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Sequences of in situ solidification experiments of Al-15% Cu alloy at different cooling rates: (a) 8 K/min; (b) 16 K/min; (c) 60 K/min. The time each image was recorded is shown in the top left of each image.
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The solute concentration profile ahead of the solid-liquid (S-L) interface and the size of the solute suppressed nucleation zone (SSNZ) were studied in situ by synchrotron X-ray radiography during the solidification of Al-15 wt% Cu alloy inoculated by Al-Ti-B. The measured solute distribution ahead of the S-L interface and the size of SSNZ were in...
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Context 1
... Gaussian filtering and the edge region was cropped because of the instability of X-ray and filtering effects. Finally, the image was normalized using values of maximum and minimum pixel intensities found over the whole sequence in order to emphasize the contrast between the solid and the liquid maintaining consistency within each experiment. Fig. 1 shows the time evolution radiographs of in situ solidification experiments of Al-15% Cu alloy at different cooling rates (8, 16 and 60 K/min). In the present research, thin samples (~250 μm) are used to limit convection effects as much as possible. The origin time of each sequence is the appearance of the first grain. In the bulk melt, ...
Citations
... For Al alloys, the methods to refine grain microstructure during solidification process are various [8], mainly including rapid solidification methods (such as high-pressure die casting and rheo-diecasting [9], liquid forging technique [10], melt spinning [11], pulsed laser melting [12]), vibration and stirring methods (such as mechanical vibration [13], electromagnetic stirring [14], ultrasonic vibration [15]), grain refiners [16][17][18] or solute elements methods [2,19,20]. From the point of practicability, economy and operability, adding grain refiners is the most widely used and effective method. ...
Al–5Ti–1B master alloy is the most widely applied grain refiner for Al alloys. However, some essential alloying elements in high-performance Al alloys, such as Si and Zr, can cause the fading or failure of the refining effect, which is called Si/Zr-poisoning. This phenomenon can seriously restrict the improvement of comprehensive performances and lead to poor processability and high rejection rate of the products. To solve the above problems, the mechanisms of Si/Zr-poisoning and the corresponding coping strategies in high-performance Al alloys are highlighted, clarified and summarized by reviewing the latest research progresses in this work. In general, the poisoning mechanisms can be classified into efficacy losing of nucleating substrate during the grain nucleation stage and weakened growth restriction capabilities during the grain growth stage. The coping strategies carried out on the industrial site mainly include increasing addition level of refiners or pre-placing it in mold/casting trough, which can bring some new problems, such as generation of coarse compounds and increase in the scrap rate. Meanwhile, the progresses or breakthroughs in academia basically focus on the development of new refiners, improving existing master alloys and micro-alloying design, which have provided a variety of feasible paths to solve the poisoning problem, among which the Al-TCB master alloy can realize anti-Si/Zr-poisoning based on the dynamically evolving TCB complex. It is believed that the anti-Si/Zr-poisoning mechanisms and coping strategies will be renewed and enhanced persistently with the characterization and detection technologies driving forward.
... It can be speculated further that because the more efficient nucleants for a-Al were removed by the prior IMC formation, the less efficient nucleants remaining resulted in a slightly higher nucleation undercooling for a-Al: 0.6 K for a-Al to nucleate on a relatively small, 1 lm Al 3 Ti particle, compared with 0.1 K on a relatively large, 5 lm Al 3 -Ti particle according to the free growth model [73]. Higher undercoolings promote the tendency for burst nucleation of a-Al that can result in finer dendrites [32,76,75]. Further work will investigate this approach quantitatively and whether it has the potential to refine both primary IMCs and a-Al in recycled alloys of significantly elevated Fe concentration. ...
Fe-rich intermetallics affect critically the mechanical properties and recyclability of aluminium alloys. Increasing effort has been spent on the inoculation of these intermetallics, hoping to promote a finer distribution. Recently Al-5Ti-1B (wt.%), originally developed to refine α-Al, has been shown to refine Al13Fe4, an intermetallic phase present in a variety of Al alloys. However, mechanisms of the formation and growth of the intermetallics on the inoculants are unclear. In this paper, Ti is added to Fe-containing Al alloys to produce a large number of potent Al3Ti particles, the active inoculant in Al-5Ti-1B. We use a combination of electron backscattered diffraction, in situ synchrotron X-ray radiography and post-solidification X-ray computed tomography to investigate the formation and growth of primary Al13Fe4 on Al3Ti inoculants, first in a model Al-Fe alloy, with key insights then confirmed in a high Fe-containing, recycled 6xxx alloy. Crystallographic orientation relationships between Al13Fe4 and Al3Ti are analysed comprehensively, and the formation and growth dynamics of Al13Fe4 on Al3Ti is also unveiled. A strong link is revealed between the formation of Al13Fe4 on Al3Ti and a twinning-related pseudo-symmetry of Al13Fe4. Finally, a potential strategy to refine both intermetallics and α-Al in recycled alloys with elevated Fe concentration is proposed.
... The time intervals between two or more grains nucleating on the same particle are always less than 3 s in our experiment due to the effect of solute suppressed nucleation zone (SSNZ) caused by the previous grain [13]. Thus, the easy detachment of grains from particles is beneficial for new grains to nucleate on the free particles. ...
... Y 2 O 3 resulted in a reduction in grain size and improvement in hardness. Furthermore, it is found an appropriate amount of rare earth elements (La, Ce, Er, Y, etc.) can purify the melt of Al-Ti-B alloy and significantly increase the long-term effectiveness of the refiner [13][14][15] . The addition of rare earth elements in Al-Ti-B grain refiner decreases the size of TiAl 3 and obtains dispersively distributed TiB 2 particles, by which the refining efficiency of Al-Ti-B is improved [16] . ...
... Although intensive research has been conducted concerning the effects of rare earth elements addition on the grain size and mechanical properties of aluminum alloys [13][14][15][16] , little attention has been paid to the effects of rare earth elements on TiB 2 morphology evolution and the modification mechanism. Therefore, to master its growth mechanism and realize the morphology control is of great significance for the preparation of high-performance grain refiner. ...
To investigate the effect of rare earth on size and distribution of TiB2 phase in aluminum alloy refiner, Al-5Ti-1B-RE master alloy was fabricated, and its microstructure and phase constitutions were investigated by the combination of first principles calculations and experimental investigation. The calculated results reveal that Ce has the most effective modification ability due to the most positive adsorption energy and the largest charge transfer value compared with other rare earth elements. Results of experimental investigation indicate that the addition of rare earth in the Al-5Ti-1B alloys can greatly refine the particle size of TiB2, improve the distribution of particles and lead to better refinement effect than that of the Al-5Ti-1B alloys without rare earth. Moreover, Ce has the best optimization effect, which agrees well with the calculated results.
... Assuming that T A gradient at the S/L interface remains stationary, both T E and (T E -T A ) raise with x unless T E reaches constant at the point of T E (C 0 ), at which the maximum compositional subcooling is achieved (T E (C 0 )-T A (C 0 )). The interdependence theory proposes that the interaction of nucleation and growth jointly affects the grain size of the final microstructure, in which the association between grain size (d gs ) and confinement factor (Q) can be expressed as [50][51][52]: ...
Innovative in-situ synthesis tactics of devising Al-Cu4/Al-Mg1 alloy inoculated with Fe80-B11-Si9 (at.%) metallic glasses were proposed to achieve comprehensive multiscale inoculation of the microstructure. The effects of inoculant on the solidification behaviors were studied, and calculated the mismatch between α-Al and Fe2B/Fe3Si according to the E2EM model. Formation of nano-Fe2B particles exerts low mismatch with α-Al was effective in stimulating heterogeneous nucleation. In the meanwhile, nano-Fe3Si particle and nano-Fe2B particle that is not trapped in α-Al dendrites hinder the movement of the S/L interface through adsorption, thus hindering the growth of α-Al dendrites. With inoculation, the grains of AlCu4 and AlMg1 got significantly refined. The manipulation of microstructure also includes weakening of element segregation and refinement of precipitation, which is ultimately accounted for the improvement of comprehensive mechanical properties. The various strengthening mechanisms of inoculated AlCu4 and AlMg1 alloy are explored, which offers an innovative idea for broadening the industrial utilization of metallic glass.
... However, it should be noted that the localized region near the mould wall is at an even lower temperature (shaded regions marked as cold zones in Fig. 12 ). At the earlier stages of solidification, external forces ap- Size and size distribution of particles, crystallographic matching, undercooling and solute and temperature gradient Activation of a potent particle when the critical undercooling is reached [ 7 , 44-47 ] In-situ studies: Validates the observations made in the conventional conditions for potent particles [182][183][184] . ...
Refinement of grains and intermetallic phases in the as-solidified alloy structure offers uniform structural properties, eliminates or minimises common solidification defects, including segregation and hot cracking, and improves thermomechanical processing of wrought alloys. Melt processing by an external field is an efficient process for achieving refinement of the solidification structure of Al and Mg alloys without altering the alloy composition. A wide range of melt processing methods and solidification studies (conventional, directional, and in-situ approaches) have been reported in the literature that explore the mechanism of refinement. Identifying the dominant grain refinement mechanism has been a focus of most investigations because significant variations exist according to the casting conditions and the type of applied external treatments. The origin of fine grains occurs through either one or a combination of heterogenous nucleation, fragmentation of dendrites and grains formed and then separated from the surface of the melt and mould wall under vibration or agitation. The first part of this review describes the prominent external field techniques and the mechanisms proposed for the origin of fine grains. The second part critically compares the current understanding of these grain refinement mechanisms to determine differences and commonalities to identify the factors that promote the formation of equiaxed zones occupying a large volume fraction of the casting.
... d Can be regarded as the size of the SDZ around the growing grain. Jia et al. [35] measured the size of the SDZ in situ by synchrotron X-ray radiography during the J Mater Sci solidification of Al-Cu alloy inoculated by Al-Ti-B master alloy. When the cooling rate is about 8 K/min, the experimental measurement value of the SDZ is about 260 lm. ...
... The study of Jia et al. [35] shows that the actual solute distribution around equiaxed grains is between initial transient growth and steady-state growth. When grain size is small, it is more appropriate to consider initial transient growth for solute distribution around the growing grain. ...
The solidification microstructure of aluminum foams is evidently different from that of bulk aluminum alloys. Generally, a single α-Al grain runs through the entire thickness of a cell-wall, and the grain morphology is irregular. However, the formation mechanism of the special microstructure in aluminum foams is unclear now, and few studies focused on optimizing the microstructure. In this study, grain refinement of aluminum foams by increasing cooling rate and inoculation were carried out, and thermal analysis was used to measure and judge solidification of aluminum foams. Experimental results show that grain refinement can be achieved by the above two methods, and the latter is more effective. The nucleation undercooling of aluminum foams decreases with the increase of Al–5Ti–1B addition. Grains can be well refined even in thin cell walls after inoculation, and the grain morphology changes from irregular to equiaxed. Under the same conditions, α-Al grains of Al foams are smaller than those of the bulk Al alloy, either with or without inoculation. An anisometric growth model of α-Al grains in foamed aluminum melt restricted by surrounding bubbles is proposed. The model shows that: (1) the size of the inhibited nucleation zone (INZ) around growing grains decreases due to the high constitutional undercooling caused by solute enrichment in cell walls, thus grains in cell walls are smaller than those of bulk Al alloy; (2) under normal solidification conditions, the shape of the INZ is irregular. Grains in aluminum foams will replicate contours of cell walls or Plateau borders, presenting irregular shapes. Increasing cooling rates and/or inoculation can reduce the size of INZ and change its shape. The grain morphology will change from irregular to equiaxed under proper conditions.
Graphical abstract
... Whereas, no immobile particle that failed to be the grain nucleus was detected in either 0.6Sc (Fig.6(c)) or 0.6Sc×10. This phenomenon indicates the lower efficient grains nucleation on Al 3 Sc particles in 1.0Sc, which can be explained by the theory of solute suppressed nucleation zone (SSNZ) [34,44], or nucleation free zone (NFZ) [6,45] that was in situ confirmed by Jia et al. in Al-15 wt.% Cu alloy [46]. In our work, a larger particle in 1.0Sc naturally led to a larger initial grain, and the nucleated grain suppressed the potential nucleation areas of its adjacent particles. ...
Grain refinement of Al alloys inoculated by rare earth elements, such as Sc, has been extensively acknowledged, while the practical behavior of how inoculant Al3Sc particles affect the refinement in solidification has not been clarified due to the non-transparency of the solidification process. Here, the microstructural evolution of primary Al3Sc particles and α-Al grains in Al-10 wt.% Cu alloy solidifications with 0.2 wt.%, 0.6 wt.%, and 1.0 wt.% Sc additions was investigated by in situ synchrotron X-ray radiography. The detailed mechanisms of curve motion of grains (CMG) and melt convection were revealed. The efficient grains nucleation, uniformly scattered small initial grains, and long duration of melt convection contribute to the best refinement in the 0.6 wt.% Sc addition sample. This work provides a deep insight into grain refinement in solidification with Sc addition, which will enlighten the composition design and casting process of Al alloys inoculated by rare earth elements.
... Recently, with the advent of highenergy, high-brilliance synchrotrons and better laboratory-based X-ray sources, together with more efficient X-ray detectors, X-ray imaging techniques have been increasingly used to investigate solidification processes in real time. These techniques have allowed important dynamic information to be obtained such as crystal formation rate [12][13][14][15][16][17][18], estimates of nucleation undercoolings [12,19], instantaneous solute distributions [12,[14][15][16]20], crystal fragmentation [21][22][23][24][25][26], and thermo-solutal convection patterns [17,20,[27][28][29]. In this paper, we give an overview of our research work using in situ X-ray radiography of Al solidification. ...
... Recently, with the advent of highenergy, high-brilliance synchrotrons and better laboratory-based X-ray sources, together with more efficient X-ray detectors, X-ray imaging techniques have been increasingly used to investigate solidification processes in real time. These techniques have allowed important dynamic information to be obtained such as crystal formation rate [12][13][14][15][16][17][18], estimates of nucleation undercoolings [12,19], instantaneous solute distributions [12,[14][15][16]20], crystal fragmentation [21][22][23][24][25][26], and thermo-solutal convection patterns [17,20,[27][28][29]. In this paper, we give an overview of our research work using in situ X-ray radiography of Al solidification. ...
In the last two decades, X-ray imaging techniques have been used increasingly to study metal solidification in real-time as, thanks to advances in X-ray sources (synchrotron and laboratory-based) and detector technology, images can now be obtained with spatio-temporal resolutions sufficient to record key phenomena and extract quantitative information, primarily relating to crystal growth. This paper presents an overview of the research conducted at the University of Oxford over the last 6 years as a partner in the UK’s Future Liquid Metal Engineering (LiME) Manufacturing Hub. The focus is on in situ X-ray radiography to investigate the solidification of Al alloys, including the formation of primary α-Al crystals, and the formation and growth of secondary intermetallic phases. Technologically, the thrust is to understand how to control as-cast phases, structures and element distributions, particularly elements associated with recycling, as a means to facilitate greater recirculation of aluminium alloys. We first present studies on refinement of primary α-Al, including extrinsic grain refinement using inoculation and intrinsic refinement based on dendrite fragmentation. Second, we describe studies on intermetallic phase formation and growth, because intermetallic fraction, morphology and distribution are frequently a limiting factor of alloy mechanical properties and recyclability. Then we present some of the latest progress in studying liquid flow during solidification and associated hot tear formation. Finally, future research directions are described.
... Unlike the aforementioned works that largely focused on X-ray imaging techniques, the current work instead presents a comprehensive review of knowledge provided by in situ X-ray imaging, for better understanding of solidification theories. Thanks to the high spatial (sub-micron) and temporal (microseconds) resolution offered by third-generation synchrotron sources, significant progress has been made in the understanding, validation and development of theories and models for near-equilibrium solidification, including solute suppressed nucleation (SSN) of both primary solid-solution α-Al [9][10][11][12][13][14] and secondary ordered intermetallics [15,16], dendritic growth of α-Al [17][18][19][20][21][22] and faceted, twin plane re-entrant (TPRE) growth of Fe-rich intermetallics [23][24][25][26], crystal fragmentation [27][28][29][30][31][32], morphological transition [33][34][35][36][37][38][39][40] and defect formation [41][42][43][44][45][46][47]. The focus of solidification research on Al alloys derives from the relatively easy-to-achieve melting temperatures of around 660°C, and excellent absorption contrast between Al and typical alloying elements such as Cu and Zn. ...
... For the past century, significant effort has been spent on understanding crystal nucleation and growth, and insightful and robust nucleation and growth theories have been developed [73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88]. Since the 1990s, and particularly during the past decade, as pioneered by Mathiesen et al. [89,90], there has been a surge in the number of in situ studies of crystal formation and growth, mostly using X-ray absorption imaging, i.e., radiography [10][11][12][13][14][15][16]18,24,26,27,29,[62][63][64][91][92][93] and tomography [21,32,35,[94][95][96][97][98][99][100][101][102][103][104][105]. ...
... A number of in situ studies have been conducted to understand better these types of solute effects on α-Al nucleation [9][10][11][12][13][14]. For example, meaningful trend-wise insights were gained into the influence of solidification conditions such as cooling rate on the size of the SSNZ and the overall grain refinement effect. ...
Synchrotron and laboratory-based X-ray imaging techniques have been increasingly used for in situ investigations of alloy solidification and other metal processes. Several reviews have been published in recent years that have focused on the development of in situ X-ray imaging techniques for metal solidification studies. Instead, this work provides a comprehensive review of knowledge provided by in situ X-ray imaging for improved understanding of solidification theories and emerging metal processing technologies. We first review insights related to crystal nucleation and growth mechanisms gained by in situ X-ray imaging, including solute suppressed nucleation theory of α-Al and intermetallic compound crystals, dendritic growth of α-Al and the twin plane re-entrant growth mechanism of faceted Fe-rich intermetallics. Second, we discuss the contribution of in situ X-ray studies in understanding microstructural instability, including dendrite fragmentation induced by solute-driven, dendrite root re-melting, instability of a planar solid/liquid interface, the cellular-to-dendritic transition and the columnar-to-equiaxed transition. Third, we review investigations of defect formation mechanisms during near-equilibrium solidification, including porosity and hot tear formation, and the associated liquid metal flow. Then, we discuss how X-ray imaging is being applied to the understanding and development of emerging metal processes that operate further from equilibrium, such as additive manufacturing. Finally, the outlook for future research opportunities and challenges is presented.
