Advanced Gradient Heating Facility (AGHF)

This section of the publication includes papers entitled: (1) Coupled growth in hypermonotectics; (2) Directional solidification of refined Al-4 wt.% Cu alloys; (3) Effects of convection on interface curvature during growth of concentrated ternary compounds; (4) Directional solidification of Al-1.5 wt.% Ni alloys; (5) Interactive response of advancing phase boundaries to particles; (6) INTeractive Response of Advancing Phase boundaries to Particles-INTRAPP; and (7) Particle engulfment and pushing by solidifying interfaces

Phase-Field Modeling of Solidification in Light-Metal Matrix Nanocomposites

The quantitative phase-field approach has been adapted to model solidification in the presence of Metal Matrix Nanocomposites (MMNCs) in a single-component liquid. Nanoparticles of fixed size and shape are represented by additional fields. The corresponding equations of motion are assumed to ensure relaxation dynamics, and can be supplemented by random forces (realizing Brownian motion) or external fields. The nanoparticles are characterized by two model parameters: their mobility and the contact angle they realize with the solid-liquid interface. We investigate the question how grain size distribution can be influenced by heterogeneous nucleation on the nanoparticles and by the front-particle interaction. We explore, furthermore, how materials and process parameters, such as temperature, density and size/shape distribution of the nanoparticles, influence microstructure evolution.Comment: in Magnesium Technology 2014, John Wiley & Sons, Inc., Hoboken, NJ, US

Direct current plasma spraying of mechanofused alumina-steel particles

Stainless steel particles (60 $\mu$m in mean diameter) cladded with an alumina shell (2 $\mu$m thick and manufactured by mechanofusion) were sprayed with an Ar-H2 (53-7 slm) d.c. plasma jet (I = 500 A, P = 28 kW, \rho_th = 56 %). Two main types of particles were collected in flight, as close as 50 mm downstream of the nozzle exit: particles with a steel core with pieces of alumina unevenly distributed at their surface and those consisting of a spherical stainless steel particle with an alumina cap. The plasma flow was modeled by a 2D steady parabolic model and a single particle trajectory by using the 3D Boussinesq-Oseen-Basset equation. The heat transfer, within the two-layer, stainless steel cladded by alumina, particle, considered the heat propagation phenomena including phase changes. The models allowed determining the positions, along the particle trajectory, where the convective movement could occur as well as the entrainment of the liquid oxide to the leading edge of the in-flight particles. The heat transfer calculations showed the importance of the thermal contact resistance TCR between alumina and steel

Prediction of nonlinear interface dynamics in the unidirectional freezing of particle suspensions with rigid compacted layer

Water freezing in particle suspensions widely exists in nature. As a typical physical system of free boundary problem, the spatiotemporal evolution of the solid/liquid interface not only origins from phase transformation but also from permeation flow in front of ice. Physical models have been proposed in previous efforts to describe the interface dynamic behaviors in unidirectional freezing of particle suspensions. However, there are several physical parameters difficult to be determined in previous investigations dedicated to describing the spatiotemporal evolution in unidirectional freezing of particle suspensions. Here, based on the fundamental momentum theorem, we propose a consistent theoretical framework to address the unidirectional freezing process in the particle suspensions coupled with the effect of water permeation. An interface undercooling-dependent pushing force exerted on the compacted layer with a specific formula is derived based on the surface tension. Then a dynamic compacted layer is considered and analyzed. Numerical solutions of the nonlinear models reveal the dependence of system dynamics on some typical physical parameters, particle radius, initial particle concentration in the suspensions, freezing velocity and so on. The system dynamics are characterized by interface velocity, interface undercooling and interface recoil as functions of time. The models allow us to reconsider the formation mechanism of ice spears in freezing of particle suspensions in a simpler but novel way, with potential implications for both understanding and controlling not only ice formation in porous media but also crystallization processes in other complex systems

Phase-field investigation on the peritectic transition in Fe-C system

We adopt a thermodynamically consistent multi-phase, multi-component phase-field model to investigate the morphological evolution of peritectic transition in carbon steel though 2-D and 3-D simulations. By using phase-field method, we rationalize the peritectic solidification in both 2-D and 3-D simulations under different liquid supersaturations as well as on the δ particle with distinct microstructures. Through the comparison between 2-D and 3-D simulation results, we clarify the reason for the different growth rate of γ phase in two and three dimensions. In 3-D simulation, we observe the unequal growth rate of γ phase in radial and axis directions. In addition, a novel measurement method is proposed to determine the dynamic contact angle. We anticipate that the simulation results can be applied to interpret the isothermal peritectic transition with a liquid supersaturation in alloys

Phase-field investigation on the microstructural evolution of eutectic transformation and four-phase reaction in Mo-Si-Ti system

By using phase-field method, we investigate the morphological evolution of three-phase eutectic transition and four-phase reaction in Mo-Si-Ti system through 2-D and 3-D simulations. For the eutectic transition, we focus on the two-phase growth of lamellar pair from an isothermally undercooled melt: $L → Ti(Mo)_5Si_3 +(Mo,Si,Ti)$, and obtain a microstructure selection map for (m_) stable, (m$_{}$) unstable, and (m$_{}$) oscillatory growth (metastable mode), in terms of the Mo-composition and lamellar spacings. The underlying reason for these three different morphologies is clarified by analyzing the growth rate of the solidification front. In addition, we scrutinize the influence of interfacial energy on the solidification morphology and observe three different types of growth mode: (g_) curving, (g$_{}$) stable, and (g$_{}$) unstable growth. For the four-phase reaction, $L + Mo(Ti)_3Si → Ti(Mo)_5Si_3 + (Mo,Si,Ti)$, we observe the remelting of $Mo(Ti)_3Si$ phase and the formation of a lamellar pair consisting of $Ti(Mo)_5Si_3 and (Mo,Si,Ti)$ on the surface of the $Mo(Ti)_3Si$ phase after an interface of the lamellae pair phases is formed. A certain orientation angle with respect to the solidification direction is obtained for the lamellar pair growth during the four-phase reaction. In both eutectic phase transformation and four phase reaction, a comparison between the 2D and 3D simulations reveals the influence of the third dimension on the development of the lamellar pair

Particle-scale structure in frozen colloidal suspensions from small angle X-ray scattering

During directional solidification of the solvent in a colloidal suspension, the colloidal particles segregate from the growing solid, forming high-particle-density regions with structure on a hierarchy of length scales ranging from that of the particle-scale packing to the large-scale spacing between these regions. Previous work has mostly concentrated on the medium- to large-length scale structure, as it is the most accessible and thought to be more technologically relevant. However, the packing of the colloids at the particle-scale is an important component not only in theoretical descriptions of the segregation process, but also to the utility of freeze-cast materials for new applications. Here we present the results of experiments in which we investigated this structure across a wide range of length scales using a combination of small angle X-ray scattering and direct optical imaging. As expected, during freezing the particles were concentrated into regions between ice dendrites forming a microscopic pattern of high- and low-particle-density regions. X-ray scattering indicates that the particles in the high density regions were so closely packed as to be touching. However, the arrangement of the particles does not conform to that predicted by any standard inter-particle pair potentials, suggesting that the particle packing induced by freezing differs from that formed during equilibrium or steady-state densification processes