For decades, grain boundary engineering has proven to be one of the most effective approaches for tailoring the mechanical properties of metallic materials, although there are limits to the fineness and types of microstructures achievable, due to the rapid increase in grain size once being exposed to thermal loads (low thermal stability of crystallographic boundaries). Here, we deploy a unique chemical boundary engineering (CBE) approach, augmenting the variety in available alloy design strategies, which enables us to create a material with an ultrafine hierarchically heterogeneous microstructure even after heating to high temperatures. When applied to plain steels with carbon content of only up to 0.2 weight %, this approach yields ultimate strength levels beyond 2.0 GPa in combination with good ductility (>20%). Although demonstrated here for plain carbon steels, the CBE design approach is, in principle, applicable also to other alloys.@en

The interaction between moving α/γ interfaces and interphase precipitated (IPd) carbides during the austenite (γ) to ferrite (α) and the ferrite (α) to austenite (γ) transformation has been systematically investigated through cyclic phase transformation experiments for a 0.1C-1.5Mn alloy containing 0.1 wt% Niobium (Nb) and its Nb-free counterpart. Shifts in the critical reaction temperatures during continuous heating and cooling are observed, which are attributed to the pinning force (PF) originating from the IPd carbides present. By applying the Gibbs energy balance (GEB) model to analyze experimental results, the PF was derived to be about 15 J/mol for the α→γ transformation and about 5 J/mol for the γ→α transformation, respectively, both of which are quite small compared to chemical driving force of phase transformations. Moreover, various modified Zener pinning equations have also been used to predict the PF, and it was found that these values are comparable with those obtained from experiments, which suggests that the classical Zener theory still has promising potential for carbide-interface interaction analysis.@en

The modification of the matrix of ductile cast irons by heat treatments has been of interest of researchers for many years. Among these treatments, in the last years the Quenching & Partitioning (Q&P) process has emerged as a viable way to produce microstructures containing controlled amounts of martensite and retained austenite, providing a good combination of strength and ductility. In this work, the different mechanisms of phase transformations occurring during the Q&P heat treatment applied to a ductile cast iron alloy is investigated. Microsegregation, inherent to cast irons, was analyzed by means of Electron Probe Microanalysis (EPMA). Microstructural characterization was performed with Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD), while kinetics of carbon redistribution and competitive reactions were studied using dilatometry and in situ synchrotron X-ray diffraction. It was found that either transition carbides or cementite precipitate in martensite depending on the partitioning temperature. Despite of carbides precipitation, evidence of carbon partitioning from martensite to austenite was obtained. Formation of bainitic ferrite occurs during the partitioning step, further contributing to carbon enrichment of austenite. The experimental results are compared with a local field model that computes the local kinetics of carbon redistribution by simultaneously considering carbides precipitation and growth of bainitic ferrite. Results showed that kinetics of carbon partitioning from martensite to austenite depends on the carbides free energy. More stable carbides do not dissolve and prevent the escape of carbon from martensite. Fast carbon partitioning occurs by dissolution of less stable carbides, but it is slowed down as growth of bainitic ferrite proceeds. This result is explained by the overlapping of the diffusion fields (soft impingement) of the carbon partitioned from martensite and the carbon rejected from growth of bainitic ferrite.@en

Quenching and partitioning (Q&P) process of a 0.3C-1.3Si-3.2Mn (wt%) steel with Mn segregation is studied experimentally and theoretically. During initial quenching a higher fraction of martensite forms in Mn-poor regions compared to Mn-rich regions. In the partitioning process, austenite in Mn-poor regions is surrounded with a higher fraction of martensite than austenite in Mn-rich regions and therefore receives a larger amount of carbon. When carbon partitioning is not sufficient to stabilize austenite, a higher fraction of martensite forms, during final quenching, in Mn-poor regions. Lowering the quenching temperature in the Q&P process reduces inhomogeneity in the distribution of phases.@en

Quenching and partitioning (Q&P) process of a 0.3C-1.3Si-3.2Mn (wt%) steel with Mn segregation is studied experimentally and theoretically. During initial quenching a higher fraction of martensite forms in Mn-poor regions compared to Mn-rich regions. In the partitioning process, austenite in Mn-poor regions is surrounded with a higher fraction of martensite than austenite in Mn-rich regions and therefore receives a larger amount of carbon. When carbon partitioning is not sufficient to stabilize austenite, a higher fraction of martensite forms, during final quenching, in Mn-poor regions. Lowering the quenching temperature in the Q&P process reduces inhomogeneity in the distribution of phases.@en

Quenching and partitioning (Q&P) process of a 0.3C-1.3Si-3.2Mn (wt%) steel with Mn segregation is studied experimentally and theoretically. During initial quenching a higher fraction of martensite forms in Mn-poor regions compared to Mn-rich regions. In the partitioning process, austenite in Mn-poor regions is surrounded with a higher fraction of martensite than austenite in Mn-rich regions and therefore receives a larger amount of carbon. When carbon partitioning is not sufficient to stabilize austenite, a higher fraction of martensite forms, during final quenching, in Mn-poor regions. Lowering the quenching temperature in the Q&P process reduces inhomogeneity in the distribution of phases.@en