Semester

Summer

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Bruce S.-J. Kang

Committee Co-Chair

Victor H. Mucino

Committee Member

Victor H. Mucino

Committee Member

Konstantinos A. Sierros

Committee Member

Zhichao Liu

Committee Member

Minking Chyu

Abstract

Additive manufacturing (AM) fabricated oxide dispersion strengthened (ODS) alloys are given high expectations for critical structural components such as the first stage turbine blade for their excellent creep strength and oxidation resistance compared to superalloys. However, the powder feedstock processing is still an open question since current state-of-the-art processes are not capable of achieving ultrafine strengthening elements such as Y2O3 in powder which leads to agglomeration issues in as-consolidated alloys. In this research, the oxidation behavior and stability of ultrafine oxide in AM-printed alloys using mechanically alloyed powders were evaluated at 1100 oC. In addition, a novel powder processing method was developed based on the mechanism of strengthening element (i.e.Y2O3) evolution in powder precursor and as-consolidated alloys with the benefit of grain refinement and improved mechanical property and oxidation resistance.

In this study, it was found that 1) the size of Y2O3 in powder precursor is of importance to achieving ultrafine Y-rich nanoparticles, both plate-like and spherical nanoparticles formed in as-consolidated alloys using mechanically alloyed powder due to non-uniform convection of molten pool, 2) nanoparticles stabilized under the protection of Al2O3 at 1100 oC and spallation of oxide layer occurs in oxide mixture, 3) high energy mixing of gas atomized alloy powder and Y2O3 can generate ultrafine nanoparticles in powder and as-printed alloys attributed to the functional composite layer formation with surface deformation in metallic powder, 4) ultrafine Y2O3 and coarser Y-Al-rich oxides (nm) were observed in AM-printed IN718, 5) exceptional strength and ductility of AM-printed ODS IN718 alloy are attributed to fine structure and oxides strengthening effects, and 6) adding 1 wt.% Y2O3 can effectively refine the sub-grain of SS316L alloy with improved Vickers hardness (HV1) from 177 to 240.

The output of this research demonstrates the mechanism of Y2O3 evolution in powder and AM process, and provides one way to cost-effective fabricate ultrafine oxide dispersed alloys with fine grain structure and improved mechanical properties.

Embargo Reason

Patent Pending

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