Off-eutectic Compositions Research Articles

Off-Eutectic Growth Model for Solidifying Alloy from an Undercooled State

Classical eutectic growth models are based on the use of eutectic composition. These models neglect the effect of primary phase formation, and their direct use in the rapid solidification process of off-eutectic (hypoeutectic and hypereutectic) alloys is absent. Combining the effect of the primary phase in the eutectic transformation and an off-eutectic composition, the solidification growth model is derived in the present work. The effect of the model and material parameters on solidification kinetics is discussed in comparison with experimental data. Computational results on the off-eutectic growth model show that the model agrees well with experimental data on the solidification kinetics of Ni–B and Ti–Si alloys.

Direct observation of concentration fluctuations in Au–Si eutectic liquid by small-angle neutron scattering

It is well-known that eutectic gold–silicon (Au–Si) alloys exhibit anomalous melting point depression, which is more than 1000 °C from the melting point of elemental Si (1414 °C). The melting point depression in eutectic alloys is generally explained in terms of a decrease of the free energy by mixing. However, it is difficult to understand the anomalous melting point depression only from the stability of the homogeneous mixing. Some researchers suggest that there are concentration fluctuations in the liquids, where the atoms are inhomogeneously mixed. In this paper, we measure the small-angle neutron scattering (SANS) of Au81.4Si18.6 (eutectic composition) and Au75Si25 (off-eutectic composition) at temperatures from room temperature to 900 °C in both solid and liquid states to observe such concentration fluctuations directly. It is surprising that large SANS signals are observed in the liquids. This indicates that there are concentration fluctuations in the liquids. The concentration fluctuations are characterized by either the correlation lengths in multiple length scales or surface fractals. This finding yields new insight into the mixing state in the eutectic liquids. The mechanism of the anomalous melting point depression is discussed based on the concentration fluctuations.

Microstructure evolution during directional solidification of off-eutectic Al-Cu-Mg alloys

Building on previous studies of invariant ternary eutectic Al-Al2Cu-Al2CuMg, this work aims to deepen our understanding of the microstructural possibilities in the Al-rich region of the Al-Cu-Mg phase diagram by considering the solidification behaviour of several off-eutectic compositions. Four Al-Cu-Mg alloys were directionally solidified at a constant growth rate (1.25 μm/s) and thermal gradient (9-14 K/mm) using a Bridgman-type furnace and analysed using a combination of scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and differential scanning calorimetry. Qualitative and quantitative analyses showed a preference for faceted Al2Cu dendrites surrounded by a ternary eutectic matrix in the two Mg-poor samples, even though one composition was not in the primary Al2Cu phase region. The two Al-poor samples consisted of large regions of three different intermetallic phases and little eutectic, making these samples very brittle.

Phase-field modelling of directional melting of lamellar and rod eutectic structures

The melting of 2D lamellar and 3D rod eutectic structures was studied by multi-phase-field simulations. A binary model eutectic system was investigated in a standard directional solidification setup. Switching from solidification to melting was realised by inverting the pulling speed of the sample. Steady state melting profiles were obtained as the long-time solutions of the evolution equations solved in the minimal representative domains of the periodic structures. Series of simulations were performed using different volume fractions of the initial solid phases, different values of the temperature gradient, and different widths of the simulation domain. It was found that melting occurs with a nearly flat interface if the average composition of the initial solid structure is equal to the eutectic composition. If the volume fraction of the solid phases is changed and the average composition becomes off-eutectic, then the melting positions of the phases decouple: the phase of sub-eutectic amount will melt near the eutectic temperature, while the phase of super-eutectic amount will melt near its liquidus temperature corresponding to the off-eutectic mean composition. The lamellar/rod spacing imposed by the width of the simulation domain has only minor effect on the interface temperature. Besides the steady states, we could observe two kinds of instabilities in the regime of non-planar melting both in 2D and 3D. By increasing the lamellar spacing, oscillations may appear around the trijunction and along the phase boundaries. By decreasing the lamellar spacing and increasing the pulling speed, the lamellae/rods of the phase that protrude deeper in the melt become thinner and may eventually break up to a series of small spherical particles before melting completely.

Modeling of stoichiometric phases in off-eutectic compositions of directional solidifying NbSi-10Ti for phase-field simulations

To meet the requirements of modern technical and scientific applications, the development and investigation of high-performance materials with locally defined properties gets more and more into focus of actual research. These materials often contain phases with at least one stoichiometric component. To investigate such materials within grand potential-based phase-field simulations, a specialized method is needed, which allows to model the required Gibbs energy formulations for the stoichiometric phases. In this work, a modeling approach is introduced, based on the equilibrium conditions of the involved phases, to generate a Gibbs energy formulation for the stoichiometric (Nb,Ti)3Si phase in Nb-Si-Ti. With this formulation, the eutectic reaction in the ternary NbSi-10Ti system is investigated within two- and three-dimensional phase-field simulations, with eutectic and off-eutectic melt compositions. The solidified microstructures are qualitatively assessed by measuring the average front undercoolings, the adjusting phase fractions and the concentrations during the simulations. The validation of the results shows the ability of the generated material model to simulate the obtained system and proves the suitability of the presented approach to model stoichiometric phases.

Calibration of a concentration-driven nucleation mechanism for phase-field simulations of eutectic and off-eutectic compositions in AlCu-5Ag

The present work is an extension of the studies conducted by Kellner et al. [M. Kellner et al., Acta Materialia, vol. 182, pp 267–277, 2020.], to show the transferability of their concentration-driven nucleation mechanism to other systems. The implementation of this method improves the adjustment process of directionally solidifying eutectic structures in two-dimensional phase-field simulations. To quantitatively evaluate the nucleation mechanism, the velocity-spacing relationships of different eutectic and off-eutectic melt compositions are investigated for AlCu-5Ag. A good agreement between simulations with and without the nucleation mechanism is found for all studied compositions.

Composition design and properties of Cu–Zr–Ti bulk metallic glass composites

(Cu50Zr50)x(Cu73Ti27)100-x (x = 0.63–0.75 at.%) alloys are designed using the proportional mixing of Cu50Zr50 and Cu73Ti27 binary alloys. Their thermal, mechanical and corrosive properties as well as the glass forming ability (GFA) are systematically investigated. The results show that the studied Cu-based glass forming alloys are bulk metallic glass composites (BMGCs) with off-eutectic compositions and display work-hardening behavior. The glass transition temperature and the reduced glass transition temperature increase and then decrease with increasing x when x ≥ 0.74 at%, while on the contrary for the critical cooling rate and supercooled liquid region. Both yield strength and fracture strength firstly increase and then decrease with decreasing x when x ≤ 0.71 at%. The plastic strain of (Cu50Zr50)0.75(Cu73Ti27)0.25 BMGC is the largest among the studied Cu-based BMGCs. Potentiodynamic polarization measurements show that all studied BMGCs exhibit a step-like anodic polarization behavior, indicating two competitive procedures of pitting corrosion and self-passivization. The corrosion behavior gradually transforms from predominant self-passivization to the pitting corrosion with decreasing x. Both corrosion current density and onset passive current density of (Cu50Zr50)0.75(Cu73Ti27)0.25 BMGCs are two orders of magnitude lower than those of the other Cu-based BMGCs. The mechanisms for the GFA and the corrosion are also discussed. The present work would shed light on how to design and develop BMGCs with superior comprehensive performances.

Effect of an abrupt change in pulling rate on microstructures of directionally solidified Al2O3-Er3Al5O12 eutectic and off-eutectic composite ceramics

Directionally solidified microstructures of Al2O3-Er3Al5O12 eutectic and off-eutectic in situ composite ceramics were explored under abrupt-change pulling rate conditions. Corresponding temperature distributions and interface locations were studied. In eutectic composition, fluctuation of eutectic spacing occurred when the pulling rate increased abruptly. A gradually increase or abrupt increase in eutectic spacing was observed when the pulling rate decreased abruptly. In hypoeutectic and hypereutectic compositions, formation of the primary phases were suppressed when the pulling rate increased abruptly from 10 µm/s to 100 µm/s, while primary phases precipitated when the pulling rate decreased abruptly from 100 µm/s to 10 µm/s. The interface altitude decreased after the pulling rate increased abruptly, but increased after the pulling rate decreased abruptly. The liquid composition restriction (around the eutectic composition) at the eutectic interface plays an important role in the suppression of the primary dendrite and coupled eutectic oxides can be obtained in off-eutectic compositions even under higher solidification rate conditions.

Mullite-zirconia-fibre/molybdenum-matrix composites: Strength and damage tolerance

Molybdenum-matrix composites reinforced with composite fibres containing mullite and zirconia in various proportions (including that corresponding to eutectic) were successfully produced by the internal crystallisation method. The composites are characterised by a high damage tolerance: the average notch sensitivity value is 0.71, while the maximum value of this parameter approaches 1. Composites with eutectic and off-eutectic fibre compositions are sufficiently strong at temperatures up to 1400 °C. The mean strength values are 178 MPa and 140 MPa at temperatures of 1300 and 1400 °C, respectively.

Pilot-scale demonstration of advanced adiabatic compressed air energy storage, Part 2: Tests with combined sensible/latent thermal-energy storage

Abstract Experimental and numerical results from the world’s first pilot-scale advanced adiabatic compressed air energy storage plant with combined sensible/latent thermal-energy storage are presented. The combined thermal-energy storage was composed of sensible and latent units with maximum capacities of 11.6 MWhth and 171.5 kWhth, respectively. The latent thermal-energy storage consisted of a steel tank with 296 stainless-steel tubes encapsulating an Al–Cu–Si alloy as phase-change material. The combined thermal-energy storage was investigated using four charging/discharging cycles with durations of about 3 h each and air inflow temperatures of up to 566 °C. The experimental results showed that the latent thermal-energy storage reduced the drop in the air outflow temperature during discharging. Minor leaks of the phase-change material were traced to the welding seams in the encapsulation as well as to holes required to insert resistance temperature detectors. Analysis of the leaked phase-change material revealed degradation and/or phase separation, which were attributed to the initial off-eutectic composition of and impurities in the phase-change material and resulted in a reduced heat of fusion. Simulations predicted the performance of the combined thermal-energy storage with good overall accuracy. Discrepancies were put down to changes in the thermophysical properties.

Effects of composition and growth rate on the microstructure transformation of β-rods/lamellae/α-rods in directionally solidified Mg-Li alloy

Mg-Li alloy is a unique system with a narrow dual-phase region and superstructure transformation. These characteristics will have significant influence on the solidification behavior. Using Bridgman-type vacuum furnace with liquid metal cooling method, the directional solidification experiments were performed on alloys with different Li contents (6.3wt.%, 6.6wt.%, 6.9wt.%, 7.2wt.%, 7.5wt.%, 7.8wt.%) at varied growth rates (3μm/s, 5μm/s, 10μm/s, 15μm/s, maximum to 21μm/s). As the Li content increases, the microstructures transform from divorced eutectic structure to lamellar structure, then to lamellae/α-Mg rod mixed structure, and finally to α-Mg rod-like structure. Fully eutectic structure is produced at a wide range of off-eutectic compositions. The effect of growth rate on the α-Mg rod-like structure is mainly to diminish the inter-rod spacing as growth rate increases, while additionally to affect the morphology of lamellar structure. High growth rate is found to be beneficial for producing straight and uniform lamellar structure. The eutectic spacing of Mg-Li system is one or two orders of magnitude lager than most other alloy systems. Moreover, a controllable three-stage transformation of β-rods/lamellae/α-rods is achieved in one single alloy system. A structure selection map is also given to define the microstructure transformation of the Mg-Li alloys.

Phase formation, glass forming ability, mechanical and thermal properties of Cu50Zr50−x Al x (0⩽x⩽11.0) glass forming alloys

Phase formation, glass forming ability, mechanical and thermal properties of Cu50Zr50−x Al x (0⩽x⩽11.0) glass forming alloys were systematically investigated. The results show that Cu50Zr47Al3 alloy has the best glass forming ability (GFA), lowest fragility index (m) and highest fracture strength. There are B2 CuZr and B19′ CuZr phases for Cu50Zr50−x Al x (0⩽x⩽5.0) alloys. B2 CuZr phase disappears and two new phases (Cu10Zr7 and CuZr2) appear when 5.0<Al<8.0. Another new AlCu2Zr phase can be clearly observed when Al ⩾8.0. The glass transition temperature (T g), the sensitivity of the T g to the heating rate (B), the activation energy for T g (E g) and the m value all increase with increasing Al content. The supercooled liquid region (ΔT x) is larger for Cu50Zr50−x Al x (x⩽5.0) than for Cu50Zr50−x Al x (x>5.0). Cu50Zr50−x Al x (0⩽x⩽4.0) alloys are off-eutectic compositions while near-eutectic compositions for Cu50Zr50−x Al x (5.0⩽x⩽11.0) alloys. The E g, E x, and E p decrease with increasing sensitive factor (β) for the studied Cu-based alloys characterized in the room-temperature brittleness. The dependence of the phase formation and GFA on the Al content is discussed.

Modeling of free eutectic growth and competitive solidification in undercooled near-eutectic alloys based on in situ measurements

Free eutectic growth and its competition with single-phase growth in solidification of undercooled near-eutectic alloys are not yet fully understood. In this paper, the historical development of eutectic growth models was reviewed. The LZ model of free eutectic growth was evaluated using recent data of eutectic growth velocities in an undercooled Ni81.3Sn18.7 eutectic composition. An excellent agreement was achieved between the LZ model and the data. Crystal growth velocities in off-eutectic Ni83Sn17 and Ni80Sn20 compositions were measured using a high-speed camera technique. The present data of the off-eutectic compositions and the recent data of the eutectic composition were modeled using the LZ model and the LKT/BCT model of free dendritic growth. The modeling revealed that the competition between the free eutectic growth and the single-phase growth is controlled by the highest interface temperature criterion. A coupled zone of the α-Ni-Ni3Sn eutectic was calculated using this criterion. The coupled zone agrees well with studies of solidified structures of undercooled samples.

Directionally solidified eutectic and off-eutectic mullite–zirconia fibres

Abstract Eutectic and off-eutectic mullite–zirconia fibres were grown by LFZ (laser floating zone) directional solidification. The microstructure of the mullite–zirconia eutectic fibres varies from planar coupled eutectic (1 mm/h pulling rate) through mullite columnar growth with coarse zirconia inclusions (10 mm/h pulling rate) to faceted mullite eutectic dendrites, which enclose a dispersion of fine zirconia fibrils (100–500 mm/h pulling rates). Near-equilibrium conditions determine the crystallization of monoclinic zirconia and the absence of any amorphous phase, whereas for higher speeds the tetragonal structure is retained and a residual liquid is kept after the eutectic solidification. Similar structural and morphological characteristics are displayed by the mullite-rich off-eutectic composition added to the development of prismatic crystals of mullite primary phase. In opposition, heavy constitutional supercooling takes place in the case of the zirconia-rich off-eutectic fibres, where equiaxed zirconia dendrites soon form as a primary phase, leaving a non-equilibrium mixture of alumina and sillimanite as interdendritic constituent.

Atomic structure and transport properties of Cu50Zr45Al5 metallic liquids and glasses: Molecular dynamics simulations

We have simulated the atomic structure and the transport properties of Cu50Zr45Al5 metallic liquids and glasses within a wide cooling temperature range from 2000 to 300 K, using molecular dynamics simulations. High fractions of Cu- and Al-centered full icosahedra and Zr-centered icosahedra-like clusters have been detected in both supercooled liquids and glasses. The heat capacity and linear thermal expansion coefficients of both liquids and glasses are also calculated, which have not been reported for this off-eutectic composition previously. The critical temperature (Tc) of Cu50Zr45Al5 liquids is determined to be 874.7 K by investigating the self-diffusivity using the mode coupling theory. A dynamics cross-over is detected in the vicinity of Tc, which can be reflected by different diffusion mechanisms and a remarkable deviation from the Einstein-Stokes relation. The results further suggest a fragile to strong transition of Cu50Zr45Al5 liquids between 1500 K and 1300 K upon cooling, which may result from a drastic increase of stable clusters within this temperature range.

Rapid solidification and fracture behavior of ternary metastable eutectic Al 2O 3/YAG/YSZ in situ composite ceramic

Directionally solidified oxide ceramic eutectic in situ composites are promising candidate materials for high-temperature structural applications in oxidizing environments. The rapid solidification characteristics of ternary metastable eutectic composition of Al 2O 3/YAG/YSZ in situ composite ceramics grown by laser zone-remelting technique are investigated. The fracture toughness and crack propagation behavior of the as-solidified composites are examined by the Vickers indentation technique. The metastable eutectic Al 2O 3/YAG/YSZ ceramic consists of Al 2O 3, YAG and YSZ phases, showing a typical ultra-fine irregular lamellar microstructure morphology with eutectic spacing as fine as 0.5 μm. Moreover, the ternary eutectic microstructures with coupled growth characteristics are also formed at the off-eutectic composition, effectively enlarging the eutectic growth range. The mean fracture toughness of the metastable eutectic Al 2O 3/YAG/YSZ ceramic is 3.9 ± 1.0 MPa m 1/2. The addition of ZrO 2 which weakens the interface bonding between YAG and Al 2O 3 phases, crack deflection, branching and microcrack toughening are the predominant toughening mechanisms.

Investigation of the solidification behavior of Al 2O 3/YAG/YSZ ceramic in situ composite with off-eutectic composition

Directionally solidified Al 2O 3/YAG/YSZ ceramic in situ composite is an interesting candidate for the manufacture of turbine blade because of its excellent mechanical property. In the present study, two directionally solidified hypoeutectic and hypereutectic Al 2O 3/YAG/YSZ ceramic in situ composites are prepared by laser zone remelting, aiming to investigate the solidification behavior of the ternary composite with off-eutectic composition under high-temperature gradient. The results show that the composition and laser scanning rate significantly influence the solidification microstructure. The ternary in situ composite presents ultra-fine microstructure, and the eutectic interspacing is refined with the increase of the scanning rate. The Al 2O 3/YAG/YSZ hypoeutectic ceramic displays an irregular hypoeutectic network structure consisting of a primary Al 2O 3/YAG binary eutectic and fine Al 2O 3/YAG/YSZ ternary eutectic. Only at low scanning rate, homogeneous ternary eutectic-like microstructures are obtained in the hypoeutectic composition. Meanwhile, the Al 2O 3/YAG/YSZ hypereutectic ceramic shows homogeneous eutectic-like microstructure in most cases and the eutectic interspacing is finer than the ternary eutectic. Furthermore, the formation and evolution mechanism of the off-eutectic microstructure of the ternary composite are discussed.

Fabrication of dense material having homogeneous GdAlO 3–Al 2O 3 eutectic-like microstructure with off-eutectic composition by consolidation of the amorphous

Off-eutectic microstructures generally have both coarse crystals of rich component and ordinary eutectic microstructures. This paper shows a new method to form a dense bulk material having homogeneous eutectic-like microstructure with off-eutectic compositions. Mixture of Gd 2O 3 and Al 2O 3 powders with the off-eutectic composition was melted and quenched rapidly to form the amorphous phase. The amorphous film was pulverized. The dense bulk material could be fabricated by the consolidation of the amorphous powder using spark plasma sintering method. Field emission scanning electron microscope (FE-SEM) observation of the material showed homogeneous fine eutectic-like microstructure without coarse crystals. This is the first case that such material was successfully prepared.

Formation of GdAlO 3–Al 2O 3 composite having fine pseudo-eutectic microstructure

GdAlO 3 and Al 2O 3 powders were mixed and pulverized using ball mills. The prepared powder was sintered by SPS at 1450 °C without holding time. SEM observation of the sintered specimen showed a eutectic-like microstructure. This is called ‘pseudo-eutectic’ in this research. The microstructure formed from a powder pulverized by a tumbling ball mill for one week was much finer than that by a planetary ball mill for 5 and 10 h. The fine homogeneous eutectic-like (pseudo-eutectic) microstructures could be formed at both eutectic and off-eutectic compositions. In case of crystallization from a melt of eutectic components, homogeneous eutectic microstructures can be formed only at restricted compositions very close to the eutectic one. Coarse primary crystals generally exist in the eutectic microstructure at off-eutectic compositions. The pseudo-eutectic microstructures can be formed at any compositions because a mixing ratio of the starting powders can be varied.

Directional solidification of Al 2O 3–Al 2TiO 5 system

Directional solidification of Al 2O 3–Al 2TiO 5 eutectic system was investigated to design in situ composites that could exhibit optimized properties of strength and toughness. Directional solidification of an alumina rich melt was expected to produce primary Al 2O 3 dendrites, these would act as load bearing component of the structure, separated by a Al 2O 3–Al 2TiO 5 eutectic matrix and. Additional aim was to utilize the differential expansion coefficient between the two phases to produce microcracks and thus weak interfaces for load deflection. Three different melt compositions were investigated; two off-eutectic compositions, at 11, 26 mol% TiO 2, and one eutectic at 43.9 mol% TiO 2. The crystallized phases were different than as one would expect from the published phase diagram. Off-eutectics were composed of Al 2O 3 dendrites separated by a Al 6Ti 2O 13 matrix. Eutectic structure was composed of Al 2TiO 5 lamellae in an aluminum titanate matrix. This matrix consisted of a superstructure made of 5 cells of Al 6Ti 2O 13 for one cell of Al 2TiO 5 along [0 0 1] direction. Al 6TiO 13 phase decomposed by a eutectoid reaction into Al 2O 3 and Al 2TiO 5. Eutectoid reaction occurred during post heat treatment at 1400 °C and decomposition of Al 6Ti 2O 13 was completed after 10 h of annealing at 1500 °C. Based on these observations, a modification of the alumina rich part of Al 2O 3–TiO 2 phase diagram was proposed. A second intermediate compound, Al 6Ti 2O 13 is introduced as a high temperature phase. Two invariant points, a peritectic (L + Al 2O 3 → Al 6Ti 2O 13) and a eutectic (L → Al 6Ti 2O 13 + Al 2TiO 5) are also added between Al 6Ti 2O 13 and Al 2TiO 5.