Directional Solidification Of Alloys Research Articles

Effect of interface anisotropy on growth direction of tilted dendritic arrays in directional solidification of alloys: Insights from phase-field simulations

We investigate the effect of interfacial anisotropy on the growth direction selection during directional solidification of alloys by using the thin-interface phase-field model. A convergence study with respect to the coupling constant λ is carried out for the tilted growth of dendritic arrays with different values of anisotropic to choose proper λ in simulations. The influence of the artificial noise at the interface on the growth direction selection is discussed. By analyzing the data from two-dimensional phase-field simulations, we discuss the dependence of the coefficients f and g in DGP law (Deschamps et al., 2008) on anisotropic strength ε4 for a wide range of misorientation angle Θ0 in order to extend the DGP law. Results confirm that the coefficient f can be expressed as f(Θ0,ε4)≡α(ε4)χ(Θ0), where α(ε4) is an increasing function of ε4 and χ(Θ0) solely depends on Θ0 with a constant coefficient β. Meanwhile, g(ε4) is a decreasing function of ε4, which can be modeled by a power-law function. Moreover, we comment on the influence of the pulling velocity on the growth direction selection for a wider range of the pulling velocity.

Near-Eutectic Ternary Mo-Si-B Alloys: Microstructures and Creep Properties

In the present work, the microstructural evolution during the solidification of different near-eutectic Mo-Si-B alloys was investigated. The alloy compositions were chosen from the vicinity of the eutectic region with respect to published liquidus projections. The aim was to identify a eutectic alloy composition in the Mo-rich region of the system, which would be suitable for directional solidification (DS). In a second step, two alloy compositions were prepared via DS and first creep results of these near-eutectic DS alloys are presented and discussed.

Numerical simulation of dendritic growth in directional solidification of binary alloys using a lattice Boltzmann scheme

A lattice Boltzmann (LB) based model is utilized to simulate three dimensional dendritic growth in directional solidification of alloys. The LB–D3Q19 lattice vectors are used to describe advancement of solid–liquid interface, coupling with a conservation equation for solute transport in solidification. After model validation, the dendritic growth under several conditions of directional solidification was investigated and a solidification entropy was proposed to quantitatively characterize solidification morphologies. The results show that the present model captures tip-splitting occurring at a relative fast solidification rate. Initial thermal undercooling and solute concentration are dominant factors influencing the final microstructure. The solidification entropy reflects complexity of dendritic morphologies and it is useful to characterize dendritic growth in solidification. A design map was proposed to predict dendritic growth with and without tip-splitting, offering potential for simulating dendritic growth and microstructure evolution in solidification of alloys.

Experimental Investigation and CALPHAD Assessment of the Eutectic Trough in the System NiAl-Cr-Mo

NiAl-based alloys are promising candidates for high-temperature structural applications but their use is restricted by the limited mechanical properties. However, an improved performance can be achieved by strengthening the NiAl matrix with embedded fibers of refractory metals, such as Cr and Mo. Such composites can be manufactured in-situ by directional solidification of alloys with eutectic composition. For these processes the location of the eutectic trough in the NiAl-Cr-Mo system has to be known. The determination of the respective compositions was achieved by combining experimental investigations with computational thermodynamics. Thus, a series of melts in the NiAl-Cr-Mo system were prepared and their microstructures were investigated after solidification. The information on the primary phases was used to model the liquidus surface of the system NiAl-Cr-Mo using the CALPHAD approach. For this purpose, the thermodynamic datasets of the respective sub-systems from the literature have been combined into a quaternary database. The database will be used in the development of NiAl-based alloys for directional solidification. The dataset allows the calculation of the eutectic temperatures, the phase fractions and further thermodynamic properties along the eutectic trough. The present dataset will provide a reliable description for the NiAl-Cr-Mo system, but other regions of the Al-Cr-Mo-Ni system have an inferior representation.

Influence of gravity level on Columnar-to-Equiaxed Transition during directional solidification of Al – 20 wt.% Cu alloys

Solidification experiments on refined and non-refined Al–Cu alloys were carried out on Earth and during two parabolic flights to investigate the influence of gravity level upon solidification dynamics. In situ and real-time monitoring of the microstructure formation was characterized by using an X-ray radiography device based on a microfocus X-ray source. This paper presents for the first time direct observation of metal alloy directional solidification under varying gravity level. By a comparative study between the experiments, the influence of gravity on fragmentation and dendrite fragments transport was enlightened. In addition, a spectacular transition from a columnar to a fully equiaxed microstructure was revealed when there is a sharp increase of gravity level. It was shown that this phenomenon is a consequence of a sudden increase of the liquid undercooling ahead of the columnar front accompanying the change in hydrostatic pressure.

Microstructure Evolution and Mechanical Properties of a Directionally Solidified Nb-Ti-Si-Cr-Al-Hf-Dy Alloy

The Nb-Ti-Si-Cr-Al-Hf-Dy alloy was prepared by conventionally casting and then directionally solidified (DS) with the optical floating zone melting technology. Microstructural evolution and mechanical properties of the as cast (AC) and DS alloys were studied by XRD, SEM, TEM, compression and bending tests. The results exhibit that α-(Nb, Ti) 5 Si 3 , (Ti, Nb) 5 Si 3 and (Nb, Ti)ss phases formed the microstructure of the AC alloy, with precipitates of Cr 2 Nb, Dy 2 O 3 and (Nb, Ti) 3 Si along phase or grain boundaries. The DS alignes (Nb, Ti)ss and α-(Nb, Ti) 5 Si 3 lamellae parallel to the growth direction and refines the eutectic cell, but coarsens the primary α-(Nb, Ti) 5 Si 3 phase. The DS alloy exhibits strong orientation of (310) (Nb, Ti)5Si3 and (110) (Nb, Ti)ss along the growth direction. TEM observation reveals that (Nb, Ti)ss and α-(Nb, Ti) 5 Si 3 phases have an orientation relationship of [110] (Nb, Ti)ss // [310] α-(Nb, Ti)5Si3 and (002) (Nb, Ti)ss // (002) α-(Nb, Ti)5Si3 . Compared with the AC alloy, the DS increases the mechanical properties of the alloy obviously and the alloy DS at 8 mm/h exhibits the best mechanical properties.

Simulation of liquid channel of Fe-C alloy directional solidification by phase-field method

In directional solidification, two characteristic parameters determine the dendritic growth: the thermal gradient and the pulling velocity. To achieve the suitable microstructure and improve the performance of casting, they are usually used to resize the pulling velocity or temperature gradient in directional solidification process. The structures obtained under different directional solidification conditions, and their associated properties both have been hot research points. It is difficult to observe the microstructure, which is usually on a micrometer scale, directly in experiment, and the phase-field method becomes a strong tool to understand the dendrite growth pattern. We mainly study the liquid channel formed after Fe-C alloy dendrite tip splitting under the specific condition of directional solidification and analyze the influence on liquid channel of pulling velocity in this paper. We choose the fixed thermal gradient G =20 K/mm which is on the order of the experimental value, and pulling velocity VP no more than 10 mm/s to keep the cooling rate in the range of low speed in dendrite growth, so that the interface kinetic effect can be neglected. Recent experimental results show the different interfacial energies in various compositions of Al-Zn alloy and Fe-C alloy, then we can investigate a series of directional solidification microstructures with fixed alloy Fe-0.5 wt.%C composition at different interfacial energies in our simulations. We find that the liquid channel is formed as a result of anisotropy competition between system and materials, the length and C concentration of liquid channel increase with the pulling velocity increasing, while the diameter of liquid channel is constant. It is interesting to find that there is a minimum of pulling velocity almost equal to 1 mm/s, the tip will not split and no liquid channel forms in the following steps either when the velocity is smaller than the minimum. We also compare the segregation caused by solute enrichment in liquid channel and solute segregation between dendrite arms in a series of simulations: the former is more serious than the latter. Then we point out the way to reduce the segregation caused by liquid phase channel by reducing the pulling velocity properly. It will be more practical to couple the flow field with other external field, such as magnetic field, in the simulation.

Effect of a transverse magnetic field on solidification morphology and microstructures of pure Sn and Sn–15 wt% Pb alloys grown by a Czochralski method

The pure Sn and Sn–15wt% Pb alloys were grown by a Czochralski method under various magnetic flux densities in this paper. The influence of thermoelectric magnetic (TEM) flows and buoyancy flows on solidification morphology, macrosegregation and microstructures had been investigated experimentally, and the velocity magnitude of TEM flows and buoyancy flows had been studied by 3D numerical simulations. The experimental results indicate that the modification of solidification morphology and microstructures is attributed to the unidirectional Pb solutes transport caused by TEM flows. The 3D numerical simulations results show that the buoyancy flows dominate the flows in the melt under a weak transverse magnetic field (B≤0.43T), and the unidirectional TEM flows at the vicinity of solid–liquid interface become the dominant flows in the melt with the increase of magnetic field. The interaction of TEM flows and buoyancy flows affecting solidification morphology and microstructures during directional solidification of alloys by the Czochralski method under various magnetic flux densities has been discussed and a corresponding simple evolution mechanism of dendritic growth has been proposed.

Monotectic Al–Bi–Sn alloys directionally solidified: Effects of Bi content, growth rate and cooling rate on the microstructural evolution and hardness

Al-based monotectic alloys can have interesting tribological characteristics with the solute acting as a solid lubricant, while the matrix provides required structural integrity. The addition of third elements can increase the alloy load capacity. The microstructural features of these alloys, such as morphology, distribution and length scale of the phases depend strongly on the parameters of their manufacture route. In the present study monotectic Al–Bi–Sn alloys were directionally solidified (DS) under a large range of experimental cooling rates, permitting a wide spectrum of microstructural scales to be examined. Experimental correlations between the microstructure interphase spacing and solidification cooling rate and growth rate are proposed. Despite a slight increase in hardness with smaller interphase spacings for regions closer to the cooled surface of the DS alloys castings having 2 and 3.2wt.%Bi, it is shown that the Bi content of the alloy has not a significant effect on hardness. It is also shown that the experimental correlations established between the cooling rate and both the interphase spacing and area fractions of droplets of the eutectic mixture can be used in the tailoring of the microstructure of Al–Bi–Sn alloys with a view to applications in the manufacture of wear resistant components.

EXPERIMENTAL AND SIMULATION STUDY OF DIRECTIONAL SOLIDIFICATION PROCESS FOR INDUSTRIAL GAS TURBINE BLADES PREPARED BY LIQUID METAL COOLING

Advanced aero and power generation industry needs high-performance gas turbine. As key parts of gas turbine directionally solidified (DS) columnar grain and single crystal (SX) blades operate in heavy stress and high temperature conditions. The continuous demand for increasing turbine inlet temperature and aggressive environment has pushed alloy designers to develop DS and SX Ni-based blade alloys that contain high amount of alloying elements. DS process of blades using such alloys has become a challenging task. The small DS and SX blades are usually produced by high rate solidification (HRS) process. However, the growth of large DS and SX blades requires directional solidification with a sustained thermal gradient along the DS direction. By increasing the thermal gradient, the dendrites are refined, which results in a mechanically-superior DS and SX with reduced defects. One method to achieve consistent and higher thermal gradients is the utilization of the liquid metal cooling (LMC) process. In this method, heat extraction from the outer surface of the mold during DS relies on heat conduction rather than radiation in the conventional HRS process. The optimization of the LMC process is difficult and costly by experimental methods, especially for the complexly shaped industry gas turbine (JOT) blades because of the complicated process parameters associated with the technique. Numerical simulation is an efficient method to solve this problem. In this work, directionally solidified industry gas turbine hollow blades were prepared by high gradient LMC process. Liquid Sn was used as cooling medium. The temperature fields, macrostructures, primary dendrite arm spacing (PDAS) at various withdrawal rates during LMC process have been calculated with ProCAST software. The impact of withdrawal rate on formation of stray grains and freckles was predicted. The calculated results and the experimental observations agreed well. The solidification rates and cooling rates were found to increase with the increase of withdrawal rate. The axial thermal gradient was high and stable during the LMC process. It was found that stray grains would not block the growth of original grains at optimized withdrawal rate. No freckles were observed in the industry gas turbine hollow blades prepared by LMC technique due to the high cooling rate. Though the mean diameters of columnar grains in LMC blades were almost identical to that observed in HRS blades, the PDAS were more than 50% refined in LMC blades than those in HRS blades.

Microstructural development and mechanical properties of a near-eutectic directionally solidified Sn–Bi solder alloy

Sn–Bi solders may be applied for electronic applications where low-temperature soldering is required, i.e., sensitive components, step soldering and soldering LEDs. In spite of their potential to cover such applications, the mechanical response of soldered joints of Sn–Bi alloys in some cases does not meet the strength requirements due to inappropriate resulting microstructures. Hence, careful examination and control of as-soldered microstructures become necessary with a view to pre-programming reliable final properties. The present study aims to investigate the effects of solidification thermal parameters (growth rate — VL and cooling rate — ṪL) on the microstructure of the Sn–52wt.%Bi solder solidified under unsteady-state conditions. Samples were obtained by upward directional solidification (DS), followed by characterization through metallography and scanning electron microscopy (SEM). The microstructures are shown to be formed by Sn-rich dendrites decorated with Bi precipitates surrounded by a complex regular eutectic mixture, with alternated Bi-rich and Sn-rich phases. Experimental correlations of primary (λ1), secondary (λ2), tertiary (λ3) dendritic and eutectic spacings (λcoarse and λfine) with cooling rate and growth rate are established. Two ranges of lamellar eutectic sizes were determined, described by two experimental equations λ=1.1 VL−1/2 and λ=0.67 VL−1/2. The onset of tertiary branches within the dendritic array along the Sn–52wt.%Bi alloy DS casting is shown to occur for cooling rates lower than 1.5°C/s.

Erratum to: Characterization of Dendritic Microstructure, Intermetallic Phases, and Hardness of Directionally Solidified Al-Mg and Al-Mg-Si Alloys

Despite the widespread application of Al-Mg-Si alloys, especially in the automotive industry, interrelations of solidification thermal parameters (cooling rate and growth rate), microstructure, and hardness are not properly established. For instance, the control of the scale of the microstructure on both Al-Mg and Al-Mg-Si alloys by adequate pre-programming of the solidification thermal parameters remains a task to be accomplished. In the present study, the directional solidification (DS) of these alloys under unsteady-state solidification conditions is investigated in an attempt to characterize the evolution of microstructural features, macrosegregation, and hardness as a function of local solidification thermal parameters along the DS castings length. Silicon addition to the Al-Mg alloy was found not to affect the sizes of primary and secondary dendrite arm spacings, but induced the onset of tertiary dendritic branches and affected also the size and distribution of intermetallic particles within the interdendritic regions. The Al-Mg-Si alloy is characterized by a more complex arrangement of phases, including binary (α-Al + Mg2Si) and refined ternary (α-Al + Mg2Si + AlFe(Si) eutectic mixtures. As a consequence, a higher Vickers hardness profile is shown to be associated with the ternary Al-Mg-Si alloy DS casting. For both alloys examined, hardness is shown to increase with the increase in the microstructural spacing according to Hall–Petch type equations.

Hot workability characteristics of Rene88DT superalloy with directionally solidified microstructure

The hot deformation characteristics of Rene88DT superalloy with directionally solidified microstructure produced by electroslag remelting continuous directionally solidification (ESR-CDS®) were studied in the temperature range of 1,040–1,140 °C and strain rate range of 0.001–1.000 s−1 by hot compression tests. Flow curves for Rene88DT alloy with initial directionally solidified (DS) microstructure exhibit pronounced peak stresses at the early stage of deformation followed by the occurrence of dynamic softening phenomenon. Rene88DT alloy with DS microstructure shows higher flow peak stresses compared with HIPed P/M superalloy FGH4096, but the disparities in peak stresses between ESR-CDSed Rene88DT and HIPed P/M superalloy FGH4096 reduce as temperature increases. The improvement of hot workability of DS alloy with columnar grains avoiding the maximum shear stress comes true. A hot deformation constitutive equation as a function of strain that describes the dependence of flow stress on strain rate and temperature is established. Hot deformation apparent activation energy (Q) varies not only with the strain rate and temperature but also with strain. The strain rate sensitivity exponent (m) map is established at the strain of 0.8, which reveals that global dynamic recrystallization (DRX) shows a relatively high m value in a large strain compression. Optimum parameters are predicted in two regions: T = 1,100–1,130 °C, $${\dot{\varepsilon}}$$ = 0.100–1.000 s−1 and T = 1,080–1,100 °C, $${\dot{\varepsilon}}$$ = 0.010–0.100 s−1, which is based on processing maps and deformation microstructure observations.

Dynamics of planar interface growth during directional solidification of alloys

The dynamics of plane front growth during directional solidification is investigated in a well-characterized system of succinonitrile–acetone, and the results show significant deviations from the predictions of existing models. This discrepancy is shown to arise from the assumption of solidification from one end in the theories that ignore the presence of an initial solute boundary layer generally present in experiments. A numerical model that relaxes this assumption is presented that gives excellent agreement with the experimental results.

Orientation-dependent morphological stability of grain boundary groove

Crystal orientation influences the morphological stability of solid—liquid interface during directional solidification of alloy, resulting in the variation of solidified microstructure. In this paper, the morphological evolution near grain boundary grooves (GBGs) with different crystal orientations in a dilute succinonitrile alloy under low temperature gradient and interface velocity is observed in situ. Under experimental conditions, the macroscopic solid—liquid interface is planar and keeps stable, while in GBGs there emerge protrusion and undulation. It is found that the morphological stability of GBG is dependent on crystal orientation. Specifically, for succinonitrile with a body-centered cubic crystal structure, GBGs around the 〈100〉 crystal orientation keep stable, while those apart from the 〈100〉 crystal orientation become unstable under the same conditions. So it is concluded that 〈100〉 crystal orientation favors the morphological stability of GBG.

Microstructural evolution and mechanical properties of as-cast and directionally-solidified Nb-15Si-22Ti-2Al-2Hf-2V-(2, 14) Cr alloys at room and high temperatures

Arc-melting (AC) and directional solidification (DS) techniques were used to prepare Nb-15Si-22Ti-2Al-2Hf-2V-(2, 14) Cr alloys (hereafter referred as to 2Cr and 14Cr alloys, respectively), and the microstructural evolution and mechanical properties, including Vickers hardness, room temperature fracture toughness and high temperature strength, of the two AC and DS alloys were compared. The results showed that with heat-treatment at 1350 °C for 50 h, the AC-2Cr alloy composed of Nb solid solution (NbSS) and α-Nb5Si3 silicide, while Laves C15-Cr2Nb phase arose in the 14Cr alloy. With two-phase NbSS/α-Nb5Si3 microstructure, the AC-2Cr alloy showed excellent room-temperature fracture toughness (KQ: 14.2 MPa m1/2) and 0.2% yield strength at 1250 °C (σ0.2: 315 MPa) and 1350 °C (σ0.2: 294 MPa), better than the AC-14Cr alloy with tri-phase NbSS/α-Nb5Si3/C15-Cr2Nb microstructure (KQ: 9.4 MPa m1/2, σ0.2: 189 MPa at 1250 °C and 87 MPa at 1350 °C). The DS technique was found not to change the phase constituent of each alloy, but it made the microstructure slightly orient to the growth direction, resulting in a significant improvement in room-temperature fracture toughness (by ∼43%) and high-temperature yield strength σ0.2 (by ∼55%), as compared with the AC samples.

Effect of directional solidification rate on the microstructure and properties of deformation-processed Cu–7Cr–0.1Ag in situ composites

The influence of directional solidification rate on the microstructure, mechanical properties and conductivity of deformation-processed Cu–7Cr–0.1Ag in situ composites produced by thermo-mechanical processing was systematically investigated. The microstructure was analyzed by optical microscopy and scanning electronic microscopy. The mechanical properties and conductivity were evaluated by tensile-testing machine and micro-ohmmeter, respectively. The results indicate that the size, shape and distribution of second-phase Cr grains are significantly different in the Cu–7Cr–0.1Ag alloys with different growth rates. At a growth rate of 200μms−1, the Cr grains transform into fine Cr fiber-like grains parallel to the pulling direction from the Cr dendrites. The tensile strength of the Cu–7Cr–0.1Ag in situ composites from the directional solidification (DS) alloys is significantly higher than that from the as-cast alloy, while the conductivity of the in situ composites from the DS alloys is slightly lower than that from the as-cast alloy. The following combinations of tensile strength, elongation to fracture and conductivity of the Cu–7Cr–0.1Ag in situ composites from the DS alloy with a growth rate of 200μms−1 and a cumulative cold deformation strain of 8 after isochronic aging treatment for 1h can be obtained respectively as: (i) 1067MPa, 2.9% and 74.9% IACS; or (ii) 1018MPa, 3.0%, and 76.0% IACS or (iii) 906MPa, 3.3% and 77.6% IACS.

Microstructure evolution of directionally solidified Nb–Si alloy

By taking the method of liquid–metal cooled directional solidification, alloys with a nominal composition of Nb–14Si–24Ti–10Cr–2Al–2Hf (at-%) were prepared under different conditions. Alloys were initially directional solidified with different withdrawal rates (R = 1·2, 6, 18 mm min−1) at 1750°C and subsequently heat treated at 1450°C for 10 h. These processes aimed to investigate the microstructure of directionally solidified (DS) and heat treated (HT) alloys by XRD, SEM, and EDS. The microstructure of DS alloy was composed of (Nb,Ti)SS, (Nb,Ti)5Si3, and Laves phase Cr2Nb, and the former two components formed (Nb,Ti)SS+(Nb,Ti)5Si3 eutectics. In addition, (Nb,Ti)5Si3 laths only presented in DS1·2 alloy. With the increasing withdrawal rates, the microstructure of alloy altered from hypereutectic into pseudo-eutectic, accompanied with the eutectic morphology transformation from petaloid into coupled. Also, the dimension of constituent phases reduced. However, after heat treatment, the constituent phases did not change. The petaloid morphology of eutectics in DS specimens disappeared and coupled eutectic transferred into network. The block or needle-like Cr2Nb gathered along the boundary between (Nb,Ti)5Si3 and (Nb,Ti)SS, and the overall alloy composition became homogenisation.

NUMERICAL SIMULATION OF AL-SI ALLOYS WITH AND WITHOUT A DIRECTIONAL SOLIDIFICATION

Numerical simulations are presented to analyze the influence of the casting process on the resulting strength of Strontium modified Al–Si alloys. A relationship is identified between the mechanical behavior and the different 3D morphologies of the eutectic silicon of the samples obtained by the die cast procedure and the directional solidification. It is shown that the mechanical behavior of the die cast alloy is isotropic in all three directions. In contrary, for the directional solidified alloy, the mechanical strength in the direction of the temperature gradient is higher than in the transverse direction. This fact has to be taken into account when analyzing structures issued from different casting processes. The volume meshes for the simulations are generated from experimental 3D FIB/SEM data sets. The influence of several levels of coarsening of the meshes as well as the order of the Lagrange element in the finite element setup are also analyzed.

Study on Microstructural Evolution of Binary Alloy Crystal Growth in Directional Solidification Process

PFM (phase field method) was employed to study microstructure evolution, and considering the effect of solute concentration to the undercooling, we developed a phase field model for binary alloy on the basis of pure substance model. In the paper, the temperature field and solute field were coupled together in the phase field model to calculate the crystal growth of magnesium alloy in directional solidification. The simulation results showed a non-planar crystal growth of planar to cellular to columnar dendrite, the comparison of different dendrite patterns were carried out in the numerical simulation, and with the increasing of the anisotropy, the second dendrite arms became more developed.