Recent Topics on the Structure and Crystallization of Al-based Glassy Alloys

Al-based glassy alloys (metallic glasses) have been of great scientific and technological interest as a high specific strength and high corrosion resistant materials. However, the low glass-forming ability (GFA) is still a choke point that greatly influences the applications. In order to further comprehend the glass formation mechanism and investigate the possibility of enhancing the GFA, it is important to obtain the definite information on the atomic scale glassy structures in correlation with properties and to study the composition dependence of crystallization and properties of Al-based metallic glasses. The purpose of this paper is to summarize the structure characteristics and crystallization behavior in conjunction with alloy component in Al-based metallic glasses and to investigate the possibility of synthesizing an Al-based bulk metallic glass with better engineering performances. This may assist to learn the structural features systematically and understand the development and current status for Al-based metallic glasses.


Introduction
Metallic glasses have been intensively investigated in conjunction with the developments of various meltquenched techniques.The first synthesis of a glassy alloy in Au-Si system by rapid quenching form melt was made by Klement et al. in 1960 1 .Since then, much attention has been paid to metallic glasses and a great large number of studies have been carried out to date.Nowadays, metallic glasses can be produced in a variety of alloy systems.These glassy alloys exhibit excellent characteristics such as high mechanical strength [2][3][4] and good corrosion resistance [5][6][7][8] due to the absences of long-range periodic atomic order and grain boundary.Tremendous effort has been devoted to pursue the bigger size of bulk metallic glasses (BMGs) 9 that are referred to the glassy alloys whose dimension is over 1 mm 10 .BMGs have attracted a definitely increasing interest because of their formation in engineering alloy systems such as Mg- [11][12] , Zr- [13][14][15][16] , Ti- [17][18][19] and Fe-based [20][21][22] .At present, the world's biggest glassy alloy is Pd 42.5 Cu 30 Ni 7.5 P 20 with a diameter of 80 mm and a length 85 mm prepared by a water-quenching method in 2012 23 .
In the development history of metallic glasses, Al-based metallic glasses have occupied special important situation because of the expectation to develop a high specific strength material.In the 1970s, Al-metalloid and Al-TM (transition metal) binary alloys prepared by splat quenching were reported to have a mixed structure of amorphous and crystalline phases [24][25][26][27][28] .In 1981, Inoue et al. 29 prepared fully amorphous alloys in Al-(Fe, Co)-B systems by melt spinning for the first time.Subsequently, they reported that an amorphous phase is formed in melt-spun Al-(Fe, Mn)-Si alloy ribbons 30,31 .However, these amorphous alloys in Al-TM-metalloid systems were not always ductile and hence no systematic data on fundamental properties were obtained.In 1988, Inoue et al. [32][33][34] and He et al. 35,36 succeeded in synthesizing Al-based amorphous alloys with good ductility in Al-ETM-LTM (ETM=IV-VI group transition metals, and LTM=VII and VIII group transition metals) and Al-RE-LTM (RE=lanthanide group metal) systems as well as in Al-RE binary system.The Al-RE-LTM glassy alloys are the most popular at present because of their higher strength, higher glass-forming ability (GFA), and the appearances of glass transition and supercooled liquid region.Al-based metallic glasses, which can be regarded as an important group of metallic glasses, have a wide range of applications 37 .Their high specific strength combined with outstanding corrosion resistance allows us to expect their application to medical fields 38 .In addition, their good ductility as well as excellent corrosion resistance behavior [39][40][41][42] makes them a promising candidate in engineering applications to structural and coating materials 43 .Al-based metallic glasses can also exhibit a degradation property as an organic water contaminant when highly reactive elements such as Ca or Mg are included 44 .
However, the GFA of Al-based metallic glasses is relatively low comparing with other metallic glasses, resulting in the limitation of applications.Such relatively low GFA has been attributed to the following two aspects: (1) their formation ranges are located in Al-rich composition range which is largely away from their eutectic point 45 ; (2) the bonding mode of Al-RE, Al-ETM and Al-LTM pairs is covalent-like so as to enable easy precipitation of Al-based compounds.Thus, the most significant issue is how to enhance the GFA of Al-based metallic glasses 46,47 .Over the past few decades, the researchers devoted themselves to look for an approach for designing and predicting the best glass-forming composition in Al-RE-LTM base systems 46,[48][49][50][51][52][53] .Although no general rules are applicable to predict the GFA, one has noticed various criterions and parameters [54][55][56][57][58][59][60] which are useful and meaningful for the study on the properties of Al-based metallic glasses.
It is well known that the outstanding properties of glassy alloys result from the structure of atomic packing.Therefore, the atomic-level understanding of the structural characteristics is important for developing a new BMG with high GFA.Since the synthesis of Al-based metallic glasses with good mechanical properties in 1988 [32][33][34][35][36] , much effort has been devoted to clarify the structural features in conjunction with the structure origin for the appearance of glass transition.The previous structural models and empirical criteria for characterizing ordinary metallic glasses do not always fit for Al-based metallic glasses 61 .For example, most of BMGs with good GFA are usually located at and/or near a eutectic composition region, but Al-RE-TM glassy alloys are formed at the compositions which are away from their eutectic points 62 .The compositional feature of Al-based metallic glasses fit the atomic size criteria employed for producing BMGs, but their compositions deviate significantly to the Al-rich side 63,64 .Besides, the minor addition improving the GFA has little effect on majority of Al-based metallic glasses 43 and not all Al-based metallic glasses have a clear glass transition phenomenon 65,66 .Hence, understanding the structure and mechanism for glass formation in metallic glasses is not an easy work.The objective of this paper is to provide a review of Al-based metallic glasses in the aspects of the local structure, thermal stability and crystallization behavior.The accurate knowledge can promote the understanding on the effect of local atomic configuration structure on fundamental properties and contribute to future development and applications of Al-based metallic glasses.

Structural features of Al-based metallic glasses
Except for the experimental approaches, computer simulations have played an equal and sometimes more significant role in the understanding of experimental data and the prospection of properties.They are also useful in understanding the dominant factors for glass formation and can be applied for investigating an isolated effect of certain factors that are otherwise difficult to study experimentally 67 .Theoretical calculation is another important method.For example, Liao et al. used the theoretical calculation method to explore the GFA 68 , since the size limitation makes difficult to investigate the critical cooling rate of Al-based metallic glasses.So far, the local structure of Al-based metallic glasses has not been completely uncovered, but several structural models have been provided for describing the structure features.These structural models not only can provide a theory foundation and instruction for further studying the structural characteristics, but also assist to look for a new Al-based metallic glass with better properties.

Structure and GFA of Al-ETM-LTM ternary metallic glasses
Al-ETM-LTM alloys such as Al-Zr-Ni and Al-V-Cu 69 glasses are one group of Al-based metallic glasses.In Al-ETM-LTM system, Zr and Hf are the most effective elements to form the amorphous phase, followed by Ti, V, Mo, Nb, Cr and then Ta 70 .
Al-Zr-LTM glassy alloys have been widely reported [71][72][73][74] , in which Al-Zr-Ni alloy is the most popular due to its high GFA.The glassy Al-Zr-Ni alloys can be made in a rather wide composition range from 10 to 30 at % Ni and 5 to 20 at % Zr 33 .In Al-Zr-Ni system, Yang et al. (75 have recognized the multiple maximum peaks in GFA in a single eutectic system, in which Al 75.5 Zr 5.5 Ni 19 and Al 82 Zr 5 Ni 13 alloys are maximum in glass formation.This demonstrates that more than one best glass former can be found in a single eutectic system and the glass formation could result from an intricate balance of kinetic, thermodynamic and topological factors.The Al-ETM-LTM glassy alloys usually show a narrow supercooled liquid region and relatively low thermal stability, limiting their usage as engineering materials.Adding rare earth elements in Al-LTM amorphous alloys can improve the thermal stability 76 .However, the addition of Pr into Al-Zr-Ni alloys can promote the GFA while declines the thermal stability 77 , suggesting the unsuitability of the coexistent Zr and RE elements because of their positive heats of mixing. By means of (X-ray diffraction) XRD, Audebert et al. 78 investigated the structure of Al-Nb-Fe glassy alloys.They illuminated that the glass formation of Al-Nb-Fe alloys results from the strong interatomic interaction between A1 and Fe.During cooling process, such a strong interaction between A1 and Fe not only decreases the interatomic distances of the Al-TM and Al-Al pairs, but also avoids TM atoms becoming neighbors, leading to the highly stable amorphous alloys.Later, the experiments by Audebert et al. 79 show that the Al-Nb-Fe alloys have higher GFA than the Al-Nb-Ni alloys.It does not agree with the thermodynamic data 80 that the Al-Ni interaction is stronger than the Al-Fe interaction.The formation of icosahedral clusters in the liquid is easy for the Al-Nb-Fe alloys.However, the local structure in Al-Nb-Ni liquid is similar to that in Al 3 Ni phase which is formed during rapid solidification.Thus, the GFA cannot be determined only by the strong atomic interaction, indicating that the topology of the clusters formed in the liquid also plays a significant role in the GFA.

Structure and GFA of Al-RE binary metallic glasses
Amorphous Al-RE binary alloys 32,81 with much higher specific strength have been formed in the ranges of 9 to 13% Y, 7 to 11% La or Ce, 10 at % Pr, 8 to 12% Nd or Gd, 8 to 16% Sm, 9 to 14% Tb, and 9-12% Dy, Ho, Er, or Yb.Inoue et al. 82 found that the as-quenched phase is an face-centered cubic (fcc) supersaturated solid solution in the RE content ranges below the glass formation ranges, amorphous + unidentified phases in the higher RE content ranges exceeding slightly the glass formation ranges, and stable fcc-Al + Al 11 RE 3 (or Al 3 RE) in the RE content ranges much higher than the glass-forming ranges.
The RE element plays a significant role in the formation of Al-RE amorphous alloys.The attractive bonding nature was thought to be the largest for Al-RE pairs 83 .The glass formation in Al-RE alloys by liquid quenching is mainly likely to a strong attractive interaction of the constituent elements as well as the volume mismatch by the difference in the atomic size ratio 84 .Enhancing the attractive interaction could lead to a much sluggish atomic rearrangement in supercooled liquid.Consequently, the nucleation and growth reactions are restrained, leading to high GFA.
The Al-Sm system has been mainly chosen for the study on the structure and GFA, because it has the widest glass formation range in Al-RE binary alloy series.Kalay et al. 85,86 studied the structures of as-quenched amorphous and liquid Al-Sm alloys by using the atom probe tomography (APT) and high-resolution transmission electron microscopy (HRTEM).In the cooling process, the quenching rate is high enough to completely bypass fcc-Al nanocrystal precipitation.The amorphous Al-Sm alloys form a skeleton-like structure that is similar to cross-linking in a polymer.The Sm-rich regions form a continuous open network which surrounds the regions of almost pure Al with a size range of 2-5 nm.Later, Kalay 87 used the reverse Monte Carlo (RMO) simulations and HEXRD experiments to construct the structural model of amorphous structures.The results show that the Smcentered clusters are always highly coordinated with 16 Al atoms in the first shell, which agrees with the results obtained by X-ray absorption fine structure (XAFS) 88 .The further investigation with Voronoi Tessellation analysis indicates that the first shell neighborhood of Sm atoms in as-quenched state is similar to the high temperature metastable Al 11 Sm 3 tetragonal phase.The structure of Al-Tb glassy alloys exhibits similar characteristics with the Al-Sm alloys 89 .Kalay also pointed out that the strongly attractive and strong bond between Al and Tb atoms lead to a strong compositional fluctuation in the liquid.These regions of Al-Tb clusters act like "superatoms" which can divide the matrix into nanoscale regions enriched with Al, as shown in Figure 1 (a).At relatively high quenching rates, the entire sample will form an amorphous phase because the structure lies below the critical nucleation size.Such amorphous structures are separated into Al-rich and Al-depleted regions as indicated in Recently, Sun et al. 90,91 identified the Sm-centered "3661" as the dominant short-range order (SRO) in Al-Sm glassy alloys by using the cluster alignment method.The Sm-centered "3661" consists of a top triangle, two consecutive hexagons and a bottom atom.Its side view and top view are shown in Figure 2 (a) and (b), respectively.Owing to the marginal glass formability in Al-Sm glassy alloys, the "3661" clusters do not form an interconnected interpenetrating network.Most "3661" clusters have only one neighbor, forming a "dimer"-

Structure and GFA of Al-RE-LTM ternary metallic glasses
In 1988, Inoue et al. 34,92 found the Al-RE-LTM (LTM = Fe, Co, Ni, or Cu) ternary metallic glasses by substitution ETM with RE.Compared with Al-ETM-LTM alloys, the Al-RE-LTM alloys have a higher probability of forming bulk metallic glasses.The glass formation is favored in the composition range of 80-92 at% Al, 3-20 at% RE and 1-15 at% LTM 93  , which makes the preparation of high-strength Al-based glassy alloy sheet feasible.This formation is an important breakthrough for Al-based metallic glasses.In 2016, Wu et al. 95 reported the modified composition Al 86 Ni 6.75 Co 2.25 Y 3.25 La 1.75 which is regarded as the best glass-forming for Al-based bulk metallic glasses so far.By combining the microalloying effect and the concept of Fermi sphere-Brillouin zone interaction, the alloy could form a glassy phase in the rod of 1.5 mm in diameter.Recently, the Al-based metallic glass with a diameter of 2.5 mm was formed at the same alloy composition by salt-fluxing treatment 96 .However, it is still lagging far behind other based metallic glasses.The improvement of GFA also seems to indicate the effectiveness of multiplication alloy components.
Through the experiments such as neutron scattering, X-ray scattering, transmission electron microscopy (TEM) and extended X-Ray absorption fine structure (EXAFS), the local structure of Al-based metallic glasses has been clarified.
The main structural information of Al-based metallic glasses has been presented in references 97,98

Is the structure of Al-based metallic glasses truly amorphous?
At the early stage, the conventional X-ray diffraction (XRD) technique was usually used to study the structure of metallic glasses.The typical characteristic of XRD patterns for metallic glasses is a broad diffuse diffraction peak.However, whether a material exhibiting a broad diffuse X-ray diffraction has a truly amorphous structure or consists of randomly oriented microcrystals 115 has ever been an intensive debate.Later, in 1990, He et al. 116 studied the structure of Al 90 Fe 5 Gb 5 by means of high-resolution electron microscopy (HREM).They did not observe any crystalline structure in the HREM image and the differential scanning calorimetry (DSC) also verified an amorphous nature of Al 90 Fe 5 Gb 5 117 .

Backbone structure
It has been indicated that alloy composition, atomic size and component mixing state have an important effect on the GFA [118][119][120][121] .During long-term research, Inoue's group 122 discovered three empirical rules for the stabilization of supercooled liquid, i.e., (1) the alloy contains at least three elements, (2) the atomic size mismatch is over about 12%, and (3) the enthalpies of mixing among the main three constituent elements are negative.It is also known that the multicomponent glassy alloys with the three component rules can have a MRO atomic configuration with highly packed state.Al-based alloys also satisfy this principle such as AlNiY and AlNiTiZr systems 123,124 .According to the atom size-composition relationships, Poon et al. 125 correlated the GFA with alloy composition and grouped the BMGs into two types.One is MSL (majority atom-small atom-large atom), referring to the system with mid-size atoms (60-70 at.%), small-size atoms and large-size atoms (~10 at.%).The other is LS (large atom-small atom) system composed with large-size atoms (~40-75 at.%) and small-size atoms (~25-60 at.%). Figure 3 shows a sketch of atomic network formed by large atoms and small atoms in the MSL class metallic glasses, in which the red balls are large atoms and the blue balls are small atoms.The large atoms and small atoms are connected to form a strong L-S network or reinforced "backbone" structures.Such structures can enhance the stability of supercooled liquid and further suppress crystallization.The optimal content of L atoms for forming BMGs is about 10 at.%.Much L atoms will tend to form the cluster, which reduces the interaction between the L atoms and the M and S atoms 126 .For the Al-RE-TM ternary metallic glasses, the RE is large-size atoms and TM is the small-size atoms.By studying the maximum thickness and mixing enthalpy between two constituents for Al 85 Ni 10 Ce 5 , Al 89 Co 6 Ce 5 , Al 90 Fe 5 Ce 5 and Al 84 Cu 9 Ce 7 , the results show that the mixing enthalpy of RE-TM is not consistent with the critical thickness 58 .This indicates that not all glass formability of Al-RE-TM can be explained by the formation of RE-TM pairs.

Efficient cluster packing (ECP) model
In 2003, Miracle and Scenkov 127 specifically proposed a model for Al-based metallic glasses in which the solute centered cluster can predict the SRO.In this model, the solute centered cluster is formed with solvent atom Al at the vertexes of the cluster due to the strong interaction of unlike atoms 63 .However, the model does not display the structure of MRO.Therefore, the efficient cluster packing (ECP) model was provided by Miracle 128,129 to account for the MRO.The ECP model obeys the basic packing principle and the atoms are regarded as hard spheres.According to the special solute/solvent atomic radius ratio, one can evaluate the expected coordination number (CN) of the solute atom, and then alloying elements can be chosen to form the high GFA compositions 107 .Figure 4 (a) displays the ECP model, where α is solute atom surrounded by 12 solvent atoms Ω, such packing structure can result in octahedral and tetrahedral sites.A secondary solute atom β occupies the tetrahedral sites and tertiary solutes γ occupy octahedral sites, which make the atom packing more effective.In this atomic packing, there exist SRO within a single solute-centered cluster, and MRO at distances of 1-1.5 nm 88

Short-to-medium-range order model
Sheng et al. 133,134 proposed the quasi-equivalent clusters to characterize Al-based amorphous alloys.They considered several types of polyhedra as quasi-equivalent clusters on the basis of their similar sizes and CN.The atoms of the clusters are treated as rigid balls.For Al-based metallic glasses, the solute TM and RE are surrounded by the solvent Al only, forming an icosahedral-like structure.These RE-centered clusters and TM-centered clusters are interconnected by sharing solvent atoms to form the medium-range order.Figure 5 shows the structure of Al 89 La 6 Ni 5 glass, in which Ni-centered clusters are as a group of quasi-equivalent cluster, and La-centered clusters are as another group of quasiequivalent clusters.These clusters overlap and interconnect via face-sharing, edge-sharing and vertex-sharing to form the Al-based metallic glass.
Based on the features of RE-and TM-centered clusters, Ma et al. 135 proposed a cluster line method to design Al-based metallic glasses.The optimum glass-forming composition can be considered as the intersection point by drawing the two clusters lines in the ternary phase diagram.Hence their intersection is assumed to indicate the best BMG-forming compositions which can be confirmed experimentally 136,137 .Figure 6 displays the structure model they used.They regarded the distance between the solvent and solute atoms as the sum of their nominal atomic radii with fixed sizes, and the RE-and TM-centered clusters are connected through Al atoms.The well-known compositions 94 firstly formed into a fully glassy rod of 1 mm were obtained by using the cluster line method.However, the experiment 138 of Liu et al. did not support the cluster line method with regard to predicting the composition with optimum GFA.In view of the interaction between different elements, Zhang et al. 139 proposed the modified cluster line method by considering the effective atomic radii.The proposed method can predict the optimum glass-forming composition of Al-Ni-RE systems more accurately.

Icosahedral supercluster MRO structure model
More recently, combined with the efficient atomic packing and electrochemical potential equalization principle, Wu et al. 140 proposed the efficient atomic packing-chemistry coupled model (EAPCC) for calculating and predicting the best glass-forming compositions in Al-RE-TM metallic glasses.The composition of Al-RE-TM alloys can be represented as (Al N -RE)TM X , where N is the coordination number in the first shell, and x is the number of TM atoms which is determined by the equalization of electrochemical potential between RE-and TM-centered clusters.The RE-centered clusters are packed as spherical periodic order (SPO) that is regarded as the basic frame of microstructure.The SPO results from the global resonance between the electronic and atomic subsystem.The TM atoms are located at the interstitial sites between RE-centered clusters, and such packing style makes the structure more efficient.
Based on the EAPCC, Wu et al. 141 further proposed an icosahedral supercluster MRO structure model to describe the connection between the clusters.A supercluster consists of 12 RE-centered clusters or 12 TM-centered clusters on the vertex of the supercluster, in which the centered TM and RE are surrounded only by Al atoms.In the center of the supercluster, there exist RE-centered clusters or TM-centered clusters.Such atomic packing will produce 20 tetrahedral interstices filled with TM or RE atoms.Figures 7 (a

The modes of crystallization
Crystallization of Al-based metallic glasses is such a process during which an amorphous phase transforms into one or more metastable or stable crystalline phases 142 .In most cases, crystallization in Al-based metallic glasses is induced by thermal treatment containing isothermal and continuous heating mode [143][144][145] .However, it may also occur when it is subject to corrosion-induced crystallization 146 , irradiation-induced crystallization 147 , and deformationinduced crystallization, including bending 148,149 , high-press torsion straining 150,151 , ball milling 152,153 , and cold rolling 154,155 .
Crystallization reaction in the metallic glasses is highly dependent on the alloy composition [156][157][158] .It is usually divided into three types, labelled as polymorphous, eutectic and primary crystallization, as illustrated for the crystallization of the amorphous Fe-B alloys as an example 159 .Figure 8(a) presents the polymorphous crystallization where a single crystalline phase is formed and its composition is the same as the amorphous matrix.No long-range diffusion is required in this process.Figure 8(b) presents the eutectic crystallization.The glass matrix transforms simultaneously into two (or more) crystalline phases.In this process, no long-range diffusion is required.Figure 8(c) presents the primary crystallization mode in which a crystal with different composition from that of the matrix is embedded in an amorphous matrix.
For the Al-based metallic glasses, these modes of crystallization have also been reported.For example, the melt-spun Al 75 Cu 15 V 10 metallic glass was transformed into a single icosahedral phase by annealing, demonstrating a polymorphous crystallization 160 .The Al 85 Y 11 Ni 4 metallic glass is subjected to the eutectic crystallization during annealing and transforms into the fcc-Al, Al 3 Y and some unidentified metastable intermetallic phases 161 .Similarly, Al 89 La 6 Ni 5 metallic glass crystallizes into fcc-Al and a metastable bcc-(AlNi) 11 La 3 -like phase 162 , indicating a eutectic type crystallization.Compared with the polymorphous or eutectic crystallization, primary crystallization is an overall slow phenomenon 163 .It is the main mode of transformation in Al-based metallic glasses.Primary crystallization occurs in Al-rich compositions, in which a high density of Al nanocrystals with diameters ranging from 7 to 20 nm dispersed in the amorphous matrix 164 .The mechanical properties of such amorphous/nanocrystal composites are strongly related to the volume fraction (V f ) of fcc-Al nanocrystals 165 .By controlling the V f of fcc-Al nanocrystals, the higher strength can be obtained than that of the corresponding amorphous alloy 166 .Usually, the nanoscale mixed structure consisting of fcc-Al embedded in an amorphous matrix can be obtained by the thermal treatment in an amorphous phase.The V f of the fcc-Al phase is correlated with annealing time 86,167,168 and can be calculated using the DSC program 169 , XRD method 170 , or transmission electron microscopic (TEM) images 171 .Comparing with the TEM method, the DSC and XRD methods are preferable, because these two methods can reflect the entire characteristics of the sample 165 .One can also obtain such structure directly during melt-spinning, when the cooling rate is controlled.The V f of the fcc-Al  phase is controlled by changing the rotation speed of wheel or the ejecting temperature of molten alloy 172,173 .Compared with the controlling of cooling rate during melt-spinning, annealing an amorphous phase with suitable composition is a more controllable method to produce an optimum volume fraction of the fcc-Al 174 .
Minor element as an addition to Al-based metallic glasses plays an important role in crystallization behavior.Here, several examples about minor element doping in the Al-based systems are given.Doping Pb into the Al-Y-Fe alloy, the crystallization onset temperature ( T x ) decreases by 20 ℃.As the extrinsic spatial heterogeneity, the insoluble Pb nanoparticles in the amorphous matrix can provide the nucleation sites for Al-fcc precipitation 175 .As a result, crystalline phase is more easily formed from the amorphous phase by heterogeneous nucleation 176 .Doping the Pb into the Al-Ni-Ce alloy is very effective in enhancing the thermal stability of metallic glasses, and the T x is greatly improved 177 .Such phenomenon is also observed when adding the Ca into Al-Y-Ni-Co system 178 .A tiny change in the alloy composition may also lead to a dramatic change in the crystallization process.Doping the Si into the Al-Ni-Nd system results in a transformation of a primary crystallization from eutectic crystallization, while the addition of Co makes a primary crystallization transform into a eutectic crystallization 179 .Crystallizing the amorphous Al-Fe-Ce by annealing at 613 K for 2h, the structure of fcc-Al + icosahedral phase + amorphous phase can be obtained and fcc-Al is a majority phase.However, icosahedral phase is dominant when adding Ti in the Al-Fe-Ce system 180 .
It is worth noting that several novel phenomena were found in Al-based metallic glasses in the last two years.In 2017, the structure consisting of amorphous and fcc-Al phase was observed for the first time by the reverse crystallization when treating the continuous heating on Al 84 Y 9 Ni 4 Co 1.5 Fe 0.5 Pd 1 metallic glass 145 .In this process, the eutectic reaction takes place firstly, then the metastable multicomponent compound phase disappears and the volume fraction of amorphous phase increases.This reverse crystallization makes the amorphous + fcc-Al phases to be formed in a wide range of high temperature, which provides a good chance for developing Al-based alloys with high elevated-temperature strength.In 2018, the fcc-Al phase was found by polymorphous crystallization when the amorphous Al 90 Y 10 and Al 84 Y 8.5 Ni 4 Co 2 Pd 1 Fe 0.5 were subjected to the cold-rolling 155 .Compared with the primary crystallization of fcc-Al embedded in an amorphous matrix, such amorphous/nanocrystal structure obtained by polymorphous crystallization could lead to the softening.It is clearly contrary to the annealing-induced crystallization that usually leads to the hardening.The discovery of this important phenomenon is beneficial to the development of Al-based metallic glasses as structural materials.

Mechanism of nanocrystallization for Albased metallic glasses.
At present, studying the primary crystallization mode for Al-based metallic glasses is the main objective.Understanding the primary crystallization and structural evolutions not only contributes to the development of the novel amorphous/ nanocrystal products, but also plays an important part to uncover the glass formation and develop fabrication processes 181,182 .In the primary crystallization, Al-based metallic glasses crystallize to an amorphous/nanocrystal composite, in which a high density of Al nanocrystals are embedded in an amorphous matrix.This process needs high nucleation frequency and low growth velocity 183 .The classical nucleation theory is difficult to explain such high nucleation density.In light of this, several models are proposed, including "quenched in nuclei", "coupled-flux model", and "phase separation in amorphous state".
During the fast cooling rate, some clusters with MRO are quenched in the amorphous alloys.These clusters are considered as the "quenched-in nuclei".They are too small at higher temperatures but become overcritical at lower temperatures.When annealing, the quenched-in nuclei provide a high density of heterogeneous nucleation sites and the subsequent growth results in the rapid crystallization 184,185 .Therefore, nanocrystallization results from an extremely high density of quenched-in nuclei.Quenched in nuclei is one possible mechanism for explaining the high density of nanocrystals.In view of the linked stochastic fluxes of interfacial attachment and diffusion in the cluster neighborhood, Russell 186 proposed a simplified model to explain nucleation.In this model, three important regions including the cluster, the neighborhood around the cluster, and the parent phase, are defined, as shown in Figure 9.The cluster population distribution correlates with not only the cluster size, but also the solute atomic number in the nearest neighbor shell around the cluster; Cluster growth is influenced by the relative exchange rates of atoms in the parent phase and the cluster. 187,188Based on this, Kelton 189 proposed a new homogeneous nucleation model (coupled-flux model) that couples the interfacial and long-range diffusion fluxes.Upon cooling, the clusters are quenched and their distribution is more representative of the steady-state distribution at high temperature.The size of the clusters is smaller than the critical size, and the composition of the shell neighborhood is closer to the crystal composition.At the annealing temperature, the clusters larger than the critical size will grow, in which the growth is initially interface limited, resulting in a rapid increase of small nuclei. 190The coupled-flux model has successful explained the formation of the nanocrystal in Al 88 Y 7 Fe 5 without phase separation during the low temperature annealing treatment.
Phase separation (also denoted as chemical decomposition) is a common phenomenon in the study of the Al-based metallic glasses.For a single amorphous phase, there always exhibit a broad diffraction peak in the XRD and diffuse halo rings in the bright-field image for TEM.In the study of the Al-Si(or Ge)-X (X = Cr, Mn, Fe, Co, Ni) metallic glasses, the split X-ray and electron haloes were first observed, demonstrating that the system is not a single amorphous phase but separates into two amorphous phases. 191,192Except for the "quenched in nuclei" and "coupled-flux model", another possible nanocrystallization mechanism is the result of nanoscale phase separation which may occur prior to   193 .For example, it was observed that the TEM image of Al 88 La 2 Gd 6 Ni 4 presents the bright regions and darker regions 194 , indicating that a phase separates into the two phases.These two regions are proved to be the Al-rich and solute-rich amorphous regions, respectively.The first nucleation of the fcc-Al nanocrystal starts preferentially near the boundaries of phase-separated regions.In the Al 92 Sm 8 and Al 91 Gd 9 metallic glasses, the nanoscrystals were observed to be formed inside the Al-rich regions 195 .Phase separation phenomenon prior to nanocrystallization is a mechanism triggering nanocrystallization 183,194,196 .The pure Al zone seems to catalyze the fcc-Al nucleation, which is responsible for the high nucleation frequency.The solute rich regions between the Al-rich regions inhibit the long-range diffusion, resulting in the low growth rates 184 .There are also some reports in several alloy systems that no phase separation phenomenon was observed, illustrating its strong dependence on alloy composition and system 144 .However, controversial results were reported even in the same materials by different analytical techniques.For example, the phase separation in Al 88 Gd 6 Er 2 Ni 4 is resulted from the TEM preparation conditions 197 .Therefore, it is still an open issue about the occurrence of phase separation in Al-based metallic glasses 198 .

Conclusions
This paper mainly introduces the features of the structure characteristics and crystallization of Al-based metallic glasses.The main conclusions of this review are summarized as follows: 1. Understanding the structure in combination with alloy compositions contributes to making the size of Al-based metallic glasses larger, which is important for applications.The structure models could give an intuitive image of structure and provide a foundation for investigating characteristic and performance of new glassy alloys.One can construct the structure model at atomic level based on the features of local atomic arrangements.After several decades of unremitting efforts, the structure models of Albased metallic glasses have been developed, and further efforts to develop new models are expected to enable the interpretation on the electronic effect on the GFA and characteristics of Al-based metallic glasses.However, the cognition of electronic structure and its influence on Al-based metallic glasses has not perfectly been obtained, and more future detailed research is required.2. The primary crystallization is strongly dependent on the solute constituent in the Al-based metallic glasses and its crystallization product consists of an ultrahigh density of fcc-Al embedded in amorphous matrix for Al-based alloys with high Materials Research Al contents above 85 at% and Al + compound for Al-based alloys with Al contents less than 85 at%.The former amorphous/nanocrystal composites have higher strength than the amorphous single phase state.At present, the advances have been obvious on the understanding of primary crystallization, but there are still several problems needed to be further solved, such as (1) the mechanism for the precipitation of very high density of fcc-Al; (2) the compositional feature and origin for phase separation; (3) micro-alloying and cold deformation effects on the pathway of crystallization.In addition, the advanced experimental and simulation techniques for analysis of local structure also need to be developed urgently, which contribute to the future development of Al-based metallic glasses as structural materials.We also need to be pay attention to the effect of multicomponent on the thermodynamic and kinetic aspects of GFA, thermal stability and crystallization, because the crystallization mode involving nucleation and growth significantly changes with solute composition.Thus, the clarification on the compositional dependence of structure, thermal stability and crystallization of metallic glasses is an important way for understanding the fundamental characteristics of Al-based glassy alloy.

Figure 1 (
b), in which Al-rich regions consist of nearly pure Al, while Al-depleted regions have nearly the Al-75 at.%Tb composition.However, some of the Al-rich regions may form the medium-range order (MRO) Al shown in Figure1(b) if the quenching rate is not high enough.

Figure 1 .
Figure 1.Schematics showing the chemical fluctuation in (a) liquid; (b) as-quenched Al 90 Tb 10 . 89 . The glass formation range is the widest for Al-RE-Ni system, followed by Al-RE-Co, Al-RE-Fe and then Al-RE-Cu.Therefore, the Al-RE-Ni alloys have attracted much attention as the basic system to search for Al-based BMGs.In 2010, Al-based metallic glasses with a diameter up to 1 mm were obtained in Al-Y-Ni-Co-Sc and Al-Y-La-Ni-Co systems on the basic alloy of Al 86 Ni 8 Y 6 94

Figure 2 .
Figure 2. "3661" short-range order surrounding Sm atoms: (a) the side view; (b) the top view; (c) interpenetration of two "3661" clusters.91 . The ECP model has provided a new insight into the designing of the potential BMG forming Albased alloys from the topological perspective.For example, the composition Al 75 V 12.5 Fe 12.5 was obtained by the ECP model130 .The ECP model can well predict the composition of RE (α atoms), but the concentrations of TM (β) and Al (Ω) cannot be predicted very accurately.Later, Wang et al.131

Figure 4 .
Figure 4. (a) ECP model 129 ; (b) modified ECP model 131 .In the figure, α is the primary cluster-forming solute, β is the secondary solute, Ω is the solvent atom, I oct is octahedral interstitial site.
) and (b) display the 2D schematics for the cluster-tetrahedral

Figure 6 .
Figure 6.A skeleton of cluster packing in Al-based MGs. 135

Figure 9 .
Figure9.Schematic of the coupled-flux model.189 nanocrystallization . Their structure features can be summarized as below: 1. TM and RE atoms are surrounded by Al atoms because the composition is very Al-rich 88,99 .2. The distance between Al and TM is shortened than the sum of radii, indicating that a strong interaction of covalent bonding results from electron orbital hybridization between the Al-3p state and TM-3d state 64,88,100-106 .Moreover, in Al 88 Ce 8 Co 4 system, Bin et al. found that the structural inhomogeneity is attributed to the diversity of Al-Co clusters, which leads to loosely and densely packed local atomic configurations and contributes to soft (liquid-like) and hard (solid-like) regions 107 .3. The distance between Al and RE atoms is close to the sum of radii of atoms 64,99,108 .In addition, the RE atoms are basically randomly distributed 109 .4. The local RE environment is in a more disordered state as compared with the TM environment 100,104,105 . 5.There exist the icosaherdral-like ordered configurations in Al-based metallic glasses 110-114 .