Transition in coarsening mechanisms of β′(NiAl) precipitates induced by Ti in Fe–Ni–Al alloys

The influence of Ti additions on the microstructural evolution, coarsening kinetics, and microhardness response in Fe–10Ni–15Al alloys was investigated during long-term aging at 800–900 °C for up to 1000 h. Thermodynamic equilibrium calculations via Thermo-Calc, coupled with HR-SEM and EDS analyses, reveal that Ti partitions preferentially to the β′ phase, increasing the equilibrium precipitate volume fraction within a stable two-phase α-Fe + β′ field and modifying the interfacial characteristics between the precipitates and the matrix. These changes are associated with morphological transitions toward elongated and irregular coalesced particles, accompanied by the development of geometric interfacial dislocation networks. While the Ti-free alloy exhibits coarsening consistent with classical Lifshitz–Slyozov–Wagner (LSW) theory, Ti-modified alloys show notable departures from fundamental LSW assumptions, suggested to be modulated by a competitive synergy among volume fraction effects, localized spatial groupings of precipitates, extensive coalescence, and interface-assisted phenomena. Vickers microhardness measurements reveal a plateau-like hardening response rather than conventional overaging; this temporary stabilization of hardness values at extended exposure times is highly likely to be enhanced by the development of geometric interfacial dislocation networks, which are expected to act as additional structural barriers against matrix dislocation glide. Ultimately, these findings indicate that titanium additions alter the coarsening pathways of β′ precipitates and delay thermal softening in these high-temperature ferritic alloys.