applied by pack cementation codeposition of B and Si. The coating structure has an outer borosilica layer followed by an inner MoSi2 and Ti5Si3 layer for the Ti-rich alloys. Tensile creep tests were carried out from 1100°C to 1200°C in air. Cyclic and isothermal oxidation tests were performed between 800°C and 1200°C for up to 3000 h. Microstructural analysis was used to identify mechanisms for the superior resistance of coated samples.

5:20 PM  
Oxidation Behavior of Silicide Coatings Produced by Molten Salt Technique on the Nb-1Zr-0.1C Alloy: Megha Tyagi1; Vishwanadh B1; S. K. Ghosh1; Raghvendra Tewari1; 1Bhabha Atomic Research Centre
    In this study molten salt technique has been used to produce silicide coating on the Nb-1Zr-0.1C alloy using NaCl–KCl–NaF–Na2SiF6–Si melt. Microstructural characterization of the coated samples were carried out using scanning electron microscopy (SEM) and X-ray diffraction (XRD) . SEM of the cross section samples showed the formation of 12μm thick NbSi2 layer on the substrate. Also, XRD pattern obtained revealed the presence of NbSi2 as a major phase. Isothermal weight gain experiments were carried out on bare Nb and coated sample at 1273 K to evaluate the oxidation performance. Microstructural characterization of all the oxidised samples was carried out using the above mentioned techniques. In addition, the various oxide phases formed on the coated Nb alloy substrate have been identified using XPS (X-Ray Photo Spectroscopy) analysis. The silicide coated samples showed better oxidation resistance than the bare Nb alloy.

5:40 PM  
Functionally Graded Tungsten/EUROFER Coating for Plasma Facing Components of Fusion Power Plants: Jarir Aktaa1; Dandan Qu1; Robert Vaßen2; Marius Wirtz2; Jochen Linke2; 1Karlsruhe Institute of Technology (KIT); 2Forschungszentrum Jülich (FZJ)
    The reduced activation ferritic martensitic steel EUROFER shall be used as structural material for the first wall (FW) of fusion power plants. The interaction between plasma and FW, especially physical sputtering will limit the FW lifetime under normal operation. Therefore tungsten coating is selected to protect the FW due to its very low sputtering yield and high melting point. However, the mismatch in thermo-physical properties between tungsten and EUROFER can lead to large residual thermal stresses and even failure. To overcome this tungsten coating with tungsten/EUROFER functionally graded (FG) interlayer on EUROFER substrate are developed and optimized. The coating and the FG interlayer are produced by vacuum plasma spraying with parameters optimized by modelling and evaluated by means of microstructural and micromechanical investigations, thermal shock, thermo-mechanical fatigue as well as fracture mechanical experiments. The results reveal the good quality of the coatings and verify the success of the approach pursued.