Cellular substructure of Invar alloy with different modifiers

Utilizing vacuum arc remelting, Ti, Nb, and Ti-Nb (0.2 wt%) oxides were incorporated into Fe-36Ni Invar alloy plates. The materials were characterized using SEM, EBSD, EPMA, TEM, and XRD. Results show that oxide modifiers effectively refine the cellular-like substructure and grain size, with the cellular substructure becoming more pronounced and forming a cross shape. This is due to the nucleation and pinning effects of the oxides and the unstable bifurcation growth mode from heat flow. The Ti-Nb-modified Invar alloy exhibits superior mechanical properties (160 GPa Young's modulus, 175 HV Vickers hardness) and a thermal expansion platform (Δ T = 150 K), which is due to refined grain (3–5 mm) and substructure sizes (20 μm to 8 μm), increased dislocation density, and low-angle grain boundaries. The study also reveals that austenite grain boundary formation lags behind the cellular substructure, indicating that eutectic cell segregation and cellular substructure precede grain boundaries.