Oxygen effect in refractory high-entropy alloys

Author(s)

Carolina Soares

• Document type: Doctoral Thesis
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Escola de Engenharia de São Carlos (EESC/SBD)
• Defense date: 2024-12-04

Examining board members

Marcelo Falcão de Oliveira; Conrado Ramos Moreira Afonso; Lauralice de Campos Franceschini Canale; Alessandro Roger Rodrigues; Guilherme Zepon

• Advisor: Marcelo Falcão de Oliveira

Abstract

The growing need for high-temperature-resistant metallic alloys has driven the development of new materials, among which refractory high-entropy alloys (RHEAs) stand out. Their first compositions were reported in 2010, and the mechanical strength of these alloys at high temperatures surpassed that of traditionally used nickel-based superalloys. RHEAs are metallic alloys composed of five or more elements, most of which are refractory, with a content ranging between 5% and 35% (at.). They have a sufficiently high mixing entropy to stabilize a single-phase solid solution microstructure, preventing the formation of intermetallic compounds. Refractory elements belonging to groups 5 and 6 of the periodic table, as well as elements from group 4 (such as Ti and Zr), commonly added to these alloys, are highly reactive with oxygen. Contamination during processing or subsequent heating is therefore quite common. However, the interference of this element is still poorly studied, and there is no consensus regarding its effect on the ductility of these alloys. Some studies claim that its presence in a solid solution can increase ductility, while others assert that oxygen is detrimental and can weaken the alloy. The objective of this work was to evaluate the effect of oxygen on the mechanical properties of the Al15Nb30Ti25V5Mo25 and Al15Nb30Ti25V5Zr25 alloys under compression at room temperature. Three levels of oxygen were used for each composition: the lowest level compatible with the manufacture of alloys with elements of lower contamination, being around 0.1% (at.) for both alloys; an intermediate level, with a content of 0.5% for the alloy with molybdenum and 1% for the alloy with zirconium; and a high level, with a content of 1% and 2%, respectively, for the alloys with molybdenum and zirconium, according to the solubility of this element supported by each alloy. According to scanning electron microscopy and X-ray diffraction analyses, these levels ensured that oxygen remained in the form of a solid solution, without the formation of oxides or other phases. Compression tests were conducted on samples measuring 9 mm in length and 6 mm in diameter, using the digital image correlation (DIC) technique to measure the displacement field and, consequently, the deformation. Distinct behaviors were observed in the Al15Nb30Ti25V5Mo25 and Al15Nb30Ti25V5Zr25 alloys. The alloy with molybdenum, which had a body-centered cubic structure, showed an increase in ductility and mechanical strength. The alloy did not exhibit plastic deformation and had an average compressive strength of 856 MPa with 0.1% oxygen; with contents around 0.5%, it showed about 5% deformation and an average strength of 4136 MPa. A subsequent increase to 1% oxygen decreased the ductility and strength, with average values dropping to 1.3% and 2744 MPa. However, the alloy with zirconium, which had a B2-type structure, showed an increase in average mechanical strength from 930 MPa to 4429 MPa and 4922 MPa, respectively, with the addition of 1% or 2% oxygen. Conversely, the total strain decreased, dropping from 14% for the base alloy to 11% and 4% with the intermediate and maximum oxygen contents. Thus, although this alloy tolerates a higher amount of oxygen, this element was detrimental to ductility. (AU)

• FAPESP's process: 20/04444-4 - Oxygen effect in high entropy refractory alloys
• Grantee: Carolina Soares
• Support Opportunities: Scholarships in Brazil - Doctorate