A new strategy to intrinsically protect refractory metal based alloys at ultra high temperatures

Abstract

Poor oxidation resistance represents one of the main shortcomings of refractory metal based alloys. This work shows an innovative way to intrinsically protect such materials. Our approach relies on the alloying with Cr and Ta allowing the formation of CrTaO4. The CrTaO4 scale formed on the novel refractory equiatomic high entropy alloy Ta-Mo-Cr-Ti-Al reveals a unique combination of the following properties: (i) easy formation in a wide temperature range from 500°C to 1500°C, (ii) excellent adherence after cooling, (iii) low growth rates, and (iv) wide stability range of the stoichiometric composition that suppresses the formation of non-protective oxides.

Current Status of Research on the Oxidation Behavior of Refractory High Entropy Alloys

“…[34,35] Melts above 675 C [2] Mo MoO 3 Evaporate above 795 C [2] W W O 3 Evaporate above 1000 C [2] Nb Nb 2 O 5 Evaporate above 1370 C [2] Ta Ta 2 O 5 Evaporate above 1370 C [2] An unexpectedly high oxidation resistance, i.e., low values of the mass change and thin external oxide scales, was recognized for alloys that form complex oxide scales such as CrTaO 4 . These scales were observed in the alloy systems Ta-Mo-Cr-Ti, [33] Ta-Mo-Cr-Ti-Al, [26,29] and Ta-Mo-Nb-Cr-Ti-Al-Si. [30] Though CrTaO 4 , which possesses the rutile-type crystal structure, has not been known as a protective oxide in the past, it obviously possesses a potential to protect RHEA as a relatively low mass gain was measured even at 1500 C. [29] Figure 5a reveals that the oxidation rate constants of the equiatomic alloy TaMoCrTiAl (red line and data) that is a CrTaO 4 former are comparable with those of commercially available Cr 2 O 3 -(black shaded area) and Al 2 O 3 -forming (green) Ni-based alloys.…”

“…An unexpectedly high oxidation resistance, i.e., low values of the mass change and thin external oxide scales, was recognized for alloys that form complex oxide scales such as CrTaO 4 . These scales were observed in the alloy systems Ta–Mo–Cr–Ti, [ 33 ] Ta–Mo–Cr–Ti–Al, [ 26,29 ] and Ta–Mo–Nb–Cr–Ti–Al–Si. [ 30 ] Though CrTaO 4 , which possesses the rutile‐type crystal structure, has not been known as a protective oxide in the past, it obviously possesses a potential to protect RHEA as a relatively low mass gain was measured even at 1500 °C.…”

“…However, a pronounced zone of internal corrosion is usually detected below the CrTaO 4 layers. [26,[29][30][31] Only one alloy oxidizes according to type 4). A fully continuous and protective Al 2 O 3 scale was observed on the alloy Nb 1.3 Si 2.4 Ti 2.4 Al 3.5 Hf 0.4 after oxidation for 100 h at 1200 C (see Figure 4d).…”

“…[30] Though CrTaO 4 , which possesses the rutile-type crystal structure, has not been known as a protective oxide in the past, it obviously possesses a potential to protect RHEA as a relatively low mass gain was measured even at 1500 C. [29] Figure 5a reveals that the oxidation rate constants of the equiatomic alloy TaMoCrTiAl (red line and data) that is a CrTaO 4 former are comparable with those of commercially available Cr 2 O 3 -(black shaded area) and Al 2 O 3 -forming (green) Ni-based alloys. [29] To avoid possible superimposed effects from evaporating Mo oxides and scale growth of other oxides such as TiO 2 and Al 2 O 3 on the oxidation rate constants calculated using thermogravimetric data, the thicknesses of CrTaO 4 scales were determined as a function of oxidation time in our very recent investigations. The results reveal that CrTaO 4 grows obeying a parabolic rate law indicating a slow growth process through solid-state diffusion of oxygen through the scale (see Figure 5b).…”

“…Several RHEA, i.e., 3.4Ta20.3Mo15.2Nb25.2Cr5.4Ti17.6Al2.9Si, TaMoCrTiAl, and TaMoCrTi, form such dense, thus, relatively protective CrTaO 4 scales. [29,30,33] Finally, RHEA oxidizing according to mechanism IV, Figure 9d, possess the highest oxidation resistance, as they form dense Al 2 O 3 scales (see Figure 4d), which serve as a reliable diffusion barrier layer. Certainly, the high Al content in this alloy seems to be decisive in the formation of a dense alumina scale.…”