The properties of the interface between solid and melt are key to solidification and melting, as the interfacial free energy introduces a kinetic barrier to phase transitions. This makes solidification happen below the melting temperature, in out-of-equilibrium conditions at which the interfacial free energy is ill defined. Here we draw a connection between the atomistic description of a diffuse solid-liquid interface and its thermodynamic characterization. This framework resolves the ambiguities in defining the solid-liquid interfacial free energy above and below the melting temperature. In addition, we introduce a simulation protocol that allows solid-liquid interfaces to be reversibly created and destroyed at conditions relevant for experiments. We directly evaluate the value of the interfacial free energy away from the melting point for a simple but realistic atomic potential, and find a more complex temperature dependence than the constant positive slope that has been generally assumed based on phenomenological considerations and that has been used to interpret experiments. This methodology could be easily extended to the study of other phase transitions, from condensation to precipitation. Our analysis can help reconcile the textbook picture of classical nucleation theory with the growing body of atomistic studies and mesoscale models of solidification.

/pdf/solid-liquid-interfacial-free-energy-out-of-equilibrium-2d83aop2ry.pdf

Solid-liquid interfacial free energy out of equilibrium

Refinement mechanism of cerium addition on solidification structure and sigma phase of super austenitic stainless steel S32654

Effects of Rare Earth elements on microstructure evolution and mechanical properties of 718H pre-hardened mold steel

The influence of Ce addition on the solidification structure and σ phase of super austenitic stainless steel S32654 was systematically investigated via microstructural characterization and thermodynamic calculation. The results indicate that a small addition of Ce could modify MgO and MnS into Ce-bearing inclusions Ce2O3 and Ce2O2S. Ce addition led to noticeable refinement of both the dendrite structure and σ phase. The refinement mechanism could be attributed to the combined actions of effective Ce-bearing inclusions and solute Ce. Effective Ce-bearing inclusions could serve as heterogeneous nucleation cores of austenite as well as σ phase, which provided a favorable prerequisite for their refinement. Solute Ce significantly enhanced the undercooling degree of the system, further promoting dendrite structure refinement. Meanwhile, solute Ce improved the eutectic precipitation conditions of σ phase and further promoted its nucleation, while the dendrite refinement limited its growth space. Finally, more fine and dispersed σ phase particles formed in S32654 with Ce addition. The refinement of dendrite structure and σ phase will reduce the temperature and time required for high-temperature homogenization, which is beneficial to the hot working of this steel. 

Revealing dendritic pattern formation in Ni, Fe and Co alloys using synchrotron tomography

Channel segregation, which is featured by the strip-like shape with compositional variation in cast materials due to density contrast-induced flow during solidification, frequently causes the severe destruction of homogeneity and some fatal damage. An investigation of its mechanism sheds light on the understanding and control of the channel segregation formation in solidifying metals, such as steels. Until now, it still remains controversial what composes the density contrasts and, to what extent, how it affects channel segregation. Here we discover a new force of inclusion flotation that drives the occurrence of channel segregation. It originates from oxide-based inclusions (Al2O3/MnS) and their sufficient volume fraction-driven flotation becomes stronger than the traditionally recognized inter-dendritic thermosolutal buoyancy, inducing the destabilization of the mushy zone and dominating the formation of channels. This study uncovers the mystery of oxygen in steels, extends the classical macro-segregation theory and highlights a significant technological breakthrough to control macrosegregation.

Inclusion flotation-driven channel segregation in solidifying steels

Different austenitizing temperatures were used to obtain medium-carbon low-alloy (MCLA) martensitic steels with different lath martensite microstructures. The hierarchical microstructures of lath martensite were investigated by optical microscopy (OM), electron backscattering diffraction (EBSD), and transmission electron microscopy (TEM). The results show that with increasing the austenitizing temperature, the prior austenite grain size and block size increased, while the lath width decreased. Further, the yield strength and tensile strength increased due to the enhancement of the grain boundary strengthening. The fitting results reveal that only the relationship between lath width and strength followed the Hall–Petch formula of. Hence, we propose that lath width acts as the effective grain size (EGS) of strength in MCLA steel. In addition, the carbon content had a significant effect on the EGS of martensitic strength. In steels with lower carbon content, block size acted as the EGS, while, in steels with higher carbon content, the EGS changed to lath width. The effect of the Cottrell atmosphere around boundaries may be responsible for this change.

Yanfei Cao

Papers

Inclusion flotation-driven channel segregation in solidifying steels

Effects of Rare Earth elements on microstructure evolution and mechanical properties of 718H pre-hardened mold steel

The Effect of Lath Martensite Microstructures on the Strength of Medium-Carbon Low-Alloy Steel

Formation mechanism of MC and M2C primary carbides in as-cast M50 bearing steel

Low-oxygen rare earth steels