Investigation of the phase stabilities in AlNiCoCrFe high entropy alloys

Abstract

High-entropy alloys (HEAs) are a developing class of materials that show great potential for a variety of commercial structural and coating applications, particularly where heat and corrosion resistance are desired. In these types of applications, the relative phase stabilities play a crucial role, since they influence the material design process and expected component lifetimes.However, a satisfactory understanding of the phase stabilities in HEAs and the ability to thermodynamically predict their behaviors is limited. This study methodically couples analytical characterization techniques with predictions based on CALPHAD models to investigate the phase equilibria and phase stabilities of a series of AlNiCoCrFe based HEAs. Heat treatments were conducted at 700°C for 1000 hours and 1050°C for 520 hours, followed by water quenching, to ensure adequate time for metastable equilibrium to be achieved. The accuracy and merit of the thermodynamic simulations in predicting the stable phases and their respective chemistries is discussed relative to the experimental observations.

Influence of Annealing on the Microstructures and Oxidation Behaviors of Al8(CoCrFeNi)92, Al15(CoCrFeNi)85, and Al30(CoCrFeNi)70 High-Entropy Alloys

“…Three bulk alloy buttons with the compositions of Al 8 15 , and Al 30 ) were arc-melted from pure elemental constituents on a water-cooled copper hearth in an inert ultra-high purity (UHP) argon atmosphere. Each button was flipped and re-melted five times to promote homogeneity.…”

“…The actual compositions were found to be in good agreement with the intended compositions. Figure 1 shows representative microstructures for the as-cast, (a-c), and annealed, (d-f), Al 8 , Al 15 , and Al 30 HEAs. All of the as-cast microstructures shown here are taken from [7,15].…”

“…Figure 1 shows representative microstructures for the as-cast, (a-c), and annealed, (d-f), Al 8 , Al 15 , and Al 30 HEAs. All of the as-cast microstructures shown here are taken from [7,15]. Figure 1a shows a backscattered electron (BSE) image of the as-cast Al 8 HEA, which consisted of a bright, high atomic number contrast (high-Z), matrix with dark, low atomic number Z contrast (low-Z), interdendritic regions.…”

“…These studies include both transition metal based [6,7,[14][15][16][17][18][19][20][21][22][23][24][25] and refractory metal based [26][27][28][29] HEA systems. In general, it was reported that elements that commonly oxidize in less-complex, conventional alloy systems (i.e., Ni, Mn, Cr, Al, Ti) also tend to preferentially oxidize in compositionally complex alloys.…”

“…However, there are few publications that attempt to systematically correlate the oxidation behavior of HEAs with existing oxide formation models [6,7,15]. There is also lack of understanding of the influence of annealing on the active oxidation mechanisms, as most published results emphasize as-cast alloys.…”