Phase nucleation, coarsening and evolution pathways of a sputtered nanostructured Inconel 725 alloy during heat treatment

Abstract

Physical vapor deposition enables the fabrication of nanostructured superalloys with unique defect architectures, yet their phase evolution pathways can differ significantly from those of conventionally processed alloys. In this study, the effects of solution and aging treatments on phase selection and precipitation behavior in sputtered Inconel 725 films with an initially uniform columnar nanotwinned structure were systematically investigated. Direct aging at relatively low temperatures promoted extensive δ-phase precipitation at twin boundaries and defect-rich regions, which depleted Nb from the γ matrix and suppressed γ&#39;/γ&#34; precipitation. In contrast, high-temperature solution treatment induced recrystallization and eliminated the nanotwinned structure, significantly reducing δ-phase precipitation and increasing Nb availability to enable the formation of ultrafine spherical γ&#39;/γ&#34; precipitates within a refined γ matrix (<1 μm). Subsequent aging treatments promoted elemental partitioning and drove the morphological evolution of γ&#39;/γ&#34; precipitates from spherical to lenticular forms, while δ precipitation became increasingly concentrated along grain boundaries. This spatial separation of intragranular γ&#39;/γ&#34; and grain-boundary δ phases enabled simultaneous precipitation strengthening and grain stabilization, resulting in hardness values approaching 9 GPa. As a whole, this study demonstrates that the initial templates provided by defect structures can govern phase selection and precipitation pathways, providing a strategy for tailoring microstructure and achieving synergistic strengthening in nanostructured superalloys.