Primary Solidification Phase Research Articles

The microstructure of high manganese TWIP steels produced via simulated direct strip casting

Three TWIP steels based on the composition Fe-23Mn-3Si-3.3Al with different carbon concentrations were examined. The alloys were produced by rapid solidification in an experimental apparatus designed to simulate direct strip casting conditions. Examination of the driving force for solidification for this alloy showed the preference for BCC and FCC solidification were very close for this composition, within 10% of one another. This factor combined with the high undercooling resulted in the change in the primary solidified phase. Chemical analysis of the as-cast strip showed inter-dendritic segregation of Si and Al, but negligible segregation of Mn. A simple heat treatment was sufficient to chemically homogenise all alloys and produce microstructures comprising a fully austenitic structure, consistent with thermodynamic predictions.

Thermodynamic modeling of CaO-SiO2-VOx system by the generalized central atom model

With the aim of modeling the vanadium partitioning behavior in iron and steelmaking process, the steelmaking slags model/database was extended for covering the vanadium. In this paper, the binary CaO-VOx, SiO2-VOx and ternary CaO-SiO2-VOx systems were assessed and optimized based upon available phase equilibrium and thermodynamic data. The Generalized Central Atom (GCA) Model was used for describing the liquid slag phase while all intermediate vanadium containing solid phases are treated as stoichiometric compounds. During the optimization, effort was focused on the slag liquidus and its primary solid phases. The obtained model parameters can represent the reliable experimental data reasonably well. The sub-solidus regions at low temperature are not well assessed.

Back diffusion resisting solidification cracking in austenitic stainless steels

Austenitic stainless steels resist solidification cracking much better if ferrite (δ), not austenite (γ), is the primary solidification phase, but why? The curve of temperature T vs. fraction of solid fS was calculated for 304, which solidifies as primary δ-ferrite, and for 310, which solidifies as primary γ. Back diffusion, being much faster in bcc (δ) than in fcc (γ), flattens the curve much more in 304 than 310. Consistent with quenched 304 mushy-zone microstructure, back diffusion narrows the freezing temperature range, depletes the residual liquid, and lets δ dendrites bond together earlier to better resist cracking. It also reduces the crack susceptibility index |dT/d(fS )1/2| near fS = 1, consistent with the much less cracking observed in 304 than 310.

Mechanical property enhancement of the Ag–tailored Au–Cu–Al shape memory alloy via the ductile phase toughening

The Au–Cu–Al biocompatible shape memory alloys (SMAs) have attracted much attention due to the requirements of biomedical applications. However, brittleness remains a critical issue in the intermetallics of Au–Cu–Al alloys. In this study, to solve the dilemma of embrittlement, Ag was introduced into the Au–Cu–Al alloy to realize the ductile phase toughening (DPT) since the ductile disordered–fcc phase was predicted to precipitate in the brittle parent β phase based on the Ag–Au–Cu phase diagram. Microstructure observations, chemical composition analysis, crystal structure identifications, and thermal analysis revealed that the Au–28Cu–16Al–28Ag (mol.%) alloy was mainly composed of the Ag–rich α1 primary solidification phase with disordered–fcc structure and the eutectic structure of the α1 phase and the β phase with L21 structure. The ductile phase was successfully inserted into the brittle Au–Cu–Al intermetallic while the martensitic transformation temperature and crystal structures remained almost uninterrupted. In the cyclic loading–unloading tensile tests, the alloy mainly composed of the β phase failed in the early elastic region due to its embrittlement; while the alloy mainly composed of the α1 phase performed 29% in the total strain deformation. The Au–28Cu–16Al–28Ag alloy, which is composed of the β phase and the α1 phase, solve the issue of embrittlement; in addition, shape recovery was also found in this alloy during unloading. According to the microstructure observations, cyclic tensile tests, and fracture surface observations, the aforementioned improvement of strengthening and enhancement of ductility were brought from the insertion of the ductile α1 phase into the brittle β/β grain boundaries.

Experimental investigation and thermodynamic description of the Fe–Mo–Zr system

The phase relations at 1273 K and liquidus surface projection of the Fe–Mo–Zr system were investigated by means of electron probe micro-analyzer (EPMA), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD) methods. The composition range of C14 Laves phase was determined at 1273 K. The maximum solubility of Mo in C15–Fe2Zr, Mo in Fe23Zr6, Fe in C15–Mo2Zr and Zr in μ phase is about 4.8, 0.6, 17.7 and 4.6 at.% at 1273 K, respectively. The isothermal section at 1273 K of the Fe–Mo–Zr system on the whole composition ranges was constructed using 30 annealed alloys. In the liquidus surface projection, the primary solidification phase regions of bcc(Fe), C15–Fe2Zr, C14, μ, R, σ, bcc(Zr), C15–Mo2Zr and bcc(Mo) were experimentally confirmed using 31 as-cast alloys. Based on the experimental data in literature and the present work, the Fe–Mo–Zr system was optimized using CALPHAD method, and a set of self-consistent reliable thermodynamic parameters was obtained.

Solidification microstructures of multielement carbides in the high entropy Zr-Nb-Hf-Ta-Cx system produced by arc melting

Multielement, high entropy carbides are a new class of refractory materials typically manufactured by solid-state synthesis. Although high entropy alloys are usually produced by solidification of a melt, the solidification of high entropy carbides has not been investigated to date. Herein, we report the first arc melting study of the solidification microstructures of the Zr-Nb-Hf-Ta-Cx system. The results highlight the presence of elemental segregation in interdendritic regions whose composition depends on the carbon load (i.e., the composition of the primary solidification phase changes with the amount of carbon in the melt). Duplex microstructures containing two distinct multielement carbides, possessing hardness as high as 30 GPa, are obtained. Interestingly, under certain conditions, there is a perfect “crystallographic continuity” (no grain boundaries and cell parameter variations) between the two phases, though their composition differs. Arc melting produces new and exotic microstructures not observed in materials processed in the solid state.

Metallurgical joining of aluminium and copper using resistance spot welding: microstructure and mechanical properties

This paper investigates the melting phenomena, joining mechanism, microstructure evolution, and mechanical properties of the Al/Cu dissimilar joints made using resistance spot welding. A metallurgical bonding between Al and Cu is achieved via the reaction-diffusion between liquid Al and solid Cu (i.e. brazing mechanism). The reaction layer was featured by the in-situ formation of an ultra-thin Cu-rich Al–Cu intermetallic compound at the joint interface, on-cooling formation of coarse and thick θ-Al2Cu as the primary phase for hyper-eutectic solidification and formation of ultrafine lamellar α-Al/θ-Al2Cu eutectic structure. It is shown that the peak load and energy absorption of the Al/Cu joints are controlled by the effective length of the solid/liquid reaction zone and the thickness of the θ-Al2Cu primary solidification phase.

Directional Solidification of AlSi7Fe1 Alloy Under Forced Flow Conditions: Effect of Intermetallic Phase Precipitation and Dendrite Coarsening

A forced flow was experimentally shown to influence the solidification microstructure of metal alloys by modifying the coarsening/ripening law. In some technical alloys (AlSi7Fe1), this flow effect can also be significantly suppressed due to the formation of intermetallic precipitates (β-Al5FeSi) that can block the flow in the mushy region. The forced flow was induced by a rotating magnetic field (RMF). Herein, a three-phase volume-average-based solidification model is introduced to reproduce the above experiment. The three phases are the melt, the primary solid phase of columnar dendrites, and the second solid phase of intermetallic precipitates. The dynamic precipitation of the intermetallic phase is modelled, and its blocking effect on the flow is considered by a modified permeability. Dendrite coarsening, which influences the permeability, is also considered. The RMF induces a strong azimuthal flow and a relatively weak meridional flow (Ekman effect) at the front of the mushy zone during unidirectional solidification. This forced flow reduces the mushy zone thickness, induces the central segregation channel, affects the distribution of the intermetallic precipitates, and influences dendrite coarsening, which in turn modifies the interdendritic flow. Both interdendritic flow and the microstructure formation are strongly coupled. The modelling results support the explanation of Steinbach and Ratke—the formed intermetallic precipitates (β-Al5FeSi) can block the interdendritic flow, and hence influence the coarsening law. The distribution of β-Al5FeSi is dominantly influenced by the flow-induced macrosegregation. The simulation results of the Si and Fe distribution across the sample section are compared with the experimental results, showing good simulation–experiment agreement.Graphic During alloy solidifications the flow can influence the mushy zone by inducing macrosegregation, modifying the solidification microstructure, and influencing the formation of intermetallic precipitates. The resulting microstructural features can in turn affect the melt flow by changing the flow intensity and flow pattern. A three-phase volume-average-based solidification model is introduced to study the flow-solidification interaction, and hence to improve the knowledge on the formation mechanism of intermetallics and their effect on solidification. (a) Schematic for the flow pattern and formation of different phases; (b) experiment–simulation comparison of macrosegregation (Fe) across the diameter of as-solidified sample.

Microstructural evolution and mechanisms in additively manufactured AlCrCuFeNix complex concentrated alloys via selective laser melting

This article discusses the influence of the Ni content on the microstructures of AlCrCuFeNix complex concentrated alloys prepared by selective laser melting. The microstructural evolution mechanisms were also uncovered. It is found that the as-built Ni1.0 alloy possesses equiaxed-columnar bimodal grain structures and basket-weave microstructure consisting of striped body-centered cubic (bcc) nano-precipitates and ordered bcc (B2) phase. Adding Ni triggers the lattice transition from B2 to face-centered cubic (fcc), leading to the change of the primary solidification phase from B2 to fcc beyond the critical content of Ni. Meanwhile, the striped bcc nano-precipitates inside the B2 lamellae gradually transform into spherical morphologies due to the consumption of Fe and Cr solutes in eutectic reaction. Furthermore, the microstructures change from hypo-eutectic (Ni2.0, Ni2.5) to near-eutectic (Ni3.0) to hyper-eutectic (Ni3.5), accompanied by a columnar-to-equiaxed transformation due to the large constitutional supercooling induced by the eutectic reaction.

Effect of Hf addition on solidification and hot isostatically pressed microstructures of the Ti-48Al-2Cr-2Nb (at%) intermetallic alloy

The microstructural evolutions during solidification and hot isostatic pressing (HIP) of Ti-48Al-2Cr-2Nb-xHf (4822-xHf, x = 0, 2, 4, 6 at%) intermetallic alloys were investigated using both experimental observations and thermodynamic calculations. The primary solidification phase of all the studied alloys was β, albeit it was predicted by Thermo-Calc to be α for the alloys containing 4% and 6%Hf. In agreement with the Thermo-Calc calculations, the solidification path of the base 4822 alloy encountered a peritectic reaction. Although alloying by 2%Hf did not change the solidification path, further increase in Hf content caused an additional eutectic reaction following the peritectic one. The eutectic microstructure was composed of two orthorhombic phases with TiAl2 and Al3Hf2 compositions and this finding was the first evidence of such phases in a quinary alloy with Cr, Nb, and Hf. It was found that Hf acted as a weak β stabilizer as it showed a low partitioning tendency in β and the β single-phase field was not formed in the high-Hf alloys. A finer dendritic microstructure was characterized in the high-Hf alloys due to the Hf solubility limit in the 4822 alloy. Interestingly, α2 laths dissolved in the Hf-containing alloys after HIP treatment and instead rode-like precipitates of Al3Hf2 emerged at newly formed γ/γ interfaces. The precipitation mechanism of Al3Hf2 particles was discussed.

σ-Phase Formation in Super Austenitic Stainless Steel During Directional Solidification and Subsequent Phase Transformations

The solidification path and the σ-phase precipitation mechanism in the S31254 (UNS designation) steel are investigated thanks to Quenching during Directional Solidification (QDS) experiments accompanied by scanning electron microscopy observations and electron backscattered diffraction (EBSD) analysis. Considering experimental conditions, the γ-austenite is found to be the primary solidifying phase (1430 °C), followed by δ-ferrite (1400 °C, ≈ 87 pct solid fraction). The σ-phase appears in the solid-state through the eutectoid decomposition of the δ-ferrite: δ → σ + γ2 (1210 °C), whereas the σ-phase is predicted to form from the austenite at 1096 °C in equilibrium conditions. The resulting temperatures of solidification path and phase transformation are compared with Gulliver–Scheil model and equilibrium calculations predicted using Thermo-Calc© software. It is shown that the thermodynamics calculations agree with experimental results of solidification path. The EBSD analysis show that the δ-ferrite has δNW2 ORs with the σ-phase.

Effects of Ni powder addition on microstructure and mechanical properties of NiTi to AISI 304 stainless steel archwire dissimilar laser welds

In this study, dissimilar laser welding of NiTi to stainless steel (SS) orthodontic archwires was investigated. As a novel and more operational approach, Ni powder was used as a weld filler metal to improve the properties of joints in laser welding of these alloys. Therefore, welding was performed in two conditions, with and without Ni powder addition, and the results were compared. Optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and focused X-ray diffraction (micro-XRD) analysis were used to study the microstructure, chemical composition and phase characteristics of joints. The Fe-Ni-Ti ternary phase diagram was used as a tool to analyze the phase changes in weld zone with chemical composition variations in two conditions. Results showed that brittle intermetallics such as Fe2Ti and Cr2Ti were formed in zones close to NiTi weld interface in joints without Ni powder addition. However, Ni powder had favorable effects on microstructure, chemical composition and mechanical properties of laser welded joints. In fact, use of Ni powder changed primary solidifying phase region from Fe2Ti to ductile phases of Ni3Ti and γ, and consequently promotes the formation of these ductile phases in the weld zone. Moreover, as a result of chemical composition dilution by Ni, the microstructure of weld zone changed from dendrite to dendrite-cellular structure by suppressing the constitutional undercooling in Ni powder added condition. Furthermore, microhardness tests revealed that the addition of Ni powder caused the average hardness of weld zone to decrease from 580 HV down to 325 HV due to aforementioned intermetallic phase replacement. On the other hand, tensile properties of joints were improved and reached to a tensile strength of 300 MPa and a fracture strain of 2.9 % by adding Ni powder. These values for the welding without Ni powder addition were 150 MPa and, 1.8 %, respectively.

Experimental investigation and thermodynamic description of the Cr–Sn–Zn ternary system

The phase equilibria of the Cr–Sn–Zn ternary system have been systematically investigated via thermodynamic modeling and experimental analysis. The isothermal sections of the Cr–Sn–Zn system at 250 and 450 °C were characterized using scanning electron microscopy coupled with energy-dispersive spectroscopy and X-ray diffraction, the results of these two isothermal sections have confirmed the existence of the ternary compound T which is stable at 250 °C and dissolved at 450 °C. The primary solidification phases were also identified, and only two different primary solidification phases were found. The phase transformations were experimentally determined by using DSC method. On the basis of the present experimental results and available literature data, we carried out the thermodynamic calculations for this system using the CALPHAD technique. A set of self-consistent thermodynamic parameters for the Cr–Sn–Zn ternary system were obtained with good agreement between the experimental and calculated results, proving the validity of the present thermodynamic description.

Phase selection in hypercooled alloys

The effect of the hypercooling limit on the crystal growth velocity v as a function of the undercooling temperature ΔT for intermetallic binary and ternary alloys remains unexplained yet. In combination with the electrostatic levitation technique, we performed in-situ high energy synchrotron X-ray diffraction measurements at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, to investigate the phase formation of the binary alloy NiTi, the Zr-based ternary alloys (NixCu1−x)Zr (x=0.3,0.4) and the Ni-based ternary alloy Ni(Zr0.5Ti0.5) for solidification events at undercooling temperatures ΔT<ΔThyp and ΔT>ΔThyp, whereas ΔThyp are the corresponding hypercooling limits of the systems. Our results indicate that the cubic B2-type (CsCl-type, space group Pm3 m #221) phase is always the primary solidifying phase for all investigated alloys. This holds for all undercoolings obtained. Thus, the deceleration effect observed in the crystal growth velocity for all investigated alloys at their hypercooling limits seems to be caused by something different than the formation of different phases.

Structure and property modulation of primary CoSn and peritectic CoSn2 intermetallic compounds through ultrasonicating liquid Sn–10%Co alloy

The effects of power ultrasound on the peritectic solidification of binary Sn–10%Co peritectic type alloy were investigated. If introducing ultrasound in the whole solidification process, primary CoSn intermetallic compound which grew as coarse strips with a preferred grain orientation under static condition was considerably refined and exhibited a random orientation distribution. Moreover, the significant increase in thickness ratio of peritectic CoSn2 to primary CoSn compounds indicated that the peritectic transformation was remarkably enhanced. To further reveal the underlying mechanism, power ultrasound was respectively applied to primary phase growth and peritectic transformation stages. As compared with the latter one, the former procedure led to a finer primary CoSn phase and a higher peritectic transformation extent. The results showed that ultrasound induced refinement of primary CoSn phase played the dominant role, while ultrasound accelerated atomic diffusion between primary and peritectic solid phases may also contributed to the enhancement of peritectic transformation. Both the yield strength and ultimate compressive strength of Sn–10%Co alloy were improved after ultrasonic solidification.

Solidification processes of as-cast alloys and phase equilibria at 1 300 °C of the Nb–Si–V ternary system

Abstract The liquidus projection and the isothermal section at 1 300 °C of the Nb–Si–V ternary system were experimentally studied. Using scanning electron microscopy, electron probe microanalysis and X-ray diffraction, the primary solidification phases and the precipitation paths in each region of the liquidus projection as well as the equilibrium relations of the phases in the isothermal section were determined. Ten primary solidification regions were found in the liquidus projection and eight three-phase equilibrium regions were observed in the isothermal section at 1 300 °C. The compounds βNb5Si3 with V5Si3 and NbSi2 with VSi2 are the phases of the same structure but different compositions and form two linear compounds βNb(V)5Si3 or V(Nb)5Si3 and Nb(V)Si2 or V(Nb)Si2, respectively The ternary linear compound (Nb,V)2Si with the stoichiometry about 2 : 1 of (Nb + V) : Si was found in both the liquidus projection and the isothermal section at 1 300 °C.

Phase diagram of Bi–In–Se ternary system

Bi–In–Se ternary system is of thermoelectric application interests and its phase diagrams are fundamentally important. Experimental measurements and Calphad-type thermodynamic modeling are carried out to determine the phase diagrams of the Bi–In–Se ternary system. Bi–In–Se ternary alloys are prepared. The equilibrium phases at 250 °C and their primary solidification phases are determined. No ternary compound is observed. The primary solidification phases are Bi2Se3, Se, In2Se3, In6Se7, InSe, In4Se3, In, ε, BiIn2, Bi3In5, BiIn, Bi, and (Bi2)m(Bi2Se3)n. In the Bi–In–Se isothermal section at 250 °C, there are seven tie-triangles which are Liquid+(Bi)+InSe, Liquid + In4Se3+InSe, (Bi)+Bi6Se7+InSe, (Bi)+In6Se7+In2Se3, (Bi)+(Bi2)m(Bi2Se3)n + In2Se3, Bi2Se3+(Bi2)m(Bi2Se3)n + In2Se3 and Liquid + In2Se3 +Bi2Se3. The temperatures of invariant reactions are determined by thermal analysis, and the reaction scheme is determined. Calphad-type thermodynamic modeling of the Bi–In–Se ternary system is carried out based on the thermodynamic models of the three binary constituent systems, and experimental results of isothermal sections and liquidus projection. The calculated and experimentally determined Bi–In–Se phase diagrams are in good agreement.

Microstructure exploration and an artificial neural network approach for hardness prediction in AlCrFeMnNiWx High-Entropy Alloys

The phase evolution of AlCrFeMnNiWx (X= 0, 0.05, 0.1, 0.5) High-Entropy Alloys (HEAs) during the solidification is understood by both thermodynamic simulation and experimental approach. The detailed structural and microstructural characterization of studied HEAs reveals the presence of BCC Fe–Cr–Mn rich (β1) primary phase and BCC Ni–Al-rich (β2) secondary dendritic phase. It is found that both primary and secondary BCC solid solution phases undergo spinodal decomposition, forming BCC_B2 (α1) and σ phases as well as BCC_B2 (α2) and BCC (γ) phases respectively. Interestingly, the hardness of the HEAs varies in the range 461–552.7 HV with alloying of W. The present investigation also reports the prediction of the hardness of AlCrFeMnNiWx (x = 0, 0.05, 0.1, 0.5) HEAs with the composition variation of tungsten by applying artificial neural network using various experimental data as input parameters. A back-propagation artificial neural network (ANN) model is used by taking the experimental data to understand the effect of alloying elements on the hardness. The ANN modeling results match well with experimental data with the accuracy of prediction 93.54% and error of the predicted value of 6.46%.

Effect of Ni foil thickness on the microstructure of fusion zone during PHS laser welding

In this present, hot-rolled Al-Si coated press hardened steel (PHS) with 1.5 mm thick was laser tailor welded using Ni foil as an interlayer. The effect of Ni foil thickness on the microstructure evolution of fusion zone (FZ) was studied. With the Ni foil thickness increased, Al had a uniform profile in the FZ but Ni had a segregation in the fusion boundary. JMatPro analysis results showed that the fraction of δ-ferrite at high temperature was decreasing and the primary solidification phase was changed from δ-ferrite to γ phase. The fusion boundary and FZ center had the similar solidification phase evolution during laser welding. The microstructure was the composition of δ-ferrite and lath martensite when the Ni foil was 30 μm, but it was full martensite with the Ni foil increased to 60 μm. Austenite was present in the fusion boundary with the Ni foil increased to 100 μm but it was still lath martensite in FZ center.

ARTEC-A furnace module for directional solidification and quenching experiments in microgravity.

A new experimental design for directional solidification experiments with high cooling rates under microgravity conditions is presented. The aerogel-based furnace module ARTEC (AeRogel TEchnology for Cast alloys) developed at DLR extends the earlier presented sounding rocket facility ARTEX by enabling a transition from low to high solidification velocities and a simultaneous operation of five independent furnaces in the same sounding rocket module. The furnaces for directional solidification are equipped with thermally insulating aerogels as a crucible material. Their optical transparency allows the control of the solidification parameters (velocity and temperature gradient) with optical methods in the lab. In ARTEC, a drastically increased solidification velocity is achieved by contacting the sample with a movable cooling-rod during processing. Therefore, a better theoretical understanding of the influence of a sudden change in solidification velocity on microstructure formation is obtained. Carrying out experiments in microgravity gives access to purely diffusive solidification conditions. Hence, convection free-growth can be compared with growth subject to natural (earth) and/or forced-convection (earth and space). Furthermore, alloys with high density differences in their alloy components and, hence, also between the primary solidifying phase and the surrounding liquid can be studied without the negative influence of fluid-flow or macrosegregation being present.