Nucleation Of Second Phase Research Articles

Characteristic Plasmon Energies for 2D In2Se3 Phase Identification at Nanoscale.

Two-dimensional (2D) materials with competing polymorphs offer remarkable potential to switch the associated 2D functionalities for novel device applications. Probing their phase transition and competition mechanisms requires nanoscale characterization techniques that can sensitively detect the nucleation of secondary phases down to single-layer thickness. Here we demonstrate nanoscale phase identification on 2D In2Se3 polymorphs, utilizing their distinct plasmon energies that can be distinguished by electron energy-loss spectroscopy (EELS). The characteristic plasmon energies of In2Se3 polymorphs have been validated by first-principles calculations, and also been successfully applied to reveal phase transitions using in situ EELS. Correlating with in situ X-ray diffraction, we further derive a subtle difference in the valence electron density of In2Se3 polymorphs, consistent with their disparate electronic properties. The nanometer resolution and independence of orientation make plasmon-energy mapping a versatile technique for nanoscale phase identification on 2D materials.

Early stages of liquid-metal embrittlement in an advanced high-strength steel

Grain-boundary degradation via liquid-metal embrittlement (LME) is a prominent and long-standing failure process in next generation advanced high-strength steels. Here we reveal, well ahead of the crack tip, the presences of nano-scale grains of intermetallic phases in Zn-infiltrated but uncracked grain boundaries with scanning- and 4D transmission electron microscopy. Instead of the often-reported Zn-rich Fe-Zn intermetallics, the nano-scale phase in the uncracked infiltrated grain boundaries is identified as the Γ-phase, and its presence reveals the local enhancement of strain heterogeneities in the grain boundary network. Based on these observations, we argue that intermetallic phase formation is not occurring after cracking and subsequent liquid Zn infiltration but is instead one of the primary nanoscopic drivers for grain-boundary weakening and crack initiation. These findings shift the focus of LME from micro- and meso-scale crack investigations to the very early stages immediately following Zn diffusion, after which secondary phase nucleation and growth emerge as the root-cause for failure.

2-dimensional polar metals: a low-frequency Raman scattering study

The intercalation of a molecule or ion in a layered structure is key to enhancing energy storage, material conductivity, intercalant structural ordering, and the formation of two-dimensional (2D) superconducting states. The process of intercalation modifies the vibrational energy of the host, which can be monitored non-invasively by Raman spectroscopy. However, the detected Raman spectral shifts may originate from a variety of phenomena, generally making the technique an indirect means of identifying intercalation success. Here, we discuss newly discovered low-frequency (LF) (<100 cm−1) Raman features due to the formation of unique 2D polar metals (Ag, Cu, Pb, Bi, Ga, In) or metal alloys (In x Ga1−x ) intercalated at an epitaxial graphene (EG)/silicon carbide (SiC) interface and demonstrate that 2D-Ag and 2D-Ga can have spatially distinct phases with their own unique Raman responses. Additionally, we establish that the 2D-Ga exhibits a structural evolution as a function of temperature, independent of the SiC and EG, that can lead to nucleation of secondary phases. The newly identified LF Raman responses discussed here lay the foundation for rapid, direct, and spatially resolved evaluation of 2D polar metals in ambient.

Microstructural, Mechanical, and Electrochemical Properties of Quenched and Partitioned 3 wt% Mn Steel

Herein, an attempt has been made to investigate effect of C and Mn partitioning during quenching and partitioning heat-treatment process on microstructure, mechanical, and electrochemical properties of experimental 3 wt% Mn steel. The quenching and partitioning heat-treatment process was applied to the experimental steel with varying partitioning time periods ranging from 15 to 120 s at a constant partitioning temperature of 425 °C. The partitioning time period of 15 s resulted in a high volume fraction of supersaturated lath martensite with a small volume fraction of retained austenite. With increasing partitioning time period to 45–60 s, diffusion of C and Mn from martensite to retained austenite occurred resulting in the formation of decreased volume fraction of martensite phase with a reduced carbon and increased volume fraction of retained austenite phase with increased carbon. Partitioning for 90 s produced a moderate volume fraction of both lath martensite and retained austenite phases with nucleation of secondary phase, epsilon carbides. This phase transformation provided an optimum combination of 21% improved Vickers hardness and threefold improved impact toughness compared to as-received steel. Electrochemical properties of quenched and partitioned 3 wt% Mn steel were also evaluated in a 3 wt% NaCl solution. The highest corrosion resistance was achieved after prolonged partitioning of 120 s, whereas slightly low corrosion resistance was achieved after partitioning of 90 s.

Eutectic Nucleation in 7xxx Series Aluminum Alloys from a Non-classical Viewpoint

The early stages of eutectic solidification in a copper-containing 7xxx series aluminum alloy (AA 7068 or AMS 4331) were studied using the two-thermocouple computer-aided thermal analysis (CATA) technique. A feature was detected on the cooling rate curve at the equilibrium solidus temperature of the alloy which persists until the peak of the subsequent final eutectic solidification. Detailed analysis of the temperature difference between the wall and the center of the thermal analysis sample, together with examination of the eutectic solidified on the walls of porosities and a study of the eutectic nucleation on the basis of the non-classical theory of adsorption heterogeneous nucleation, indicated how the feature can be related to the faceting of the atomic structure of the solid/liquid (S/L) interface. The solidification of the remnant liquid after the faceting transition at the equilibrium solidus point depends on the interfacial undercooling and proceeds via either primary phase re-nucleation or secondary phase nucleation by adsorption. The eutectic solidification is affected by the presence of the primary phase which acts like an adsorbent.

D band filling and magnetic phase separation in transition metal-doped Mn3SnC

The structural and magnetic properties of transition metal-doped Mn3SnC are studied with an aim to understand the effect of transition metal atom on magnetostructural properties of the antiperovskite compound. The doped Mn2.8T0.2SnC (T = Cr, Fe, Co, Ni and Cu) compounds show a distinctly different magnetic behavior which can be related to electronic filling of the d band of the transition metal atom. While Cr and Fe doped Mn3SnC show properties similar to that of Mn3SnC, the Co, Ni and Cu doped compounds exhibit nucleation of secondary phases which are devoid of carbon and having Heusler and DO19 type hexagonal structure. A strong magnetic interaction is observed between the impurity phases and the major antiperovskite phase leading to a sharp decrease in magnetostructural transition temperature of the antiperovskite phase and a cluster glassy ground state.

Application of positron lifetime annihilation spectroscopy for characterization of friction stir welded dissimilar aluminum alloys

Positron annihilation lifetime spectroscopy (PALS) was utilized to characterize dissimilar aluminum alloy workpieces of 2017A-T451 and 7075-T651 joined by friction stir welding. Since the interphase boundaries between the aluminum matrix and second phase particles provided open volumes to trap the positrons, positron lifetimes in the welds and in the heat-affected and thermomechanically-affected zones corresponded with the distribution of secondary phases. The 7075 alloy displayed higher positron lifetimes than the 2017A alloy. The positron lifetime profiles across the weld showed numerous local maxima and minima on the advancing and retreating sides, which also paralleled the hardness behavior. These variations in positron lifetime and hardness away from the weld center stemmed from the temperature distribution in these areas relative to the critical temperatures for secondary phase nucleation and/or dissolution in the two alloys. Weld temperatures on the advancing side were relatively higher than those on the retreating side and consequently promoted more precipitation on the advancing side away from the weld center. This behavior was reflected by the higher positron lifetime on the advancing side compared to the retreating side, at the same distances from the weld center.

Uncoupling Growth Mechanisms of Binary Eutectics during Rapid Solidification

Eutectic solidification under rapid solidification conditions has enormous applications, as it can produce microstructure-refined and interface-stable combined composite structures with low cost. However, investigations reported that the coupled interfaces will be destroyed under the condition of large undercoolings. For further understanding of the mechanisms of uncoupling growth, we investigated the uncoupling growth process of binary eutectics during rapid solidification using atomistic simulations. We find that both the nucleation rate and the crystallization velocity for the first phase of eutectics are very high; however, nucleation of the second phase is very inactive: its nucleation rate is low and nucleation incubation time is very long. As the nucleation of the eutectic second phase is severely suppressed during rapid solidifications, the crystallization of the second phase lags far behind, therefore we speculate that the uncoupling growth of eutectics during rapid solidification is nucleation-i...

Distinct surface and bulk charge density waves in ultrathin 1T−TaS2

We employ low-frequency Raman spectroscopy to study the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition in 1T-TaS2 ultrathin flakes protected from oxidation. We identify new modes originating from C phase CDW phonons that are distinct from those seen in bulk 1T-TaS2. We attribute these to CDW modes from the surface layers. By monitoring individual modes with temperature, we find that surfaces undergo a separate, low-hysteresis NC-C phase transition that is decoupled from the transition in the bulk layers. This indicates the activation of a secondary phase nucleation process in the limit of weak interlayer interaction, which can be understood from energy considerations.

The impact of thermo-mechanical controlled processing on structure-property relationship and strain hardening behavior in dual-phase steels

We elucidate here the impact of thermo-mechanical controlled processing (TMCP) in governing nucleation of second phase in Nb-Ti microalloyed dual-phase steels. A wide range of mechanical properties were obtained through change in the microstructure, depending on the TMCP cooling schedule. Polygonal ferrite matrix with martensite as the second phase exhibited highest initial strain hardening rate, lowest yield strength, and highest tensile strength as compared to the situation when bainite or bainite-martensite was the second phase. The Crussard–Jaoul (C-J) strain hardening behavior of granular bainite and granular bainite-martensite as the second phase exhibited one-stage characterized by linear and parabolic behavior, respectively. On the other hand, two stages were observed in steel containing martensite as the second phase, viz., linear stage I and parabolic stage II.

Dynamic strain-induced transformation: An atomic scale investigation

Phase transformations provide the most versatile access to the design of complex nanostructured alloys in terms of grain size, morphology, local chemical constitution etc. Here we study a special case of deformation induced phase transformation. More specifically, we investigate the atomistic mechanisms associated with dynamic strain-induced transformation (DSIT) in a dual-phased multicomponent iron-based alloy at high temperatures. DSIT phenomena and the associated secondary phase nucleation were observed at atomic scale using atom probe tomography. The obtained local chemical composition was used for simulating the nucleation process which revealed that DSIT, occurring during load exertion, proceeds by a diffusion-controlled nucleation process.

Peculiarities of Phase Transitions and Structure Formation in a Ternary Al-Cu-Ni Alloy with Four-Phase Peritectic Reaction

The structure formation in peritectic Al-4.5at.%Cu-11at.%Ni ternary alloy with four-phase peritectic reaction was investigated using the quantitative phase-field model of eutectic growth. This model, extended to an arbitrary number of phases, guarantees the stability requirements on individual interfaces. The thermal noise terms disturb the stability and produce the heterogeneous nucleation of secondary phases in accordance to the energetic and concentration conditions. In our recent work it was shown that in differential thermal analysis (DTA) experiments specific microstructure parts in Al-4.5at.%Cu-11at.%Ni alloy with a four-phase peritectic reaction were observed, which cannot be explained by Scheil calculation or simple phase-field modeling. In this work, it was found by numerical experiments that, due to the formation of anisotropic quasi-primary Al3Ni2 crystals and the suppression of the nucleation of (Al) phase, the eutectic-like coupled growth of Al3Ni2 and Al3Ni phases can be observed. In addition, at further cooling the anisotropic shape of quasi-secondary Al3Ni crystals promotes the nucleation of the (Al) phase. The simulated final structure is comparable to the experimental one which is characterized by large Al3Ni2 crystals enveloped by the Al3Ni phase.

Catalytic Effect of Nanoparticles on Primary and Secondary Phase Nucleation

Nanoparticles were shown to catalyze nucleation of primary and secondary phases in metal matrix nanocomposites (MMNCs). This catalysis is important as it contributes to the mechanical property enhancement in the MMNCs. Primary aluminium grain refinement was demonstrated in A356 matrix nanocomposites. Various types and sizes of nanoparticles (SiC, TiC, γ-Al2O3; 10-96 nm) were used to make these MMNCs and in all cases the MMNCs had smaller, more equiaxed grains compared to the reference A356. Using the droplet emulsion technique, undercoolings were shown to be significantly reduced. Undercoolings in the MMNCs were in good general agreement with the undercooling necessary for free growth, suggesting the applicability of this model to nucleation on nanoscale catalysts. Secondary phase nucleation catalysis was demonstrated in a zinc alloy AC43A MMNC and a binary Mg-4Zn MMNC. In AC43A, secondary phase nucleation was catalyzed with the addition of various nanoparticles (TiC, SiC, γ-Al2O3). The secondary phase nucleation catalysis in AC43A coincided with ductility enhancement. In Mg-4Zn, SiC nanoparticle addition changed the secondary phases that formed. MgZn2 was formed in the MMNC at relatively high temperatures consuming the Zn and reducing the amount of the low temperature Mg2Zn3 phase that formed in the reference alloy. The change in secondary phase formation coincided with significant enhancement in strength and ductility.

Second-phase nucleation on an edge dislocation

A model for nucleation of second phase at or around a dislocation in a crystalline solid is considered. The model employs the Ginzburg–Landau theory of phase transitions comprising the sextic term in the order parameter (η6) in the Landau free energy. The ground state solution of the linearised time-independent Ginzburg–Landau equation is derived, through which the spatial variation of the order parameter is delineated. Moreover, a generic phase diagram indicating tricritical behaviour near and away from the dislocation is depicted. The relation between classical nucleation theory and the Ginzburg–Landau approach is discussed, for which the critical formation energy of the nucleus is related to the maximum of the Landau potential energy. A numerical example illustrating the application of the model to the case of nucleation of hydrides in zirconium alloys is provided.

Formation, Dynamics, and Characterization of Nanostructures by Ion Beam Irradiation

Ion beam irradiation is a potential tool for phase formation and material modification as a non-equilibrium technique. Localized rise in temperature and ultra fast (∼10−12 s) dissipations of impinging energy make it an attractive tool for metastable phase formation. As a matter of fact, a major component of materials science is dominated by ion beam methods, either for synthesis of materials or for its characterization. The synthesis of nanostructures, and their modification by ion beam technique will be discussed in this review article. Formation of nanostructures using ion beam technique will be discussed first. Depending on species (e.g., mass and charge state) and energy range, there are various modes for an energetic ion to dissipate its energy. The role of the electron will also be covered in this article as a basic principle of its interaction with matter, which is same as for an ion. By using a simple reactive ion beam or electron induced deposition, a secondary phase can be nucleated by ion beam mixing techniques, either by using inert gas irradiation or reactive gas implantation on any desired substrate. Nucleation of secondary phase can also be executed by electron irradiation and direct implantation of either negative or positive ions. Post implantation annealing processes are required for the complete growth of clusters formed in most of these ion irradiation techniques. Implantation processes being inherently a non-equilibrium technique, defects always have a role to play in phase formation, amorphization, and beyond (blister formation). When implanted with large energy, even electrons, one of the lightest charged particles, also manifest these properties. Electronic and nuclear energy losses of the impinging charged particle play a crucial role in material modification. Doping a nanocluster, however, is still a controversial topic. Some light will be shed on this topic with a discussion of focused ion beam.

Artificial aging and creep of an Al–Ge–Si alloy

Abstract The microstructural stability of a peak-aged (24 h at 160°C) Al–Ge–Si alloy, containing incoherent GeSi precipitates with high interfacial energy, has been examined under different creep conditions, with temperatures varying from 120 to 140°C and initial stresses varying from 65 to 85 MPa. A combination of transmission electron microscopy (TEM) and X-ray diffraction revealed that under all creep conditions, fresh nucleation of second-phase precipitates occurs in the alloy. TEM and quantitative image analysis demonstrated that exposure at 120°C with and without stress has no observable effect on the precipitate size, while increasing the creep temperature to 140°C coarsens the precipitates. Furthermore, prolonging the isothermal aging time at 160°C coarsens the second-phase precipitates considerably. These results are discussed in terms of the known factors responsible for the coarsening of precipitates in agehardenable alloy systems.

Effect of high-energy heavy ion irradiation on the crystallization kinetics of Co-based metallic glasses

Differential scanning calorimeter (DSC) is employed to study the crystallization kinetics of irradiated (at three different fluences with high-energy heavy ion; Ni11+ of 150 MeV) specimens of two Co-based metallic glasses. It is found that the crystallization process in both the glasses is completed in two phases. The DSC data have been analysed in terms of kinetic parameters viz. activation energy (E c), Avrami exponent (n), dimensionality of growth (m), bdusing two different theoretical models. The results obtained have been compared with that of virgin samples. The lower activation energy in case of second crystallization occurring at higher temperature indicates the easier nucleation of second phase. The abnormally high value of Avrami exponent in Co-Ni glass indicates very high nucleation rate during first crystallization.

Simulation of microsegregation and microstructural evolution in directionally solidified superalloys

A two-dimensional model for solidification and secondary phase precipitation in directionally solidified superalloys is presented, which makes use of a shape function approximation for the isothermal cross-section of the primary dendrites. The phase boundary is represented by a diffuse interface on a finite difference grid, as in phasefield methods. Thus, two-dimensional multicomponent diffusion can be calculated for all phases. Via a Fortran interface, the model is coupled to thermodynamic databases assessed using the ‘calculation of phase diagrams’ (Calphad) approach. In this way, for each time step actual data are available for partitioning, dendritic growth, and nucleation of secondary phases. The model is applied to Ni–Al–Cr as a ternary model system and to the commercial superalloy IN706.

Crystallographic Relationships between .DELTA.-Ferrite and .GAMMA.-Austenite during Unidirectional Solidification of Fe-Cr-Ni Alloys.

The crystallographic orientation relationships between δ-ferrite and γ-austenite during unidirectional solidi-fication of Fe–Cr–Ni alloys have been evaluated with EBSPs (Electron Back Scattering Patterns). The preferre dorientation relationship between δ and γ agrees with the Kurdjumov–Sachs (K-S) relationship in both rod and lamellar eutectic structures obtained by Bridgman experiments. It is thought that the preferred relationship is developed by nucleation of secondary phase. The orientation relationship between primary γ-dendrites and primary δ-dendrites during dissimilar alloy welding (primary γ to δ-dendrites) by a laser experiment also agrees with the K-S relationship. The transition from primary γ to δ-dendrites is thought to be controlled by heterogeneous nucleation of ferrite at the growing γ-dendrite front. It is suggested that the nucleation also plays an important role in the microstructure formation.

The effect of vanadium and grain refiner additions on the nucleation of secondary phases in 1xxx Al alloys

High purity Al–0.3 wt% Fe–0.1 wt% Si alloys with different Si, V and grain refiner contents were melt spun to produce microstructures of submicron secondary phases entrained in a higher melting point Al matrix. On reheating, a dispersion of eutectic liquid droplets forms that represents an exaggerated version of the liquid puddles that solidify pinched-off between Al dendrite arms during conventional casting. The subsequent resolidification of the droplets, analysed using differential scanning calorimetry (DSC), allows the nucleation-controlled aspects of secondary phase selection to be studied. The droplets solidify as the metastable FeAl m phase in ribbons containing ≃500 ppm V or ≃100 ppm V plus Al–Ti–B, Al–Ti–C or Al–B grain refiner. This phase contributes to the “fir-tree” surface defect in commercial sheet products. This work suggests that the combination of V and Al–Ti–B promotes FeAl m in commercial ingots, and confirms that solidification rate and bulk Si content also influence phase content.