Primary Solidification Research Articles

Phase diagrams of Bi–Sb–Se–Te system

Bi–Sb–Se–Te is one of the most important material systems for thermoelectric applications. Phase diagrams of its constituent binary and ternary systems are reviewed and assessed. The Bi–Sb–Se–Te isothermal section tetrahedron at 400 °C and liquidus projection tetrahedron were proposed. Ternary compounds are only found in the Bi–Sb–Se system. There are eight three-phase regions at 400 °C and seven primary solidification phases in the Bi–Sb–Se system, including (Bi,Sb), (Bi2)m(Bi2Se3)n, Bi2Se3, Se, Sb2Se3, Bi3Sb5Se2, Bi3Sb12Se15. In the Bi–Sb–Te system, there are four three-phase regions at 400 °C. The (Bi,Sb)2Te3 and (Bi,Sb) are continuous solid solutions. There are six primary solidification phases, including (Bi,Sb)2Te3, (Te), γ, δ, (Bi,Sb), and (Bi2)m(Bi2Te3)n. In the Bi–Se–Te system, there is one three-phase region. (Bi2)m(Bi2Se3)n and (Bi2)m(Bi2Te3)n form a continuous solid solution phase at 400 °C. There are four primary solidification phases, including Bi, (Bi2)m(Bi2(Se,Te)3)n, Bi2(Se,Te)3 and (Se,Te). In the Sb–Se–Te system, there are five three-phase regions and six primary solidification phases, including (Sb), δ-(Sb2Te), γ-(SbTe), Sb2Te3, and (Se,Te). A wide range of (Bi,Sb)2(Se,Te)3 single-phase region was observed in the Bi–Sb–Se–Te quaternary system.

Phase diagrams of the thermoelectric Bi–Sb–Se system

Bi–Sb–Se is an important thermoelectric material system. However, a phase diagram of the entire compositional range is not available in the literature. This study determines the Bi–Sb–Se liquidus projection and its phase equilibria isothermal section at 400 °C. Ternary Bi–Sb–Se alloys are prepared. Their primary solidification phases and invariant reactions are determined. There are seven primary solidification phases, (Se), Bi2Se3, Sb2Se3, (Bi,Sb), (Bi2)m(Bi2Se3)n, Bi3Sb5Se2 and Bi3Sb12Se5. Both Bi3Sb5Se2 and Bi3Sb12Se5 are newly found ternary compounds. In the Bi–Sb–Se isothermal section at 400 °C, there are eight three-phase regions. Besides these two ternary compounds, the other single phases are, Sb2Se3, Bi2Se3, (Bi2)m(Bi2Se3)n, Liquid (Bi,Sb), Liquid (Se), and (Bi,Sb) phases. It has been found the solubilities of Sb in the (Bi2)m(Bi2Se3)n and Bi2Se3 compounds are significant.

Experimental investigation and thermodynamic optimization of the Co–Ta–Zr system

This work is focused on a deeper understanding of the solidification microstructure of the as-cast alloys, as well as the phase constituents and phase fractions of annealed samples in the Co–Ta–Zr system. Nine primary solidification regions were experimentally determined in the liquidus surface projection, and three primary solidification regions were deduced based on binary phase diagrams. Six three-phase equilibria were observed in the isothermal sections at 1373 and 1273 K. The maximum solubilities of Zr in μ and CoTa2 were determined to be ∼ 6.1 and ∼ 6.7 at% at 1373 K, and ∼ 3.9 and ∼ 5.9 at% at 1273 K, respectively. The maximum solubilities of Ta in Co23Zr6 and CoZr were determined to be ∼ 3.3 and ∼ 14.1 at% at 1373 K, and in Co23Zr6, CoZr and CoZr2 were determined to be ∼ 2.6, ∼ 13.5 and ∼ 11.9 at% at 1273 K, respectively. The Co–Ta–Zr system was optimized by CALculation of PHAse Diagram (CALPHAD) method based on the experimental data. Four solution phases, liquid, bcc, fcc, and hcp, were described as substitutional solution. The thermodynamic models of intermetallic phases Co11Zr2, Co23Zr6, CoZr2, Co3Ta, CoTa2, λ1, λ2 and λ3 were treated as (Co)11(Ta,Zr)2, (Co)23(Ta,Zr)6, (Co,Zr)(Ta,Zr)2, Co3(Co,Ta), (Co,Ta,Zr)(Co,Ta,Zr)2, (Co,Ta)2(Co,Ta), (Co,Ta,Zr)2(Co,Ta,Zr) and (Co,Ta)2(Co,Ta) using a two-sublattice model, respectively. CoZr3 was described as (Co,Zr)(Co,Zr)Zr2 by a three-sublattice model. μ phase was modeled as (Co,Ta,Zr)1(Ta,Zr)4(Co,Ta,Zr)2(Co,Ta,Zr)6 using a four-sublattice model. And CoZr with a B2 structure was described as an ordered phase of bcc_A2, and the model was treated as (Co,Ta,Zr,Va)0.5(Co,Ta,Zr,Va)0.5(Va)3. A set of self-consistent thermodynamic parameters of the Co–Ta–Zr system was obtained using CALPHAD method. The calculated solidification paths and phase fractions can provide theoretical guidance for material design and thermal processing.

Study of the Structure and Properties of 30CrMnNiMo Steel Subjected to Vibration and Heat Treatment

The paper considers the effect of vibration exposure on changes in the structure of 30KhGSNMA steel after steel treatment by vibration during primary crystallization. Studies have shown that after vibration exposure under certain modes, the contamination index decreases, the grain score decreases, the structure becomes more homogeneous. The second part of the work is devoted to the study of the possibility of using normalization as a replacement for the improvement of 30KhGSNM steel subjected to vibration during primary crystallization. Melt samples of 30KhGSNM steel were subjected to vibration and then heat treatment, which consisted of normalization under different modes. At the end of the heat treatment, the hardness, tensile strength, and impact strength were evaluated. The microstructure of the samples was also studied using the Thixomet Pro software. The analysis showed that, depending on the cooling medium, the dispersion of perlite changes during normalization, which affects the hardness. Based on the studies carried out, it was concluded that for steel subjected to vibration during the primary solidification process, it is possible to use normalization with air-water cooling instead of improvement. The properties of the prototypes are comparable to those after improvement

Investigations on microstructure, oxidation, and tribological behavior of NiCoCrAlYTa/Y2O3 blade tip protective coating produced by electro spark deposition over a wide temperature range

NiCoCrAlYTa/Y2O3 coatings were prepared on two nickel-based superalloys by electro spark deposition (ESD). The ESD coatings “inherited” the microstructure of the substrate, which depended on the orientation of the substrate and the primary solidification phase of the coating. Columnar crystal structure had fewer grain boundaries, better high-temperature bearing capacity, and a more uniform “glaze” layer than equiaxed crystal structure, which allowed it to exhibit better oxidation resistance and wear resistance. The competition between the lubrication/wear reduction of the “glaze” layer and the plowing caused by hard oxide particles influenced the tribological behaviors of both. The difference in cutting performance between the two coatings is related to the oxidation behavior of the coating and the structure of the “glaze” layer.

Thermodynamic reassessment of Fe–Nb–V system

Thermodynamic reassessment of the ternary Fe–Nb–V system was carried out after a critical review of the available experimental and ab initio data. The primary solidified phases of Fe–Nb–V alloys were investigated using scanning electron microscopy and X-ray diffraction analysis. The C14 Laves phase exhibits a wide primary solidification field in the Fe-rich region, which is in contradiction with the experimental observation in the literature. Ab initio calculations were performed to obtain the enthalpies of formation of three intermetallic compounds (i.e., C14 Laves, μ, and σ), which were used to estimate the thermodynamic parameters. The end-member parameters for the three phases were improved in comparison with the previous description. The presently obtained thermodynamic description of the ternary system can reproduce the wide homogeneity range of the C14 Laves phase and the phase equilibria between the intermetallic compounds and bcc phase.

Experimental investigation and thermodynamic description of the Fe–Hf–Nb system

According to solidification microstructure of as-cast alloys and phase constituents of annealed samples, the liquidus surface projection and phase relationships at 1373 and 1273 K in the Fe–Hf–Nb system were constructed. A ternary phase τ with AlCuHf-structure was found, and the composition ranges of Nb in τ at 1373 and 1273 K were ∼ 20.5–30.8 at.% and ∼ 20.8–30.2 at.%, respectively. Moreover, the maximum solubilities of Nb in FeHf2 was determined to be ∼ 16.9 at.% and ∼ 16.0 at.% at 1373 and 1273 K, and the maximum solubilities of Hf in μ was measured to be ∼ 4.6 at% and ∼ 3.3 at.% at 1373 and 1273 K. Fe2Hf and Fe2Nb with the same C14 structure formed a homogeneous solid solution from Fe2Hf to Fe2Nb at 1373 and 1273 K. Seven primary solidification regions in the liquidus surface projection and two three-phase regions and seven two-phase regions in the isothermal sections at 1373 and 1273 K were experimentally determined. On the basis of experimental data, the thermodynamic parameters of individual phases in the Fe–Hf–Nb system were optimized by CALculation of PHAse Diagram (CALPHAD) method. Four solution phases (liquid, bcc, fcc, and hcp) were described as the substitutional solution. The intermetallic phases FeHf2, λ1, λ2, λ3 and τ were modeled as (Fe)1(Hf,Nb)2, (Fe,Hf,Nb)2(Fe,Hf,Nb)1, (Fe)2(Hf)1, (Fe)2(Hf,Nb)1, and (Fe,Hf,Nb)1(Fe,Hf,Nb)1 by a two-sublattice model, respectively. μ was described as (Fe,Hf,Nb)1(Hf,Nb)4(Fe,Hf,Nb)2(Fe,Hf,Nb)6 by a four-sublattice model. A set of self-consistent thermodynamic parameters of the Fe–Hf–Nb system was obtained.

Control of microstructure and shape memory properties of a Fe-Mn-Si-based shape memory alloy during laser powder bed fusion

• Microstructure modification by varying the laser scan velocity is demonstrated for a Fe-Mn-Si based shape memory alloy fabricated by laser powder bed fusion. • Pronounced Mn evaporation affects the primary solidification phase and, thus, the phase fraction in the samples. • Phase-transformation bcc-δ → fcc-γ induced by the intrinsic heat treatment refines the microstructure and reduces the crystallographic texture. • Microstructural changes result in variation of the alloy's shape memory and mechanical properties. Fe-Mn-Si shape memory alloys are materials, whose functional properties strongly depend on microstructural factors such as grain size and orientation, phase fraction and chemical composition. The present study demonstrates the possibility of microstructure modification via process parameter variation on a Fe-Mn-Si based shape memory alloy fabricated by laser powder bed fusion. By varying the scan speed, samples characterized by coarse elongated grains with strong <001> orientation along the build direction or by finer equiaxed grains without preferential orientation can be fabricated. Changes in the volume phase fraction of bcc-δ ferrite and fcc-γ austenite are also introduced by selective Mn evaporation. A direct correlation between the generated microstructure and achieved mechanical and shape memory properties is found.

Synergistic effect of Ti, B, Si, and C on microstructure and mechanical properties of as-cast Al0.4Co0.9Cr1.2Fe0.9Ni1.2(Si, Ti, C, B)0.375 complex concentrated alloy

The synergistic effect of Ti, B, Si, and C on microstructure and mechanical properties of as-cast Al0.4Co0.9Cr1.2Fe0.9Ni1.2(Si, Ti, C, B)0.375 complex concentrated alloy (CCA) was studied. The as-cast alloy was prepared by vacuum induction melting in a ceramic crucible followed by a tilt casting to a graphite mould. The effect of alloying elements on the primary solidification phase, solidification path, and phase transformation temperatures is evaluated. The solidification of the alloy starts with the growth of columnar FCC(A1) (face-centred cubic crystal structure A1) dendrites and is finalised by the formation of lamellar FCC(A1) + (Ni,Co,Fe)16(Ti,Cr)6(Si,Al)7 eutectic in the interdendritic region. During solidification, Al, Ti, B, Si, and C segregate preferentially to liquid, which leads to the formation of BCC(B2) (ordered body-centred cubic crystal structure B2), BCC(A2) (body-centred cubic crystal structure A2), FCC(A1), (Ni,Co,Fe)16(Ti,Cr)6(Si,Al)7, Cr2B, and TiC phases in the interdendritic region. The highly anisotropic microstructure of tensile specimens prepared from the as-cast alloy consists of columnar FCC(A1) dendrites (89 vol%) oriented at an angle ranging from 75 to 90 degrees to the longitudinal axis of specimens and multiphase interdendritic region (11 vol%). The tensile deformation curves show only the strain hardening stage at test temperatures up to 700 °C. The strain hardening coefficient decreases from 0.20 to 0.17 by increasing the test temperature from 20° to 700°C. At temperatures ranging from 725° to 900°C, the tensile curves show the strain hardening stage, which is followed by the strain-softening stage up to a fracture. The strain softening stage results from dynamic recrystallisation of FCC(A1) dendrites and fracture of brittle high elastic modulus phases in the interdendritic region.

Segmentation of tomography datasets using 3D convolutional neural networks

Dendritic microstructures are ubiquitous in nature and are the primary solidification morphologies in metallic materials. Techniques such as X-ray computed tomography (XCT) have provided new insights into dendritic phase transformation phenomena. However, manual identification of dendritic morphologies in microscopy data can be both labor intensive and potentially ambiguous. The analysis of 3D datasets is particularly challenging due to their large sizes (terabytes) and the presence of artifacts scattered within the imaged volumes. In this study, we trained 3D convolutional neural networks (CNNs) to segment 3D datasets. Three CNN architectures were investigated, including a new version of FCDenseNet which we extended to 3D. We show that using hyperparameter optimization (HPO) and fine-tuning techniques, both 2D and 3D CNN architectures outperform the previous state of the art. The 3D U-Net architecture trained in this study produced the best segmentations according to quantitative metrics (intersection-over-union of 95.56% and a boundary displacement error of 0.58 pixels), while 3D FCDense produced the smoothest boundaries and best segmentations according to visual inspection. The trained 3D CNNs are able to segment entire 852 × 852 × 250 voxel 3D volumes in only ∼60 s, thus hastening the progress towards a deeper understanding of phase transformation phenomena such as dendritic solidification.

Computational simulation of thermo-mechanical field and fluid flow and their effect on the solidification process in TWIP steel welds

The primary austenitic solidification mode of Twinning Induced Plasticity (TWIP) steel weld joints is one of the most important issues limiting its weldability. In order to gain knowledge on this topic, a comprehensive study was carried out through numerical simulation of diffusive-convective phenomena of fluid flow and heat transfer of the melted material in the weld pool of TWIP steel weld joints. Weldments were performed using different heat inputs and constraint conditions. The predictions of convective motion, thermal history and cooling rates estimated by finite element (FE) and finite volume (FV) simulations were correlated with chemical species segregation (Mn, C, Al and Si) and the solidification mode in the fusion zone. The microstructural characterization of weld joints and the numerical results provided a full analysis of the weld pool solidification process. Results revealed a relationship between both the dendrite size and Mn segregation with the estimated velocity field of liquid material in the melted zone. Moreover, the presence of δ-ferrite was experimentally detected in some weld joints and associated with the convective movement of melted material and chemical segregation. The cellular dendritic growth mode found in some weld joints contributed to the formation of δ-ferrite and promoted the transition of the solidification mode from austenitic to ferritic-austenitic. As well δ-ferrite allowed to mitigate hot-cracking and interrupted the Mn-enriched liquid film migration towards the heat-affected zone, avoiding the formation of liquation cracks. The presence of δ-ferrite phase maintained a high mechanical strength providing improved elongation regarding the welded material mechanical properties.

Characterization of the microstructure and mechanical properties of activated tungsten inert gas welded and hot-wire tungsten inert gas welded super austenitic stainless steel AISI 904L

Super austenitic stainless steel AISI 904L is a premium stainless steel variant known for its superior properties and ability to retain the same in highly challenging conditions. This study investigates the feasibility of welding 10-mm thick plates of AISI 904L through activated tungsten inert gas welding and hot-wire tungsten inert gas welding methods. The fabricated joints are then subjected to solidification mode study, ferrite number analysis, microstructural characterization, and mechanical tests. Results of the solidification mode study indicated primary austenitic solidification in both the weld joints. The ferrite number measurement revealed that the hot-wire tungsten inert gas weld joint had a higher δ-ferrite content of 1.58 over 0.99 of the activated tungsten inert gas weld joint. A microstructure study confirmed the presence of refined equiaxed grains in the fusion zone of the hot-wire tungsten inert gas weld joint, while the activated tungsten inert gas weld joint displayed equiaxed grains crammed between the columnar grains and dendrites. Liquid penetrant inspection of bend-tested samples proved the soundness and quality of both weld joints. The hot-wire tungsten inert gas weld joint excelled over the activated tungsten inert gas weld joint with a higher microhardness value. During tensile tests, activated tungsten inert gas and hot-wire tungsten inert gas weld joints displayed average ultimate tensile strength values of 521 and 514 MPa, respectively. The impact toughness of the activated tungsten inert gas weld joint was 248 J, while it was 220 J for the hot-wire tungsten inert gas weld joint. Field emission scanning electron microscope analysis equipped with energy-dispersive spectroscopy and elemental color mapping provided details about weld microstructure and the reasons for the material's failure. The study proved the efficiency and soundness of the 10-mm thick joints of AISI 904L fabricated by activated tungsten inert gas and hot-wire tungsten inert gas welding techniques.

Experimental investigation and thermodynamic description of the Co–Nb–Ti system

The solidification microstructures of as-cast alloys and the equilibria microstructure of annealed alloys at 1273 K in the Co–Nb–Ti system were determined by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD), and the liquidus surface projection and partial isothermal section at 1273 K were constructed. Nine primary solidification regions, bcc(Nb,Ti), fcc(Co), Co3Ti, CoTi, CoTi2, λ1, λ2, λ3, and μ and five ternary invariant reactions, liq. → λ1 + λ2 + μ, liq. → CoTi + λ2 + μ, liq. → bcc(Nb,Ti) + CoTi + μ, liq. + fcc(Co) + λ3 → Co3Ti, and liq. + CoTi → bcc(Nb,Ti) + CoTi2 were experimentally determined or deduced in the liquidus surface projection. Three three-phase regions, CoTi + λ2 + μ, bcc(Nb,Ti) + CoTi + μ, and bcc(Nb,Ti) + CoTi + CoTi2 and six two-phase regions, Co3Ti + λ3, λ2 + μ, CoTi + λ2, CoTi + μ, bcc(Nb,Ti) + μ, and bcc(Nb,Ti) + CoTi at 1273 K were experimentally determined. The continuous compound λ2 with a C15 structure from the Co–Nb side to Co–Ti side was formed at 1273 K. The solubilities of Ti in μ and Nb in CoTi were determined to be ~11.1 at% and ~18.4 at% at 1273 K, respectively. And no new phases were found. According to the available experimental data from the present work and literatures, the Co–Nb–Ti system was optimized by CALPHAD (CALculatiuon of PHAse Diagram) method. Four solution phases (liquid, bcc, fcc, and hcp) were described using the substitutional solution model. A two-sublattice model (Co,Nb,Ti)2(Co,Nb,Ti)1 was used to described three compounds λ1, λ2, and λ3 according to their crystal structures and homogeneity ranges. Co7Nb2 and CoTi2 were treated as Co7(Nb,Ti)2 and Co(Nb,Ti)2 by a two-sublattice model, respectively. μ was treated as (Co,Nb,Ti)1(Nb,Ti)4(Co,Nb,Ti)2(Co,Nb,Ti)6 by a four-sublattice model. Co3Ti with L12 structure and CoTi with B2 structure were treated as the ordered phases of fcc and bcc solutions using the thermodynamic models (Co,Nb,Ti)0.75(Co,Nb,Ti)0.25Va1 and (Co,Nb,Ti,Va)0.5(Co,Nb,Ti,Va)0.5Va3, respectively. A set of self-consistent thermodynamic parameters of the Co–Nb–Ti system was obtained.

Effects of processing conditions on the solidification and heat-affected zone of 309L stainless steel claddings on carbon steel using wire-directed energy deposition

Directed energy deposition and laser cladding technologies are suitable advanced manufacturing techniques for applying corrosion-resistant claddings to large carbon steel components. In this work, we clad 309L stainless steel wire onto carbon steel substrates and examine the effects of processing parameters (laser power and travel speed) on metallurgical bonding and microstructures. Wire dripping defects correlate to excessive heat input, while cracking, stubbing and delamination defects are linked with insufficient heat input. Spectroscopic scans of the weld microstructure show the amount of base metal mixing in the weld is a function of processing parameters and the number of cladding layers. In the first cladding layer, the weld metal is diluted with base metal ranging from 1.7 to 45.6 %; this amount dramatically decreases in the second layer (0–8.6 %), promoting primary ferrite solidification, which decreases solidification cracking susceptibility. Martensite forms in the heat-affected zone from the high cooling rates associated with laser processing. Subsequent laser passes temper the martensite and carbon diffuses from the substrate into the first cladding layer. Cladding two or more layers is beneficial for mitigating defects, lowering dilution, and producing a highly alloyed surface suitable for corrosion protection.

Experimental determination of the isothermal sections and the liquidus surface projection of the Mo-Si-Cr ternary system

The isothermal sections at 1200 and 1000 ℃ and the liquidus projection of the Mo-Cr-Si system have been experimentally determined based on the microstructures, the phase structure analyses and the phase constitutions of the as-cast and the isothermally heat treated alloys in whole composition range, using scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques. In the isothermal section at 1000 and 1200 °C, five three-phase regions and ten two-phase regions were derived, and no ternary compounds were observed. Nine primary solidification regions and six univariant reaction lines were deduced based on the study on the solidification processes of the as-cast alloys in the liquidus surface projection. In addition, the stoichiometry compounds Mo5Si3 and αCr5Si3 are the phases of the same structure (W5Si3-type) but different compositions, forming two intermetallic compounds Mo-rich Mo(Cr)5Si3 and Cr-rich Cr(Mo)5Si3. The present experimental results could be used for the further thermodynamic assessment of the Mo-Si-Cr ternary system, and be helpful for the database development of the multi-components Nb/Mo-Si-based superalloy system.

Phase equilibria and liquidus projection determination of the Hf–Si–V ternary system

The phase equilibria including the isothermal sections at 1100 and 1300 °C as well as the liquidus surface projection of the Hf–Si–V ternary system were experimentally determined. The microstructures, crystal structures and phase constitutions of the annealed and the as-cast alloys in whole composition range were investigated by means of scanning electron microscopy equipped with energy dispersive spectroscopy, X-ray diffraction and electron probe microanalysis, respectively. The isothermal sections of the Hf–Si–V system at 1100 and 1300 °C were accordingly constructed. Fifteen three-phase regions were confirmed, the ternary compound HfVSi was identified, and the solubilities of the third element in the binary compounds were determined. In the meanwhile, based on the XRD data obtained in the present work and by means of the JADE software package, the lattice parameters of each compound are calculated. Furthermore, the solidification paths of the Hf–Si–V as-cast alloys were analyzed to verify the primarily solidified phases from the liquid phase. Fifteen primary solidification regions were finally confirmed in the liquidus surface projection. Particularly, transmission electron microscopy results revealed that the binary stoichiometric compound V6Si5 extended into the Hf–Si–V ternary system, forming the line compound (V,Hf)6Si5. With the decrease of temperature, the line compound (V,Hf)6Si5 can decompose into two phases, one of which V(Hf)6Si5 has the composition near V6Si5, another near Hf2V4Si5, but with the same structure type. The Hf2V4Si5 phase is proposed to form due to the miscibility gap of the line compound (V,Hf)6Si5. The present experimental results are useful for the thermodynamic assessment of the Hf–Si–V ternary system, and the database development for the Nb/Mo–Si–V–Hf containing multi-component superalloy materials.

Solidification behavior and enhanced properties of semi-solid Al–8Si–0.5Fe alloys fabricated by rheo-diecasting

Compared with the high pressure diecasting, rheo-diecasting has an advantage in decreasing defects, refining microstructure and improving properties. In the present work, an Al–8Si–0.5Fe alloy with a good combination of mechanical property and thermal conductivity was obtained by rheo-diecasting. The solidification behavior, mechanical properties, and thermal conductivity of the alloy under rheo-diecasting were discussed. The results demonstrated that the chilling effect and shearing vibration not only affected the primary solidification on the vibrating contraction sloping plate, but also the secondary solidification in the remaining liquid during rheo-diecasting. The rheo-diecasting alloy possessed spherical primary α1-Al grains and secondary α2-Al grains, small-sized Al5FeSi phases, eutectic Si phases, and low porosities, and thus exhibited the high mechanical properties and thermal conductivity. With decreasing pouring temperature from 690 °C to 670 °C, the size of primary α1-Al grains, secondary α2-Al grains, Al5FeSi phases and eutectic Si phases was decreased due to high cooling rate, which further improved the mechanical properties and thermal conductivity of the alloy.

Fast scanning calorimetry study of Al alloy powder for understanding microstructural development in laser powder bed fusion

We examined the variation in the solidification microstructure of AlSi10Mg alloy powder over a wide range of cooling rates controlled by using the fast scanning calorimetry (FSC) technique. Two exothermic peaks derived from the primary solidification of the α-Al phases and the α-Al/Si eutectic reaction were detected. These exothermic peaks became broader and shifted to lower temperatures at higher cooling rates. Focus ion beam (FIB) milling enables microstructural characterization of AlSi10Mg powder solidified at controlled cooling rates above 102 °C·s−1 in the FSC equipment. The sample solidified at 4 × 104 °C·s−1 exhibited a fine microstructure consisting of a primary α-Al phase surrounded by lamellar-shaped Si phases. The fraction of the Si phase continuously decreased with increasing cooling rate, indicating higher content of solute Si element in the α-Al phase solidified at a higher cooling rate. The secondary dendrite arm spacing (λ) of the primary α-Al phase decreased in the sample solidified at a higher cooling rate (dT/dt). The relation follows a general equation of λ = A (dT/dt)-n (A: 38.4, n: 0.33) in wide range of cooling rate (10-1 ∼ 104 °C·s−1). The results were utilized to discuss the cooling rate of Al alloys during the laser powder bed fusion (L-PBF) process.

Experimental study and thermodynamic modeling of the Ti–Cu–B system

The Ti–Cu–B ternary system has been systematically studied by experimentation and thermodynamically assessed combining the CALPHAD (CALculation of PHAse Diagram) approach and first-principles calculation. Using seven equilibrated alloys, the isothermal section at 1073 K was established by electron probe micro analyzer equipped with wavelength dispersive spectrometer and X-ray diffraction. Differential scanning calorimetry analysis was performed on all the samples to determine the phase transformation temperatures. Primary solidification phases were also identified from the micrographs of the as-cast samples. Based on all the experimental data obtained in this work, the thermodynamic description of the Ti–Cu–B system was reasonably developed and a set of self-consistent thermodynamic parameters was obtained. The calculated isothermal section, vertical section and liquidus projection account for the experimental data satisfactorily.

Experimental investigation and thermodynamic calculation of the Co–Sn–Bi ternary system

Phase relations of the Co–Sn–Bi ternary system have been studied experimentally for the whole composition range at 250 and 450 °C by means of scanning electron microscopy (SEM) coupled with wave dispersive spectrometer (WDS) and x-ray diffraction (XRD). Five three-phase regions have been confirmed in this system at 250 °C, and three three-phase regions exist in 450 °C. No ternary compound can be observed at these two temperatures. Moreover, Bi has almost no solubility in the binary compounds αCo3Sn2, CoSn, CoSn2 and CoSn3. The primary solidification phases were also distinguished. Based on the experimental results, the thermodynamic calculations for Co–Bi and Co–Sn–Bi system were conducted by using the CALPHAD technique. A set of self-consistent thermodynamic parameters for the Co–Sn–Bi ternary system was obtained, and the experiment results are in good agreement with the calculated results.