Excess Interfacial Free Energy Research Articles

Influence of ternary solute additions on nanocrystalline stability and mechanical behavior

The addition of a ternary solute to reduce the accompanying loss in hardness in a Ni-Cu-P alloy is reported. Here, P serves as the stabilizing addition by its partitioning to the grain boundaries (GBs). In a Ni-40Cu-0.6P (at.%) alloy, nanocrystalline (NC) stability is achieved thermodynamically, i.e., the P solute rapidly diffuses from the matrix to wet the GBs reducing the excess interfacial free energy. In a Cu-19.4Ni-0.6P and a Cu-19.8Ni-0.2P alloy, the P precipitates nanoscale Ni3P in the GBs but such kinetic pinning precipitates where less effective in providing NC stabilization as both the phosphate phase and grains coarsened. For a Ni-40Cu-0.3P alloy, the Ni3P phase also precipitated in the GBs but did not coarsen. Rather, as the GBs broke free from these phosphate precipites they became incorporatedwithin the grains’ interior as evidenced by in situ TEM annealing characterization studies. Nanoindentation confirmed that the addition of the tenary element was effective in mitigating the loss of strength as the stabilizing solute partitions to the GBs as compared to a binary stabilized NC alloy using the same stabilization solute. The collective outcomes of this work concretely demonstrate the need to achieve a sufficient concentration of the stabilizing solute to the GBs and their associated mechanisms by which NC stability is achieved; the influence of a tenary element on solute partitioning diffusivity; and the mechanical tunability offered through ternary alloying in NC alloys.

Molecular dynamics study of the thermodynamic and kinetic properties of the solid-liquid interface in FeMn

The solid-liquid phase equilibria as a function of temperature and composition has been computed for the Fe-Mn system using a semi-grand canonical Monte Carlo technique. Interaction between atoms were modeled using a second nearest neighbor modified embedded atom method potential. In addition, the excess solid-liquid interfacial free energy (γ) and its anisotropy has been computed from the capillary fluctuation method. The results indicate that there is very little variation in γ with temperature/composition. We also propose a new MD simulation method for computing solute trapping behavior that is able to access relatively low interface velocities. The segregation coefficient as a function of velocity for the Fe-Mn system was determined for three temperatures and the results are discussed within the framework of previously proposed theoretical models.

Adsorption transparency of supported graphene

It remains controversial whether the substrate affects the wettability of a single graphene sheet. In this manuscript, we investigate if adsorption transparency exists for graphene, analogous to the contact angle transparency previously reported. The adsorption transparency of graphene supported by different metals is characterized by experimentally measuring the adsorption behavior of water on graphene. By comparing the adsorption isotherm, the heat of adsorption, and the excess interfacial free energy, we find that physisorption on graphene is largely controlled by the supporting substrate. Adding graphene to Pt and Al decreased the interfacial free energy by about 10% when the samples were exposed to water vapor from high vacuum to 5 Torr. This value was larger for graphene on Au (ca. 30%). While we chose to use graphene and water for this study, the results have broader implications to 2D materials in general. This adsorption transparency has applications in a range of technologies where adsorption and wetting are important: molecular electronics, heat transfer, catalysis, ice-nucleation, additive manufacturing, and corrosion control.

Method for computing short-range forces between solid-liquid interfaces driving grain boundary premelting

We present a molecular dynamics based method for accurately computing short-range structural forces resulting from the overlap of spatially diffuse solid-liquid interfaces at wetted grain boundaries close to the melting point. The method is based on monitoring the fluctuations of the liquid layer width at different temperatures to extract the excess interfacial free energy as a function of this width. The method is illustrated for a high-energy Sigma9 twist boundary in pure Ni. The short-range repulsion driving premelting is found to be dominant in comparison to long-range dispersion and entropic forces and consistent with previous experimental findings that nanometer-scale layer widths may be observed only very close to the melting point.

A Thermodynamically Stable Nanophase Material

Nanoparticles are metastable relative to the equivalent bulk material due to the positive excess interfacial free energy (IFE). Previous studies have shown that, with increasing surface interaction strength, the IFE diminishes but remains positive. We describe an experimental multicomponent system in which a nanoscale ZnS material is thermodynamically favored and can be formed at the expense of bulk ZnS. In 17 M sodium hydroxide solution, at 230 degrees C, both 3 nm ZnS nanoparticles and bulk ZnS are transformed into sheetlike nanocrystals with a ZnS polytype structure. Our results are theoretically compatible with the concept of a negative IFE, although not with the assumption of constant interface composition. We clarify the meaning of an effective negative IFE and present the necessary conditions for strong chemical surface interactions to stabilize nanoscale inorganic materials relative to bulk matter. Our results show that synthesis methods employing thermodynamic controls can produce nanomaterials with novel morphology.

Thermodynamic and atomistic modeling of irradiation-induced amorphization in nanosized metal–metal multilayers

A brief summary is firstly presented concerning up-to-date experimental studies of the formation of amorphous alloys (metallic glasses) in binary metal systems by ion beam mixing of nanosized metal–metal multilayers. Secondly, under the framework of Miedema's theory, thermodynamic modeling of crystal-to-amorphous transition is described with special consideration of the effect of the excess interfacial free energy on the alloy phase formation. Thirdly, in some representative systems, glass-forming ranges are calculated directly from their realistic interatomic potentials by determining the critical solid solubilities of the systems by molecular dynamics simulations. Finally, a comparison between irradiation and thermally induced amorphization in the metal–metal multilayers is also discussed.

Experimental investigation of contact angle, curvature, and contact line motion in dropwise condensation and evaporation

Image-analyzing interferometry is used to measure the apparent contact angle and the curvature of a drop and a meniscus during condensation and evaporation processes in a constrained vapor bubble (CVB) cell. The apparent contact angle is found to be a function of the interfacial mass flux. The interfacial velocity for the drop during condensation and evaporation is a function of the apparent contact angle and the rate of change of radius of curvature. The dependence of velocity on the apparent contact angle is consistent with Tanner's scaling equation. The results support the hypothesis that evaporation/condensation is an important factor in contact line motion. The main purpose of this article is to present the experimental technique and the data. The equilibrium contact angle for the drop is found experimentally to be higher than that for the corner meniscus. The contact angle is a function of the stress field in the fluid. The equilibrium contact angle is related to the thickness of the thin adsorbed film in the microscopic region and depends on the characteristics of the microscopic region. The excess interfacial free energy and temperature jump were used to calculate the equilibrium thickness of the thin adsorbed film in the microscopic region.

Role of interface in forming non-equilibrium hcp phase by ion mixing in an immiscible Au–Co system

In an equilibrium immiscible Au–Co system characterized by a positive heat of formation of +11 kJ mol−1, a non-equilibrium Au–Co phase of hcp structure was formed by 200 keV xenon ion mixing at 77 K in the Au50Co50 multilayered films. Based on the free energy calculation, the excess interfacial free energy stored in the Au–Co multilayered films could provide adequate thermodynamic driving force for alloying between Au and Co and forming the non-equilibrium Au–Co hcp phase. Besides, the average magnetic moment per Co atom in the newly formed hcp structure was reduced by 22% of its equilibrium value, within a measuring error of 8%.

Solid-state crystal-to-amorphous transition in metal‐metal multilayers and its thermodynamic and atomistic modelling

In this review article, first a brief summary is presented concerning the formation of amorphous alloys (or metallic glasses) in binary metal systems by solid-state reaction of metallic multilayers. Secondly, under the framework of Miedema's model, thermodynamic modelling of crystal-to-amorphous transition is developed with special consideration of the excess interfacial free energy in metallic multilayers. Thirdly, the results of molecular dynamics simulations in some representative systems are presented, revealing the detailed kinetics of the crystal-to-amorphous transition on the atomic scale, such as the temperature/time dependence of interfacial reactions, the asymmetric growth of amorphous interlayers, and the nucleation and/or presence of growth barriers resulting from the interfacial texture. Fourthly, the critical solid solubilities of some representative systems are directly determined from the inter-atomic potentials through molecular dynamics simulations and then correlated with the metallic-glass-forming ability of the systems as well as their asymmetric growth during solid-state amorphization observed in experiments and/or simulations.

Interface assisted formation of a metastable hcp phase by ion mixing inan immiscible Ag-Ni system

A metastable Ag-Ni phase was formed by 200 keV xenon ion mixing at 77 K in theAg80Ni20 multilayered films, in which the excess interfacial freeenergy provided partial driving force for alloying in an equilibrium immiscibleAg-Ni system characterized by a large positive heat of formation. Themetastable phase was identified to be of D019 hcp structure with astoichiometry of Ag3Ni and can therefore be considered as a Hume-Rothery 7/4electron compound. A thermodynamic calculation showed that when the multilayersconsisted of 8 or more bilayers, the interfacial free energy could elevate theirinitial energy levels up to a state higher than that of the hcp phase, and thatwhile the multilayers composed of 6 or less bilayers, it was thermodynamicallynot favoured to form the hcp phase. Kinetically, the metastable hcp phase wasgrown from the fcc Ag lattice through a fast sliding mechanism. Furthermore, anab initio calculation showed that a minimum total energy of the Ag3Niphase did correspond to the above observed metastable state, indicating that thestability of the metastable Ag-Ni hcp phase was correlated with its electronicstructure.

Metastable phases induced by ion irradiation in the equilibrium immiscible Fe–Cu system

Anomalous structural evolution was induced by room-temperature 200 keV xenon ion irradiation and it results in the formation of various new metastable phases in the equilibrium immiscible Fe–Cu system. First, nanosized quasicrystals were formed in an amorphous matrix through a two-step transition of crystal to amorphous to quasicrystal in Fe70Cu30 multilayered films. The real compositions of the amorphous matrix and quasicrystals were determined to be close to Fe70Cu30 and Fe50Cu50, respectively. Moreover, the same icosahedral phase was also obtained in another similarly designed Fe50Cu50 multilayered sample upon 850 °C thermal annealing, confirming the existence of such a metastable state. Second, amorphous alloys were formed in a composition range of 30–50 at. % of Cu. Third, a Cu-based face-centered-cubic (fcc) solid solution was formed at an alloy composition of about Fe30Cu70 and, interestingly, another fcc structured metastable crystalline phase was obtained at a composition very close to that of Fe50Cu50. A Gibbs free energy diagram of the Fe–Cu system was calculated, with special consideration of excess interfacial free energy stored in the multilayered films, within the framework of Miedema’s model and it gives a reasonable thermodynamic explanation to the metastable phase formation observed. The amorphous-to-quasicrystal transition is also discussed in terms of the similarity in the atomic configuration between the icosahedral and amorphous short-range orders. Furthermore, molecular dynamics study showed that Fe and Cu could be mixed on an atomic scale in the metastable phases observed.

Sequential disordering during ion-induced amorphization in the Mo–Fe multilayered films

Amorphous alloys were formed within an extended broad composition range from 26 to 67 at.% of Mo by room temperature 200 keV xenon ion mixing of multilayered films in the Mo–Fe system with a small negative heat of formation of −3 kJ/mol, provided the multilayered films were properly designed to include sufficient fraction of interfacial atoms, thus possessing enough excess interfacial free energy. Moreover, a sequential disordering of Mo and Fe lattices was observed, for the first time, in the process of ion-induced amorphization in the Fe–Mo multilayered films. The sequential behavior can be attributed to the fact that Fe acted as a fast diffuser into Mo lattice, leading to an earlier disordering of the Mo crystalline structure and might be also associated with a difference in Debye temperature of the two metals.

Synthesis of metallic glasses in an immiscible Y–W system by solid-state reaction of multilayered films

The Y–W system has a positive heat of mixing of +24 kJ/mol, in which no metallic glass has so far been obtained. Beginning with multilayers, which consist of a number of interfaces, by solid-state reaction it is thus possible to synthesise some new alloys in this immiscible system. Considering the excess interfacial free energy, some Y–W metallic glasses were indeed obtained by thermal annealing in the specially designed multilayered films. Thermodynamically, a Gibbs-free-energy diagram was constructed by calculating the free energy curves of the amorphous phase and the multilayers with various fractions of interfaces. It turned out that while including a sufficient fraction of interfacial atoms, the initial energetic state of the multilayers was elevated to be higher than that of the amorphous state, giving a relevant explanation to the above unusual glass-forming behavior. It is expected that the above-described multilayer technique is able to synthesize new metallic glasses in other immiscible systems.

Irradiation-induced alloying in immiscible Mo–Cu system through multilayer technique

Room temperature 200 keV xenon-ion irradiation of multilayers, designed under interfacial free energy concern, did result in the formation of Mo–Cu metallic glasses only within a narrow Mo-enriched composition range. In addition, some Cu-based alloys of metastable fcc solid solutions were also formed at different irradiation stages. To obtain a possible thermodynamic interpretation, a Gibbs-free-energy diagram was calculated based on the semiquantitative Miedema’s model and the related methods proposed by Alonso. The free energy of the Mo–Cu multilayers including excess interfacial free energy was also estimated and was compared to that of the amorphous state. It turned out that the interfacial free energy stored in the multilayered films could raise the energy level of the multilayers to a state higher than that of the metastable phases and thus played an important part, in comparison to the energy deposited by irradiation, in inducing alloying in this immiscible system.

Observation of ion-induced alloy phase formation in an immiscible YTa system

Amorphization was achieved by room temperature 200 keV xenon-ion mixing in an immiscible YTa system, which has a positive heat of formation exceeding +40 kJ/mol. In addition, three new metastable crystalline phases, i.e., an fee phase (fee-I) in Y-rich films, an hep and another fee phase (fee-II) in Ta-rich films, were observed at different ion mixing stages. To give a possible explanation to these anomalous alloying behaviors, a Gibbs-free-energy diagram of the system was calculated based on Miedema's model of heat of formation and a method suggested by Alonso. The free energy of the YTa multilayered films containing excess interfacial free energy was also estimated and added into the diagram. It was found that the excess interfacial free energy stored in the multilayered films could increase the initial energetic state of the multilayers and therefore played an important role in ion-induced alloying in this immiscible system.

A thermodynamic approach to the wetting and dewetting of grain boundaries

An idealized geometry is adopted in order to calculate the equilibrium configuration of a constant volume of liquid initially partially wetting a grain boundary intersecting a free surface. To minimize the excess interfacial free energy, competition exists between the liquid as it dewets creating additional solid-solid interface (on the grain boundary) and increasing both the solid-liquid and liquid-vapor contact areas on the free surface. When the surface is flat only two possible non-trivial equilibrium configurations exist: either the liquid is all on the surface or it is all on the grain boundary. The transition between these two configurations is rather abrupt. The grain boundary wetting parameter k b plays a significant role in governing the equilibrium configuration, whereas the surface wetting parameter k s plays only a minor role. When the boundary intersects a deeply grooved surface, a third configuration is possible, one in which both the surface and the grain boundary are partially wetted. The implications of the calculations include the observed stability of remnant liquid phases in liquid phase sintered ceramics and the spontaneous wetting of metal grain boundaries by molten metals, such as gallium.

Amorphous alloys formed by ion mixing or solid-state reaction in immiscible Y-refractory metal systems and their elastic properties

This paper presents the formation of amorphous alloys in the Y-refractory metal systems, which have a rather positive heat of formation, by ion mixing or solid-state reaction of multilayers and the elastic properties of the amorphous alloys. An estimation showed that the excess interfacial free energy stored in the multilayered films elevated the initial energetic state of the films to a level higher than that of the amorphous phase in a certain range, thus providing a driving force for alloying. Accordingly, the multilayers were designed to include enough fraction of interfacial atoms versus the total atoms, and amorphous alloys were indeed obtained by either room-temperature 200-keV xenon-ion mixing or solid-state reaction. Moreover, the elastic modulus and hardness changes were characterized by nanoindentation for the formed amorphous films. The change of elastic property upon thermal annealing also served as an evidence in support to the free volume model proposed for the amorphous structure.

Chemical and interface effects on glass forming ability under ion mixing in ZrTa system of positive heat of formation

Amorphous alloy films were formed by room temperature 200 keV xenon ion mixing of multilayers in a relatively broad composition range in the ZrTa system, which has a positive heat of formation of +4 kJ mol −1. A free energy diagram was constructed to include the free energy curves of the amorphous phase and the multilayers, to which the previously missing term of the excess interfacial free energy was calculated and added. It turned out that the interface plays a role in influencing the alloying behavior in such system and the calculated diagram provided a reasonable interpretation of the observed amorphization. Two new metastable crystalline phases of both fcc structures were obtained.

Amorphous films formed by solid-state reaction in an immiscible Y–Mo system and their structural relaxation

Amorphous films of 300 nm thick were obtained by solid state reaction in an immiscible Y–Mo system with a positive heat of mixing being +26 kJ/mol. The experimental results revealed that the Y51Mo49 multilayers, including 19% interfacial atoms, were vitrified by thermal annealing at 300 °C for one and half hours, and that the formed amorphous films recrystallized at about 600 °C. These confirmed that the excess interfacial free energy stored in the multilayered films elevated the initial energetic state of the multilayers to a level higher than that of the amorphous phase, providing a major driving force in solid state amorphization. The elastic modulus and hardness changes of the formed amorphous films upon annealing were characterized by nanoindentation, and the results were in accordance with the proposed free volume model for the amorphous structure. ]

Ion-mixing-induced amorphization in an immiscible Au - Mo system with a small size difference

Amorphization in immiscible Au - Mo multilayered films with a very small atomic size difference was achieved by 200 keV xenon ion mixing at 77 K. The experimental results revealed that Au/Mo multilayered films can be amorphized in the Mo content range 75 - 85 at.% upon ion mixing. Thermodynamically, the excess interfacial free energy stored in multilayered films was responsible for the formation of amorphous alloys.