Heterogeneous Nucleation and Microstructure Formation: From a Model Systems to Applied Metal Physics

Abstract

Most physical and chemical properties of metallic alloys are determined to a large degree via their microstructure morphology. One example is the crystal structure of a ferritic and an austinitic steel, respectively, in which the desired mechanical, electrical, and magnetic properties are strongly tied to a likewise desired specific microstructure of the steel. As many of today’s industrial relevant metallic alloy systems are processed from the molten state, a most quantitative understanding of solidification as an essential primary processing step is a premise for a most quantitative tailoring of such desired microstructures. Nonetheless, heterogeneous nucleation and successive microstructure formation as initial solidification steps are still far from being comprehensively understood. However, the comparison of available advanced models and recent experimental results brings about controversial conclusions. To progress these scientific discussions, from 2007 to 2013, the German Research Foundation (DFG) funded the scientific priority program 1296 related to the heterogeneous nucleation and initial microstructure formation, aiming at a fundamental understanding of the basic mechanisms underlying heterogeneous nucleation as well as the subsequent development of the nucleus into a specific heterogeneous microstructure. To that end, a systemand method-spanning scientific approach was developed, in which the simplest types of model systems for heterogeneous crystalline orders, pure metals, binary metal alloys, and colloids were investigated in a comparative manner. Moreover, they were assessed with complementary experimental as well as simulation techniques from the atomic to the microscale to obtain a comprehensive view across all relevant time and length scales, as depicted in Fig. 1. Some questions explored in detail in the context of the priority program were as follows: With respect to heterogeneous nucleation, what does a critical nucleation grain look like? Does the classic concept of a contact angle make sense for heterogeneous nucleation? Can claims made about the dominant contributions to the nucleation barrier for heterogeneous nucleation, such as they can be obtained from molecular simulations, be reconciled with data gained via the phase-field method? With respect to transition from nucleus to microstructure, how does a microstructure develop from a nucleus in the interplay between crystallization and segregation depending on the precise reference point in the phase diagram? How stable are those scenarios with respect to changes of that reference point? With respect to microstructure development, what kind of consequence results from the new understanding of nucleation for the initial development of the microstructure, e.g., in terms of new kinetic scaling laws? What kind of kinetic rules does the initial growth of the solidifying microstructure follow? An overview of the most important results of that priority program has recently been presented in Ref. 5. In this section of JOM we go a step further and present some of the more applied recent results acquired in the priority program, which build the bridge from fundamental investigations in model systems toward industrially more relevant multicomponent and multiphase metallic systems. The more applied specific questions related to heterogeneous nucleation and initial microstructure formation in such systems addressed here range from the identification of the quantitative influence of inoculation process parameters on the amount of achievable grain refinement, over a comprehensive assessment of the kinetics in four-phase peritectic reactions, pointing out differences in different types of peritectica, to a quantitative assessment of nucleation energies in solid–solid systems based on newly established simulation methodologies. In toHeike Emmerich is the guest editor for the Phase Transformations Committee of the TMS Materials Processing & Manufacturing Division, and coordinator of the topic Heterogeneous Nucleation and Initial Microstructural Formation in this issue. JOM, Vol. 66, No. 8, 2014