The Effect of Al on the Formation of a CrTaO4 Layer in Refractory High Entropy Alloys Ta-Mo-Cr-Ti-xAl

Abstract

In this study, the effect of Al on the high temperature oxidation of Al-containing refractory high entropy alloys (RHEAs) Ta-Mo-Cr-Ti-xAl (x = 5; 10; 15; 20 at%) was examined. Oxidation experiments were performed in air for 24 h at 1200 °C. The oxidation kinetics of the alloy with 5 at% Al is notably affected by the formation of gaseous MoO3 and CrO3, while continuous mass gain was detected for alloys with the higher Al concentrations. The alloys with 15 and 20 at% Al form relatively thin oxide scales and a zone of internal corrosion due to the formation of dense CrTaO4 scales at the interface oxide/substrate. The alloys with 5 and 10 at% Al exhibit, on the contrary, thick and porous oxide scales because of fast growing Ta2O5. The positive influence of Al on the formation of Cr2O3 followed by the growth of CrTaO4 to yield a compact scale is explained by getter and nucleation effects.

High‐Temperature Ternary Oxide Phases in Tantalum/Niobium–Alumina Composite Materials

“…CrNbO 4 and CrTaO 4 have been found to form dense, protective layers in refractory high-entropy alloy oxides scales. [28][29][30][31] Investigation of AlTaO 4 in this regard may be promising, whereas no rutile-type compound is known in the Al-Nb-O system.…”

Microstructure tailoring of Al-containing compositionally complex alloys by controlling the sequence of precipitation and ordering

“…Many alloy compositions consisting only of refractory elements exhibit (i) high densities and (ii) insufficient oxidation resistance, precluding their use for high-temperature application in air [8,9,10]. Therefore, many RCCA reportedly contain some lighter elements, which are known to form protective oxide scales, e.g., Cr, Al [11,12], and increase the specific material properties, e.g., Ti or Al [13]. Experimental results further promote the idea that some carefully selected RCCA exhibit excellent oxidation resistance [10,14,11,15,16,17].…”

Compositional Modifications to Alter and Suppress Laves Phases in AlxCrMoTayTi Alloys

“…[13,14] Yu et al developed Al-containing refractory superalloys with dramatic elevated temperature strength improvements over MarM247, [14] and Stepanov et al [11] and Senkov et al [13] reported substantial specific strength improvements over a wide temperature range compared to many state-of-the-art precipitation hardened alloys, albeit with multiphase microstructures observed in some instances.A single-phase structure was maintained in the Al 0.4 Hf 0.6 NbTaTiZr alloy [9] after solidification, annealing, and deformation. This was in partial agreement with thermodynamic predictions for that system, although some lower-temperature equilibrium phases that were predicted were in fact not observed.Despite the exceptional oxidation resistance demonstrated by the Al-Cr-Mo-Ta-Ti alloy system, it is known to quite preferentially form a Cr-Ta Laves phase, [15] and recent substantial modifications to Al did not drastically alter the prevalence of the Laves phase, [16,17] while significant Ta modifications were necessary to rid the system of grain boundary Laves phase presence after…”

“…Despite the exceptional oxidation resistance demonstrated by the Al-Cr-Mo-Ta-Ti alloy system, it is known to quite preferentially form a Cr-Ta Laves phase, [15] and recent substantial modifications to Al did not drastically alter the prevalence of the Laves phase, [16,17] while significant Ta modifications were necessary to rid the system of grain boundary Laves phase presence after…”

“…Previous modifications to the AlCrMoTaTi alloy system revealed that less than 15 at% Al reduces the potency of CrTaO 4 formation during oxidation at 1200 °C, whereby rapid oxidation of Al at or near the surface allows for easier diffusion of Cr and formation of Cr 2 O 3 and critical CrTaO 4 layers. [17] In addition, Ta modifications demonstrated that <10 at% is also critical to oxidation at 1200 °C, in which MoO 3 evaporation can accelerate and deficiencies of Ta do not support sustained growth of the CrTaO 4 intermediate layer, which itself slows diffusion and additional ingress of oxygen into the AlCrMoTaTi metal substrate. [18] Thus, this research reports on the optimization of modified compositions in the AlCrMoTaTi alloy system.…”