A critical review of chemical heterogeneity-driven microstructure control in steels

This paper reviews a recent microstructure control strategy for steels based on intentional chemical heterogeneity, referred to as "chemically heterogeneous steels" or "chemically patterned steels." This approach introduces controlled chemical inhomogeneity, which has traditionally been avoided, in order to diversify microstructural design and enhance mechanical properties. The strategy involves a three-step process: (i) introducing compositional variations in a multiphase region, (ii) preserving these variations through rapid austenitization, and (iii) controlling microstructural evolution during subsequent cooling. Chemical heterogeneity is introduced via austenite reversion, carbide precipitation, or pearlitic microstructures, enabling precise control of concentration gradients and characteristic length scales. Building on this processing route, the review emphasizes two primary mechanisms for property improvement. First, localized elemental partitioning stabilizes a higher fraction of retained austenite with a broad stability spectrum, inducing a progressive transformation-induced plasticity (TRIP) effect that enhances ductility. Second, the resulting "chemical boundaries" act as physical barriers to dislocation motion and crack propagation, increasing strength and resistance to hydrogen embrittlement. Finally, the paper suggests extending this concept beyond Mn to other alloying elements such as Cr and Ni, as well as its potential application to titanium alloys and implementation in industrial-scale production lines.