New Opportunities in Refractory Alloys
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New Opportunities in Refractory Alloys N.R. PHILIPS, M. CARL, and N.J. CUNNINGHAM For decades, the promise of refractory alloys (robust structural materials for use above 1500 K (1200 C)) has been hampered by concurrent requirements for fabricability via ingot metallurgy and oxidation resistance. These constraints have thus far proven impossible to meet in a single material. The work herein demonstrates that the advent of economical powder feedstocks and consolidation methods, such as additive manufacturing (AM), effectively removes the constraints preventing widespread utilization of high-strength refractory alloys into aerospace, defense, and space access applications. High-purity, spherical refractory-alloy powders (e.g. the Nb-10Hf-1Ti alloy ATI C103) were made via both atomization and plasma spheroidization and consolidated using a variety of methods. Consolidated material shows comparable and sometimes superior high temperature performance to equivalent wrought material. The availability of cost-effective powder synthesis techniques enables the elimination of critical barriers in fabrication: namely the requirement for hot and cold working. The availability of AM and hot isostatic pressing expands the design space for high-strength refractory alloys and makes the adoption of new paradigms in alloy design, such as refractory complex concentrated alloys, plausible. Going forward, the most difficult challenge, oxidation, can be tackled with the new approaches that are now available. https://doi.org/10.1007/s11661-020-05803-3 The Minerals, Metals & Materials Society and ASM International 2020
I.
INTRODUCTION
THE golden age of refractory alloy development began in the 1950s, driven by the increasing demands of turbine engines. Early in this period the foundational refractory alloys (e.g. Moly TZM) and key techniques for production (e.g. vacuum arc melting) were developed. The discovery of pyrochlore deposits in Canada and Brazil allowed the relatively lightweight niobium to transition from a rare metal to an engineering material and complete the structural refractory alloy family: Nb, Ta, Mo, W. The timing, concurrent with the ‘‘space race,’’ allowed substantial resources to be directed to the development of high-strength alloys for high-temperature service, primarily in connection with the orbital and reentry applications that were the major development drivers for refractory metals. Since the beginning of spaceflight it has been clear that very high power levels can only practically be achieved with nuclear power; thus, the peculiar need for very high temperature materials has been, in large part,
N.R. PHILIPS, M. CARL, and N.J. CUNNINGHAM are with the Allegheny Technologies Incorporated, Albany, OR 97321. Contact email: [email protected] Manuscript submitted on January 10, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
driven by the space-based nuclear reactor programs.[1] The other major demand signal for refractories has historically been linked to the aerothermal heating associated with high
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