Microstructure Selection Map Research Articles

Phase Field Simulation of Al-Fe-Mn-Si Quaternary Eutectic Solidification

This study investigates the eutectic equilibrium phases in a multicomponent system through 3-D multi-phase-field simulations. Emphasizing the directional solidification process, the work examines the growth dynamics of intermetallic phase Al13Fe4, a lamellar structure (FCC-A1), and a quaternary phase beta-AlMnSi from the liquid that is solidified at a specific temperature. The eutectic transformation, described by the four phase reaction L→Al13Fe4+FCC-A1+beta-AlMnSi, is analyzed to develop a microstructure selection map. This map correlates stable growth modes with initial system composition and lamellar spacing. The results provide detailed insights into the segregation behaviour of alloying elements and their influence on transformation kinetics, enhancing the understanding of eutectic microstructure evolution in complex alloy systems.

Microstructure evolution in AlSi10Mg alloy fabricated by laser-based directed energy deposition

Investigation of the influence of processing parameters on the microstructure evolution of the melt pool is important for microstructure control in laser-based directed energy deposition (DED-LB) processes. Single-track experiments of DED-LB AlSi10Mg alloy were conducted in this work, processing with two laser powers (900 and 1800 W) at a fixed scanning speed (20 mm/s). The grain morphology was changed from columnar to equiaxed towards the center of each melt pool. Meanwhile, the morphology of the α-Al cells in the grains varied from elongated to equiaxed along the same direction due to the increase of cooling rate. A more equiaxed microstructure can be obtained in the melt pool produced at a larger laser power. The scales of the microstructures and the grain morphologies in the melt pools were in accord with the microstructure selection map and grain morphology selection map in temperature gradient (G)–growth velocity (V) space constructed by solidification models. Since G and V are directly related to laser power and scanning speed, the G–V maps can provide the guidance for processing optimization to tailor microstructures in DED-LB Al–Si alloys based on the temperature field simulations of melt pools.

Constructing solidification microstructure selection map for AlSi10Mg alloy processed by laser powder bed fusion

The cellular structures, which consist of cells and eutectic networks at the cell boundaries, determine the mechanical properties of the Al–Si alloys fabricated by laser powder bed fusion (L-PBF). Therefore, it is scientifically interesting and technologically important to establish the relationship between cellular structures and processing parameters, which is applicable to all kinds of processing methods involving rapid directional solidification, such as L-PBF. In this work, a rapid solidification microstructure selection map (SMSM) of the Al–10Si system in temperature gradient (G)–solidification velocity (V) space is constructed by a synergistic calculation with dendritic and eutectic growth models compatible to the L-PBF process. Considering that the divorced eutectic reaction should occur as the width of the liquid film entrapped in the α-Al cell boundaries becomes smaller than the Al–Si eutectic spacing, the G–V domain suitable for acquiring the cellular structures decorated with divorced eutectic Si networks is determined on the SMSM. The microstructure evolution trends predicted by the SMSM are well consistent with those in the L-PBFed AlSi10Mg melt pools with different processing parameters. The length scales of actual microstructures observed by the experiments from this work and the literature are quantitatively compared to those estimated by the SMSM, and the experimental results are basically in accord with the SMSM. It is demonstrated that divorced eutectic Si networks prefer to form at the bottom of the melt pool, where G is large (> 5 × 106 K/m) and V is small (3 × 10−4–6 × 10−3 m/s) in the SMSM. The outcomes of this work may provide fundamental guidance for tailoring the microstructures of the L-PBFed Al–Si alloys for achieving optimal mechanical properties.

Phase-field investigation on the microstructural evolution of eutectic transformation and four-phase reaction in Mo-Si-Ti system

By using phase-field method, we investigate the morphological evolution of three-phase eutectic transition and four-phase reaction in Mo-Si-Ti system through 2-D and 3-D simulations. For the eutectic transition, we focus on the two-phase growth of lamellar pair from an isothermally undercooled melt: L→Ti(Mo)5Si3+β(Mo,Si,Ti), and obtain a microstructure selection map for (mi) stable, (mii) unstable, and (miii) oscillatory growth (metastable mode), in terms of the Mo-composition and lamellar spacings. The underlying reason for these three different morphologies is clarified by analyzing the growth rate of the solidification front. In addition, we scrutinize the influence of interfacial energy on the solidification morphology and observe three different types of growth mode: (gi) curving, (gii) stable, and (giii) unstable growth. For the four-phase reaction, L+Mo(Ti)3Si→Ti(Mo)5Si3+β(Mo,Si,Ti), we observe the remelting of Mo(Ti)3Si phase and the formation of a lamellar pair consisting of Ti(Mo)5Si3 and β(Mo,Si,Ti) on the surface of the Mo(Ti)3Si phase after an interface of the lamellae pair phases is formed. A certain orientation angle with respect to the solidification direction is obtained for the lamellar pair growth during the four-phase reaction. In both eutectic phase transformation and four phase reaction, a comparison between the 2D and 3D simulations reveals the influence of the third dimension on the development of the lamellar pair.

Understanding cellular structure formation in rapidly solidified Al–Ni alloys by constructing solidification microstructure selection maps

Understanding cellular structure formation in rapidly solidified Al–Ni alloys by constructing solidification microstructure selection maps

Constitutional supercooling and corresponding microstructure transition triggered by high magnetic field gradient during directional solidification of Al-Fe eutectic alloy

Directional solidification experiments of eutectic AlFe alloys were carried out under different high magnetic field gradients. High magnetic field gradient changed the microstructure selection during solidification process, induced the solidification microstructure to undergo eutectic instability, cellular transition, single-phase instability, and dendrite transition, which is similar to the planar-cellular-dendritic crystal growth pattern transition usually induced by an increasing growth velocity without magnetic field. The single phase was proved to be formed through an independent nucleation mechanism during single-phase instability. The effect of high magnetic field gradient on solidification was analyzed. Through the coupling effect of magnetic force and Lorentz force on solute migration and diffusion during solidification, high magnetic field gradient triggered a constitutional supercooling at the front of solid-liquid interface, which led to the growth pattern transition. A solidification model of eutectic alloy under high magnetic field gradient was proposed, and the microstructure selection map of AlFe alloy under gradient magnetic field was qualitatively drawn for the first time. This work proved that an alloy material with a microstructure usually obtained at high growth velocity can be obtained at a low growth velocity by applying a high magnetic field gradient, and suggested a new potential method for the future controlling of alloy structures and properties.

Microstructure selection map for rapidly solidified Al-rich Al–Sr alloys

Abstract The effect of rapid solidification on the microstructure of Al-5Sr, Al-10Sr and Al-23Sr (wt.%) alloys has been investigated using X-ray diffraction and transmission electron microscopy. The phases present in the melt-spun Al –Sr alloys are the equilibrium phases a-Al and tetragonal Al4Sr. The microstructure of the melt-spun Al –Sr alloys is quite different from that of the ingot-like alloys. Microstructure selection map is established for the rapidly solidified Al-rich Al –Sr alloys, in light of the microstructure together with the estimated cooling rates. The undercooling obtained through quenching at ≈ 9.5 × 105 K s–1 is sufficient for the Al-10Sr alloy but insufficient for the Al-23Sr alloy to completely suppress the formation of primary Al4Sr. Rapid solidification has a significant effect on the morphology of primary Al4Sr and the higher level of undercooling enhances the formation of the Al4Sr dendrites with 45° branches in the melt-spun Al-23Sr alloy.

An In Situ High‐Energy Synchrotron X‐Ray Diffraction Study of Directional Solidification in Binary TiAl Alloys

Phase formation and microstructure selection during solidification in γ‐TiAl alloys are highly relevant, both with respect to the microstructure and texture of cast alloys and to directional solidification, which allows to obtain a unique combination of properties. However, during cooling to room temperature, the solidifying phases transform to low‐temperature phases, which mask the prior situation during solidification. A new inductive crucible‐free zone melting furnace is used in this work, which is specially designed for in situ investigations of solidification using synchrotron radiation. Herein, alloys with 45 and 48 at% Al are studied, and it can be shown that with varying withdrawal rate, a change from α solidification to synchronous β + α solidification occurs in Ti–48Al, as it is predicted in the literature. Furthermore, it is observed that by increasing the withdrawal rate, a preferred <001> growth of the β phase occurs in alloys with 45 at% Al, which is interpreted as a transition to dendritic solidification. The observations are compared with a microstructure selection map calculated according to the nucleation and constitutional undercooling (NCU) model proposed in the literature.

Modified Microstructure Selection Map and Growth Conditions of the Band Structure in Directionally Solidified Hypo-Peritectic Alloys

Considering the variation of the solid/liquid interface morphology with a growth condition, microstructure selection map in peritectic system was modified by comparing growing α (primary phase) interface tip temperature and β (peritectic phase) nucleation temperature. The results were proved experimentally in hypo-peritectic Sn–Cd alloys by delicate directional solidification experiment under the diffusive growth condition using capillary thin tube. When β-phase took over α-phase and grew a short distance as the leading phase, the instability of β-phase was first observed experimentally. This phenomena was explained by constitutional undercooling, which was present temporarily in liquid ahead of the β/l interface.

Revisiting solidification microstructure selection maps in the frame of additive manufacturing

Understanding microstructural development in additive manufacturing under highly non-equilibrium cooling conditions and the consequent effects on mechanical properties of the final component is critical for accelerating industrial adoption of these manufacturing techniques. In this study, simple but effective theoretical solidification models are recalled to evaluate their ability to predict of microstructural features in additive manufacturing applications. As a case study, the resulting solidification microstructure selection maps are created to predict the stable growth modality and the columnar to equiaxed transition (CET) of an Al-10Si-0.5Mg alloy processed via Selective Laser Melting. The potential of this method in microstructural predictions for additively manufactured products, as well as outstanding challenges and limitations, are discussed.

Microstructure and electrochemical anodic behavior of Inconel 718 fabricated by high-power laser solid forming

Improvements to the surface of high-power laser solid formed Inconel 718 by use of electrochemical methods were systematically investigated. Formation features (surface microstructure characteristics and surface morphology) and the corresponding electrochemical anodic behavior (electrochemical dissolution behavior and surface levelling mechanism) were analysed. The electrochemical results show that the surface energy difference between the horizontal section and vertical section causes no difference to their transpassive current densities in 10 wt.% NaNO3 solution up to 2.5 V, owing to the existence of a passive film. Analysis of micro-morphologies indicates that the high current density leads to a smooth micro-morphology due to the higher detachment rates of the interdendritic phases and surface products. Numerical simulations suggest that surface peaks dissolve faster than depressions during the electrochemical levelling process owing to the higher local current density on the surface peaks. Importantly, a detailed microstructure selection map for as-deposited Inconel 718 and an anodic current-density-potential map were developed, and the electrochemical levelling mechanism of its wavy surfaces was proposed. This work has the potential to integrate understanding of laser solid forming-process conditions, formation features, and electrochemical anodic behavior.

Microstructure and deformation behavior of Ti-6Al-4V alloy by high-power laser solid forming

This work investigated the microstructure and tensile deformation behavior of Ti-6Al-4V alloy fabricated using a high-power laser solid forming (LSF) additive manufacturing. The results show that the post-fabricated heat-treated microstructure consists of coarse columnar prior-β grains (630–1000 μm wide) and α-laths (5–9 μm) under different scanning velocities (900 and 1500 mm/min), which caused large elongation (∼18%) superior to the conventional laser additive manufacturing Ti-6Al-4V alloy. The deformation behavior of the LSF Ti-6Al-4V alloy was investigated using in situ tensile test scanning electron microscopy. The results show that shear-bands appeared along the α/β interface and slip-bands occurred within the α-laths, which lead to cracks decaying in a zigzag-pattern in the LSF Ti-6Al-4V alloy with basket-weave microstructure. These results demonstrate that the small columnar prior-β grains and fine basket-weave microstructure exhibiting more α/β interfaces and α-laths can disperse the load and resist the deformation in the LSF Ti-6Al-4V components. In addition, a modified microstructure selection map of the LSF Ti-6Al-4V alloy was established, which can reasonably predict the microstructure evolution and relative grain size in the LSF process.

Microstructure selection in thin-sample directional solidification of an Al-Cu alloy: In situ X-ray imaging and phase-field simulations

We study microstructure selection during directional solidification of a thin metallic sample. We combine in situ X-ray radiography of a dilute Al-Cu alloy solidification experiments with three-dimensional phase-field simulations. We explore a range of temperature gradient G and growth velocity V and build a microstructure selection map for this alloy. We investigate the selection of the primary dendritic spacing Λ and tip radius ρ. While ρ shows a good agreement between experimental measurements and dendrite growth theory, with ρ∼V−1/2, Λ is observed to increase with V (∂Λ/∂V>0), in apparent disagreement with classical scaling laws for primary dendritic spacing, which predict that ∂Λ/∂V<0. We show through simulations that this trend inversion for Λ(V) is due to liquid convection in our experiments, despite the thin sample configuration. We use a classical diffusion boundary-layer approximation to semi-quantitatively incorporate the effect of liquid convection into phase-field simulations. This approximation is implemented by assuming complete solute mixing outside a purely diffusive zone of constant thickness that surrounds the solid-liquid interface. This simple method enables us to quantitatively match experimental measurements of the planar morphological instability threshold and primary spacings over an order of magnitude in V. We explain the observed inversion of ∂Λ/∂V by a combination of slow transient dynamics of microstructural homogenization and the influence of the sample thickness.

Evaluation of an Al-Ce alloy for laser additive manufacturing

This study focuses on the development of a design methodology for alloys in AM, using a newly developed Al-Ce alloy as an initial case study. To evaluate the candidacy of this system for directed energy deposition processes, single-line laser melts were made on cast Al-12Ce plates using three different beam velocities (100, 200, and 300 mm/min). The microstructure was evaluated in the as-melted and heat treated conditions (24 h at 300 °C). An extremely fine microstructure was observed within the melt pools, evolving from eutectic at the outer solid-liquid boundaries to a primary Al FCC dendritic/cellular structure nearer the melt-pool centerline. The observed microstructures were rationalized through the construction of a microstructure selection map for the Al-Ce binary system, which will be used to enable future alloy design. Interestingly, the heat treated samples exhibited no microstructural coarsening.

Phase and Microstructure Selection During Directional Solidification of Peritectic Alloy Under Convection Condition

A phase and microstructure selection map used for peritectic alloy directionally solidified under convection condition was presented, which is based on the nucleation, constitutional undercooling criterion (NCU criterion), and the highest interface temperature criterion. This selection map shows the relationships between the phase/microstructure, the G/V ratio (G is the temperature gradient, V is the growth velocity), and the alloy composition under different convection intensities and nucleation undercoolings. Comparing with the results from directional solidification experiments of Sn–Cd peritectic alloys, this selection map was generally in agreement with the experimental results.

Laser Rapid Forming of Ti-Ni Functionally Graded Alloy

Ti-Ni based functionally graded alloy is a kind of the promising material, which has potential to be used in aero engines. Using laser rapid forming, a Ti-Ni graded alloy with a continuous compositional gradient from pure Ti to Ti-50wt.%Ni were fabricated. On comparison with the graded alloy, a series of homogenous deposits with the typical composition between pure Ti to Ti-50wt.%Ni were also laser rapid formed. The phase evolution along the compositional gradient direction in the graded alloy is: α+β→β+Ti2Ni →(TiNi +Ti2Ni)+ TiNi; and the phase evolution in the corresponding compositional homogeneous deposit is: α+β→ β+(β+Ti2Ni)→ β+Ti2Ni+(β+Ti2Ni)→ (TiNi+Ti2Ni)+TiNi. The phase transformation and microstructural evolution along the compositional gradient were analyzed by using the microstructure selection map.

Solidification microstructure selection of the peritectic Nd-Fe-B alloys

Bridgman directional solidification and laser remelting experiments were carried out on Nd11.76Fe82.36B5.88 and Nd13.5Fe79.75B6.75 alloys. Microstructure evolutions along with solidification parameters (temperature gradient G, growth velocity V and initial alloy composition C0) were investigated. A solidification microstructure selection map was established, based on the consideration of solidification characteristics of peritectic T1 phase. In Bridgman directional solidification experiments, with the increasing growth velocities, the morphology of T1 phase changed from plane front or faceted plane front to dendrites. In laser remelting experiments, a transition from primary γ-Fe dendrites to T1 dendrites was found. Theoretical predictions are in good agreement with experimental results.

Microstructure Evolution of Ti-Al Peritectic System during the Initiated Stage of Directional Solidification

The microstructure evolution of Ti-Al peretectic system in transient stage and steady state in directional solidification was predicted via theoretical analysis. The solute distribution controlled by diffusion at and ahead the solid-liquid interface will determine whether the properitectic and peritectic phases can nucleate and grow ahead of the opposing solid phase. The formation of banding structure is possible in a certain composition range. At the steady state, a microstructure selection map was set up based on interface response function model. The microstructure of TiAl alloys with different aluminum content was studied with Bridgman directional solidification method. Some evidence in the experiment has been found to support the theoretical prediction.

Microstructural Characterization of the Emulsified Cu&lt;sub&gt;47&lt;/sub&gt;Ti&lt;sub&gt;33&lt;/sub&gt;Zr&lt;sub&gt;11&lt;/sub&gt;Ni&lt;sub&gt;6&lt;/sub&gt;Sn&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;1&lt;/sub&gt; Alloy Powder

Phase selection and microstructural morphology change of the Cu47Ti33Zr11Ni6Sn2Si1 alloy were investigated through the droplet emulsion technique(DET). The emulsified Cu47Ti33Zr11Ni6Sn2Sil alloy powders showed several different microstructures depending on the amount of undercooling. The amount of undercooling of the powders was monitored by differential thermal analysis and was matched with the microstructures. The phase transition of Cu47Ti33Zr11Ni6Sn2Sil alloy powders according to the increase of undercooling proceeds by the process Cu4Ti3 +CuTi +Cu2Ti +Cu51Zr14 → Cu4Ti3 + CuTi + Cu2Ti + Cu51Zr14 + CuTi2 → Cu2Ti + Cu51Zr14 + CuTi2 → Cu51Zr14 +CuTi2. Specifically, the morphology and scale of the CuTi2 phase were examined by SEM observation, and area fraction measurement using an image analyzer, transmission electron microscopy studies, and microhardness tests showed that the amorphous phase could be synthesized by DET. A microstructure selection map of Cu47Ti33Zr11Ni6Sn2Sil alloy powders for tailored solidification was also suggested.

Microstructural characterization of droplet emulsified Mg 60Cu 30Y 10 powders

The phase selection and microstructural morphology changes of Mg 60Cu 30Y 10 (numbers indicate at.%) alloy were investigated through the droplet emulsion technique (DET). The emulsified Mg 60Cu 30Y 10 alloy powders showed several different microstructures depending on the amount of undercooling. The amount of undercooling of the powders was monitored by differential thermal analysis and was matched with the microstructures. Both the transmission electron microscopy study and the microhardness test showed that a nanocrystalline embedded amorphous phase could be synthesized using the DET. The use of a microstructure selection map for the tailored solidification of Mg 60Cu 30Y 10 alloy powders was also suggested.