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The current understanding of the formation mechanism and kinetics of the σ phase in DSS remains ambiguous. This study investigates the in-situ evolution of the σ phase in DSS through a multi-faceted approach. Distinct σ phase formation mechanisms were identified at 850 °C and 950 °C, with short-range diffusion of eutectoid decomposition at 850 °C and long-range diffusion of nucleation and growth at 950 °C. In-situ X-ray diffraction (XRD) revealed that the lattice parameter of the γ phase peaks at ∼ 600 °C. The thermal expansion coefficient (CTE) of the γ phase is 1.8 times higher than that of the α phase at low temperatures. Interestingly, the convergence of the CTEs of the α and γ phases above 600 °C triggers σ nucleation. The strain field diagram shows maximum impinging strain at the lattice parameter maxima just before the σ formation range. The adverse effects of the σ phase on DSS heat transport properties were also explored, showing an initial increase in thermal conductivity with an increasing γ volume fraction, followed by an 18 % decrease upon σ evolution. First-principle calculations highlight the correlation between Ni enrichment and lattice parameter maxima, supporting these findings.

The high-temperature phase stability of the NbCr2 Laves phase has not yet been established beyond disagreement. The well accepted C14 phase field has been eliminated from the Nb-Cr phase diagram in a recent study that claimed that the NbCr2 alloy shows the C15 structure up to melting. The present study was conducted to address these ambiguities. The experimental investigations show the presence of C14, C15, and C36 structures in the as-cast alloy. Differential scanning calorimetry studies of the as-cast alloy show the transformation of the eutectic mixture of bcc + NbCr2 to single-phase NbCr2 at 1338 K. An in-situ X-ray diffraction (XRD) study of as-cast alloy shows a phase transformation of C36 → C14 below 1273 K. Density functional theory (DFT) calculations confirm two phase transformations: (a) a metastable transformation of C36 → C14 at 783 K and (b) an equilibrium transformation of C15 → C14 at 1971 K, which was validated by high temperature DTA and TEM studies. Further, extended DFT calculations on supercells with four compositions of NbCr1.9, NbCr2, NbCr2.1, and NbCr2.2 also corroborated the presence of the C14 phase, and consequently, the Nb-Cr phase diagram is modified. This finding resolves the controversy regarding the existence of an equilibrium C14 phase field before melting.