Effect of heat treatment on grain boundary character and strength-ductility synergy in additive-manufactured Hastelloy X

Nickel-based superalloys fabricated by selective laser melting often suffer from a strength-ductility trade-off due to non-equilibrium microstructures. Here, we demonstrate that this limitation can be overcome by grain boundary engineering through multi-stage heat treatments. In Hastelloy X, solution treatment at 1175°C followed by furnace cooling triggers complete recrystallization, producing an optimal microstructure with a high fraction of Σ3 annealing twins (22.1%) and high-angle grain boundaries (64.6%). This unique configuration, along with uniformly distributed grain boundary carbides, promotes strain-accommodating boundary sliding and alleviates dislocation pile-up. Consequently, the material achieves an exceptional strength-ductility synergy (ultimate tensile strength: 663.5 MPa; ductility: 43.7%), representing a 30.4% enhancement in ductility over conventional forged material. We reveal that the enhanced performance stems from a transition in the dominant strengthening mechanism, where solid solution strengthening effectively compensates for the reduced contribution from grain boundaries. This work provides a microstructural design strategy to unlock the full potential of additive manufacturing for high-performance alloys.