Solidification Pathways Research Articles

Laser based directed energy deposition system for operando synchrotron x-ray experiments.

The adoption of metal additive manufacturing (AM) has tremendously increased over the years; however, it is still challenging to explain the fundamental physical phenomena occurring during these stochastic processes. To tackle this problem, we have constructed a custom metal AM system to simulate powder fed directed energy deposition. This instrument is integrated at the Cornell High Energy Synchrotron Source to conduct operando studies of the metal AM process. These operando experiments provide valuable data that can be used for various applications, such as (a) to study the response of the material to non-equilibrium solidification and intrinsic heat treatment and (b) to characterize changes in lattice plane spacing, which helps us calculate the thermo-mechanical history and resulting microstructural features. Such high-fidelity data are made possible by state-of-the-art direct-detection x-ray area detectors, which aid in the observation of solidification pathways of different metallic alloys. Furthermore, we discuss the various possibilities of analyzing the synchrotron dataset with examples across different measurement modes.

Morphological Heredity of Intermetallic Nb5Si3 Dendrites in Hypereutectic Nb-Si Based Alloys via Non-Equilibrium Solidification

For hypereutectic Nb-Si based alloys, primary Nb5Si3 phases typically grow in a faceted mode during equilibrium or near-equilibrium solidification, which damages the ductility and toughness. To address this issue, here we artificially manipulate the growth morphology of Nb5Si3 using electron beam surface melting (EBSM) and subsequent annealing treatments. Results show that such a non-equilibrium solidification pathway enables the transition from faceted growth to non-faceted dendritic growth of Nb5Si3, along with evident microstructure refinement, generation of metastable β-Nb5Si3 phases and elimination of chemical segregation. The transformation from β-Nb5Si3 to α-Nb5Si3 and Nb solid solution (Nbss) particles is triggered by the annealing treatment at 1450 °C for 5 h. Also, we find the annealing-mediated formation of inherited Nb5Si3 dendrites that maintain the dendritic morphology of the original as-solidified β-Nb5Si3 dendrites. This work thus provides a feasible routine to obtain thermally stable and refined α-Nb5Si3 dendrites in hypereutectic Nb-Si based alloys.

Composition dependence of thermophysical properties for liquid Zr–V alloys determined at electrostatic levitation state

The thermophysical properties of liquid Zr–V alloys covering a whole composition range were systematically measured by an electrostatic levitation technique. A series of maximum undercoolings from 150 to 386 K (0.2 TL) was achieved for 11 different liquid alloys under containerless state and radiative cooling conditions, where Zr83.5V16.5 and Zr20V80 alloys displayed the strongest undercooling ability. The densities of liquid Zr–V alloys were measured over a wide temperature range from overheated to undercooled states, and the results exhibited a linear dependence on temperature for all 11 compositions. Two typical solidification pathways were observed for hypoeutectic alloys. Except for a slowing down of decreasing tendency near a eutectic Zr57V43 alloy, the liquid densities of Zr–V alloys almost decrease linearly with increasing V content. Accordingly, the thermal expansion coefficients of Zr–V alloys were also derived from containerless measurements, and they showed an increasing tendency with V content. Since thermal radiative dominated the heat transfer process, the ratio of isobaric specific heat to hemispherical emissivity was directly deduced from the thermal balance equation, leading to a quadratic relationship with temperature. It was found that the increase of V content enhanced the ability of radiative heat dissipation below 16.5 at. % V content.

Preparation of Ti-46Al-8Nb Alloy Ingots beyond Laboratory Scale Based on BaZrO3 Refractory Crucible

The high Nb-containing TiAl-based alloy ingot beyond laboratory scale with a composition of Ti-46Al-8Nb (at.%) was prepared by a vacuum induction melting process based on a BaZrO3 refractory crucible. A round bar ingot with a diameter of 85 mm and a length of 430 mm was finally obtained, and the chemical composition, solidification pathway, microstructure and tensile properties of the ingot were investigated. The results show that the deviations of Al and Nb content along a 430 mm long central part of the ingot are approximately ±0.39 at.% and ±0.14 at.%, and the oxygen content in the ingot can be controlled at around 1000 ppm. The structure of the alloy ingot is a full lamellar structure composed of γ and α2 phases, and the thickness of the lamellae is approximately 0.53 μm. In case of the α2 phase, the surface content of the ingot is higher than the middle region and the centrical region; also, it indicated a decreasing trend. During cooling, the alloy solidified from a peritectic reaction (L + β→α) rather than the solidified via β phase (β→α). In addition to Al segregation and Nb segregation, β-phase particles associated with γ phase at the triple junction of the colonies were observed. Moreover, the tensile properties of the longitudinal-cut sample in the ingot is significantly better than those of the transverse-cut sample, with a tensile strength of up to as high as 700 MPa and a corresponding fracture elongation of 1.1%. However, the tensile strength of the transverse-cut sample is only 375 MPa, and the fracture elongation is 0.52%.

Evolution of the Microstructure during Hot Working of Gamma-Based TiAl Alloys

Abstract Cast products of two-phase titanium aluminides suffer from microsegregation of the alloying elements, which arises from the peritectic solidification pathway. The resulting variation in composition and microstructure occurs on a length scale in the range of 1 mm and, thus, cannot be tolerated for the anticipated applications of the materials. A significant structural and chemical consolidation can be achieved by homogenization and hot-working procedures, however, the processing experience is limited. This problem is addressed in the present paper in that the evolution of the microstructure of two-phase alloys after different stages of wrought processing is characterized by structural and chemical microanalysis involving scanning electron microscopy and energy dispersive X-ray analysis. The investigations involve different two-phase alloys in order to assess the effects of ternary and higher alloying elements on the finally obtained microstructure.

Re-Examination of the Microstructural Evolution in Undercooled Co-18.5at.%B Eutectic Alloy.

The undercooling (∆T) dependencies of the solidification pathways, microstructural evolution, and recalescence behaviors of undercooled Co-18.5at.%B eutectic alloys were systematically explored. Up to four possible solidification pathways were identified: (1) A lamellar eutectic structure consisting of the FCC–Co and Co3B phase forms, with extremely low ΔT; (2) The FCC–Co phase primarily forms, followed by the eutectic growth of the FCC–Co and Co2B phases when ΔT < 100 K; (3) As the ΔT increases further, the FCC–Co phase primarily forms, followed by the metastable Co23B6 phase with the trace of an FCC–Co and Co23B6 eutectic; (4) When the ΔT increases to 277 K, the FCC–Co phase primarily forms, followed by an FCC–Co and Co3B eutectic, which is similar in composition to the microstructure formed with low ΔT. The mechanisms of the microstructural evolution and the phase selection are interpreted on the basis of the composition segregation, the skewed coupled zone, the strain-induced transformation, and the solute trapping. Moreover, the prenucleation of the primary FCC–Co phase was also detected from an analysis of the different recalescence behaviors. The present work not only enriches our knowledge about the phase selection behavior in the undercooled Co–B system, but also provides us with guidance for controlling the microstructures and properties practically.

Solidification Pathways in Highly Undercooled Co79.3B20.7 Alloy

Solidification Pathways in Highly Undercooled Co79.3B20.7 Alloy

The Effect of Fe Content on the Solidification Pathway, Microstructure and Thermal Conductivity of Hypoeutectic Al–Si Alloys

In the present study, the solidification pathway, microstructure and thermal conductivity of Fe-rich Al–x Si (x = 5, 7, 9, wt%, the same below) alloys were systematically investigated. The results showed that the thermal conductivity was firstly improved and then decreased with Fe content increasing. The improvement and decreasing rate of thermal conductivity was related to the Si content. The change of thermal conductivity was closely related to the change of microstructure and solidification pathway. The addition of Fe element resulted to the precipitation of ternary β (Al5SiFe) phase (βT) and binary β phase (βB). The precipitation of βT was benefited to improve the thermal conductivity. On the contrast, the precipitation of βB was prone to deteriorate the thermal conductivity. The precipitation amount of βT phase and βB phase was related to the critical value of Fe content (Fecrit). The calculation results showed that the Fecrit of Al–x Si alloys was about 0.20% (5% Si), 0.29% (7% Si) and 0.38% (9% Si), respectively. When the Fe content was less critical value (Fecrit), the precipitation amount of βT phase was increased by Fe increasing. Furthermore, when the Fe content was more than Fecrit, the precipitation amount of βT phase was changed slightly, and the precipitation amount of βB phase continued to increase with Fe content increasing.

Experimental investigation of phase equilibria in the Mg-rich corner of Mg-Nd-Sc system

Phase equilibria in the Mg-rich corner of Mg–Nd–Sc ternary system was studied by using the equilibrated alloys method. The partial isothermal sections at 500 °C, 530 °C, and 550 °C were constructed by analyzing the phase constitution and the chemical compositions of the present phases in 19 alloy samples. The Mg3(Mg, Nd, Sc) phase exhibited noticeable ternary solubility introduced by the solid solution of Mg and Sc in the Mg3Nd lattice. (Mg_hcp) showed a narrow homogeneity range paralleling to Mg–Sc binary boundary, whereas Mg41Nd5 showed negligible ternary solubility at all three temperatures. The three-phase equilibrium region of (Mg_hcp) + Mg41Nd5 + Mg3(Mg, Nd, Sc) dominated the Mg-rich corner and was surrounded by three two-phase equilibrium regions constituted by any two of above three phases. The primary crystals and the solidification pathways of alloys in the Mg-rich corner were analyzed via the as-cast microstructure. The matrix phases of (MgSc_bcc) and/or (Mg_hcp) together with the precipitations of Mg3(Mg, Nd, Sc) and other ordering structures could provide additional space to optimize microstructure and properties of Sc-alloyed Mg–Nd alloys.

Effect of La Addition on Solidification Behavior and Phase Composition of Cast Al-Mg-Si Alloy

The current study focusses on the phase composition, solidification path, and microstructure evaluation of gravity cast Al-4Mg-0.5Si-xLa aluminum alloy, where x = 0, 0.1, 0.25, 0.5, 0.75, and 1 wt.% La. A computational CalPhaD approach implemented in Thermo-Calc software and scanning electron microscopy technique equipped with electron microprobe analysis (EMPA) was employed to assess its above-mentioned characteristics. The thermodynamic analysis showed that the equilibrium solidification path of La-containing Al-Mg-Si alloys consists of only binary phases LaSi2 and Mg2Si precipitation along with α-Al from the liquid and further solid-state transformation of this mixture into α-Al + Al11La3 + Mg2Si + Al3Mg2 composition. Scheil–Gulliver simulation showed a similar solidification pathway but was accompanied by an increase in the solidification range (from ~55 °C to 210 °C). Furthermore, microstructural observations were congruent with the calculated fraction of phases at 560 °C and related to α-Al + LaSi2 + Mg2Si three-phase region in terms of formation of La-rich phase having both eliminating effect on the eutectic Mg2Si phase. Quantitative EMPA analysis and elemental mapping revealed that the La-rich phase included Al, La, and Si and may be described as Al2LaSi2 phase. This phase shows a visible modifying effect on the eutectic Mg2Si phase, likely due to absorbing on the liquid/solid interface.

Dynamic observation of MoSiBTiC alloy phase transitions using in situ ultrahigh-temperature X-ray diffraction measurement

In situ ultrahigh-temperature XRD measurement of a MoSiBTiC alloy was carried out using a laser-heated diamond anvil cell and synchrotron radiation. This technique revealed the complicated dynamic behavior of phase transitions in the MoSiBTiC alloy on heating above 1950 °C. The MoSiBTiC alloy initially consisted of Mo solid solution (Moss), Mo5B2Si (T2), TiC, Mo2B, Mo3Si, and Mo2C phases. The Mo2B, Mo3Si, and Mo2C phases disappeared on heating at ~1647 °C. The Moss grains coarsened during heating and the Moss, TiC, and T2 phases were present >1950 °C. The obtained results verified the solidification pathway of the MoSiBTiC alloy.

Influence of nano-Y2O3 addition on microstructure and tensile properties of high-Al TiAl alloys

The effect of nano-Y2O3 addition on microstructure and tensile properties of high-Al TiAl alloys prepared by induction skull melting is investigated. The result shows that the addition of nano-Y2O3 affords an obvious refinement in lamellar colony due to the heterogeneous nucleation. A crystallographic model is used to predict the α and γ nucleation potencies of Y2O3 particles during solidification. Besides, Y2O3 induces the solidification pathway of TiAl alloys into high-Al side, resulting in the increase of γ phase. Owing to fine grain strengthening and second phase strengthening, the tensile strength and elongation of TiAl–Y2O3 alloys are enhanced significantly at room temperature.

A rational interpretation of solidification microstructures in the Mg-rich corner of the Mg–Al–La system

A range of alloys in the Mg rich corner of the Mg–Al–La ternary system have been cast to experimentally analyse the solidification microstructure. Discrepancies between our experimental observations and thermodynamic liquidus projection predictions were observed and revised solidification pathways have been confirmed in this study. The range of compositions that solidify with Mg as the primary phase is significantly smaller than what has been calculated by Pandat database (Mg 2018) but quite similar to that predicted by FactSage (FTlite). The revised solidification pathways are shown to be a useful tool for practical Mg–Al–La alloy design and avoiding the presence of the Mg17Al12 phase which has been associated with poor creep resistance of Mg–Al alloys.

Solidification pathways and phase equilibria in the Mo–Ti–C ternary system

AbstractThe liquidus surface projection and isothermal section at 1,800°C in the Mo–Ti–C ternary system are examined using arc-melted alloys. A ternary transition peritectic reaction (L + Mo2C → Mo + TiC) takes place during solidification, which is apparently different from the ternary eutectic reaction (L → Mo + TiC + Mo2C) observed in a previous report. Since the composition of the eutectic reaction (L → Mo + TiC) shifts toward the Mo–Ti binary line with increasing Ti concentration, the volume fraction of the Mo phase and the interlamellar spacing of the Mo and TiC phases increase in the eutectic microstructure. At 1,800°C, the TiC phase in equilibrium with the Mo phase can contain more than 28 at% Mo and a Mo/TiC/Mo2C three-phase region exists at around Mo–15Ti–10C.

Cooling curve thermal analysis, phase constitution, and solidification characteristics of cast Mg–5Gd–2Y–xNd–Zr alloys

Mg–RE alloys are high-strength magnesium alloys with great application potential. The solidification pathways and microstructure formation of most Mg–RE alloys are still under discussion. In the present research, phase constitution and solidification characteristics of Mg–5Gd–2Y–xNd–Zr (x = 0, 2, 4, 6) alloys were investigated using a Computer-Aided Cooling Curve Analysis (CA-CCA) system, together with microstructure observation and phase identification methods including SEM, EDS, and XRD. The results show that the solidification microstructure of Mg–5Gd–2Y–Zr alloy was composed of α-Mg and Mg24RE5 phase. And the addition of Nd leads to the formation of β phase through a eutectic reaction, L → Mg24RE5 + α-Mg + β. Combining the CA-CCA and microstructure observation results, the non-equilibrium solidification route and solid fraction–time relationship of Mg–5Gd–2Y–xNd–Zr (x = 0, 2, 4, 6) alloys was summarized. Besides, hot tearing susceptibility of the investigated alloys was evaluated according to their solid fraction–time relationship using Clyne–Davies’ criterion.

Liquid-liquid phase separation in immiscible Cu-Co alloy

In this study, the solidification pathways and the generation of nano-sized Co-rich phase in metastable immiscible Cu-Co alloys with different undercoolings were systematically elucidated. For undercooling above the metastable miscibility gap, solidification commenced in a normal way with the nucleation and growth of primary α-Co dendrites. For undercooling below the metastable miscibility gap, primary LLPS and even secondary LLPS took place. Nano-sized Co-rich phase was found to be easy precipitated from the Cu-rich phase after solidification in both cases. The results provide a possibility for the adjustment of immiscible alloys possessing desired properties by nano-sized phases.

Cost-affordable, high-performance Ti–TiB composite for selective laser melting additive manufacturing

Abstract

Mechanism exploration on the aluminum supplementation coupling the electrokinetics-activating geopolymerization that reinforces the solidification of the municipal solid waste incineration fly ashes

Traditional cementation technique is insufficient in making municipal solid waste incineration (MSWI) fly ashes meet the permissible leaching threshold. Aluminum supplementation and electrokinetic (EK) activation were combinedly incorporated into the traditional solidification pathway to enhance the geopolymerization and the immobilization of Pb and Cd in MSWI fly ashes in this study. The aluminum addition remarkably affected the geopolymer formation. The minimum toxicity leaching as well as the maximum compressive strength were achieved at the combination of the voltage gradient of 1.0 V/cm, the proposing time of 48 or 72 h, the mass ratio of alkali activator to fly ash of 11.5%, and the modulus of 2.1. Chloride reduction in the mortar obtained during the EK process increased the negative charge relativity of oligomers. The leaching concentrations of Pb and Cd from the geopolymer were successfully predicted by a linear model based on the compressive strengths at 28 d. Higher reaction degree was found in the EK-activated mortar in the geopolymerization kinetics. Aluminum supplementation had induced the production of some amorphous aluminosilicate minerals including Al6Si2O13, CaAl2Si2O8·4H2O, Ca2Al3(Si3O12)OH, Ca2Al(OH)7·3H2O, and Ca4Al2O6Cl2·10H2O during the EK process. Larger particles observed in the EK-treated specimen directly verified the EK-activated pozzolanic reactions.

Dynamic Observation of MoSiBTiC Alloy Phase Transitions Using &lt;i&gt;In&lt;/i&gt; Situ Ultrahigh-Temperature X-Ray Diffraction Measurement

In situ ultrahigh-temperature XRD measurement of a MoSiBTiC alloy was carried out using a laser-heated diamond anvil cell and synchrotron radiation. This technique revealed the complicated dynamic behavior of phase transitions in the MoSiBTiC alloy on heating above 1950 oC. The MoSiBTiC alloy initially consisted of Mo solid solution (Moss), Mo5B2Si (T2), , Mo2B, Mo3Si, and Mo2C phases. The Mo2, Mo3Si, and Mo2C phases disappeared on heating at ~1647 °C. The Moss grains coarsened during heating and the Moss, TiC, and T2 phases were present >1950 °C. The obtained results verified the solidification pathway of the MoSiBTiC alloy.

Study of solidification pathway of a MoSiBTiC alloy by optical thermal analysis and in-situ observation with electromagnetic levitation

MoSiBTiC alloys are promising candidates for next-generation ultrahigh-temperature materials. However, the phase diagram of these alloys has been unknown. We have developed an ultrahigh-temperature thermal analyser based on blackbody radiation that can be used to analyse the melting and solidification of the alloy 67.5Mo–5Si–10B–8.75Ti–8.75 C (mol%). Furthermore, electromagnetic levitation (EML) was used for in-situ observation of solidification and microstructural study of the alloy. On the basis of the results, the following solidification pathway is proposed: Mo solid solution (Moss) begins to crystallize out as a primary phase at 1955 °C (2228 K) from a liquid state, which is followed by a (Moss+TiC) eutectic reaction starting at 1900 °C (2173 K). Molybdenum boride (Mo2B) phase precipitates from the liquid after the eutectic reaction; however, the Mo2B phase may react with the remaining liquid to form Moss and Mo5SiB2 (T2) as solidification proceeds. In addition, T2 also precipitates as a single phase from the liquid. The remaining liquid reaches the (Moss + T2 + TiC) ternary eutectic point at 1880 °C (2153 K), and the (Moss + T2 + Mo2C) eutectic reaction finally occurs at 1720 °C (1993 K). This completes the solidification of the MoSiBTiC alloy.