Metastable Solid Solution Research Articles

Metastable Solid Solutions Formed by Metal Nanoparticles

Metastable Solid Solutions Formed by Metal Nanoparticles

Influence of Liquid Composition and Discharge Energy on Process Performance, Composition, and Properties of Particles Formed During Electric Discharge Erosion of WC-5TiC-10Co Alloy

Increasing the hardness and wear resistance of powder alloys and coatings through the use of ultrafine-grained powders and metastable phases is a promising way in powder metallurgy. This paper presents the results of the studies of the process of obtaining ultrafine powders by the electric discharge erosion of the cemented carbide waste WC-5TiC-10Co on a special installation. An empirical model that describes the dependence of the productivity of the process on the discharge energy and properties of the liquid is provided. The dependence of the chemical and phase compositions of the obtained powder on the composition of the used liquid and specific energy consumption has been investigated. The effect of the discharge energy on the morphological composition and average particle diameter has been examined. It is revealed that the formation of a metastable solid solution (W, Ti)C and a decrease in the concentration of cobalt induce an increase in the hardness of the resulting spherical particles from 1410HV0.05 to 2540HV0.05.

Spinodal decomposition introduces strain-enhanced thermochromism in polycrystalline V1-xTixO2 thin films.

Processes of self-organization play a key role in the development of innovative functional nanocomposites, allowing, in particular, the transformation of metastable solid solutions into multilayers by activating spinodal decomposition instead of layer-by-layer film growth. We report the formation of strained layered (V,Ti)O2 nanocomposites in thin polycrystalline films using a spinodal decomposition. Already during the growth of V0.65Ti0.35O2 films, spinodal decomposition was detected while producing atomic-scale disordered V- and Ti-rich phases. Post-growth annealing enhances compositional modulation, arranges the local atomic structures of the phases, and yields periodically layered nanostructures that resemble superlattices. The coherent interfacing of the V- and Ti-rich layers results in the compression of the V-rich phase along the c-axis of the rutile structure and enables strain-enhanced thermochromism. The latter is characterized by a simultaneous decrease in the temperature and width of the metal-insulator transition in the V-rich phase. Our results provide proof-of-concept for an alternative strategy to develop VO2-based thermochromic coatings by introducing strain-enhanced thermochromism into polycrystalline thin films.

Yttrium incorporation in Cr2AlC: On the metastable phase formation and decomposition of (Cr,Y)2AlC MAX phase thin films

AbstractHerein we report on the synthesis of a metastable (Cr,Y)2AlC MAX phase solid solution by co‐sputtering from a composite Cr–Al–C and elemental Y target, at room temperature, followed by annealing. However, direct high‐temperature synthesis resulted in multiphase films, as evidenced by X‐ray diffraction analyses, room‐temperature depositions, followed by annealing to 760°C led to the formation of phase pure (Cr,Y)2AlC by diffusion. Higher annealing temperatures caused a decomposition of the metastable phase into Cr2AlC, Y5Al3, and Cr‐carbides. In contrast to pure Cr2AlC, the Y‐containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y‐content dependent and was increased by ∼200°C for 5 at.% Y compared to Cr2AlC. Calculations predicting the metastable phase formation of (Cr,Y)2AlC and its decomposition are in excellent agreement with the experimental findings.

Microstructural evolution and thermal stability in a nanocrystalline lightweight TiAlV0.5CrMo refractory high-entropy alloy synthesized by mechanical alloying

• Lightweight refractory high-entropy alloy was synthesized by mechanical alloying. • The milled powder exhibited a homogeneity chemical distribution. • The average grain size of milled powder is less than ∼8 nm. • The milled powder maintained BCC phase and nanosized grains at 1200 °C. The lightweight TiAlV 0.5 CrMo refractory high-entropy alloy with nanocrystalline structure has been fabricated by mechanical alloying. Microstructure, morphology and thermal stability of the powder under various milling time and annealing temperatures were investigated. The 30 h milled powder consists of a single body-centered cubic (BCC) solid solution structure with an average grain size of less than ∼8 nm. The single solid solution structure transformed into BCC and Laves phases after annealing at 1000 °C, mainly due to the phase transformation of the metastable supersaturated solid solution. It is noticeably interested that the mechanically alloyed powder exhibited excellent thermal stability when annealed at 600, 800 and 1200 °C.

A facile method to produce rodlike β‐Si3N4 seed crystallites for bimodal structure controlling

Abstractβ‐Si3N4 rodlike seed crystallites were successfully produced by single‐step heat treatment of commercial α‐Si3N4 powder at 1900°C for 20 h under an N2 gas pressure of 980 kPa. The average diameter, length, and aspect ratio of the seed crystallites were 0.73 μm, 1.37 μm, and 1.86, respectively. The α‐ ⇀ β‐Si3N4 phase transformation proceeded mainly at 1900°C, and this temperature was lower than the theoretical α‐Si3N4 dissociation temperature (1933°C) under N2 gas pressure of 980 kPa. The formation of metastable solid solution due to the dissolution of O impurity into the α‐Si3N4 crystal lattice was suggested as the driving force for the present oxide additive‐free α‐ ⇀ β‐Si3N4 phase transformation. β‐Si3N4 ceramics were fabricated by liquid phase sintering promoted by an additive system of 1 wt% MgO with 3 wt% Gd2O3. Starting α‐Si3N4 powder with 10 vol% rodlike β‐Si3N4 seed crystallites prepared in this study and an extended sintering time for up to 20 h at 1950°C resulted in the formation of bimodal microstructure composed of fine matrix grains and large elongated grains originated from the seed crystallites. The β‐Si3N4 ceramics exhibited improved fracture toughness and thermal conductivity of 5.9 ± 0.8 MPa m−1/2 and 109.3 ± 0.4 W m−1 K−1, respectively, retaining a high fracture strength of about 1 GPa.

Effects of substrate rotation during AlSi-HiPIMS/Ti-DCMS co-sputtering growth of TiAlSiN coatings on phase content, microstructure, and mechanical properties

A combined high-power impulse and dc magnetron co-sputtering (HiPIMS/DCMS) technique is used to deposit Ti0.6Al0.32Si0.08N films with 1-fold substrate table rotation. Layers are grown at two different substrate-target separations, two different rotational speeds, and with different values of substrate bias. The aim is to study the role of (1) overlap between ion and neutral fluxes generated from HiPIMS and DCMS sources, respectively, and (2) the subplantation range of low-mass ions. Results from X-ray diffractometry highlight the necessity of flux intermixing in the formation of the metastable B1-structured TiAlSiN solid solutions. All films grown at short target-to-substrate distance contain the hexagonal AlN phase, as there is essentially no overlap between HiPIMS and DCMS fluxes, thus the Al+ and Si+ subplantation is very limited. Under conditions of high flux intermixing corresponding to larger target-to-substrate distance, no w-AlN forms irrespective of rotational speed (1 or 3 rpm) and bias amplitude (120 or 480 V), indicating that the role of Al+/Si+ and Ti flux overlap is crucial for the phase formation during film growth by HiPIMS/DCMS with substrate rotation. This conclusion is further supported by the fact that the reduction of the bilayer thickness with increasing the target-to-substrate distance (hence increasing flux overlap) is larger for films grown with higher amplitude of the substrate bias, indicative of more efficient Al+/Si+ subplantation into the c-TiN phase. Single-phase films with the hardness close to that of layers grown with stationary substrate table can be achieved, however, at the expense of higher compressive stress.

(Battery Division Research Award) Structure-Property Relationships: A Journey From LFP to Lithium Metal

Conceptual importance of “Matastable Phase Diagram” and “Liquid Madelung Potential” will be demonstrated by picking up some of my mechanism-solving works on LFP and lithium metal.In Li x FePO4, the ground state phase separation and metastable solid solution are energetically competitive, and the solid solution phase is induced in a non-equilibrium electrochemical reaction, playing critical role for fast kinetics. However, there had been no discussion or experimental report of the crystal structure, optical and transport properties of the metastable solid solution phase due to the extreme technical difficulties; the solid-solution phase Li x FePO4 is metastable during electrochemical process and disappears within a few seconds. We overcame this limitation by quenching Li x FePO4 (x = 2/3) at 350 °C to room temperature. This quenched phase remained stable for a couple of weeks, which enabled sufficient time to measure the several intrinsic properties, including polaron-crystallized superstructure, optical properties with emerging inter-valence charge transfer absorption, and enhanced conductivity over 103 in order.Since the Li/Li+ redox potential (E Li/Li+) positions outside of the thermodynamical potential window of the present electrolyte, suppressing side reactions is essential, which can be realized not only by kinetic suppression by SEI but also by thermodynamic stabilization viz. upshift of E Li/Li+. Remarkable improvement of Coulombic efficiency is exemplified in (locally) salt-concentrated electrolyte, where E Li/Li+ largely upshifts over 0.6 V. We revealed that the unusual upshift of E Li/Li+ is primarily dominated by Coulombic energy penalty of Li+ in the electrolytes rather than the simple Nernst equation with extremely high Li+ activity coefficient.[1] Overall energetics was verified and directly quantified by introducing the “liquid Madelung potential (E LM)” analogous to the conventional concept in solid-state science.[1] Namely, Li+ is electrostatically more stabilized by electron-localized solvents, while relatively less stabilized by electron-delocalized larger sized anion, thus decreasing the overall Coulombic energy gain of Li+ under progressive anion coordination.[1][1] N. Takenaka, S. Ko, A. Kitada, and A. Yamada, submitted (2022) Figure 1

Forced Disorder in the Solid Solution Li3P-Li2S: A New Class of Fully Reduced Solid Electrolytes for Lithium Metal Anodes.

All-solid-state batteries based on non-combustible solid electrolytes are promising candidates for safe energy storage systems. In addition, they offer the opportunity to utilize metallic lithium as an anode. However, it has proven to be a challenge to design an electrolyte that combines high ionic conductivity and processability with thermodynamic stability toward lithium. Herein, we report a new highly conducting solid solution that offers a route to overcome these challenges. The Li–P–S ternary was first explored via a combination of high-throughput crystal structure predictions and solid-state synthesis (via ball milling) of the most promising compositions, specifically, phases within the Li3P–Li2S tie line. We systematically characterized the structural properties and Li-ion mobility of the resulting materials by X-ray and neutron diffraction, solid-state nuclear magnetic resonance spectroscopy (relaxometry), and electrochemical impedance spectroscopy. A Li3P–Li2S metastable solid solution was identified, with the phases adopting the fluorite (Li2S) structure with P substituting for S and the extra Li+ ions occupying the octahedral voids and contributing to the ionic transport. The analysis of the experimental data is supported by extensive quantum-chemical calculations of both structural stability, diffusivity, and activation barriers for Li+ transport. The new solid electrolytes show Li-ion conductivities in the range of established materials, while their composition guarantees thermodynamic stability toward lithium metal anodes.

Thin film combinatorial sputtering of Al-Ce alloys: Investigating the phase separation of as-deposited solid solutions and determining the coefficient of thermal expansion

AlxCe100−x thin films with a composition range of ~75.0 < x < 99.5 at% (36.5 < x < 97.5 wt%) were synthesized via combinatorial co-sputtering from an Al and an Al50Ce50 target. The crystal structure, phase fraction, film morphology, electrical resistivity, and temperature-dependent coefficients of thermal expansion (CTE) are all correlated to the AlxCe100−x composition. The as-deposited films form a metastable solid-solution, and annealing leads to the formation of the thermodynamically stable two-phase system of Al and the α-Al11Ce3 intermetallic. Temperature dependent x-ray diffraction (XRD) reveals that the two phases expand independently of one another, and the thin film Al temperature-dependent CTE is similar to bulk Al. The thin film Al11Ce3 intermetallic phase has a nearly constant CTE of ~1.5 × 10−5/°C within the temperature range studied (25–550 °C). To confirm the thin film Al11Ce3 results, bulk stoichiometric Al11Ce3 and +/- 1 wt% Ce samples were prepared and the CTE of each was measured with the same conditions. A Rietveld analysis of the bulk data enabled an estimation of the CTE in each of the 3 orthorhombic lattice parameters, which displayed anisotropic behavior. The thin film and bulk CTE measurements were in very good agreement. Estimations of the temperature dependent CTE of the two-phase alloys are made via the Reuss and Voigt models. By demonstrating the efficacy of the approach, more complex multi-component rapid materials discovery of low CTE Al-alloys can be pursued via the combinatorial thin film synthesis and XRD measurement.

Producing ultrastable Ni-ZrO2 nanoshell catalysts for dry reforming of methane by flame synthesis and Ni exsolution

Commercial dry reforming of methane (DRM) is limited by catalyst deactivation through metal sintering and coking. New catalyst synthesis methods are needed to overcome such challenges. Here, we report a unique flame aerosol synthesis and exsolution method for producing hollow Ni-Zr oxide nanoshells as a highly stable and efficient DRM catalyst. This process incorporates immiscible elements into single-phase hollow nanoshells by rapid particle formation and quenching. Ni nanoparticles are then uniformly exsolved from the metastable solid solution to provide strong metal-support interactions that limit metal sintering. Rapid synthesis under reducing conditions produces oxygen vacancies in ZrO 2 that favor carbon removal during the DRM reaction. As a result, our catalyst maintained 98% CH 4 conversion for more than 500 h, without sintering or coking, dramatically outperforming conventional catalysts. This method of metastable solid solution nanoshell formation followed by active site exsolution could provide durable catalysts for many high-temperature reactions. • Incorporation of NiO and ZrO 2 into a single phase by rapid flame synthesis • Exsolution of active Ni nanoparticles leads to stable catalyst formation • High sintering and coking resistance of Ni-ZrO 2 nanoshell catalyst • Maintains 98% CH 4 conversion over more than 500 h for dry reforming of methane Dry reforming of methane, which converts two greenhouse gases to useful chemical feedstocks, has been regarded as a promising strategy to address global climate change and sustainable energy development. Design of effective nonnoble metal catalysts is key to achieving its industrial implementation. However, conventional Ni-based catalysts often suffer from deactivation by carbon coking and metal sintering, while complex catalyst synthesis approaches inevitably lead to high cost. This work presents a continuous, one-step, vapor-phase synthesis route to produce a highly stable Ni/ZrO 2 nanoshell catalyst with excellent resistance to both coking and sintering. The methods proposed in this work are expected to expand routes to heterogeneous catalyst synthesis with impact on both scientific and industrial communities. A highly stable Ni-ZrO 2 nanoshell catalyst is produced by a continuous flame aerosol process and Ni exsolution. Its notable characteristics include abundant oxygen vacancies and strong metal-support interactions, which lead to both coking and sintering resistance, along with high activity, for dry reforming of methane.

A sustainable ultra-high strength Fe18Mn3Ti maraging steel through controlled solute segregation and \u03b1-Mn nanoprecipitation

The enormous magnitude of 2 billion tons of alloys produced per year demands a change in design philosophy to make materials environmentally, economically, and socially more sustainable. This disqualifies the use of critical elements that are rare or have questionable origin. Amongst the major alloy strengthening mechanisms, a high-dispersion of second-phase precipitates with sizes in the nanometre range is particularly effective for achieving ultra-high strength. Here, we propose an alternative segregation-based strategy for sustainable steels, free of critical elements, which are rendered ultrastrong by second-phase nano-precipitation. We increase the Mn-content in a supersaturated, metastable Fe-Mn solid solution to trigger compositional fluctuations and nano-segregation in the bulk. These fluctuations act as precursors for the nucleation of an unexpected α-Mn phase, which impedes dislocation motion, thus enabling precipitation strengthening. Our steel outperforms most common commercial alloys, yet it is free of critical elements, making it a new platform for sustainable alloy design.

Plasma spraying Ti–Al–C based composite coatings from Ti/Al/graphite agglomerates: Synthesis, characterization and reaction mechanism

This study explored the feasibility of fabricating Ti–Al–C based composite coatings using 2Ti/xAl/0.5C (x = 0.2–1.5) agglomerated powders by plasma spraying technique. The effect of Al content on microstructure and mechanical properties of spray coatings was studied, and the reaction mechanism was revealed. Results showed that the typical structure of agglomerates developed from C@Ti to C@Ti–Al and isolated Al as increasing Al additions. All the as-sprayed coatings consist of TiC, (Ti,Al)Cx (metastable solid solution formed by Al replacing Ti site in TiC), Ti2AlC and some TixAly phases. Ti2AlC phase grew tightly surrounded by TiC and (Ti,Al)Cx nanoparticles embedded in the TixAly regions. With elevated Al additions, the Ti2AlC phase content increases first and then decreases at x = 1.0. Correspondingly, the hardness of the composite coatings first increased and then decreased. The study proved the possibility of reaction synthesis of Ti2AlC phase by plasma spraying element powder mixtures and its formation mechanism was proposed.

Making Ultra-Tough Nanoceramics by Columnar Submicrocrystals with Three-Level Micro-Nano Structures.

The low fracture toughness of equiaxed nanocrystalline ceramics is the main bottleneck of its wide range of commercial applications. Here, the authors report a method to overcome this limitation for preparing ultra-tough nanoceramics from using amorphous and supersaturated Al2 O3 /ZrO2 solid solution micro-powders, which is fabricated by Al-O2 ultrahigh-temperature combustion synthesis assisted rapid water cooling. The Al2 O3 /ZrO2 micro-powders containing amorphous and metastable dendritic solid solutions can induce the three-level micro-nano structure (submicro/nano/supra-nano) of the high-content (up to 70-90%) columnar submicro-crystals accompanied with high-density nanoprecipitation after sintering or annealing, which makes the fracture toughness of Al2 O3 /ZrO2 ceramics with a unique combination of high-strength and high-hardness increased by 50-100%. This method is beneficial to microstructural design of high-performance ceramics and can be widely applied to various ceramic systems, coupled with simplicity, low-cost, and high-efficiency, making it suitable to industrially produce large-sized nanoceramics with specific grain geometry in large quantities.

Thermal Stability of Microstructure of High-Entropy Alloys Based on Refractory Metals Hf, Nb, Ta, Ti, V, and Zr

In the present work, a series of high-entropy alloys based on refractory metals Hf, Nb, Ta, Ti, V, and Zr with various compositions have been systematically investigated. Our study revealed that a bcc single-phase solid solution of a Hf-Nb-Ta-Ti-V-Zr system is thermodynamically stable only at high temperatures above 1000 °C. At lower temperatures, the phase separation into disordered bcc phases with slightly different chemical compositions occurs. Despite the phase separation, a single-phase random solid solution can be saved at room temperature as a metastable phase by rapid cooling of the sample from high temperature. The microstructure of a single-phase metastable random solid solution was characterized and compared with the microstructure of the as-cast state. Furthermore, the mechanical properties of annealed and as-cast alloys were compared. Interestingly, both states exhibit comparable mechanical properties. It indicates that from the point of view of practical applications, a mechanical mixture of disordered bcc solutions is as good as single-phase random solid solution.

Formation of Catalytically Active Nanoparticles under Thermolysis of Silver Chloroplatinate(II) and Chloroplatinate(IV).

The thermal behaviour of Ag2[PtCl4] and Ag2[PtCl6] complex salts in inert and reducing atmospheres has been studied. The thermolysis of compounds in a helium atmosphere is shown to occur in two stages. At the first stage, the complexes decompose in the temperature range of 350–500 °C with the formation of platinum and silver chloride and the release of chlorine gas. At the second stage, silver chloride is sublimated in the temperature range of 700–900 °C, while metallic platinum remains in the solid phase. In contrast to the thermolysis of Ag2[PtCl6], the thermal decomposition of Ag2[PtCl4] at 350 °C is accompanied by significant heat release, which is associated with disproportionation of the initial salt to Ag2[PtCl6], silver chloride, and platinum metal. It is confirmed by DSC measurements, DFT calculations of a suggested reaction, and XRD. The thermolysis of Ag2[PtCl4] and Ag2[PtCl6] compounds is shown to occur in a hydrogen atmosphere in two poorly separable steps. The compounds are decomposed within 170–350 °C, and silver and platinum are reduced to a metallic state, while a metastable single-phase solid solution of Ag0.67Pt0.33 is formed. The catalytic activity of the resulting nanoalloy Ag0.67Pt0.33 is studied in the reaction of CO total (TOX) and preferential (PROX) oxidation. Ag0.67Pt0.33 enhanced Pt nano-powder activity in CO TOX, but was not selective in CO PROX.

Tetragonal distortion in magnetron sputtered bcc-W films with supersaturated carbon

Carbon has a low solid solubility in bcc tungsten at equilibrium. However, metastable supersaturated solid solutions can be synthesized with magnetron sputtering. Here, we present a systematic study on the phase stability and mechanical properties of such supersaturated W–C solid solutions. Θ–2θ scans show a split of the 200/020 and the 002 peaks for supersaturated films which is explained by a tetragonal distortion of the bcc structure. This split increases with increasing C content and is maximized at 4 at.% C, where we observe an a/b axis of 3.15–3.16 Å and a c-axis of 3.21–3.22 Å. We performed first-principles calculations of lattice parameters, mixing enthalpies, elastic constants and polycrystalline elastic moduli for cubic and tetragonal W–C solid solutions. Calculations show that tetragonal structure is more stable than the bcc supersaturated solid solution and the calculated lattice parameters and Young’s moduli follow the same trends as the experimental ones as a function of C concentration. The results suggest that supersaturated films with lattice distortion can be used as a design approach to improve the properties of transition metal films with a bcc structure.

An ab initio investigation of the temperature-dependent energetic barriers towards CrAlB and (Mo,Cr)AlB formation in a metastable synthesis scenario.

The orthorhombic CrAlB MAB phase has not been synthesized so far and was shown to be energetically unstable vs. the competing Cr2AlB2 phase in previous theoretical reports, which, however, did not explicitly investigate the magnitude of the energetic barrier towards CrAlB formation as a function of temperature. Temperature-dependent Gibbs energies of formation, obtained from density-functional-theory-based lattice dynamics simulations performed in this study, reveal that this barrier is very small (around 10 kJ mol-1 ≈ 0.008 eV per atom, on average) and may readily be overcome during high-energy synthesis scenarios, likely resulting in metastable phase formation. Furthermore, the electronic structures of MoAlB, a phase synthesized experimentally both in bulk and thin film form, and CrAlB are shown to be similar in direct comparison, with MoAlB exhibiting a higher electronic stability due to a local DOS minimum in proximity to the Fermi level, and quaternary compositions lying between the ternaries. Likewise, bonding characteristics are qualitatively very similar between both phases, with the transition metal-boron bonds being the dominant interaction in the entire unit cell, even though individual B-B bonds are stronger; quantitatively, all interactions are again stronger in MoAlB compared to CrAlB. It is reasonable to assume that, considering the successful synthesis of phase-pure MoAlB and known formation of metastable phases during physical vapor deposition, direct synthesis of metastable CrAlB thin films is possible due to the aforementioned small energy barrier. Furthermore, stability is enhanced upon alloying with Mo as this lowers the energy of formation, with a Mo/Cr ratio of approx. 0.33 sufficient to stabilize the Cr-rich (Mo,Cr)AlB solid solution vs. the primary competing phases, allowing for deposition of Mo-concentration-dependent stable and metastable (Mo,Cr)AlB solid solution phases.

Synthesis and thermal properties of the heterometallic nickel–ruthenium complex: a potential precursor for catalytically active nanosized Ni–Ru alloy

Thermal decomposition of the novel heterometallic complex [RuNO(NO2)4OHNi(En)2] results in a single-phase metastable solid solution Ni0.5Ru0.5. The catalytic activity of the prepared nanoalloy in the CO methanation reaction was demonstrated.

Atomic scale understanding of phase stability and decomposition of a nanocrystalline CrMnFeCoNi Cantor alloy

High entropy alloys (HEAs) provide superior mechanical and functional properties. However, these advantages may disappear when a metastable single-phase solid solution decomposes at low temperatures upon long-term annealing. Therefore, understanding the underlying phase separation mechanisms is important for the design of new HEAs with controlled properties. In the current work, the thermal stability of a nanocrystalline CrMnFeCoNi HEA was investigated at different annealing conditions using a combinatorial processing platform, involving fast and parallel synthesis of nanocrystalline thin films, short annealing time for a rapid phase evolution, and direct characterization by atom probe tomography. The microstructural features of the decomposed CrMnFeCoNi alloy as well as its decomposition process were analyzed in terms of elemental distributions at the near-atomic scale. The results show that the segregation of Ni and Mn to grain boundaries in the initial single-phase alloy is a prerequisite and is observed to be the only occurring physical process at the early stage of phase decomposition. When the concentrations of Ni and Mn reach a certain value, phase decomposition starts and a MnNi-rich phase forms at grain boundaries. Next, two Cr-rich phases form at the interface between the MnNi-rich phase and the matrix. Meanwhile, a FeCo-rich phase forms in the grain interior. Based on these observations, the underlying mechanisms involving nucleation, diffusivity as well as thermodynamic considerations were discussed.