New insights of the nucleation and subsequent phase transformation in duplex stainless steel

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Materials Characterization


The solidification behavior, especially involving multi-phase, of duplex stainless steel (DSS) directly determines its microstructures. However, thermodynamic calculations and other reports, which indicates that δ-ferrite is always thermodynamically stable as the primary phase, cannot precisely describe the nucleation and subsequent phase transformations in DSS. Both high temperature confocal microscopy (HTCM) and differential thermal analysis (DTA) are well-established tools for studying solidification behaviors and phase transformation. Here, an in-house build apparatus was employed to investigate the solidification process of DSS. It enables contemporaneous in-situ observation of the surface and thermal analysis of the bulk thermal events. Meanwhile, N2 and Ar are used as protective gas. Dual-peak DTA signals of solidification were recorded under an N2 atmosphere. Furthermore, through the real-time comparison between the DTA signals and the in-situ observations, a solid-state phase transition was accurately located. We propose the following new insight into the solidification mechanism for DSS, by combining experimental results, thermodynamic arguments (Gibbs free energy), and microstructural characterization using electron back scattered diffraction (EBSD): The γ nucleates and grows via a competitive nucleation process, and completely transforms into δ-ferrite in a solid-state phase transformation. After that, part of the δ-ferrite transforms into γ, and the duplex structure remains (L → γ → δ → δ + γ). Heterogeneous nucleation of γ on the liquid surface was also observed, which is quite different from the nucleation and growth pattern of δ-ferrite. These new insights contribute to the improvement of solidification mechanisms of high nitrogen stainless steels and, in turn, microstructures and physical properties of steels in the course of materials production processes.

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