Interface engineering enables fast bainitic transformations with minimal alloying

Fine microstructures in steels are commonly produced by the low-temperature decomposition of austenite. Bainite is one such microstructure, combining high strength and excellent toughness, but its formation kinetics is sluggish. Here, we demonstrate a strategy to accelerate bainite formation by coupling the introduction of prior athermal martensite with selective solute segregation. A fraction of 0.2 prior martensite increases by tenfold the density of interfaces available for bainite nucleation. Moreover, boron is added to suppress the formation of softer, coarser phases at high temperatures by segregating to austenite grain boundaries. Yet, boron does not segregate to the freshly formed martensite/austenite interfaces. This selective segregation facilitates bainite nucleation at martensite/austenite interfaces and reduces transformation time by 66%. The proposed interface engineering strategy overcomes the traditional trade-off between hardenability control and bainite kinetics, enabling the design of compositionally leaner bainitic steels, and provides a general pathway for enhancing phase transformation kinetics in metals.