Retained Free Energy with Enhanced Nucleation during Electrostatic Levitation of Undercooled Fe-Co Alloys

Douglas M Matson,Justin E Rodriguez,Olga Shuleshova,Xuanjiang Liu,Sangho Jeon

doi:10.3390/cryst11070730

Douglas M Matson, Justin E Rodriguez + Show 3 more

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https://doi.org/10.3390/cryst11070730

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Abstract

Double recalescence in many ferrous alloy systems involves rapid solidification of metastable ferrite from the undercooled melt with subsequent transformation to stable austenite. Containerless processing is used to monitor the process using pyrometry and high-speed cinematography such that delay behavior can be predicted based on the application of the retained damage model (RDM). When comparing Fe-Cr-Ni alloys to Fe-Co alloys, the cluster attachment rate is enhanced while free energy retention is reduced. These trends are tied to specific alloy properties. A retained free energy criterion is proposed based on the ratio of thermophysical properties used to define the transformation driving force such that the thermodynamic limit for energy retention may be predicted. Surprisingly, at long delay times, healing occurs such that much of the retained free energy is not available to enhance the transition from metastable to stable phases. At delay times less than one second, no healing is observed and the RDM correctly predicts transformation delay behavior over a wide range of alloy compositions.

Highlights

In order to control the phase selection and subsequent microstructural evolution in solidification processes involving rapid heat extraction, such as additive manufacturing, spray forming, and welding, the thermodynamic limits that define transformation kinetics must be understood
Previous work showed that the influence of undercooling alone may be evaluated by investigation of transformation behavior during processing using electrostatic levitation (ESL), where convection during undercooling is negligible
This contrasts the behavior during electromagnetic levitation (EML) processing where convection and undercooling simultaneously impact the delay

Summary

Introduction

In order to control the phase selection and subsequent microstructural evolution in solidification processes involving rapid heat extraction, such as additive manufacturing, spray forming, and welding, the thermodynamic limits that define transformation kinetics must be understood. The retained damage model (RDM) is used to predict the delay, or incubation time, between primary and secondary recalescence events during rapid solidification, where transformation from molten metallic alloy liquid passes through a metastable intermediary primary phase before conversion to a stable secondary phase. Previous work showed that the influence of undercooling alone may be evaluated by investigation of transformation behavior during processing using electrostatic levitation (ESL), where convection during undercooling is negligible. This contrasts the behavior during electromagnetic levitation (EML) processing where convection and undercooling simultaneously impact the delay. By applying the results from ESL to the EML results, it is possible to isolate the impact on transformation delay due to convection alone. No previous modeling effort has successfully been employed to numerically distinguish between these effects

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