Mechanisms of eutectic lamellar destabilization upon rapid solidification of an undercooled Ag-39.9 at.% Cu eutectic alloy

The eutectic Ag-Cu alloys exhibiting fine Ag-Cu lamellar eutectic structure formed upon rapid solidification have great potentials being used in various engineering fields.However,the desired fine primary lamellar eutectic structure(PLES)is usually replaced by a coarse anomalous eutectic structure(AES)when the undercooling prior to solidification exceeds a certain value.The forming mechanism of AES in the undercooled eutectic Ag-Cu alloy has been a controversial issue.In this work,the undercooled Ag-39.9 at.% Cu eutectic alloy is solidified under different cooling conditions by using techniques of melt fluxing and copper mold casting.The results show that the coupled eutectic growth of this alloy undergoes a transition from a slow eutectic-cellular growth(ECG)to a rapid eutectic-dendritic growth(EDG)above a undercooling of 72 K,accompanying with an abrupt change of the distribution and amount of AES in as-solidified microstructures.Two kinds of primary lamellar eutectic structures are formed by ECG and EDG during recalescence,respectively.The destabilization of PLES that causes the formation of AES is ascribed to two different mechanisms based on the microstructural examination and theoretical calculations.Below 72 K,the destabilization of PLES formed by slow ECG is caused by the mechanism of"termination migration"driven by interfacial energy.While above 72 K,the destabilization of PLES formed by rapid EDG is attributed to the unstable perturbation of interface driven by interfacial energy and solute supersaturation.

Effects of dealloying and heat treatment parameters on microstructures of nanoporous Pd

Microstructures of nanoporous Pd are essentially important for its physical and chemical properties.In this work,we show that the microstructures of nanoporous Pd can be tuned by adjusting compositions of the precursor alloys,and dealloying and heat treatment parameters.Both the ligament and pore sizes decrease with increasing the electrochemical potential upon dealloying and the concentration of noble component in the precursor alloys.Heat treatment causes coarsening of the nanoporous structure.Above a critical temperature,the nanoporous structures are subjected to significant coarsening.Below the critical temperature,surface diffusion is believed to dominate the coarsening process.Above the critical temperature,the nanoporous structure coarsens remarkably at a rather high rate,which is ascribed to a multiple-mechanism controlled process.

Nucleation/growth design by thermo-kinetic partition

Departing from nucleation and growth involved in phase transformations (PTs) and/or plastic deformations (PDs), thermodynamics, kinetics, and thermo-kinetic partition are described. It has been shown that the thermo-kinetic partition reflects the scale of the so-called thermo-kinetic correlation, and by combining with the reference state of PT or PD, corresponds to so-called generalized stability. Regarding the universality of nucleation/growth and thermo-kinetic partition, the principle of high thermodynamic driving force-high generalized stability has been reinterpreted by integrating nucleation and growth, separating nucleation and growth, and designing so-called negative driving forces, respectively. As such, the current materials design is classified, summarized, and prospected. This work is helpful to realize high strength and high plasticity by designing nucleation and growth.

On the mechanism of Si-promoted destabilization of TiC_(x)particles in Al alloys

TiC_(x)is an excellent composite strengthening particle and grain refiner for Al alloys.However,the stability of TiC_(x)is poor when solute Si exists in Al alloy melts,which significantly depresses its strengthening and grain refining effects.In this work,the destabilization mechanisms of the TiC_(x)particles in Al-Si alloy melt with a composition of Al-7Si-7.5TiC were explored via experiments,first-principles calculations and thermodynamic calculations.The experimental results show that Si atoms diffuse into TiC_(x)and Ti atoms are released into the Al melt to form a Ti-rich transition zone during the insulation of TiC_(x)in Al-Si melt,and the TiAlySiz and Al_(4)C_(3)phases are solidified in the Ti-rich zone and at Ti-rich zone/TiC_(x)interface,respectively.The first principles calculations show that the low formation energy of C vacancies facilitates the rapid diffusion of Si atoms in TiC_(x),while the doping of Si atoms reduces the energy barrier of diffusion of Ti atoms in TiC_(x)and promotes the formation of Ti-rich zones.The thermodynamic calculations show that the wide crystallization temperature range of the destabilized product TiAlySiz phase is the key to continuous decomposition of TiC_(x)particles.In addition,the driving force of the main destabilization reaction of TiC_(x)in the Al-Si alloys is about 44 times higher than that in the Al alloys without Si addition.This indicates that the presence of solute Si remarkably promotes the subsequent decomposition process of TiC_(x)in the Al-Si alloy melts.