Materials for High Temperature Applications: Next Generation Superalloys and Beyond: Refractory Metals
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

Tuesday 2:00 PM
February 28, 2017
Room: Pacific 16
Location: Marriott Marquis Hotel

Session Chair: Martin Heilmaier, KIT Karlsruhe; Nobuaki Sekido, Tohoku University

2:00 PM  Invited
High Temperature Oxidation Behavior of Mo-Si-B-Ti-Based Alloys: Bronislava Gorr1; 1University Siegen
    Mo-Si-B-based alloys represent perspective materiales for high temperature applications primarily due to their excellent mechanical properties. The decisive shortcoming of these materials is their poor oxidation resistance at intermediate temperatures. It was found that the macroalloying with Ti leads to a substitution of the Mo3Si phase exhibiting poor oxidation resistance by the (Mo,Ti)5Si3 phase showing excellent oxidation behaviour in a wide temperature range. The improved oxidation resistance can be attributed to the formation of a duplex layer consisting of a silica matrix with embedded TiO2 particles. Based on thermodynamic calculations, it was found that the formation of the (Mo,Ti)5Si3 phase can be stabilized by the addition of 2 at.% Fe that causes a widening of the composition region that comprises the required phases, i.e. Mo(ss), Mo5SiB2 and (Mo,Ti)5Si3. Moreover, recent experimental results reveal that the B-doped two-phase Mo-Si-Ti alloys possess high oxidation resistance at both, moderate and high temperatures.

2:30 PM  
Design and Production of bcc Titanium-molybdenum-based Alloys Strengthened by Ordered Intermetallic Precipitates: Alexander Knowles1; Nick Jones2; Neil Jones3; Howard Stone2; David Dye1; 1Imperial College London; 2University of Cambridge; 3Rolls-Royce plc
    Reinforcement of alloys with ordered intermetallic precipitates is an effective strategy for obtaining high strengths alongside damage tolerance, exemplified by nickel-based superalloys. There has been a desire to exploit this strategy in bcc-based systems; with success in some maraging steels. However, design and production challenges have limited the utilisation of ordered intermetallic precipitates within refractory-metal-based or titanium-based alloys. This work has developed alloys with a bcc (A2) titanium-molybdenum matrix strengthened by B2 structured TiFe ordered intermetallic precipitates, by two-step heat treatments. Instructed by study of the phase equilibria in the Ti-Fe-Mo system, alloys were designed within the extensive A2-B2 phase field. Detailed characterisation showed that these contained ultra-fine A2-B2 microstructures, while compression testing has demonstrated exceptional strengths. The prospects for extending the capability of these alloys to higher temperatures will be discussed. This work was supported through the EPSRC-DARE project (EP/L025213/1) and Rolls-Royce/EPSRC Strategic Partnership (EP/H022309/1 and EP/H500375/1).

2:50 PM  
The Influence of Titanium on the Phase Equilibria in Mo-Si-B Alloys: Daniel Schliephake1; Martin Heilmaier1; 1Karlsruhe Institute of Technology
    Despite some drawbacks regarding fracture toughness and oxidation resistance it is now well-established that Mo-Si-B alloys offer considerable potential for ultrahigh temperature structural engineering materials. A trade-off in combining the optimum single phase properties would virtually lead to a multiphase material exhibiting α-Mo + Mo5Si3 + Mo5SiB2 microstructure. The suppression of the otherwise present Mo3Si phase can be accomplished by the addition of Titanium is a major alloying element. In essence, we will show how Titanium promotes the eutectoid decomposition of Mo3Si into Mo and Mo5Si3. Hence, Mo-Si-Ti alloys with and without additional Boron were produced by repetitive arc-melting in protecting argon atmosphere, followed by homogenization treatment at high temperatures. The phases were identified by XRD, SEM and EDS/ESMA analysis. Additionally, the creep resistance of those alloys was determined at temperatures ranging from 1100 to 1300 °C to estimate their capability for high-temperature use.

3:10 PM  
Microstructure and Mechanical Behavior of Nb-based Nb-Al-Fe Alloys: Frank Stein1; Noah Philips2; 1Max-Planck-Institut für Eisenforschung; 2ATI Specialty Alloys and Components
    High-melting Nb-based alloys hold significant promise for the development of novel high-temperature materials for structural applications. In order to understand the effect of alloying elements Al and Fe and to evaluate the potential of respective alloys, the Nb-rich part of the ternary Nb-Al-Fe system was investigated. A series of Nb-rich ternary alloys was synthesized from high-purity Nb, Fe, and Al metals by arc melting and heat treatments were performed at temperatures up to 1600 °C. Microstructures were characterized metallographically and type and composition of phases were analysed by electron probe microanalysis and, where needed, by X-ray diffraction. The solidification paths were identified from inspection of the as-cast microstructures and the results of differential thermal analysis. Combining all experimental data enabled us to establish the Nb-corner of the ternary phase diagram. Finally, some mechanical tests were performed in order to get a rough idea on the mechanical behavior of the material.

3:30 PM Break

3:50 PM  
Phase Evolution and Creep Properties of Nb-rich Nb-Si-Cr Eutectics: Florian Gang1; Alexander Kauffmann1; Martin Heilmaier1; 1Karlsruhe Institute of Technology
    Nb-Si based alloys are promising novel materials for high temperature structural applications. Good mechanical strength and oxidation resistance can be achieved simultaneously by e.g. combining different phases in a very fine eutectic microstructure. Solely the ternary system Nb-Si-Cr exhibits a Nb-rich ternary eutectic. In this system, silicon is known to improve the creep properties by formation of silicides. Chromium is added to improve the oxidation resistance due to Cr2Nb Laves phase formation. However, the nominal compositions stated in literature are contradictory and report the presence of different phases. Therefore, the Nb-rich eutectic in the ternary system Nb-Si-Cr is determined experimentally. The phase evolution during solidification of different (near-)eutectic alloys is studied as well as the phase stability when employing heat treatments up to 1500 °C for several hundred hours. The resulting microstructures are comparatively assessed. Compressive creep test at 1200 °C give insights into acting creep deformation mechanisms.

4:10 PM  
On the Design of Nb Silicide Based Alloys with a Balance of Properties: Panayiotis Tsakiropoulos1; 1University of Sheffield
    Nb silicide based alloys (or Nb in situ silicide composites) have the potential to replace Ni based superalloys in high temperature aero-engine applications. The two most important phases in the microstructures of these new alloys are the bcc Nb solid solution (Nbss) and the tetragonal Nb5Si3 silicide. Different types of solid solution can form in the new alloys. The room, intermediate and elevated temperature mechanical properties and oxidation of Nb silicide based alloys critically depend on the chemistry, volume fraction and distribution of the bcc Nbss as well as the properties of the intermetallic(s) present in their microstructures. This presentation will propose and discuss parameters that can guide alloy design for achieving creep and oxidation properties in Nb silicide based alloys that meet the property goals for ultra-high temperature alloys with capabilities beyond those of Ni based superalloys.

4:30 PM  
Powder Route Processing of Nb Silicide Based Alloys: Claire Utton1; Panayiotis Tsakiropoulos1; Edward Gallagher1; 1University of Sheffield
    Alloys with capabilities beyond those of Ni based superalloys include Nb silicide based alloys (or Nb in situ silicide composites). The literature on Powder Metallurgy (PM) Nb silicide alloys is limited compared with that on process – microstructure – properties of cast alloys. Few PM Nb silicide based alloys have been produced using elemental or pre-alloyed powders and processing routes that include hot isostatic pressing (HIP) and spark plasma sintering (SPS). A major challenge for PM Nb silicide based alloys is to achieve the desirable microstructures with controlled interstitial content and type, volume fraction and distribution of required phases. The latter include the bcc Nb solid solution and tetragonal Nb5Si3 as well as Laves and A15 phases. In this presentation PM processing of model and latest generation developmental alloys will be discussed with emphasis on the effects of power type (elemental or pre-alloyed) and processing parameters on microstructure and properties.

4:50 PM  
Solidification Processing of Nb-silicide Based Alloys: Nicola Tankov1; Claire Utton1; Panayiotis Tsakiropoulos1; 1University of Sheffield
    Ni based superalloys are reaching their maximum intrinsic temperature capability and thus new high temperature alloys are needed. Nb silicide based alloys are candidate alloys because they can meet property goals. These alloys obtain their properties from two main phases that are stable from room temperature up to high operating temperatures, namely the Nb5Si3 silicide that provides high temperature strength, creep and oxidation resistance, and the bcc Nbss that provides room temperature toughness and damage tolerance. Achieving the property goals requires balancing the morphology, size, distribution, volume fraction and properties of phases that critically depend on alloy chemistry and optimised processing. This presentation will compare the solidification processing and properties of a latest generation developmental alloy that has been produced using cold hearth melting and directional solidification. Phase selection/stability and microstructure development will be discussed in relation to solidification conditions and segregation phenomena.

5:10 PM  
Accelerated Discovery and Development of Intermetallic-containing Refractory-based Multi-principal-component Alloys: Michael Titus1; Hauke Springer2; Fritz Körmann2; Blazej Grabowski2; Dierk Raabe2; 1Purdue University; 2Max-Planck-Institut für Eisenforschung
    Recently, new refractory-based high entropy alloys (HEAs) have been investigated for potential use as high temperature structural alloys, and some alloys exhibit excellent high temperature strength and ductility. While the high entropy alloy community is generally concerned with obtaining single phase solid-solution phases, secondary strengthening phases are usually required to achieve an adequate balance of mechanical and physical properties for structural applications. An elemental palette of Mo-Nb-Hf-Ta-Ti-V-W-Zr was chosen in order to promote the formation of a single body-centered cubic (BCC) solid-solution phase upon solidification, which facilitates homogenization heat treatments. Al, Cr, and Si were also included to promote secondary phase formation. Silicide and Laves phases were observed to be the most prevalent intermetallic phases. Preliminary compression and tensile tests of selected alloys will be presented along with our recent progress towards rapid alloy prototyping using unique suction casting processes.

5:30 PM  
Deformation Behavior and Solid Solution Hardening of Al-containing Refractory High-entropy Alloys: Hans Chen1; Alexander Kauffmann1; Bronislava Gorr2; Daniel Schliephake3; Christoph Seemüller; Julia Wagner4; Hans-Juergen Christ2; Martin Heilmaier3; 1Karlsruhe Institute of Technology ; 2University of Siegen; 3Karlsruhe Institute of Technology; 4University of Stuttgart
     High temperature structural materials have to provide good oxidation behavior, a high melting point and outstanding specific mechanical properties. Al-containing refractory high-entropy alloys may fulfill these requirements. The high-melting refractory metals Nb and Mo, the passivating metals Cr, Ti and Al as well as the low density of the last two elements perfectly meet the needs. Hence, we present the microstructural and mechanical characterization of the equiatomic alloy Nb-Mo-Cr-Ti-Al. A quasi-homogeneous microstructure was achieved by arc-melting and subsequent homogenization at 1300 °C for 20 h. Mechanical properties at high temperatures were obtained by compression tests at 800 °C-1200 °C. The altering of the microstructure during deformation was investigated by SEM and EBSD, which aid to reveal the underlying deformation mechanism. In addition, the impact of the number of elements and changing atomic size differences on the deformation behavior is compared to quaternary and senary alloys such as Mo-Cr-Ti-Al or Zr-Nb-Mo-Cr-Ti-Al.