High-throughput compositional mapping of phase transformation kinetics in low-alloy steel

Abstract Knowledge of phase transformation kinetics is a key point in designing steel grades, in particular modern high-performance grades, highly sought-after in energy and transportation applications. The design space for these grades is highly multi-dimensional given the numerous potential alloying elements. The characterization techniques that are usually relied on to assess transformation kinetics, such as metallography or dilatometry, are highly time consuming, due to their limitation to either a single transformation time or a single composition per experiment. The high-throughput approach showcased here overcomes those limitations by combining compositionally graded samples with time- and space-resolved in situ X-ray diffraction, yielding full kinetic records over a range of compositions in a single run. Its application to low-alloy steel required addressing specific challenges related to the reactivity and thermodynamics of the material. The transformation of austenite into ferrite was chosen to illustrate its benefits. Using the rich resulting database, the transformation mechanism was examined quasi-continuously across sections of the composition space. Neither the paraequilibrium, nor the local equilibrium with negligible partitioning model, nor a transition from the former to the latter is applicable over the whole range of investigated conditions. Instead, the observed kinetics were explained by accounting for the solute drag exerted on the mobile interface. This work is a major contribution in accelerating the design of future low-alloy steel grades, involving the transformation of austenite into ferrite or any other transformation to which the present high-throughput methodology can be adapted.