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Characterization of a High Strength, Refractory High Entropy Alloy, AlMo0.5NbTa0.5TiZr

Jensen, Jacob K
http://rave.ohiolink.edu/etdc/view?acc_num=osu1492175560975813

Abstract Details

2017, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
High entropy alloys (HEAs) are a relatively new class of materials that have garnered significant interest over the last decade due to their intriguing balance of properties including high strength, toughness, and corrosion resistance. In contrast to conventional alloy systems, HEAs are based on four or more principal elements with near equimolar concentrations and tend to have simple microstructures due to the preferential formation of solid solution phases. HEAs appear to offer new pathways to lightweighting in structural applications, new alloys for elevated temperature components, and new magnetic materials, but more thorough characterization studies are needed to assess the viability of the recently developed multicomponent materials. One such HEA, AlMo0.5NbTa0.5TiZr, was selected to be the basis for this characterization study in part due to its strength at elevated temperatures (s0.2 = 1600 MPa at T = 800 ºC) and low density compared with commercially available Ni-based superalloys. The refractory element containing HEA composition was developed in order to balance the high temperature strength of the refractory elements with the desirable properties achieved by the high entropy alloying design approach for potential use in aerospace thermal protection and structural applications. Ingots of AlMo0.5NbTa0.5TiZr were cast by vacuum arc melting followed by hot isostatic pressing (HIP) and homogenization at 1400 ºC for 24 hrs with a furnace cool of 10 ºC/min. The resulting microstructure was characterized at multiple length scales using x-ray diffraction (XRD), scanning transmission electron microscopy (SEM), conventional and scanning transmission electron microscopy (TEM and STEM), and x-ray energy dispersive spectroscopy (XEDS). The microstructure was found to consist of a periodic, coherent two phase mixture, where a disordered bcc phase is aligned orthogonally in an ordered B2 phase. Through microstructural evolution heat treatment studies, the nanoscale interpenetrating microstructure was discovered to form via a conditional spinodal reaction pathway involving a congruent ordering transformation preceding spinodal decomposition. In order to gain a comprehensive understanding of the true morphology of these phases and obtain a novel perspective of 3D elemental segregation in the HEA, STEM-high angle annular darkfield (HAADF) micrographs and XEDS spectral images were utilized in the tomographic reconstruction of the microstructure, which was inherently difficult to observe through conventional characterization techniques. The microstructure of the alloy was ultimately refined by incremental variations to the base alloy composition in an effort to remove deleterious intermetallic phases adversely affecting ductility. Despite the excellent compressive strength across a wide range of temperatures and the ability to tailor the microstructure by compositional modifications, microstructural and phase transformations in the desired operating temperature range indicate that the AlMo0.5NbTa0.5TiZr alloy may not be a suitable material for high temperature aerospace structural components.
Hamish Fraser (Advisor)
Michael Mills (Committee Member)
Yunzhi Wang (Committee Member)
William Brantley (Committee Member)
227 p.
Materials Science
high entropy alloys, materials characterization, SEM, TEM, EBSD, EDS, tomography, 3D reconstructions, light metal alloys, superalloys, refractory high entropy alloys

Recommended Citations

Citations

  • Jensen, J. K. (2017). Characterization of a High Strength, Refractory High Entropy Alloy, AlMo0.5NbTa0.5TiZr [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492175560975813

    APA Style (7th edition)

  • Jensen, Jacob. Characterization of a High Strength, Refractory High Entropy Alloy, AlMo0.5NbTa0.5TiZr. 2017. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1492175560975813.

    MLA Style (8th edition)

  • Jensen, Jacob. "Characterization of a High Strength, Refractory High Entropy Alloy, AlMo0.5NbTa0.5TiZr." Doctoral dissertation, Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492175560975813

    Chicago Manual of Style (17th edition)

osu1492175560975813
839
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This open access ETD is published by The Ohio State University and OhioLINK.