Rational design and multi-stage formation mechanisms of FeCrNiAl medium-entropy alloy strengthened by multi-scaled dual phases

Abstract

Constructing multimodal microstructure in high/medium-entropy alloys (H/MEAs) is expected to further improve the performance via introducing heterostructure strengthening mechanism. In this study, FeCrNiAl-based MEAs were rationally designed to form a special microstructure with trimodal B2 nanoparticles and rod-shaped Laves phase into body-centered-cubic (BCC) structured matrix, via the CALPHAD method and composition adjustment. To systematically exhibit the microstructure evolution, phase formation and stabilization mechanisms, we reported the representative Fe61.93Al15.62Cr13.88Ni6.25Nb0.97Mo1.16Ta0.19 (at.%) alloy. It is found that the multi-scaled B2 particles with distinct mean radii of 225 nm, 15 nm, 1–5 nm precipitate in the melt solidification, aging and cooling processes in turn, and the Laves phase also forms in the aging stage. The primary and secondary B2 particles precipitate in the classical nucleation and growth pathway, while the ultrafine B2 particles follow a spinodal decomposed mechanism. By co-alloying Mo, Nb, and Ta elements, the thermal stability of B2 and Laves phase is significantly improved, resulting in a much lower coarsening rate (k = 8 × 10−27 m3/s) than that in common B2-strengthened BCC alloys at 800 °C. This ultra-stable and heterostructured alloy should have good application potential at elevated temperature, and should be a paradigm for future development of multi-modal precipitate strengthened alloys.