Fundamental Effects of Al and Ta on Microstructure and Phase Transformations in the Al–Cr–Mo–Ta–Ti Refractory Complex Concentrated Alloy System

Abstract

The effect of aluminum and tantalum concentrations on a refractory metal complex concentrated alloy is reported, particularly with respect to their effect on microstructure and phase composition of the alloy in cast and annealed form. Alloys with an equiatomic composition, (AlCrMoTaTi), an aluminum‐lean composition (Al0.75CrMoTaTi), and a tantalum‐lean composition (AlCrMoTa0.75Ti) are produced via arc melting. The alloys exhibit multiphase structures, confirmed by X‐ray diffraction, microstructural characterization, and thermal analysis. The minor off‐equiatomic adjustments of aluminum and tantalum in this alloy system did not drastically alter the prevalence of the Cr–Ta‐based Laves phase. Correlations between thermodynamic predictions and observed phase transformations via thermal analysis are improved upon refinement of calculations removing impractical intermediate phases. Experimental findings provide information for the refinement of thermodynamic modeling and deliver additional insight into the optimization of alloy compositions within this five‐component system.

Compositional Modifications to Alter and Suppress Laves Phases in AlxCrMoTayTi Alloys

“…homogenization and cooling. [18] However, reasonable correlation was found between thermodynamic predictions and observed phase transformations, [16] leading to confidence in further potential optimization of the system. Tuning Al and Cr ratios in the AlCrMoTaTi system was demonstrated, whereby sophisticated ordered structures could be directed, although these modifications did not demonstrate a disordered solid solution.…”

“…Initial thermodynamic calculations in this system focus on refined approaches based on CALPHAD results from prior work, [16] as well as other work that specifically evaluated the dependencies of Al, Cr, and Ta. [17][18][19] In this research, a critical driver for the success of the alloy includes long-term environmental and microstructural stability at 1000 °C.…”

“…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 homogenization and cooling. [ 18 ] However, reasonable correlation was found between thermodynamic predictions and observed phase transformations, [ 16 ] leading to confidence in further potential optimization of the system. Tuning Al and Cr ratios in the AlCrMoTaTi system was demonstrated, whereby sophisticated ordered structures could be directed, although these modifications did not demonstrate a disordered solid solution.…”

“…[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…”