Spinodal-like Decomposition Research Articles

Coupling of alloy chemistry, diffusion and structure by grain boundary engineering in Ni–Cr–Fe

The diffusion–microstructure correlations for grain boundaries (GBs) in the technologically-relevant Ni-based 602CA alloy are investigated. Prolonged annealing treatments up to 744 h create distinct GB complexions with specific segregation–precipitation–structure states. Globular M23C6-type carbides at straight GBs and plate-like carbides together with NiAl-enriched (γ′-type) particles at hackly GBs are found to co-exist. Moreover, an atomic-scale GB spinodal-like decomposition, especially at straight GBs, is observed. The co-existence of the two distinct states of general high-angle GBs, indicated by tracer diffusion experiments and verified by a detailed structure examination, is explained via state-of-the-art measurements of local elastic strains. In a course of annealing at 873 K, the relatively “fast” diffusivities are found to increase by a factor of 10 or more as a result of a coupled evolution of the GB plate-like precipitates and the irregular GB structures, whereas the relatively ”slow” diffusivites remained practically unchanged representing the contributions of straight interfaces with spherical precipitates. Thus, the diffusion properties of high-angle GBs evolve together with characteristic changes of GB complexions distinguished by a growth of carbide- and γ′-type precipitates and a concomitant generation of GB dislocation networks. The obtained results provide novel insights into grain boundary tailoring by utilizing structure – kinetics correlations involving segregation, precipitation and the evolution of interface defects.

2430% Superplastic strain in a eutectic Au-Sn alloy with micrometer-sized grains maintained by spinodal-like decomposition

Polycrystalline metals with fine grains are highly possible to possess superplasticity via grain boundaries sliding (GBS). However, the mechanism of stabilizing the fine grains and releasing grain boundary strain incompatibilities to maintain GBS has puzzled the researchers for several decades. Here, a classic example is represented by the Au-Sn eutectic (ζ-Au5Sn + δ-AuSn) alloy with micrometer-sized equiaxed grains, which achieved 2430% tensile strain at 473 K. Characterization at nanoscale reveals, unexpectedly, the universal occurrence of spinodal-like decomposition in one of the eutectic phases (δ-AuSn phase) in the Au-Sn alloy. The occurrence of the spinodal-like decomposition is energetically favorable. Therefore, the system's free energy was minimized by the spinodal-like decomposition rather than grain growth. Micrometer-sized grains were thus maintained at relatively high temperatures for maintaining GBS. In addition, the stacking faults (SFs) were generated in the spinodal-like decomposed substructures during deformation. SFs coordinated the strain incompatibilities during GBS and contributed to the plastic flow during superplasticity. In the present Au-Sn alloy, the spinodal-like decomposition is the root cause to stabilize the fine grains, eventually leading to outstanding superplasticity. This study enriches our fundamental understanding of the relation between spinodal-like decomposition and mechanical performance. It also provides new insights into the design of polycrystalline metals to achieve superplasticity.

Ultrahigh hardness in nanostructured dual-phase high-entropy alloy AlCrFeCoNiNb developed by high-pressure torsion

High-entropy alloys (HEAs) usually show higher hardness compared to conventional alloys due to the presence of five or more principal metals in the solid-solution form, but there are significant efforts for further enhancing the hardness of these alloys. In this study, three strategies are combined to achieve one of the highest hardness levels reported for metallic alloys: (i) solution hardening by the concept of multi-principal element alloys, (ii) grain refinement by severe plastic deformation via the high-pressure torsion method, and (iii) introduction of dual phases to hinder dynamic recrystallization and enhance interface hardening. An ultrahigh hardness of 1030 Hv is achieved by the introduction of nanograins in a dual-phase (cubic + hexagonal) HEA, AlCrFeCoNiNb. Such a high hardness is not only due to the formation of nanograins with an average size of 10 nm, but also due to the generation of dislocations, interfaces and spinodal-like elemental decomposition.

A Sequence of Phase Transformations and Phases in NiCoFeCrGa High Entropy Alloy

The present investigation is directed to phase transitions in the equimolar NiCoFeCrGa high entropy alloy, which is a mixture of face-centered cubic (FCC) and body-centered cubic (BCC) crystalline phases. The microstructure of the samples was investigated by using scanning electron microscopy (SEM), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), transmission electron microscopy-based energy-dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), as well as X-ray diffraction (XRD) measurements. Based on the phases observed in different temperature ranges, a sequence of the phase transitions can be established, showing that in a realistic process, when freely cooling the sample with the furnace from high to room temperature, a microstructure having spinodal-like decomposition can also be expected. The elemental mapping and magnetic behaviors of this decomposed structure are also studied.

On spinodal-like phase decomposition in U–50Zr alloy

Finely dispersed two phase microstructures resulting from a spinodal decomposition are of interest as they are associated with enhanced mechanical properties and excessive interfaces to mitigate defect related behavior. This study reports a spinodal-like phase decomposition in a U–50Zr alloy by thermal annealing at 620 °C and ion irradiation at 550 °C, with the latter temperature too low to initiate pure thermal phase transformation. The results hold broad impact for U–Zr alloy systems and its application as advanced nuclear fuel.

Thermodynamic origin of solute-enriched stacking-fault in dilute Mg-Zn-Y alloys

We have investigated thermodynamic behaviors of dilute Mg-Zn-Y ternary alloys to form a unique solute-enriched stacking-fault (SESF), which is an intrinsic-II type stacking-fault (I2-SF) enriched by the Zn and Y atoms and represents the structural-unit of the long-period stacking/order (LPSO) phase. SESF in the hexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc) environment, and hence our thermodynamic analysis is based on the Gibbs energy comparison between hcp and fcc phases over the Mg-Zn-Y ternary composition ranges, using the calculation of phase diagrams (CALPHAD) method aided by the first principles calculations. We find that the Zn/Y co-segregations at the SESF provide a remarkable condition that the fcc layers become more stable than the hcp-Mg matrix. Furthermore, within the SESF, the following spinodal-like decomposition into the Mg-rich solid-solution and the Zn/Y-rich L12-type order phase causes a significant reduction of the total Gibbs energy of the system. These spontaneous thermodynamic behaviors explain why the fault layers can be remarkably stabilized in the LPSO-forming ternary Mg alloys, and also support a phenomenological origin of the Zn-Y clustering with the L12-type short-range order, which is known to occur for the LPSO phases and has been confirmed for the present SESF by electron microscopy experiments.

Thermodynamic Origin of Solute-Enriched Stacking-Fault in Dilute Mg-Zn-Y Alloys

We investigate thermodynamic behaviors of dilute Mg-Zn-Y ternary alloys to form a unique solute-enriched stacking-fault (SESF), which is an intrinsic-II type stacking-fault (I2-SF) enriched by the Zn and Y atoms and represents the structural-unit of the long-period stacking/order (LPSO) phase. SESF in the hexagonal-close-packed (hcp) Mg matrix forms a local face-centered-cubic (fcc) environment, and hence our thermodynamic analysis is based on the Gibbs energy comparison between hcp and fcc phases over the Mg-Zn-Y ternary composition ranges, using the calculation of phase diagrams (CALPHAD) method aided by the first principles calculations. Segregation behaviors of solute Zn/Y atoms into the SESF are firstly estimated according to the Hillert's parallel tangent law, followed by the possible disorder-order phase transformation within the SESF using the multiple-sublattice model. We find that the Zn/Y co-segregations at the SESF provide a remarkable condition that the fcc layers become more stable than the hcp-Mg matrix. Besides, within the SESF, the following spinodal-like decomposition into the Mg-rich solid-solution and the Zn/Y-rich L12-type order phase causes a significant reduction of the total Gibbs energy of the system. These thermodynamic behaviors explain fairly well a phenomenological origin of the Zn-Y clustering with the L12-type short-range order, which is known to occur for the LPSO phases and also confirmed for the present SESF by electron microscopy experiments. Therefore, strong Zn-Y interactions even in dilute conditions play a key role to stabilize firmly the SESF in the Mg-Zn-Y alloys.

On Spinodal-Like Phase Decomposition in U-50 Zr Alloy

Finley dispersed two phase microstructures resulting from a spinodal decomposition are of interest as they are associated with enhanced mechanical properties and excessive interfaces to mitigate defect related behavior. This study reports a spinodal-like phase decomposition in a U-50Zr alloy by thermal annealing at 620°C and ion irradiation at 550°C, with the latter temperature too low to initiate phase transformation pure thermally. The results hold broad impact for U-Zr alloy systems and its application as advanced nuclear fuel.

Enhancement of open circuit voltage in InGaAsP-inverted thin-film solar cells grown by solid-source molecular beam epitaxy

Because InGaAsP alloys grown on InP substrates with 1.05eV bandgap are often susceptible to compositional fluctuations owing to spinodal-like decomposition due to the miscibility gap, the corresponding open circuit voltage (VOC) for InGaAsP solar cells is typically smaller than the empirically expected value. In this study, we investigate the impact of the device structure on the VOC of In0.83Ga0.17As0.36P0.64 solar cells grown by solid-source molecular beam epitaxy. In comparison to the upright homojunction cell as a baseline, a reverse saturation current density (J0) for the upright heterojunction cell was effectively reduced from 3.2×10−4 to 1.5×10−6mA/cm2 using the n-InP emitter with higher electron mobility, resulting in an enhancement of VOC from 0.557 to 0.568V. Owing to the reduction of J0 to 2.8×10−7mA/cm2, the VOC was further enhanced to 0.590V for the inverted heterojunction cell. For the reduction of J0, a longer radiative lifetime of 20.2ns obtained for the inverted heterojunction cell by photoluminescence decay at room temperature, presumably caused by light trapping, was responsible. Consequently, the efficiency was enhanced from 9.5% for the upright homojunction cell to 12.1% for the inverted heterojunction cell. Using the inverted InGaAsP cell, we demonstrated the mechanically stacked InGaP/GaAs//inverted-InGaAsP (// denotes the bonded interface) triple-junction solar cells with enhanced VOC of 2.64V.

Diffusional Decomposition and Glass Forming Ability of Solidifying Ternary Liquids

ABSTRACTAn innovative model based on spinodal-like decomposition of supercooled multi-component liquids was developed for prediction of the glass forming ability (GFA) of ternary alloys. The up-hill diffusion in the initially homogeneous freezing ternary liquid was considered as a necessary condition for solidification of crystalline phases. New generalized criteria of spinodal decomposition of ternary alloys for time/space correlated fluctuations were formulated. These criteria take into account both thermodynamic and kinetic properties of the system. The introduced criteria were found to provide adequate GFA evaluation of different compositions in the Mg-based ternary alloys.

A Spinodal Decomposition Model for the Prediction of the Glass-Forming Ability of Ternary Mg Alloys

The glass-forming ability (GFA) of two alloy systems, Mg-Y-La and Mg-Zn-Nd, was investigated using thermal and microstructural analysis. Rapid solidification was found to lead to microstructural refinement and partial amorphization of the most investigated alloys. The addition of Cu to the Mg-Y-La group was found to increase its tendency to undergo amorphization during rapid solidification, exemplified by the Mg86Y9.5Cu2.5La2 alloy exhibiting a pronounced crystallization peak in the differential scanning calorimetry trace. Two Mg-Zn-Nd alloys, Mg71Zn28Nd and Mg73.6Zn22.1Nd4.3, were found to exhibit significant amorphous behavior, with the former alloy being more amorphous than the latter. An innovative model predicting the GFA of alloys based on spinodal-like decomposition of supercooled alloys is formulated herein. New generalized thermo-kinetic criteria for spinodal decomposition of ternary alloys for time/space-correlated fluctuations were formulated. The time-dependent amplification factor of concentration fluctuations in ternary systems was found to provide adequate GFA evaluation for the compositions of both alloy systems: Mg-Y-La and Mg-Zn-Nd. The model was able to pinpoint the most amorphous alloy in each alloy system, and comparison between both systems pointed to Mg71Zn28Nd as having the best GFA, while also recognizing that it has a lower GFA than the widely known and highly glass-formable Mg65Cu25Y10 alloy. This model is expected to predict the GFA of any envisaged composition, thereby avoiding cumbersome trials.

“Colossal” interstitial supersaturation in delta ferrite in stainless steels—I. Low-temperature carburization

Low-temperature carburization has been successfully used to surface harden 17-7 precipitation-hardening (PH) and 2205 duplex stainless steels. After carburization, the delta ferrite grains in both alloys near the free surface show a uniform weak contrast under conventional transmission electron microscopy (TEM). Spatially resolved compositional analysis shows that these delta ferrite grains possess enormous carbon contents (as high as 18at.%) in solid solution, but structurally there is no detectable tetragonality (<5%) or evidence of carbide formation. Near the interface between the interstitially hardened layer and bulk material, weak-contrast plates with significant carbon concentrations were observed in ferrite grains in 17-7 PH stainless steel. A carbon-induced spinodal-like decomposition of delta ferrite to the nanometer-scale Cr-rich and Fe-rich alpha ferrite phases is observed. Carbon is enriched in Cr-rich ferrite due to the high affinity between C and Cr, which introduces lattice mismatch between the Cr-rich and Fe-rich regions. The weak contrast is believed to be the result of overlapping strain fields of these Cr-rich and Fe-rich phases. As the binding energies of carbon interstitials to dislocations in body-centered cubic Fe-based alloys are greater than the binding energy of C to Fe in possible carbides, segregation to dislocation cores is expected. The extremely high dislocation density we observe in high-resolution scanning TEM is consistent with the hypothesis that carbon segregation to dislocation cores effectively delays carbide precipitation and makes possible the “colossal” carbon supersaturation.

Structural and mechanical evolution of reactively and non-reactively sputtered Zr–Al–N thin films during annealing

The influence of reactive and non-reactive sputtering on structure, mechanical properties, and thermal stability of Zr1−xAlxN thin films during annealing to 1500°C is investigated in detail. Reactive sputtering of a Zr0.6Al0.4 target leads to the formation of Zr0.66Al0.34N thin films, mainly composed of supersaturated cubic (c) Zr1−xAlxN with small fractions of (semi-)coherent wurtzite (w) AlN domains. Upon annealing, the formation of cubic Zr-rich domains and growth of the (semi-)coherent w-AlN domains indicate spinodal-like decomposition. Loss of coherency can only be observed for annealing temperatures above 1150°C. Following these decomposition processes, the hardness remains at the as-deposited value of ~29GPa with annealing up to 1100°C. Using a ceramic (ZrN)0.6(AlN)0.4 target and sputtering in Ar atmosphere allows preparing c-Zr0.68Al0.32N coatings with a well-defined crystalline single-phase cubic structure combined with higher hardnesses of ~31GPa. Due to the absence of (semi-)coherent w-AlN domains in the as-deposited state, which could act as nucleation sites, the decomposition process of c-Zr1−xAlxN is retarded. Only after annealing at 1270°C, the formation of incoherent w-AlN can be detected. Hence, their hardness remains very high with ~33GPa even after annealing at 1200°C. The study highlights the importance of controlling the deposition process to prepare well-defined coatings with high mechanical properties and thermal stability.

The Influence of Compositional Changes on the Glass-Forming Ability of Y-Containing Mg Alloys

ABSTRACTThe effect of compositional changes on the glass forming ability of Mg alloys containing Y was studied. Four rapidly-solidified Mg-alloys were investigated: Mg91Y7.5La1.5, Mg85Y12La3, Mg86Y9.5Cu2.5La2 and Mg82Y11La4Eu3. XRD and DSC spectra revealed that the Mg86Y9.5Cu2.5La2 was the most amorphous out of the investigated alloys. A model based on a spinodal-like decomposition of a supercooled liquid alloy was developed. The model provides qualitative and quantitative explanation for the variation in glass forming ability.

Micro-phase Separation via Spinodal-like Decomposition in Hexamethylynediisocyanate (HDI)-polyurea

Micro-phase separation in hexamethylynediisocyanate-polyurea was studied using small-angle X-ray scattering and infrared absorption. It was found that phase separation in this system followed spinodal-like decomposition on a 3–4 nm size scale with phase separation occuring on a time scale of days.

Stability of polydihydrosilane liquid films on solid substrates

The quality of polydihydrosilane liquid films is a key factor in the fabrication of solution-processed silicon films. This study investigates the stability of polydihydrosilane liquid films with a thickness L of ~40nm on solid substrates by a comparison between the observed optical microscope images and the values of the Hamaker constant AALS for the air/liquid (polydihydrosilane)/solid substrate systems. AALS values for a series of SiO2-based substrates were determined by adopting a simple spectrum method. We found that the micrographs of the polydihydrosilane films provide direct evidence of stability in accordance with the sign of AALS; a stable liquid film with AALS>0 showed a continuous figure, while an unstable film with AALS<0 exhibited an array of dots caused by the rupture of the film. The array of dots in the unstable liquid films has a slight orderly distribution with a period λ that is in accord with the characteristic wavelength of the undulation related to the spinodal-like decomposition in van der Waals unstable liquid.

Irradiation-Driven Compositional Changes in Alloys

ABSTRACTA model to study the formation of compositional patterns in concentrated binary alloys under irradiation is presented. In this model, six atomic and defect species are considered: regular lattice atoms (A and B), dumbbell interstitials (AA, BB and AB) and vacancies. In addition to long range diffusion of these species, local defect-defect (vacancy-interstitial recombination) and defect-atom (dumbbell-atom) reactions have also been considered. The model tracks simultaneous evolution of all the species in reaction-diffusion formalism. Irradiation events are modeled as a stochastic point process which changes the concentration of all the species instantaneously in a random fashion. In each irradiation event, a spatial distribution of point defects (core-shell distribution: core dominated by vacancies and shell by interstitials) has been introduced in the system which drives the kinetics of diffusion and reaction. The model has been non-dimensionalized with respect to intrinsic length and time scales of the material system and solved numerically using finite difference technique. Applying this model to CuAu solid solution, we have shown that the alloy exhibits spinodal-like decomposition with specific steady state wavelengths that depend on the irradiation conditions.

Deep eutectic solvents as both precursors and structure directing agents in the synthesis of nitrogen doped hierarchical carbons highly suitable for CO2 capture

Deep eutectic solvents (DESs) have been used in the synthesis of nitrogen-doped carbons exhibiting a hierarchical porous structure. The CO2 sorption capacity of these solid sorbents was extraordinary because of their relatively high nitrogen content and their bimodal porous structure where micropores provide high surface areas (ca. 700 m2 g−1) and macropores provide accessibility to such a surface. DESs were composed of resorcinol, 3-hydroxypyridine and choline chloride in 2 : 2 : 1 and 1 : 1 : 1 molar ratios. Polycondensation of resorcinol and 3-hydroxypyridine (with formaldehyde) promoted DES segregation in a spinodal-like decomposition process by the formation of a polymer rich phase and a depleted polymer phase. Thus, DESs played a multiple role in the synthetic process; the liquid medium that ensured reagents homogenization, the structure-directing agent that is responsible for the achievement of the hierarchical structure, and the source of carbon and nitrogen of the solid sorbent obtained after carbonization. Interestingly, the homogeneous incorporation of nitrogen at the solution stage of the synthetic process (rather than by post-treatment of the preformed carbon) allowed the achievement of significant nitrogen contents even in carbons obtained at relatively high temperatures (e.g. 8–12 at% for 600 °C and ca. 5 at% for 800 °C). It is worth noting that, despite thermal treatments at high temperatures tend to decrease the nitrogen content, the high surface area of the solid sorbents obtained at 800 °C contributed to a significant enhancement of CO2 capture while providing superior selectivity, recyclability and stability.

Influence of surface strain on the MOVPE growth of InGaP epitaxial layers

In this paper, we present a study of the influence of strain on the composition uniformity, phase separation and ordering of compressive and tensile strained InGaP epitaxial layers grown on GaP and GaAs substrates. The paper demonstrates how strain, caused by lattice mismatch, influences the spinodal-like decomposition and ordering in InGaP layers. We concentrated our efforts on an alloy with the lattice mismatch close to a value of Δa/a=10-2. Transmission electron microscopy (TEM), low temperature photoluminescence and atomic force microscopy (AFM) were used to demonstrate that epitaxial layers grown under compressive strain have additional properties in comparison to those ones grown under tensile strain. The presence of ordering, spinodal-like decomposition and compositional fluctuations was studied in relation to the sign of strain.

Role of the V–III ratio and growth rate in decomposition of In 0.27Ga 0.73P/GaP grown by MOVPE

In this paper, we report on a study of the spinodal-like decomposition of In x Ga 1− x P grown on (1 0 0) GaP substrates at 740 °C by metalorganic vapor phase epitaxy (MOVPE). We concentrated our efforts on an alloy with the InP mole fraction close to x=0.27, at which the indirect-to-direct band-gap structure crossover occurs. We used the V/III ratio and growth rate for the limitation of diffusion length of adatoms before their incorporation into an epitaxial layer. Transmission electron microscopy (TEM) together with energy dispersive X-ray analysis (EDX) and low-temperature photoluminescence were used to demonstrate that a spinodal-like decomposition of such InGaP layers can partially be suppressed if the V/III ratio is increased from a starting value of 75–350. Next improvement in the quality of the epitaxial layer may be achieved by an increase of the growth rate v g up to value higher as 1.1 μm/h.