Primary Solidification Phase Research Articles

Acid transformation of bauxite residue: Conversion of its alkaline characteristics

Bauxite residue (BR) is a highly alkaline solid hazardous waste produced from bauxite processing for alumina production. Alkaline transformation appears to reduce the environmental risk of bauxite residue disposal areas (BRDAs) whilst potentially providing opportunities for the sustainable reuse and on-going management of BR. Mineral acids, a novel citric acid and a hybrid combination of acid-gypsum treatments were investigated for their potential to reduce residue pH and total alkalinity and transform the alkaline mineral phase. XRD results revealed that with the exception of andradite, the primary alkaline solid phases of cancrinite, grossular and calcite were transformed into discriminative products based on the transformation used. Supernatants separated from BR and transformed bauxite residue (TBR) displayed distinct changes in soluble Na, Ca and Al, and a reduction in pH and total alkalinity. SEM images suggest that mineral acid transformations promote macro-aggregate formation, and the positive promotion of citric acid, confirming the removal or reduction in soluble and exchangeable Na. NEXAFS analysis of Na K-edge revealed that the chemical speciation of Na in TBRs was consistent with BR. Three acid treatments and gypsum combination had no effect on Na speciation, which affects the distribution of Na revealed by sodium STXM imaging.

Change in the primary solidification phase from fcc to bcc-based B2 in high entropy or complex concentrated alloys

An examination of a compositionally graded AlxCuCrFeNi2 high entropy alloy (HEA) or complex concentrated alloy (CCA), revealed that marginally increasing Al content from x=0.8 to x=1.0 (+6at.%) changes the primary solidification phase from a simple disordered-fcc to a bcc-based ordered-B2 phase. Subsequently, a second solidification product forms, a disordered-bcc in case of x=0.8 and a disordered-fcc in case of x=1.0. Solid-state decomposition within these phases results in fcc+L12 and bcc+B2 products, accompanied by compositional partitioning. These results provide new insights into the influence of Al on the primary solidification product, and have been rationalized using a computational thermodynamic approach.

Assessment of structural, thermal, and mechanical properties of portlandite through molecular dynamics simulations

The structural, thermal, and mechanical properties of portlandite, the primary solid phase of ordinary hydrated cement paste, are investigated using the molecular dynamics method. To understand the effects of temperature on the structural properties of portlandite, the coefficients of thermal expansion of portlandite are determined in the current study and validated with what reported from the experimental tests. The atomic structure of portlandite equilibrated at various temperatures is then subjected to uniaxial tensile strains in the three orthogonal directions and the stress-strain curves are developed. Based on the obtained results, the effect of the direction of straining on the mechanical properties of portlandite is investigated in detail. Structural damage analysis is performed to reveal the failure mechanisms in different directions. The energies of the fractured surfaces are calculated in different directions and compared to those of the ideal surfaces available in the literature. The key mechanical properties, including tensile strength, Young's modulus, and fracture strain, are extracted from the stress-strain curves. The sensitivity of the obtained mechanical properties to temperature and strain rate is then explored in a systematic way. This leads to valuable information on how the structural and mechanical properties of portlandite are affected under various exposure conditions and loading rates.

A Novel Method for Semi-Solid Casting of Hypereutectic Gray Cast Iron in Expendable Mold

In the present research, the possibility of single stage semi-solid casting of a hypereutectic gray cast iron in a sand mold was investigated. For this purpose, a specially designed chilled sprue sand mold rigged to a vertical centrifugal casting machine was used. The molten cast iron was poured in the chilled sprue sand mold at 1275 oC at a rotation speed of 400 rpm. Two control samples were also poured at 1275 oC under gravity and centrifugal casting conditions in the same mold without sprue chilling. Microstructures and mechanical properties of the specimens were then characterized. The results showed that the average length and thickness of the primary solid phase, i.e. the Kish graphite, and those of the eutectic flake graphite were smaller in semi-solid centrifuged castings and their hardness were higher than those of the control samples. The results are discussed in terms of the shearing and chilling effects of the designed sprue during the initial stages of pouring.

Microstructual evolution and stability of second generation single crystal nickel-based superalloy DD5

The microstructual evolution and stability of a second generation single crystal (SC) nickel-based superalloy DD5 with minor grain boundary (GB) strengthening elements (C, B and Hf) were studied as a function of as-cast, heat treatment and thermal exposure. The microstructure and composition of the alloy were investigated by optical microscopy, scanning electron microanalysis (SEM), electron probe microanalysis (EPMA), energy dispersive spectrometry (EDS) and extraction analysis. In the as-cast condition, the microstructure observations and composition analysis showed that γ phase was the primary solidification phase and there were three microsegregations in the metal matrix. The morphology of these microsegregations depended on element segregations. After heat treatment, the dendrite cores contained fine and cuboidal-shaped γ′ particles with an average edge length of about 0.5 μm, while interdendritic regions contained irregularly-shaped γ′ particles and MC/M23C6 carbides. The mass fraction of γ′ phases was 61.685%. After exposure at 980 °C for 1000 h, no TCP phase was observed in both dendritic and interdendritic regions, indicating a good microstructual stability of the DD5 alloy at 980 °C.

Microstructures and mechanical properties of Co2MoxNi2VWx eutectic high entropy alloys

In this paper, Co2MoxNi2VWx (x=0.5, 0.6, 0.8, 1.0, 1.5, and 1.75 in molar ratio) eutectic high entropy alloys (EHEAs) were designed to achieve a combination of strength and ductility. The effects of Mo and W elements on the microstructures and mechanical properties of the alloys were investigated, and results showed that by decreasing the Mo and W contents to 0.8 in Co2MoxNi2VWx alloy, a fully eutectic microstructure was obtained. By increasing the Mo and W contents to 1.75, a primary BCC solid solution phase plus eutectic microstructure were obtained. The eutectic microstructure consisted of a mixture of FCC solid solution phase and μ phase. The Co2Mo0·8Ni2VW0.8 alloy with the largest volume fraction of eutectic microstructure exhibited the highest compression strength of 2364MPa, while the Co2Mo1·75Ni2VW1.75 alloy with the largest volume fraction of BCC phase exhibited the highest yield strength of 1607MPa.

Liquidus projection of Co-Sb-Yb ternary system

Co-Sb-Yb alloys at the Sb-rich corner were prepared. Their primary solidification phases were determined. Co-Sb-Yb liquidus projection is proposed based on the experimental results at the Sb-rich corner and the phase diagrams of its three constituent binary systems. There is no ternary phase in the Co-Sb-Yb system. The primary solidification phases along the Co-Sb, Sb-Yb and Yb-Co binary sides are Co, CoSb, CoSb2, CoSb3, Sb, YbSb2, YbSb, Yb11Sb10, Yb4Sb3, α-Yb5Sb3, β-Yb5Sb3, Yb, Yb2Co17, YbCo3 and YbCo2, respectively.

Liquidus Projections of Bi-Se-Ga and Bi-Se-Te Ternary Systems

This study determines the liquidus projections of both Bi-Se-Ga and Bi-Se-Te ternary systems which are constituent ternary systems of promising Bi-Se-Te-Ga thermoelectric materials. Ternary Bi-Se-Ga and Bi-Se-Te alloys are prepared. Their primary solidification phases are experimentally determined, and thermal analysis experiments are carried out. The liquidus projections are determined based on the ternary experimental results and phase diagrams of constituent binary systems. The Bi-Se-Ga system includes seven primary solidification phases, Bi, Ga, GaSe, Ga2Se3, Se, Bi2Se3, and (Bi2)n(Bi2Se3)m. In the Bi-Se-Te system, there are five primary solidification phases, Bi, (Bi2)n(Bi2Te3)m, Bi2(Se,Te)3, (Se,Te), and (Bi2)n(Bi2Se3)m. Both the (Bi2)n(Bi2Te3)m and (Bi2)n(Bi2Se3)m phases are not a single phase, but a collection of series undetermined phases. Large miscibility gaps are observed in the Bi-Se-Ga system. The temperatures of the invariant reactions, Liquid + Bi + GaSe = Ga and Liquid + Ga2Se3 = Bi + GaSe, are at 495 K (222 °C) and 533 K (260 °C), respectively.

Geochemical characterization of engineered coal-combustion byproducts (CCBs): Occurrence and mobility of trace elements, implications for interactions with acidic and ambient groundwater

Fixed scrubber sludge (FSS), is a coal-combustion byproduct composed of flue gas desulphurization sludge, fly ash, and quick lime. FSS has potential usages as structural fill and capping materials in mine land reclamation because of its low permeability and acid-neutralization capacity. Sediment cores extracted from an abandoned mine land reclamation site where FSS was used as a hydrologic barrier to groundwater recharge were analyzed in an effort to quantify the chemical phases (both primary and secondary) that constitute the FSS, and to evaluate any evidence of weathering that has occurred during the 18years since emplacement. The primary solid phase is amorphous spheroids. Four different types were identified and their quantitative compositions were obtained, with SiO2, Al2O3, and FeO being the primary components. Of special concern was the occurrence and mobility of trace elements within the solid phases. Sequential extractions using different chemical reagents were conducted to analyze the concentrations of As, B, Ba, Cd, Cr, Cu, Ni, Pb, and Zn in different fractions as indications of mobility. The highest concentration of the trace elements was contained in the residual fraction, which is consistent with the amorphous spheroids being very resistant to chemical weathering. The results also suggest that the mobility of the trace elements decrease in the following order: B, Cu>Cd, Zn, Pb, Cr>Ba>Ni>As. Comparison of samples collected from the weathered edges and unweathered portions of the FSS layer reveal that a more aggressive weathering environment exists at the upper boundary of the FSS, where it is in contact with oxygenated soil water. The quantitative chemical compositions, distribution of trace elements, as well as evidence of how trace elements are released provide background data for the development a reactive transport model to simulate long-term effects of placing FSS in the natural environment; and particularly in areas that are being subjected to abandoned mine land reclamation.

Liquidus projection and isothermal section of the Pb–Sb–Se ternary system

Pb–Sb–Se and its base alloys are promising thermoelectric materials. Its liquidus projection and the 400 °C (673.2 K) isothermal section are determined. There are eight primary solidification phases. In addition to the three terminal phases, (Pb), (Sb) and (Se), there are PbSe, Sb2Se3, PbSb2Se4, Pb6Sb6Se17, and PbSb5Se6 phases. The PbSb5Se6 compound was found for the first time in this study. There are nine invariant reactions involving liquid. The reactions with the lowest and highest reaction temperatures are Liquid = PbSe + (Sb) + (Pb) at 250 °C (523.2 K) and Liquid + PbSe + (Sb) = PbSb5Se6 at 578.3 °C (851.5 K), respectively. There are nine tie-triangles in the 400 °C isothermal section including PbSe + (Sb) + Liquid, PbSe + PbSb2Se4 + (Sb), PbSb2Se4 + PbSb5Se6 + (Sb), PbSb2Se4 + PbSb5Se6 + Sb2Se3, PbSb5Se6 + Sb2Se3 + (Sb), PbSe + Pb6Sb6Se17 + PbSb2Se4, PbSe + Pb6Sb6Se17 + Liquid, Pb6Sb6Se17 + PbSb2Se4 + Sb2Se3, and Pb6Sb6Se17 + Sb2Se3 + Liquid.

The system Na2CO3–CaCO3–MgCO3 at 6 GPa and 900–1250°C and its relation to the partial melting of carbonated mantle

ABSTRACTIn order to constrain the Na2CO3–CaCO3–MgCO3 T–X diagram at 6 GPa in addition to the binary and pseudo-binary systems we conducted experiments along the Na2CO3–Ca0.5Mg0.5CO3 join. At 900–1000°C, melting does not occur and isothermal sections are presented by one-, two- and three-phase regions containing Ca-bearing magnesite, aragonite, Na2CO3 (Na2) and Na2(Ca1–0.9Mg0-0.1)3-4(CO3)4-5 (Na2Ca3-4), Na4(Ca1–0.6Mg0–0.4)(CO3)3 (Na4Ca), Na2(Ca0-0.08Mg1–0.92)(CO3)2 (Na2Mg) phases with intermediate compositions. The minimum melting point locates between 1000°C and 1100°C. This point would resemble that of three eutectics: Mgs–Na2Ca3–Na2Mg, Na2Mg–Na2Ca3–Na4Ca or Na2Mg–Na4Ca–Na2, in the compositional interval of [45Na2CO3·55(Ca0.6Mg0.4)CO3]–[60Na2CO3·40Ca0.6Mg0.4CO3]. The liquidus projection has seven primary solidification phase regions for Mgs, Dol, Arg, Na2Ca3, Na4Ca, Na2 and Na2Mg. The results suggest that extraction of Na and Ca from silicate to carbonate components has to decrease minimum melting temperature of carbonated mantle rocks to 1000–1100°C at 6 GPa and yields Na-rich dolomitic melt with a Na# (Na2O/(Na2O + CaO + MgO)) ≥ 28 mol%.

Determination of the enthalpy of fusion and thermal diffusivity for ternary Cu60−xSnxSb40 alloys

The liquidus and solidus temperatures, enthalpy of fusion, and the temperature dependence of thermal diffusivity for ternary Cu60−xSnxSb40 alloys were systematically measured by DSC and laser flash methods. It is found that both the liquidus temperature and the enthalpy of fusion decrease with the rise of Sn content, and their relationships with alloy composition were established by polynomial functions. The thermal diffusivity usually drops from the solid toward the semi-solid state. The undercoolability of those liquid Cu60−xSnxSb40 alloys with primary Cu2Sb solid phase is stronger than the others with primary β(SnSb) intermetallic compound, and the increase of cooling rate facilitates further undercooling. Microstructural observation indicates that both of the primary Cu2Sb and β(SnSb) intermetallic compounds in ternary Cu60−xSnxSb40 alloys grow in faceted mode, and develop into coarse flakes and polygonal blocks.

Microstructure and Thermal Analysis of As-Cast Ag-Bi-Ni alloys

The calculated liquidus projection of the Ag-Bi-Ni ternary system has been experimentally examined. Alloys were prepared by induction melting, and their microstructure studied by scanning electron microscopy coupled with energy dispersive x-ray spectroscopy. Of the primary solidification phases, (Ni) solidifies over the largest concentration range, although it was found to be narrower than calculated. The range in which Bi3Ni is the primary solidification phase was found to be broader than calculated. Also, the liquid miscibility gap is broader than predicted from assessed thermodynamic parameters. Differential thermal analysis was used to study temperatures of phase transitions of as-cast alloys, and recorded temperatures of melting of Bi3Ni and BiNi phases in ternary alloys agree well with those calculated.

Co-In-Sb Ternary System (I): Isothermal Sections and Liquidus Projection

The Co-In-Sb ternary system has thermoelectric application interests. Co-In-Sb ternary alloys are prepared in this study. Their equilibrium phases at 1123.15 K and 923.15 K (850 °C and 650 °C) as well as their primary solidification phases and univariant reaction temperatures are determined. The isothermal sections at 1123.15 K and 923.15 K (850 °C and 650 °C) and the liquidus projection of the Co-In-Sb ternary system are determined based on the ternary experimental results, the phase diagrams of its three constituent binary systems, and the limited phase equilibria literature. No ternary compounds are found. The phase relationships at both 1123.15 K and 923.15 K (850 °C and 650 °C) are the same. There are five phases, seven two-phase regions, and three tie-triangles which are Co-CoSb-liquid, CoSb-CoSb2-liquid, and CoSb2-CoSb3-liquid. In the liquidus projection, there are nine primary solidification phase regions, Co, CoSb, CoSb2, CoSb3, Sb, InSb, In, CoIn2, and CoIn3. There are seven invariant reactions which are two Class I, four Class II, and one Class III.

Effect of Chromium on Microstructure and Mechanical Properties of Nb–Si Hypoeutectic and Eutectic Alloys

The niobium silicide based alloys have been found to be the best alternative over Ni-based superalloys as they provide the balance between both ductility and fracture toughness at low temperature and strength and creep resistance at elevated temperature. The concept of making Nb-silicide based alloys is to form hard silicide phase which is thermodynamically stable and imparts adequate strength to the material at high temperature, within the primary ductile solid solution phase `of niobium. However, the main concern of using these alloys at high temperature is their low oxidation resistance and strength at high temperature. To overcome these limitations, alloying elements can be added, which will improve mechanical properties as well as high temperature oxidation resistance of the alloys. Our present investigation is to study the effect of chromium on the microstructure of Nb–Si hypoeutectic and eutectic alloys. Cylindrical rods of pre-set composition are synthesized by vacuum-arc-melting-cum-suction casting using argon atmosphere. The binary hypoeutectic alloys show the presence of primary (Nb)ss dendrites and lamellar eutectic between (Nb)ss and Nb3Si whereas primary Nb3Si along with eutectic (Nb)ss/Nb3Si are observed in the eutectic binary alloy. In the hypoeutectic Nb–Si–Cr ternary alloys, primary dendritic (Nb)ss, silicide (Nb3Si and/β-Nb5Si3), Laves (Cr2Nb)(depends on chromium concentration) and their eutectic phases are observed whereas Laves phase is found in the microstructure of eutectic Nb–Si–Cr ternary alloys even at very low concentration of Cr (2 at.%). At room temperature, compressive properties of these rods have been reported.

Phase diagrams of the Ag–In–Se photovoltaic material system

Ternary Ag–In–Se alloys are promising photovoltaic materials. The liquidus projection and the 450 °C isothermal section of the ternary Ag–In–Se system are determined. The primary solidification phases include three terminal solid solution phases, (Ag), (In), and (Se), nine binary intermediate phases, β-(Ag3In), ζ-(Ag3In), γ-(Ag9In4), φ-(AgIn2), β-(Ag2Se), In2Se3, In6Se7, InSe, and In4Se3, and two ternary compounds, AgInSe2 and AgIn5Se8. There are fourteen invariant reactions involving liquid, and seven of them are experimentally determined in this study. The reactions with the highest and lowest reaction temperatures are L + (Ag) ↔ AgInSe2 + β-(Ag2Se) at 631.7 °C and L ↔ φ-(AgIn2) + InSe + (In) at 142.8 °C, respectively. In the 450 °C isothermal section, there are eleven tie-triangles, which are (Ag) + β-(Ag2Se) + AgInSe2, (Ag) + AgInSe2 + InSe, (Ag) + ζ-(Ag3In) + InSe, ζ-(Ag3In) + InSe + Liquid, In4Se3 + InSe + Liquid, AgInSe2 + InSe + AgIn5Se8, InSe + In6Se7 + AgIn5Se8, AgIn5Se8 + In6Se7 + In2Se3, AgIn5Se8 + In2Se3 + Liquid, AgInSe2 + AgIn5Se8 + Liquid and β-(Ag2Se) + AgInSe2 + Liquid.

Formation of Icosahedral Phase in Bulk Glass Forming Ti-Zr-Be-Cu-Ni Alloy

Formation of an icosahedral phase in the bulk glass forming alloy during crystallization from amorphous phase and solidification from melt is investigated. The icosahedral phase with a size of 10 to 15 nm forms as a thermodynamically stable phase at intermediate temperature during the transformation from amorphous to crystalline phases such as Laves and -(Ti-Zr) phases, indicating that the existence of the icosahedral cluster in the undercooled liquid. On the other hand, the icosahedral phase forms as a primary solidification phase even though the Laves phase is stable at high temperature, which is can be explained based on the high nucleation rate of icosahedral phase relative to that of competing crystalline Laves phase due to lower interfacial energy between icosahedral and liquid phases.

Liquidus projection of Cu–In–Se photovoltaic material system

Cu–In–Se (CIS) and its based alloys are among the most important solar cell materials. However, there are only limited Cu–In–Se phase equilibria data available. This study determines the liquidus projection of the Cu–In–Se system. Cu–In–Se alloys were prepared from pure constituent elements. The primary solidification phases of as-cast Cu–In–Se alloys were determined. The univariant lines of Cu–In–Se liquidus projection were constructed according to the experimental results determined in this study, those in the literature and the phase diagrams of its three binary constituent systems. There are 15 primary solidification phases in the ternary Cu–In–Se liquidus projection: β1-Cu2Se, Cu, β2-Cu4In, γ-Cu7In3, η′-Cu2In, Cu11In9, In, In4Se3, InSe, In6Se7, In2Se3, Se, CuSe2, CuSe and CuInSe2 phases. A large miscibility gap extends from the Cu–Se side to the In–Se side. There are 16 ternary invariant reactions in this Cu–In–Se liquidus projection involved in the liquid phase. The temperatures of invariant reactions were determined using differential thermal analysis.

Solidification process and effect of thermal treatments on Ni–Co–Mn–Sn metamagnetic shape memory alloys

The solidification process of three metamagnetic Ni–Co–Mn–Sn alloys has been investigated by DTA and DSC calorimetry, X-ray diffraction, optical and electron microscopy. The as-cast microstructures are composed of three phases: austenite L21/6-layered martensite, D03 phase and γ phase (disordered fcc). Subsequent annealing treatments at 900°C completely dissolve the D03 phase in the three alloys and also the γ phase in alloys with low Co content (6.4 and 7.2at.%). However, for 9at.% Co the stability of γ-phase extends to temperatures close to the melting point and becomes very sensitive to the Mn and Sn content. No traces of the B2–L21 order–disorder transition have been obtained in the alloys investigated. The multiphase solidification can be interpreted as a peritectic process where D03 is the primary solidified phase with dendritic growth, followed by the peritectic reaction of this phase with the residual liquid to form the austenite L21, whereas the γ-phase precipitates from the austenite L21 upon further cooling.

Experimental study and thermodynamic assessment of the Zr–Al–Gd system

The Zr–Al–Gd system, as a key constituent ternary in Cu-, Gd- and Zr-based bulk metallic glasses, was thermodynamically studied in this work. Employing equilibrated alloys, the isothermal section at 873K was established by means of scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray diffraction. Phase transformation temperatures were determined by performing differential scanning calorimetry analysis on selected samples. Moreover, the microstructure of the as-cast samples was observed to identify primary solidification phases. Based on all experimental information of this work, the Zr–Al–Gd system was thermodynamically optimized with the assistance of the first-principles calculation. The calculated isothermal section, vertical sections and liquidus projection account for experimental observations satisfactorily.