Daniel dos Santos Avila

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.

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.

In situ composite manufacture is an approach to improve interfacial adhesion between matrix and reinforcements, in which reinforcements are synthesized along composite processing itself. In situ powder metallurgy route, in particular, offers alternatives to some shortcomings found in other techniques. This work aims not only to review the state of the art on metal matrix composites (MMCs)—including cermets—obtained in situ by powder metallurgy, but also to dissect key aspects related to the development of such materials in order to establish theoretical criteria for decision making before and along experiments. Aspects regarding the design, raw material selection, and processing of such composites were observed and divided between concept, intrinsic, and extrinsic parameters. That way, by means of material databases and computational thermodynamics applied to examples of the reviewed literature, we aim at providing tools in both conducting leaner experiments and richer discussion in this field.