A Modified Embedded-Atom Method Potential for a Quaternary Fe-Cr-Si-Mo Solid Solution Alloy

Ferritic-martensitic steels, such as T91, are candidate materials for high-temperature applications, including superheaters, heat exchangers, and advanced nuclear reactors. Considering these alloys' wide applications, an atomistic understanding of the underlying mechanisms responsible for their excellent mechano-chemical properties is crucial. Here, we developed a modified embedded-atom method (MEAM) potential for the Fe-Cr-Si-Mo quaternary alloy system-i.e., four major elements of T91-using a multi-objective optimization approach to fit thermomechanical properties reported using density functional theory (DFT) calculations and experimental measurements. Elastic constants calculated using the proposed potential for binary interactions agreed well with ab initio calculations. Furthermore, the computed thermal expansion and self-diffusion coefficients employing this potential are in good agreement with other studies. This potential will offer insightful atomistic knowledge to design alloys for use in harsh environments.

A Phase-Field Model for In-Space Manufacturing of Binary Alloys

The integrity of the final printed components is mostly dictated by the adhesion between the particles and phases that form upon solidification, which is a major problem in printing metallic parts using available In-Space Manufacturing (ISM) technologies based on the Fused Deposition Modeling (FDM) methodology. Understanding the melting/solidification process helps increase particle adherence and allows to produce components with greater mechanical integrity. We developed a phase-field model of solidification for binary alloys. The phase-field approach is unique in capturing the microstructure with computationally tractable costs. The developed phase-field model of solidification of binary alloys satisfies the stability conditions at all temperatures. The suggested model is tuned for Ni-Cu alloy feedstocks. We derived the Ginzburg-Landau equations governing the phase transformation kinetics and solved them analytically for the dilute solution. We calculated the concentration profile as a function of interface velocity for a one-dimensional steady-state diffuse interface neglecting elasticity and obtained the partition coefficient, k, as a function of interface velocity. Numerical simulations for the diluted solution are used to study the interface velocity as a function of undercooling for the classic sharp interface model, partitionless solidification, and thin interface.

Grain boundary-induced premelting and solid ↔ melt phase transformations: effect of interfacial widths and energies and triple junctions at the nanoscale

The strong effect of dry grain boundary (GB) and isolated solid-melt interface widths and energies and the triple junction energy on GB-induced premelting and the transformations between solid, premelt, and melt at the nanoscale are studied using a thermodynamically consistent original phase field approach of Ginzburg-Landau type. One order parameter is considered for describing solid ↔ melt transformation and another N- 1 independent order parameters are used to describe N grains in the samples. A variable energy of the GB regions is assumed which depends on the order parameter related to solid ↔ melt transformation, and the energy of the GBs reduces from their dry state energy γ_gb in solid to zero at melt. The complex 'phase diagrams', showing the variation of the transformation temperatures with the non-dimensional width ratio (ratio of dry GB width and isolated solid-melt interface width), are plotted for 2D bicrystals with a planar GB and a circular GB for the cases when (i) solid-melt interface width is fixed and GB width varies, and (ii) GB width is fixed and solid-melt interface width varies. A bistable region, where the premelt spontaneously appears or transforms back to the solid, is shown indicating that these transformations accompany hysteresis. The effect of the width ratio on the effective melt layer thickness and the disjoining potential between the solid-melt interfaces on premelting/melting is also studied. The effect of triple junctions on the transformations is studied in tricrystals having three planar GBs meeting at the interior of the sample with equal dihedral angles. A premelt/melt pocket appears at the junction in a jump-like manner at temperatures significantly lower than the temperatures at which the GBs premelt. The premelt/melt that appeared at the junction eventually wets the adjacent GBs, and finally, the entire sample melts at the melting point. The results show a plausible reason for differences in the premelting and melting temperature ranges of the GBs observed in various experimental and atomistic studies.

Kinetics of the γ-δ phase transition in energetic nitramine-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

The solid, secondary explosive nitramine-octahydro-1,3,5,7-tetranitro-1,3,5,7 or HMX has four different stable polymorphs which have different molecular conformations, crystalline structures, and densities, making structural phase transitions between these nontrivial. Previous studies of the kinetics of the β-δ HMX structural transition found this to happen by a nucleation and growth mechanism, where growth was governed by the heat of fusion, or melting, even though the phase transition temperature is more than 100 K below the melting point. A theory known as virtual melting could easily justify this since the large volume difference in the two phases creates a strain at their interface that can lower the melting point to the phase transition temperature through a relaxation of the elastic energy. To learn more about structural phase transitions in organic crystalline solids and virtual melting, here we use time-resolved X-ray diffraction to study another structural phase transition in HMX, γ-δ. Again, second order kinetics are observed which fit to the same nucleation and growth model associated with growth by melting even though the volume change in this transition is too small to lower the melting point by interfacial strain. To account for this, we present a more general model illustrating that melting over a very thin layer at the interface between the two phases reduces the total interfacial energy and is therefore thermodynamically favorable and can drive the structural phase transition in the absence of large volume changes. Our work supports the idea that virtual melting may be a more generally applicable mechanism for structural phase transitions in organic crystalline solids.

Structural transformation in monolayer materials: a 2D to 1D transformation

The excess energy of surface atoms along with the surface stresses can be the source of structural instabilities in low dimensional materials, which here we revealed the 2D to 1D transformation.