Research progress on solidificatio structure of alloys by synchrotron X-ray radiography: A review

Yongio Wng, Sensen Ji, Minggung Wei, Liming Peng, Yujun Wu, Xintin Liu

a Henan Key Laboratory of Intelligent Manufacturing of Mechanical Equipment, Mechanical and Electrical Engineering College, Zhengzhou University of Light Industry, Zhengzhou 450002, China

b School of Material Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

Abstract The synchrotron radiation technology has recently emerged as a powerful tool to characterize the real-time microstructure evolution during solidificatio of alloys.Compared with other methods, the synchrotron radiation technology, along with its unique advantages of strong brightness, high energy, excellent resolution, and good monochromaticity, allows for capturing the dendrite evolution behavior of alloys in real time and can be dynamically coordinated with high-resolution CCD (Charge-coupled Device) imaging systems.This paper briefl reviews the recent advances in developing synchrotron radiation for solidificatio of alloys with low, medium, and high melting points, and under the external electric, magnetic, and ultrasonic fields Furthermore, a series of microstructural features and behaviors such as dendrite morphology,growth orientation,dendrite fracture,and rotation are described in detail.Finally,the development trends and application prospects of synchrotron radiation technology in alloy solidificatio are forecasted.

Keywords: Solidification Synchrotron radiation; Dendritic growth; External field

1.Introduction

The solidifie microstructure is the bridge connecting the composition and properties of alloys.An accurate understanding of the formation mechanism, governing factors, and control approaches of solidificatio microstructures of alloys is conducive to the precise control and design of material organization structure to improve the comprehensive properties of materials and optimize the performance of alloys [1].However, due to the opacity, micro-nanoscale of the solidifi cation structure and the high-temperature environment during solidification traditional characterization techniques such as metallographic microscope observation, rapid quenching, and molten liquid dumping cannot observe the whole solidifi cation process dynamically, completely, and in real time.As a result, some crucial information in the solidificatio process can be missing.In order to obtain the entire growth information of solidifie structure before and after solidifi cation, in the 1980s, the dynamic growth of microstructures during the solidificatio of transparent organic compounds(such as NH4Cl aqueous solution, succinonitrile-acetone,etc.) was used to obtain the solidificatio behavior of similar metals and some useful research results were obtained [2-5].However, the physical and chemical properties of organic compounds are quite different from those of alloys, which cannot fully reflec the solidificatio law of actual alloys.The rapid development of computer simulations in recent years has enabled the use of numerical simulation technology to simulate the dendrite growth process, which has gradually become an essential means to study the evolution of metal solidificatio microstructures [6-10].Nevertheless, real-time dynamic experimental observation is still essential to compare with and verify simulation results and predictions.

The emergence and development of high energy X-ray provide a new characterization method for the study of the solidificatio structure of alloys [11-16].Especially, the appearance of synchrotron radiation in situ imaging technology makes it possible to observe the dynamic evolution of metal solidificatio in real time, and the technology also becomes the key to open the door of metal crystallization.Synchrotron radiation high-energy radiation belongs to electromagnetic radiation just like visible light and X-ray, but the difference is that it is generated in the synchrotron by the curvilinear motion of electrons near the speed of light in the magnetic field Compared with conventional light sources, high-energy X-rays in synchrotron radiation have many characteristics such as strong energy source, high brightness, good penetration, etc.[17-19].Meanwhile, the CCD (Charge-coupled Device) imaging system with high-speed reading and writing ability has the advantage of high resolution, which can meet the requirement of real-time dynamic imaging observation of metal dendrite growth.At present, synchrotron radiation in situ imaging technology has become an important means for real-time and dynamic observation of solidificatio structure growth behavior of metal alloys [20].

Starting from the solidificatio behavior of alloys with different melting points under external physical fields this paper describes in detail the state-of-the-art of synchrotron radiation imaging technology in alloy solidificatio microstructures observation, the significan problems existing in this field and the perspective on the future development.

2.Synchrotron radiation in situ imaging of dendrite growth in low melting alloys

At present, low melting point alloys, such as Sn-Bi and Sn-Pb alloys, are widely used in machinery, aviation, automobile, and other industrial fields The mechanical properties of alloy castings to a great extent lie on the microstructure formed during solidification Dendrite is the most common morphological feature during solidification For low melting point alloys, the microstructure is easy to observe because of the relatively lower solidificatio temperature.The solidificatio structure of these alloys was characterized by synchrotron radiation in situ imaging, from which one can obtain a series of microstructural evolution processes, such as dynamic dendrite growth, fracture, and transformation.This information will play an important guiding role when predicting the mechanical properties of alloys.In the following,we will present two examples.

2.1.Sn-Bi alloy

Sn-Bi alloy has a low melting point and is widely used in industry as solder alloys.The quality of solder to a great extent depends on the microstructure after remelting.Therefore, how to reliably control the grain size and distribution is critical.Based on the European Synchrotron Radiation Facility (ESRF), Mathiesen et al.[21] successfully obtained two-dimensional imaging of the microstructure growth of binary metal alloys Sn-Bi and Sn-Pb.A series of morphological evolutions and dynamic growth behaviors of columnar crystal-equiaxed crystal transformation, dendritic arm fracture, and free and new dendrites were observed clearly during the solidificatio of the alloy.This study is the pioneering work to investigate solidificatio of low melting point alloys using synchrotron radiation imaging technology.Yasuda et al.[22] observed the fragmentation phenomenon of dendrite arm under directional solidificatio of Sn-21 wt%Bi alloy by using Japanese synchrotron radiation device Spring-8.The columnar cell crystals of Sn-Bi alloy began to neck gradually from the root and finall completely fractured from the root.After in-depth analysis, it was found that the concave space was encountered during the growth of the dendrites of Sn-Bi alloy, the dendrite arms were prone to break if the separated dendrite grow again before the dendrite tip reaches the dendrite arm and the separated dendrite becomes stray dendrite.Based on their observation, the necking theory of dendritic tip growth has been proposed [23,24].

Subsequently, Li et al.[25-27] studied the dendrite growth and coarsening process in Sn-13% Bi alloy by synchrotron radiation.The results show that, under different temperature gradients and cooling rates, the phenomenon of dendrite arm remelting, coalescence, and fragmentation will occur.When the temperature gradient is constant, local remelting and dendrite arm solidificatio occurs, and the dendrite of alloy moves along the temperature gradient from low to high.At the same time, they found that the coarsening of the dendrite arm mainly occurs in the initial solidificatio stage.In contrast, at the middle and late stages, the dendrite arm roughening is no longer obvious.The main reasons for dendrite arm coarsening are the remelting,fracture and polymerization of dendrite arm.

The in-situ characterization of solidificatio microstructural evolutionary process of alloys by synchrotron radiation starts very recently in China.Xu et al.[28] firstl analyzed the coarsening of secondary dendrite arm of Sn-Bi alloy by using Beijing light source device.As shown in Fig.1, when the cooling rate is low, competitive growth of dendrites,columnar-equiaxed transformation and fracture and dissociation of dendrite arm occur in Sn-Bi alloy.Meanwhile, it is also found that in the early stage of solidification the competitive growth and dissociation of dendrites have a significan influenc on the coarsening of secondary dendrite arm, and dendrite arm coarsening occurs mainly in the early stage of dendrite growth.Based on Wang’s et al.research,Xu et al.[29] used Shanghai Synchrotron Radiation Facility,the third generation synchrotron radiation source, to image the dendrite growth process of Sn-12 wt%Bi alloy at three different cooling rates.The results show that the spacing of the secondary dendrite arms decreases as the cooling rate increases.The increase of the distance between secondary dendrite arms mainly occurred in the early solidificatio stage while the spacing remains basically constant during the intermediate and late stages of solidification Moreover, the nucleation rate increased with the increase of cooling rate.

Fig.1.(a) Sn-Bi alloy dendritic growth competition (b) Sn-Bi alloy columnar crystal-equiaxed crystal transformation (c) Sn-Bi alloy dendrite arm fracture phenomenon [28].

2.2.Sn-Pb, Sn-Ni alloys

In addition to the Sn-Bi alloy, many scholars have also investigated the synchronization radiation characterization of other solder alloys.Wang et al.[30,31] used the way of Xray diffraction enhanced imaging to probe the crystallization of the solder Sn-50 wt%Pb alloy in real time.They successfully obtained a series of dynamic images of dendrite growth behavior and morphology evolution, which provided direct experimental data for studying the crystallization theory of metal alloys.It is found that the Sn-50 wt%Pb alloy with near eutectic composition exhibited equiaxed crystal growth at firs and columnar crystal growth subsequently under a certain temperature gradient and cooling rate.Later,Zhu[32] studied the dendritic growth behavior of alloys under the control of a DC electric field They successfully found that after the application of a DC electric field the phenomenon ofself-poisoningof dendrites caused by gravity weakened and the dendritic morphology evolved from an irregular equiaxed crystal to an approximately regular equiaxed crystal.Moreover, by comparing the dendrite growth rates before and after applying a DC electric field one can see that the growth of dendrite tips is inhibited by the effect of Joule heating when it is in the free growth stage.At the interaction stage, the action of the electric fiel caused the solute at the leading edge of the dendrite to spread rapidly.As a result, the solute enrichment layer in the fina solidificatio region becomes thicker,each of the growth of single dendrite is inhibited, and the interaction between dendrites is enhanced.Finally, the dendrite growth under the fina application of a DC electric fiel will stop before the dendrite growth without applying a DC electric field

In many alloy systems containing an eutectic point, there can be competition between a stable and metastable eutectic reaction.At present, there is little known of the competition between the stable and metastable eutectics in the Sn-Ni system.Hou et al.[33]observed the unidirectional curing process of Sn-Ni plating alloy by synchrotron radiation in situ imaging technology.The results show that during the solidificatio process, the steady-state phase Sn-Ni3Sn4and the metastable phase Sn-NiSn4competed with each other, and synchrotron radiation technology reproduces the dynamic transition process between the two eutectics.At the same time, the study also found that the Sn-Ni3Sn4phase has a highly irregular morphology,while the metastable phase Sn-NiSn4has a more regular fracture layer state.These in-situ imaging results provide the most direct data for the inspection and improvement of the crystallization kinetics model of metal alloys.

In summary, due to the relatively low difficult in observing low melting point alloys, the study of dendrite growth during solidificatio of low melting point alloys is carried out by using synchrotron radiation imaging technology.A series of evolutionary behaviors have been characterized,such as transformation from columnar crystal to equiaxed crystals, dendrite arm fracture, and detachment during the solidificatio process of such alloys,which is not only helpful for understanding the formation mechanism of microstructure and controlling its influencin factors but also improves the simultaneous radiation research of low-melting alloys.

3.Synchrotron radiation in situ imaging of dendritic growth of medium and high-temperature melting point alloys

Structural material such as medium- and high-temperature alloys have been playing an indispensable role in the field of aviation, aerospace, automobile, electronics, machinery, and defense.The microstructure (crystal morphology, grain size,alloy phase distribution) of the alloy solidificatio structure directly affects the mechanical properties of the casting.How to observe the behavior of solidificatio microstructure evolution in real and effective is the focus of current research.As an important means to observe the evolution process of alloy microstructure, synchrotron radiation in situ imaging technology has opened an avenue to the research of medium and high-temperature alloys.

3.1.Al-Cu alloy

In order to capture the growth process of Al-30 wt%Cu alloy with near eutectic composition under directional solidification Mathiesen and Arnberg [34] used the European Synchrotron Radiation Facility (ESRF) to study the dendrite growth of medium-high temperature Al-Cu alloys.It is found that the normal of the eutectic interface varies slightly along the horizontal direction,indicating that the isotherm is not entirely perpendicular to the applied thermal gradient.Since the direction of the temperature gradient is parallel to the gravity,the growth front is almost on the same horizontal line.During the solidificatio process, the difference in liquid-solid relative ray absorption provides good contrast for imaging.Using this different contrast, the solute of the columnar dendritic growth front can be directly observed.Subsequently,Mathiesen et al.[35-37] further conducted an in-depth study on the directional solidificatio of Al-20 wt%Cu alloy and observed the columnar crystal-equiaxed crystal transformation, i.e., CET transformation process.Meanwhile, they also captured the breakage and detachment of the dendrite arm.They found that during the CET transition, when the solidificatio direction is opposite to the direction of gravity,the fracture of dendrite arm is prone to occur, and the broken dendrite arm continues to grow in the liquid phase and blocks the growth of the columnar crystal.It is also found that the remelting and fracture of dendrite roots are primarily dependent on buoyancy generated by solidificatio convection.On this basis, Mathiesen et al.[38] tried to add Al-Ti-B refine in the solidificatio process of Al-20 wt%Cu alloy.The results show that the introduction of refine resulted in the continuous growth of equiaxed nuclei in the liquid phase,while the growth of columnar crystals is inhibited.

Fig.2.Velocity profile measured for three y-abscissae y1, y2, and y3 during transient solidificatio (Al-4 wt%Cu, G = 35.5 K cm?1) for: (a)RH=RC=0.1 K min?1 (VP=0.47 μm/s); (b) RH=RC=0.2 K min?1 (VP=0.94 μm s?1); (c) RH=RC=0.3 K min?1 (VP=1.4 μm s?1).The arrows indicate the times at which the solidificatio fronts destabilize (at y1 firs and then at y2), the solid lines are the velocity profile predicted by the Warren-Langer model and the dashed lines are power down velocities define as the ratio of the cooling rate to the average temperature gradient [42].

In order to study the effect of different solute content on the evolution of microstructures, Yasuda and co-workers [39-41] carried out the in-situ characterization of Al-(5%, 10%,15%) Cu alloys with different solute contents.It is found that nucleation and fragmentation of dendrite arms are often observed in 15% and 10% Cu alloys when unidirectional solidificatio is carried out from the plane interface, while nucleation and fragmentation are rarely observed in 5% Cu alloys.Convection plays an important role in the evolution of alloy structure during solidification Therefore, Bogno et al.[42]studied the effect of convection on the dynamic evolution of solid-liquid interface and solute distribution during solidificatio of Al-4%Cu alloy.They successfully observed that lateral solute segregation caused by flui fl w resulted in significan deformation of the solid-liquid interface.In addition,a new quantitative image analysis technique is used to measure the longitudinal solute distribution in the initial transition stage.The effects of convection on growth rate and characteristic parameters of solute boundary layer are discussed and compared with Warren and Langer(WL)models.As shown in Fig.2, it can be seen that at least at the early stage of growth(t <50 min), there is a reasonable consistency between WL model and experimental measurements, and then there is a significan difference.It is noteworthy that the plateau values aty2andy3are lower than the WL curve,because the convective rolling results in solute accumulation.During the same period, Bogno et al.[43,44] also analyzed the growth and interaction of equiaxed grains by synchrotron radiation.It is found that grains in the early stage of equiaxed solidification the solutal self-poisoning by flui fl w plays an important role in the growth of primary dendrite arm.Through the analysis of experimental data, it is found that PELC’s model can well describe the initial growth stage of dendrite arm.This study provides a good guidance for the analysis of equiaxed solidificatio structure at different cooling rates.However,Zimmermann et al.[45] and Rakete et al.[46] studied the formation of grain structure during directional solidificatio of Al-10%Cu alloy.They found that the debris separates continuously from the dendritic tip region and moves slowly to the solid-liquid interface before they grow.Because the density difference between the debris and the surrounding melt is tiny, the debris overgrows.As a result, equiaxed crystal growth occurs in the large 3D sample with low solidificatio rate.

Fig.3.Sequence of in situ radiographs showing the diffusion behavior around the interface during the melting and solidificatio of Al/Cu bimetal: (a-e) the melting process(t=0 s is assigned to the onset of melting); (f-j) the solidificatio process, heating rate 20 K min?1, cooling rate 5 K min?1.The blue curve in Fig.2(c, e, h, i) show the gray level of each pixel along the red vertical line.The orange curve in Fig.2(j) shows the content variation of Cu at the interfacial transition zone.The inset in Fig.2(j) is a BEI image of transition zone IV [54].

Fig.4.Image analysis algorithm showing extracted grain area envelopes overlaid on FOV.Blue contour lines show the temporal grain envelope evolution.Star-dashed line profile indicate equiaxed grain nucleation and primary arm growth direction.Al-20 wt%Cu-0.1 wt%GR alloy, cooling rate of ?0.025 K/s.The dashed red boundary represents the measurement ROI [55].

Most recently, Luo et al.[47] developed a new method to study equiaxed grain growth of Al-20 wt%Cu alloy by melt superheating.They found that with the increase of melt superheat, the nucleation rate and average grain size increase rapidly in the early solidificatio stage, and even reached the fina grain size.Two reasons cause the above phenomenon:one is to increase the superheat of the melt to cause an increase in nucleation and supercooling, and the other is caused by a decrease in the growth rate and the duration of the free growth phase.Later, Li et al.[48-53] made use of the Shanghai light source platform to study the dendrite evolution of Al-Cu alloy during the solidificatio process.The growth of axon crystals (twins) and twin crystals has been successfully discovered.Furthermore, the growth and evolution of dendrites under the solidificatio space of the cross-section and T-structure are studied for the firs time.The results show that, when the solute boundary layer at the front of the solid-liquid interface is thin, the separation and floatin of the dendritic tip fragments is a prelude to the transformation.However, the triangular tip of the dendrite is a fracturesensitive area, indicating that a new dendritic morphology will be gestating and growing as long as the conditions are appropriate.Meanwhile, Wang et al.[54] studied in situ the interdiffusion of Al/Cu interfacial elements and the evolution of microstructure.As shown in Fig.3, Al and Cu elements inter-diffused at the interface and form a clear diffusion front edge.The concentration gradient resulting from the interdiffusion of elements would lead to the growth ofα-Al dendrites formed by the solidificatio of Al side samples towards the interface and the fina microstructure of the diffusion region.It is composed of theα-Al dendrite, the eutectic structure(α-Al+Al2Cu), Al2Cu, and an intermetallic compound layer.Murphy et al.[55] carried out near-isothermal equiaxed solidificatio experiments on Al-Cu system by laboratory radiography technology.An automatic algorithm for measuring solid evolution of in-situ X-ray image sequences is also used,as shown in Fig.4, the dashed red line define a manually selected internal region-of-interest (ROI), outside which any growth was ignored.The blue contours indicate the extracted grain envelope temporal evolution during solidification Also shown are four pre-selected primary dendrite arm growth axes, denoted by the star-dashed lines.By supplying the grain nucleation center and the growth direction, the algorithm can track grain growth by calculating the intersection between the grain envelope and the specifie line segment.Meanwhile, Bedel et al.[56] took the lead in studying the micro-morphology of Al-4.5 wt% copper droplets prepared by pulsed atomization.They found that the development of the dendrite arms occurs in some droplets along<111>crystallographic axes instead of the usual<100>directions observed in conventional casting for the same alloy.It has been observed that such an unusual growth direction of the dendrites is directly related to the solidificatio velocity.

With the development of the third-generation light source and the new generation CCD (Charge-coupled Device), CT(computed X-ray tomography) tomography fast scanning three-dimensional reconstruction technology, it is possible to realize in-situ visualization of three-dimensional crystal growth behavior of metallic alloys [57-59].Limodin et al.[60,61] performed three-dimensional in situ imaging of the dendrite growth process of Al-Cu alloy on the ID19 beamline of European ESRF.As shown in Fig.5, they successfully observed the real-time evolution process of a single dendrite.In addition to solidificatio growth, at least two roughing mechanisms are observed for the dendritic arms.The firs roughening mechanism involves the remelting of the secondary dendrite arms to benefi adjacent large dendritic arms.The other coarsening mechanism is the combination of adjacent dendrite arms, which seems to dominate in high solids components.It causes the space between the dendritic arms to be fille gradually, and the adjacent dendritic arms fuse near the tip.

By extending three-dimensional imaging to the “dynamic”four-dimensional characterization (4D) of the solidifie microstructure, Ludwig et al.[62] took the lead in the study of solid/liquid evolution during solidificatio of Al-Cu alloy.Because of the slow cooling rate, the measured solid volume fraction agrees well with Gulliver-Scheil model and the reverse diffusion model.Moreover, studying the macroscopic shrinkage quantitatively in the solidificatio process, it was found that the macroscopic shrinkage increased linearly with the increase of solid volume fraction.Later, Cai et al.[63] also took advantage of the four-dimensional (4D)method to probe the growth of columnar crystals in the directional solidificatio process of Al-15 wt%Cu alloy.They used the power-down mode to cool while capturing the process of cell growth, the dendritic transformation between cells, and the columnar dendrite growth.Fig.6 shows the three-dimensional morphology evolution of columnar dendrites during the directional solidificatio of Al-Cu alloy,from which one can see the blockage of dendrites around the SD side branches.The nearby dendrites are consistently deviating from SD, which provides space for the development of their secondary dendritic arms.In addition, the adjacent dendritic has no secondary arm in the surface, and the surface is smooth.This findin indicates that the lateral branches of SD cause these adjacent dendrites to tilt away from SD to a large extent, thus inhibiting the growth of dendritic.

Fig.5.Computerized three-dimensional reconstruction of Al-Cu alloy dendrite growth [60,61].

3.2.Al-Ni, Al-Si, Al-Bi, Al-Sn alloys

Compared with the study of Al-Cu alloys, other Al-based alloys have been less studied.In order to investigate the microstructure evolution behavior of Al-Ni alloy during solidification Reinhart and co-workers [64-66] refine the Al-Ni alloy and found that when a large number of equiaxed crystals appeared at the front of the columnar crystal and grew,the growth of the columnar crystals is hindered.Finally,the columnar crystals are transformed into equiaxed crystals.Later, Nguyen-Thi and co-workers [67,68] attempted to add Al-Ti-B refine during the directional solidificatio of Al-Ni alloy.The change of dendritic type under different pull-down rates was in situ observed.Specificall , the columnar crystals in the microstructure are equiaxed and finall grow in the form of equiaxed crystals, as shown in Fig.7.The equiaxed grains appear in the dark liquid layer around the columnar structure.Some of these particles precipitate and are captured by the eutectic front due to gravity.After a period of time,columnar growth is blocked (Fig.8(c)), which marks the beginning of equiaxed grain growth.Furthermore, they also investigated the effect of buoyant fl w on the growth of columnar dendrites.It is worth noting that the eutectic front apparent in the X-ray photograph of Fig.8, the eutectic plane can be seen because the bright line near the horizontal direction is mainly caused by the phase contrast caused from the average density difference between the solid and liquid regions with roughly the same eutectic composition.

Fig.6.Three-dimensional topological evolution of the columnar dendritic structure of Al-Cu alloy [63].

Fig.7.The equiaxed and equiaxed crystal growth of columnar crystals of Al-Ni alloy [67,68].

In addition, Nogita and co-workers [69,70] successfully observed the phenomenon of the nucleation, growth, and eutectic solidificatio of Al grains using the Al-1 wt%Si and Al-4 wt%Si alloy.They found that within a few seconds after the grain nucleation, the total number of grains for each sample is fi ed and no further nucleation (no new solid grains)is observed.Note that the number of grains is slightly higher in the Al-1Si sample (around 18 grains in the observed area compared to around 10 in the Al-4Si), which may relate to the higher concentrations of Ti and B.In the firs few seconds, the newly formed grains are highly mobile, and some rotation and drifting of the grain location under gravity are observed.Around 5 s after nucleation, the grains had formed a coherent network, and no further grain movement was noted.In their study in 2009, Schaffer et al.[71,72] took a different approach to the solidificatio process of percrystalline and monotectic alloys when studying the solidificatio of monotectic alloys.It is found that during the solidificatio process of the metamorphic Al-Bi alloy, the liquid phase is divided into two non-phase solution phases.With the decrease of temperature, the metamorphic reaction occurs.The Al phase enriched by high melting point liquid firs solidifies while the Bi phase enriched by low melting point precipitates in the form of small droplets, and finall solidifie in the low melting point phase.Finally, it is concluded that the direction of directional solidificatio has a significan influenc on the distribution of small droplets in the enriched phase.In order to study the effect of solidificatio crack on alloy, Aveson et al.[73] observed in situ the solidificatio cracks of a thin sample of Al-15 wt%Sn alloy.During the experiment, solidificatio cracking was seen to occur during natural cooling of the sample at a solid fraction of ～95%, between directionally agglomerated dendritic networks.And three stages of crack growth were observed: crack initiation, crack propagation,crack coalescence.These studies provide effective guidance for improving the properties of aluminum alloy castings.

Fig.8.X-ray radiographs showing planar eutectic and columnar dendritic growth during directional solidificatio parallel to gravity in Al-30 wt%Cu.Top row: every fift image from a part of the sequence representing the microstructure evolution at times t0, t0+2.25 s, and t0+4.5 s.The images also display the local liquid concentration, Cl (Cu), in a relative manner (color contrast relates to greyscale as indicated by the vertical bars to the right).Bottom row:examples of quantitative information extracted by image processing: (a) liquid compositional contour map at t0+4.5 s, starting at Cl ≤33 wt% Cu (black)and with increments of ?0.5 wt% Cu; (b) velocities for the eutectic front and columnar tips 1 to 3, enumerated from left to right, normalized against the sample pulling velocity, vs p ≈22.5 μm/s; (c) time evolution of the 2D solid-liquid interface projections over the full video sequence [68].

3.3.Mg-based alloys

Different from the research of Al-based alloys, the research on Mg-based hexagonal alloys by synchrotron radiation imaging technology started very recently.Aiming at the texture evolution of Mg-6%Al in 3D research, Eiken et al.[74] successfully discovered that the texture evolves mainly due to the retarded growth in basal<0001>orientations.While the grains with the fast growth along<1120>orientation closest to the temperature gradient usually dominated,process dependent along other orientations of the basal plane(between<1120>and<1010>) may also coexist.Later,Wang et al.[75] studied the dendritic morphology ofα-Mg during the solidificatio of magnesium-based alloys, and successfully observed the dendritic morphology formed during the solidificatio of magnesium alloys.This study has deepened the understanding of the solid-liquid interface mode and solidificatio morphology evolution in hcp structural alloys.

For Mg-RE alloys, Wang et al.[76,77] studied the morphological evolution of dendritic directional solidificatio at different low cooling rates by synchrotron X-ray imaging.The real-time evolution process of the microstructure of directional solidificatio of Mg-Gd alloy was successfully obtained, when studying the effects of different cooling rates on the fi ed thermal gradient, it is found that the growth direction of the columnar dendrites gradually rotates toward the thermal gradient as the cooling rate increases due to the difference in subcooling.At the same time, with the increase of interfacial velocity,the average dendrite spacing decreases the morphology changes with the influenc of solute segregation.This study provides a new perspective for dendritic morphology evolution and directional solidificatio of magnesium and rare-earth alloys.Casari et al.[78] studied the microstructural evolution of industrial Mg-Nd-Gd-Zn-Zr alloy Elektron 21 during solidificatio under near isothermal conditions using synchrotron radiation technique.It is found that at the beginning of the sequence,α-Mg dendrites with typical six-folded symmetry nucleate and grow in the center of the FOV (Field of View).The growth leads to solute enrichment in Zn and RE in the liquid surrounding the dendrites, which appears darker due to the increased X-ray absorption.Subsequently atT=0.0125 K/s, a fine microstructure (marked “I”) is observed to develop at much higher growth rate and with a distinctively different morphology than the original dendrites in the region, as shown in Fig.9.

In addition, Yang et al.[79,80] successfully simulated the growth morphology of Mg dendrites in three dimensions using the KKS model.On this basis, they combined the simulation with synchrotron radiation three-dimensional tomography and used EBSD (Electron Backscattered Diffraction) technology to study the dendritic morphology and growth orientation of Mg.It is found that the dendrite of magnesium alloys with different additional elements, including Sn, Ba, Ga, Al,Y and Gd, all exhibited an 18-branch morphology with six primary arms growing along<1120>and twelve growing along<1123>directions.For Mg-Zn alloys, the phenomenon termed dendrite orientation transition (DOT) took place.Under the case with 20 wt% or less Zn, theα-Mg dendrite grew along both<1120>and<1123>directions, achieving an 18-branch morphology, whereas, with 45 wt% or more Zn,theα-Mg dendrite would only grow along<1123>directions with a 12-branch morphology.Seaweed structures were observed for dendrites of alloys in intermediate compositions between 20 wt% and 45 wt% Zn, in which case the preferred growth orientation was shown to be<1121>.Subsequently,Jing et al.[81]and Wang[82]also used synchrotron radiation X-ray microtomography and phase fiel simulation to study the three-dimensional dendrite morphology and growth orientation ofα-Mg in magnesium alloys.Finally, the dendrite growth model of magnesium alloy is established, and its evolution law is elaborated.Based on the microstructure of magnesium alloy obtained by experiments, a phase fiel model is established to simulate the microstructure of magnesium alloy.Through the combination of experimental methods and phase-fiel simulations, the three-dimensional microstructure,dendritic characteristics and microstructure evolution mechanism of cast magnesium alloy were described and predicted in detail, in order to perfect the dendrite theory and provide a reliable reference for controlling the solidificatio structure.In 2017, Du et al.[83] studied the relationship between dendrite growth direction and solute types of magnesium alloys.Based on density functional theory and HCP lattice structure,the directionally dependent surface energy and subsequent crystal anisotropy are further studied.It is found that for most binary magnesium alloys, the preferred growth direction of theα-Mg dendrite in the basal plane is always<1120>,which is independent of the type or concentration of additional elements.In particular, for Mg-Zn alloys, with the change of zinc content, the direction changes from<1123>to<2245>or<1122>, as shown in Fig.10.Later, Du et al.[84] also studied the three-dimensional (3D) growth pattern and preferred growth direction of magnesium alloy dendrites by means of synchrotron X-ray tomography and electroback scattered diffraction techniques.They got the same results as Yang et al.[79,80], the typical 3D morphology of the a-Mg dendrite exhibits an 18-primary-branch pattern, with six along the<1120>basal direction, and the other twelve along the<1123>non-basal direction, as shown in Fig.11.

Fig.9.Images taken from a sequence of X-ray radiograms showing the time evolution of the α-Mg primary phase in the Elektron 21 alloy under nearly isothermal conditions at T=0.0125 K/s.The initial α-Mg dendrites appear as bright, while the secondary morphologies are distinctively darker.The gray levels in the liquid phase relate to its constitution, where enhanced solute concentration gives darker image pixels.The regions marked with I, II, III and IV represent the spatial locations where a morphologically different structure appeared [78].

In order to study the evolution of dendrite structure during solidificatio of Mg-15 wt%Sn alloy more truly, Shuai et al.[85,86] quantifie the firs 4D (3D plus time) observations of magnesium alloy solidificatio based on synchrotron radiation imaging techniques.The effects of cooling rate on these amounts and nucleation of primary phase were studied from the aspects of solid phase volume fraction, dendritic tip growth rate, interface specifi surface area and surface curvature.It is found that the lower cooling rate of 3 °C/min results in a structure that is more globular in nature, as compared to the thinner dendritic structures when the cooling rate is 4 times faster (12 °C/min).For both cooling rates,the initial dendritic microstructure is observed to change significantl during the free growth stage (a1-a2, c1-c2).Besides, as the temperature decreases and the solid evolves,the dendrites become thicker and the curvatures reduce, as shown in Fig.12.Later, Guo et al.[87] used synchrotron radiation three-dimensional tomography to directly quantify the evolution of dendritic structure length scale of Mg-Sn dense-packed hexagonal alloy during 4D coarsening.It showed that the coarsening of dense hexagonal dendrites is mainly caused by the remelting of fin dendrites and the combination of adjacent dendrites through three-dimensional observation.It has also been found that the solute concentration has a significan influenc on the morphology of the obtained microstructures and that increasing the solute solubility will result in dendritic and algae-type particles.

Fig.10.EBSD analysis on the crystallographic orientations of (a,b) Mg-30 wt%Zn, and (c,d) Mg-45 wt%Zn alloys, respectively [83].

Fig.11.Growth pattern of the hcp a-Mg dendrite reconstructed from the synchrotron X-ray tomography experiments.(a) and (b) show the 3D growth pattern of multi-dendrites viewed from different perspectives, (c) shows the typical 3D morphology of the hcp a-Mg dendrite grain, (e, f) show the dendritic morphology viewed from <0001>, <1010> and <1120>, respectively.(g) shows the preferred growth directions of the hcp a-Mg dendrite analyzed by the EBSD experiments [84].

Fig.12.2D slices and 3D surface rendering of dendrite evolution during solidificatio of Mg15 wt%Sn for the cooling rates of 3 °C/min (a1-a3, b1-b3) and 12 °C/min (c1-c3, d1-d2) [85,86].

3.4.Fe-based alloys

Fig.13.Morphology evolution of Fe-Si alloys [88].

The evolution of microstructure in the solidificatio process of steel has always been the focus of scholars.However,due to the high requirements for steel materials for samples and heating equipment, the research on synchrotron radiation of Fe-based alloys started relatively late.Until 2008, Yasuda et al.[88] took the lead in the study of Fe-10Si-0.5Al alloy.They clearly observed the dendrite growth and crystal morphology evolution ofδ-Fe, as shown in Fig.13.The white area in the figur is due to solidificatio shrinkage of the metal as it solidifie in the sheet area, indicating that the fl w of the melt in the mushy zone occurs at the fina stage of solidification During the solidificatio process,the dendrites are significantl coarsened, and the secondary dendrite arm spacing at the initial stage of solidificatio is about 20 μm.At the late solidificatio stage, the secondary dendrite arm spacing becomes 60 μm.Moreover, in the late stage of solidification most of the primary dendrite arms and secondary dendritic arms are difficul to distinguish in the figure After that, for the firs time, they carried out imaging experiments on the solidificatio of pure iron and low carbon steel, which made it possible to observe the solidificatio process of various alloy steels in situ [89].When the Fe-C alloy was observed in situ by synchrotron radiation, it is found that the coarsening of the second arm in the Fe-0.3 wt% C alloy is divided into three stages: (I)secondary arm selection, (II) coalescence occurs, resulting in a rapid increase in arm spacing, and (III) liquid phase is separated by solids [90,91].Studying the melt transition from stage I to stage II and from the stage II to stage III is helpful to determine the permeability of the melt in the paste region and the mechanical properties of the alloy after solidification

With the deepening of the research on synchrotron radiation of medium and high-temperature alloys, more and more studies no longer limit the observation of solidifica tion microstructure of alloys on a two-dimensional scale but attempted to probe the evolution process of solidifica tion microstructures of alloys at 3D and 4D scales.These studies have taken into account the different orientations,thermodynamics, and dynamics.In the aspects of the CET transformation process, fracture and dissociation of dendrite arm, dendrite evolution of columnar dendrite dynamic growth, many new discoveries have been obtained.The solidificatio mechanism and dendrite evolution law of the alloy have been adequately studied, which provides reliable theoretical guidance for the improvement and change of various properties of the alloy materials.

4.Synchrotron radiation in situ imaging of alloy dendrite growth under an external fiel

The addition of external physical field has obvious effects on refinin alloy grains and eliminating component segregation.The physical field currently studied mainly include:(1) electric field (direct current, alternating current, pulse current) (2) magnetic field (traveling wave magnetic fields pulsed magnetic fields rotating magnetic fields etc.) and (3)ultrasonic fields Based on these aspects, many researchers have used synchrotron radiation imaging technology to characterize in situ a series of microstructural evolution behavior of alloy grains such as dendrite growth, fracture,and transformation.Through the interaction of metals and physical fields the nucleation and growth process of metals are affected, and the solidificatio structure can be improved.The application of physics field is a new way to observe the microstructure evolution behavior of the alloy during the solidificatio process, and finall , optimize the performance of the control alloy.

4.1.Microstructure evolution of alloy solidificatio under electric field

The current changes the convection mode, solute diffusion,temperature transfer and interface energy state inside the melt through electromagnetic force, electromigration, and electro-heating effect, thereby affecting the thermodynamic and dynamic processes of grain growth.Wang and co-workers[92-95] studied the evolution of dendrite morphology during solidificatio of Sn-Bi alloys using the third generation Shanghai synchrotron radiation source under the action of DC and pulse current.The mechanism of current inhibiting dendrite branching and promoting dendrite refinemen were successfully revealed.Besides, the current-induced dendrite tip splitting was revealed.

Fig.14.Synchrotron X-ray diffraction patterns of Cu-Cr-Zr alloys (a) cold rolled, (b) aged at 450 °C for 4 h, and (c) aged at 450 °C for 4 h with DC pulses [100].

In 2015, He et al.[96] and Wang and Lin [97] studied the change of crystal structure of Sn9Zn alloy under current stress by in-situ synchrotron radiation X-ray diffraction(XRD) analysis.The results show that the XRD orientation peaks of tin and zinc crystals decay rapidly under current stress.Later, Yang et al.[98,99] used in situ synchrotron X-ray imaging technology to study the effect of DC on grain nucleation and growth of Sn-50 wt%Pb alloy.It is observed that the application of direct current during solidificatio could effectively promote the nucleation of grains and inhibit their growth.However, Wang et al.[100] investigated the response of Cu-Cr-Zr alloys to aging under direct current(DC) pulses.They used synchrotron radiation X-ray diffraction analysis to confir the retarding effect of DC pulses on the precipitation process, which is the reason for a slight decrease in conductivity.It is found that the synchrotron radiation X-ray diffraction experiment can provide better statistical data, so it has a clearer peak than the traditional X-ray diffraction, as shown in Fig.14, it presents synchrotron X-ray diffraction patterns of Cu-Cr-Zr alloys under three conditions:(a) aged at 450 °C for 4 h without DC pulses,(b) aged at 450 °C for 4 h with DC pulses and (c) cold rolled state.At the same time, Xuan et al.[101] used the synchrotron radiation technique to observe the structural evolution behavior of the peritectic solidificatio of Sn-Cu alloy under direct current (DC) fiel and perform real-time imaging.It is found that when a DC fiel of 20 A/cm2or 200 A/cm2is applied during the solidificatio of the Sn-10%Cu alloy, the peritectic phaseηis directly precipitated from the parent liquid phase without undergoing a corresponding peritectic reaction.This observation suggests that the DC electric fiel has changed the phase change sequence and suppressed the nucleation of the primaryεphase.

Fig.15.Fragmentation maps from a sequence during the solidificatio of Al-15 wt%Cu with thermal gradient G = 48 K mm?1 and no external PEMF.In each image the location of a fragmentation event within the 30 s time frame is identifie by a yellow circle [102,103].

4.2.Microstructure evolution of alloy solidificatio under magnetic field

As a cast alloy, Al-Cu alloy is a kind of aluminum alloy which has the earliest application and the highest heat resistance.In-situ observation of microstructure evolution of Al-Cu alloy under physical field emerges very recently.Liotti et al.[102,103] used the synchrotron radiation imaging technique to observe the fracture and separation behavior of alloy dendritic arms under the action of a pulsed magnetic fiel (static magnetic fiel +pulse current).It is pointed out that solute enrichment is the main factor leading to remelting of dendritic roots and dendrite arm fracture.In the absence of PEMF (Pulsed Electromagnetic Field), as shown in Fig.15,it shows a sequence fragmentation map for one of the experiments without a PEMF.Each map represents a period of 30 s and each yellow circle represents the location of a fragmentation event that occurred within that 30 s segment of the experiment.Unlike the former, Li et al.[104,105] studied the effect of high-intensity magnetic fiel on the directional solidificatio structure of Zn-Cu peritectic alloy in the axial direction.The experimental results show that the magnetic fiel is an important factor to induce the instability of the solid-liquid interface and the formation of banded structure.Furthermore, the magnetic fiel also causes the splitting of the columnarη-Zn andε-Zn5Cu dendrites.With the increase of magnetic fiel intensity, the dendrites undergo a transition from a columnar crystal to an equiaxed crystal.Cao et al.[106] studied the effect of travelling magnetic fiel (TMF) on solute distribution and dendrite growth during solidificatio of Sn-Pb alloy based on Shanghai light source, as shown in Fig.16, it shows the schematic of the interaction between the forced melt fl w and the dendrites with different inclination angles to illustrate the mechanism of the growth behavior of the secondary arms under TMF.They get that the Sn-rich melt at the left side of dendrite II and the right side of dendrite IV are not easy to be transported into the melt with melt fl w from left to right.As a consequence, the growth of the secondary arms in these areas is suppressed due to the solute enrichment.While the Sn-rich melt transportation is enhanced by the forced melt fl w,which facilitates the growth of secondary arms.Because the tin-rich melt on both sides of primary dendrite is easily washed away, the growth behavior of secondary arms on both sides of dendrite is similar.

Fig.16.Schematic of the interaction between the forced melt fl w and the dendrites with different inclination angles [106].

4.3.Microstructure evolution of alloy solidificatio in the ultrasonic fiel

In order to explore the evolution behavior of alloy solidificatio microstructure under the action of an ultrasonic field Lee and co-workers [107-110] firs took advantage of synchrotron radiation imaging technology to study quasisteady-state cavitation bubble dynamics in Sn-Bi melt with a low melting point.The dynamics of ultrasonic bubbles in liquid metals and their interaction with solidifie solids in transparent alloys have been in situ captured.This study provides a more quantitative analysis of how ultrasound/bubbles affect the growth of dendritic grains.It is found that dendrite grain refinemen is accelerated by ultrasound/bubbles.Later,Huang et al.[111,112] made use of in-situ synchrotron radiation technology to study the acoustic cavitation behavior of ultrasonically processed Al-Cu alloys in situ.A quantitative study was carried out on the characteristic of size distribution,quantity density, and volume fraction of acoustic cavitation bubbles, and the size of acoustic cavitation zone.Combined with the small-angle scattering of synchrotron radiation, the structural changes after melt sonication was further analyzed,which provided evidence for acoustic cavitation-induced nucleation.After that, Nagira et al.[113] observed in situ the evolution of dendritic morphology of Sn-Bi alloy under melt convection and oscillation induced by ultrasound.He found that after applying ultrasonic vibration, the ultrasonic wave immediately caused cyclic convection at the tip of the dendrite while the size of the convection is almost the same as the sample.Finally, it is concluded that both cyclic convection and oscillation can promote solute transport in the mushy region.Xu and co-workers [114-116] studied the in situ behavior of the formation and evolution kinetics of ultrasonic cavitation bubbles in Al-Cu alloy melts and quantitatively analyzed the average diameter,size distribution,and growth rate of the bubbles.Recently, Wang et al.[117] made use of in-situ observation of the phenomenon of the fragmentation and refinemen of Al2Cu intermetallic compound dendrites in hyper-melting treatment of hypereutectic Al-35%Cu alloy during ultrasonic melting treatment.That is, the acoustic cavitation and acoustic streaming fl w progressively break the intermetallic dendrites into small fragments.Most of these small fragments can survive and then act as nuclei for the subsequent solidificatio of intermetallic phases, consequently leading to intermetallic refinemen in the solidifie microstructure.

At the same time, Wang et al.[118] conducted system experiments taking advantage of ultrafast synchrotron X-ray imaging (up to 271,554 frames per second) technology.In order to study the dynamic interaction between ultrasonic bubbles and acoustic currents and the solidifie phase of Bi-8%Zn alloy, the results show that the ultrasonic bubbles and enhanced sound fl w are obvious for the fracture of the solidifie Zn phase and the damage of the liquid-solid interface.The calculated results are in good agreement with the theoretical ones.In this study, unambiguous experimental evidence and robust theoretical explanations were used to clarify that ultrasound is the primary mechanism of microstructural fragmentation and solidificatio tissue refinement

The physical fiel has a significan influenc on the low-melting Sn-based alloy, the medium-high melting point alloys such as Al-Cu and Al-Si, even the high-melting steel solidificatio structure and dendrite growth.Combined with the physical field the in-situ imaging characterization of the solidificatio microstructure of different alloys can obtain a series of microstructure evolution processes such as dynamic dendrite growth, fracture, transformation, solid-liquid interface transformation, solute distribution, and acoustic cavitation bubble formation.The application of the external fiel enriches the process of microstructure evolution of the alloy under synchrotron radiation conditions and improves the research fiel of dendrite evolution.

5.Summary and outlook

The rapid development of synchrotron radiation 2-D/3-D imaging technology based on the third generation of synchrotron radiation sources has brought new opportunities for the research of metal materials.The real-time, in-situ,dynamic, and non-destructive characterization ability at high resolution reveal the phenomena of columnar-equiaxed transformation, free growth of equiaxed crystals, directional growth of columnar crystals, fracture, and dissociation of dendritic arms and competitive growth between dendrites in solidificatio structures of low-temperature and medium-hightemperature alloys.These imaging results provide not only direct experimental evidence for verifying the solidificatio theory of metal alloys but also provide technical support for further improving and enriching the solidificatio theory.Therefore, using synchrotron radiation imaging technology to analyze crystal morphology, solute distribution, growth rate, and other aspects can obtain a large number of accurate data which cannot be obtained by post-analysis method.At present, synchrotron radiation imaging technology has become the most advanced and powerful tool for studying the solidificatio process.Although synchrotron radiation imaging technology has greatly promoted the study of alloy microstructures, its potential in the solidificatio process,especially in solid phase transformation, has not been fully exploited because the technology is still in the developing stage.There are mainly the following problems to be solved.

(1) At present, the research of synchrotron radiation mainly focuses on Sn-based and Al-based alloys with low melting points, while the research on superalloys widely used in aerospace and other extreme environmental field has been less carried out.

(2) Under the traditional experimental conditions, the in-situ study of various alloys has been carried out, while the insitu study of the synchrotron radiation of alloy dendrites with an electric field magnetic field ultrasonic field and other physical field is relatively rare.Synchrotron radiation imaging technology has great potential in the application of an external fiel to study alloys, which needs to be further explored.

(3) In the study of dendrite growth, two-dimensional imaging observation is mainly used.Although three-dimensional reconstruction technology has been applied in the study of alloy dendrite growth, it is still static three-dimensional imaging.The dynamic three-dimensional characterization(4D) of the solidifie microstructure is just getting started.How to use synchronous radiation imaging technology to obtain real-time 3D or 4D dynamic image results of alloy solidificatio structure is our future research direction.

(4) At present, synchrotron radiation is used to study the solidificatio microstructures of alloys in a single form,while it has not been yet integrated with other characterization techniques.Synchrotron radiation technology has great potential in combination with EBSD, numerical simulations, and other characterization technologies, which need further development.

In conclusion, in situ synchrotron radiation imaging technology has significan advantages in the study of alloy structure, and it will become a powerful technical means in the fiel of material science and engineering.For instance,this technology has also been well applied in the research of the growth kinetics of amorphous and quasicrystalline materials and the initiation and propagation of microcracks.In particular, synchrotron radiation imaging technology has unique advantages in the three-dimensional reconstruction,which is impossible to achieve by conventional electron microscope analysis technology.Therefore, we expect that the synchrotron radiation multi-dimensional in-situ imaging technology will become one of the most powerful technical means in the fiel of material science and engineering.For example, a synchrotron radiation technique can be used to observe the evolution of solidificatio microstructures of alloys at high rotating speed in real time.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

Acknowledgments

The author would like to thank financia support of the National Natural Science Foundation-Youth Science Foundation Project (51901208, 51771113), Henan University Key Scientifi Research Project (20B430020), the Key scientifi and technological projects in Henan Province(202102210016,202102210272), and Zhengzhou University of Light Technology Doctoral Research Initiation Fund (JDG20190098).