Phase-Dependent Magneto-Elastic Responses of iron allotropes under extreme multiaxial Stress: Fundamental Insights for extreme environment Applications

The structural integrity of next-generation nuclear reactor components relies fundamentally on the performance of Body-Centered Cubic (BCC, e.g., ferritic/martensitic steels) and Face-Centered Cubic (FCC, e.g., austenitic stainless steels) iron-based alloys. Under severe neutron irradiation and thermal gradients, these materials accumulate extreme multiaxial residual stresses. This study systematically investigates the phase-dependent magneto-elastic divergence of idealized BCC and FCC iron allotropes using first-principles calculations, validated by macroscopic experimental characterization of a pre-stressed Fe-Si proxy. Our multiscale analysis reveals a critical mechanical bifurcation: under profound compression, the BCC lattice intrinsically stiffens across all elastic moduli, demonstrating robust microstructural resilience. In stark contrast, the FCC lattice (initialized in its high-spin ferromagnetic state) exhibits a catastrophic elastic instability under compression, with its Young’s modulus decreasing drastically to a near-zero minimum (∼0.36 GPa) at −6 GPa. This severe shear softening is driven by a pressure-induced Invar-like magnetovolume effect, wherein the collapse of the localized high-spin state structurally destabilizes the lattice. Furthermore, under tension, the FCC phase displays an atypical stiffening and a nonlinear magnetic surge driven by enhanced exchange splitting. These theoretical findings establish an idealized atomistic baseline for understanding the fundamental phase stability and divergent resistance to stress-driven microstructural degradation between BCC and FCC iron-based structural materials.