Precipitation kinetics, microstructure, and equilibrium state of A2 and B2 phases in multicomponent Al_2.75CoCrFeNi alloy

Al_xCoCrFeNi high-entropy alloys have received significant attention recently because of their promising mechanical and corrosion-resistance properties. These alloys tend to form a mixture of fcc and bcc phases, where the latter has an important role in material hardening. In many cases, the bcc phase is a mixture of disordered bcc (A2), which is an Fe- and Cr-rich phase, and ordered bcc (primitive cubic, B2), which is an Al- , Ni- , and Co-rich phase. Although phase diagrams above x = 2 are somewhat consistent, they unfortunately contain no valuable data about the mole fraction and phase composition. Moreover, Al_x > 2CoCrFeNi alloys suffer from a lack of systematic experimental investigation into the kinetics of the phase transformation. To clarify these points, the present study investigates the phase relations and precipitation kinetics of A2 from the B2 matrix in Al_2.75CoCrFeNi. The results show that the compositions of the A2 and B2 phases are temperature-dependent and that, with increasing temperature in the B2 phase, the Al content decreases while the Cr content increases, which correlates with thermodynamic calculations. In addition, the equilibrium composition and phase content lead to a reduced lattice distortion parameter compared with that of the nominal alloy. Concerning the kinetics of phase transformation, the results suggest that, to precipitate in the solid state, the A2 phase within the B2 matrix must overcome the internal stresses that are due to the different lattice parameters of the two phases. Furthermore, the diffusion activation energy is estimated and its implications are discussed from the perspective of sluggish diffusion in multicomponent systems. Finally, the coefficients of thermal expansion of Al_2.75CoCrFeNi alloy and of Al–Ni–Co- and Cr–Fe-rich alloys (both alloys containing Al, Co, Cr, Fe, Ni) were measured and are discussed in relation to phase transformation.