Elucidating hierarchical microstructures in high entropy superalloys: An integrated multiscale study

Abstract

In this study, we examine a high entropy superalloy (HESA-Y1: Ni_49.37Co₂₀Cr₇Fe₄Al_11.6Ti₆Re₁Mo_0.5W_0.5Hf_0.03 at%), focusing on hierarchical microstructure formation and its effects on mechanical properties. Thermodynamic modeling using Thermo-Calc predicts equilibrium phase fractions, compositions, and transition temperatures, which are validated by experimental data from differential scanning calorimetry (DSC). Transmission electron microscopy (TEM) reveals that secondary aging induces nanometer-sized γ particles within γ' precipitates, forming a hierarchical γ/γ' microstructure. Atom probe tomography (APT) confirms supersaturation of γ' precipitates with γ-forming elements (Co, Cr, Fe), driving γ particle formation, and measures interfacial widths between γ' and γ phases. Partitioning coefficients derived from APT align with Thermo-Calc predictions for most elements. Vickers microhardness testing shows an increase of about 50 HV in the hierarchical microstructure compared to the conventional one. In situ synchrotron X-ray diffraction (XRD) from 25 to 750 °C determines a small, negative lattice misfit δ between γ and γ' phases, suggesting enhanced microstructural stability, consistent with Thermo-Calc calculations. Our methodological approach enables measurement of the unconstrained lattice parameter of phase-extracted γ' in a single-crystal XRD setup. Due to their small size and low volume fraction, γ particles do not produce distinct reflections in the X-ray diffractogram. Elucidating hierarchical microstructures across multiple scales, we establish that the presence of Re and Hf and controlled aging processes lead to enhanced mechanical properties, offering valuable insights for the design of advanced high entropy superalloys.