Abstract
Ternary Fe-Cr-Ni stainless steel alloys often exhibit a multi-step transformation known as double recalescence where primary ferrite converts to austenite during rapid solidification processes such as casting and welding. In addition to the volume free energy associated with undercooling between the phases, the free energy driving the transformation comes from two additional sources that are retained within the metastable solid—one from the primary phase undercooling and one from melt shear. A new physical model is proposed based on accumulation of defects, such as dislocations or tilt boundaries, and lattice strain. A dimensionless analysis technique shows that the free energy associated with metastable solidification is conserved and the contribution from melt shear can be predicted based on a modification of the Read-Shockley dislocation energy equation. With these additional terms the incubation time between nucleation events becomes inversely proportional to the total free energy squared for bulk diffusion and cubed for grain boundary diffusion mechanisms. In the case of the ferrous alloys studied, the grain boundary mechanism provides a better fit and when the model is applied the delay time behavior collapses to a single master-curve for the entire alloy family.
Highlights
Molten metal processes such as casting or welding are commercially important manufacturing operations with casting shipments alone accounting for over $198 trillion/year worldwide.1 The US Materials Genome2 has identified improving modeling capabilities as an industry priority in order to significantly reduce product development time
Since the incubation time is reduced with increased undercooling or melt shear the metastable solid must retain additional free energy to drive the transformation
Experimental observations show that both primary undercooling tests spanning a broad range of melt shear conditions, from laminar to turbulent flow, across multiple test facilities
Summary
Molten metal processes such as casting or welding are commercially important manufacturing operations with casting shipments alone accounting for over $198 trillion/year worldwide. The US Materials Genome has identified improving modeling capabilities as an industry priority in order to significantly reduce product development time. The US Materials Genome has identified improving modeling capabilities as an industry priority in order to significantly reduce product development time. Investigating process improvement options using electronic means through varying key parameters virtually, instead of conducting expensive trials, allows industry to be flexible in adapting to a changing market while increasing quality and profitability. Modeling increases speed to market, reduces development costs and allows the user to investigate company strategic position. The generation of models will include phase selection, localized solidification stress relaxation, and microstructural evolution for defect control. A missing piece of the puzzle is understanding how convection, through melt shear, influences microstructural evolution and this paper concentrates on developing a model to describe transformation kinetics in an important class of structural materials by looking at control of phase selection in Fe-Cr-Ni alloys during rapid solidification
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