A review of Gum Metal: Developments over the years and new perspectives

Gum Metals are β-Ti alloys that present only non-toxic elements in their composition, low elastic modulus, good formability, and excellent biocompatibility. The composition of these alloys needs to fulfill some electronic criteria to present their typical properties. In addition, during their processing, Gum Metal must be significantly deformed, which can be obtained by cold rolling or swaging. Over the last years, several new processing routes and alloy compositions have been proposed to meet these requirements, expanding the range of properties for Gum Metal, and increasing their extent of application. Although being relatively new, Gum Metal has been extensively studied. However, there is a lack of a review that encompasses the findings obtained over the years, as well as its future perspectives. This review aims to critically discuss and analyze Gum Metal development, with a special focus on new compositions, processing routes, and applications. Moreover, future perspectives are explored as well.


Introduction
Ti-based alloys are widely used in a broad range of applications due to the unique properties presented by this class of metallic material, ranging from applications in the aerospace field to the biomedical area [1][2][3][4]. Ti alloys present low specific mass combined with high mechanical strength, low elastic modulus, high corrosion resistance, and excellent biocompatibility [3,[5][6][7]. These alloys have a relatively high cost, however, their special properties make them a highly competitive material for engineering applications [1,3]. This high competitiveness of Ti new β-Ti alloys possess a great perspective of application in the biomedical area, due to their excellent biocompatibility combined with the low elastic modulus typical of the β phase [7,9,10,13,14,16,17].
Thus, β-Ti alloys present themselves as possible candidates to replace the widely used Ti-6Al-4 V alloy, whose cytotoxicity has been questioned in recent years [14,16,[18][19][20]. The high cytotoxicity of V as well as the association of Al ions (possibly released into the bloodstream) with the development of neurological diseases has originated several discussions about the real biocompatibility of these alloys [21,22]. Likewise, shape memory alloys based on Ti-Ni (Nitinol®), also widely used in the biomedical area, such as in stents, have had their use questioned since Ni may cause hypersensitivity [23][24][25]. This creates a market opportunity for β-Ti alloys that have non-toxic and nonallergic elements in their composition. In this context, several studies have been observed, mainly aiming biomedical applications, such as stents and implants. However, these alloys can also fulfill the requirements of other fields such as the aerospace area.
Among the β-Ti alloys currently studied, those based on the quaternary Ti-Nb-Ta-Zr (TNTZ alloys), known as Gum Metal, have drawn attention in the literature. These alloys correspond to a reasonably new metallic alloy family that was first reported in 2003 by Saito et al. [26]. The Gum Metal has very interesting properties as elastic modulus lower than 70 GPa, high tensile strength, superelasticity, and superplasticity at room temperature. Over the last two decades, an increase in interest in Gum Metal alloys has been noted, as can be seen in the graphs depicted in Fig. 1(b). It can be seen a higher number of published articles on this topic in the recent years. Several developments, both in chemical composition design and processing Invited Feature Paper-Review routes, were reported in the literature in the last decade, which are driven based on the creation and improvement of these alloys for potential new applications. Despite the growing and stable interest observed in recent years, there is no work in the literature so far that presents the general developments of Gum Metal over the last decade in a deep and comprehensive way.
Based on that, the present review aims to present in a synthesized way an overview of general aspects of Gum Metal, followed by a more detailed review of the recent developments in processing routes, compositions, and applications. Lastly, we present the future perspectives and conclusions remarks.

General aspects of Gum Metal
Gum Metal corresponds to a new metallic alloy family that was first reported in literature in 2003 by Saito et al. [26]. In his pioneering work, it was stated that these alloys present "super" properties, such as a very low elastic modulus, around 55 GPa, high strength, superelasticity, and a perfect plastic behavior with no hardening effect, combined with high formability [26].
The composition of the first Gum Metal processed was Ti-23Nb-0.7Ta-2Zr-1.2O (%at.), presenting only non-toxic and β-stabilizing elements. Although Zr may be considered a neutral element, it tends to become a β stabilizer if there is a considerable amount of other β-stabilizing elements in the alloy composition [9]. According to Saito et al. [26] the Gum Metal properties could be observed for other compositions of the same system, if they fulfill three electronic parameters: (i) electron/atom ratio (e/a) of about 4.24; (ii) binding order ( Bo ) around to 2.87; and (iii) electron-orbital energy level ( Md value) of about 2.45 eV. Besides that, the Gum Metal properties are only reached if a severe cold mechanical deformation, of about 90% area reduction takes place during the processing of these alloys [26,27]. The pioneer Gum Metals were produced by a route consisting of sintering metallic powders followed by forging, solubilization heat treatment, and cold swaging with a 90% area reduction [26]. Variations of these processes have been proposed in the literature, particularly in the final deformation, which can be replaced by different processes [28]. Attempts have also been made to produce these alloys by routes involving casting, as will be discussed in further sections.
A third and last requirement to obtain the Gum Metal's typical properties is the need for a specific amount of oxygen in a solid solution. It is reported that an oxygen level between 0.7 and 3.0%at. is required [29]. Since this value is presented in atomic percentage, it tends to vary according to the alloy composition. For the alloys studied by Saito, this value would be around 1600 to 6900 ppm for the Ti-12Ta-9Nb-3 V-6Zr alloy (%at) and around 1873 to 8126 ppm for the Ti-23Nb-0.7Ta-2Zr alloy (%at.). A minimum value close to 2000 ppm is usually recommended in the literature.
Since Gum Metal has only the addition of β-stabilizing elements into the Ti matrix, it is expected only the β phase in the microstructure. However, other phases can also be present, such as α, αʺ, and ω phases. The volume fraction of each phase will depend on the chemical composition as well as the influence of possible thermomechanical treatments. The α phase (space group:P6 3 /mmc ) has a hexagonal structure and is formed by diffusion from the β phase (space group: Im3m ) during cooling. For rapid cooling, the transformation of β into α will be suppressed, resulting in a martensitic transformation. The type of martensite formed will depend on the content of alloying elements in the chemical composition. For low contents, the structure produced will be αʹ (space group: P6 3 /mmc ), which presents a crystallographic structure identical to that observed for the hexagonal α phase. For higher solute contents, the martensitic αʺ (space group: Cmcm ) orthorhombic phase will be formed, which presents an intermediate structure between the α hexagonal and the β BCC [30]. The αʺ phase is the one that has the lowest lattice symmetry and the one in which the lattice parameters are most influenced by the addition of alloying elements. The very close structural relationship between αʹ and αʺ suggests that both form from the β phase in a very similar way. On the other hand, the ω phase corresponds to an equilibrium phase at high pressures or can appear in metals and their alloys in a metastable manner at normal pressures [31]. This phase can be formed by diffusion, thermally activated, giving rise to the isothermal ω phase ( ω iso ), or induced by stress, forming the athermal ω phase ( ω ath ) . The ω ath usually forms in small volumes with dimensions of around a few nanometers, being homogeneously distributed throughout the β matrix. During aging, these particles of ω ath give rise to the isothermal phase, which diffusively grows and can reach dimensions of about 100 nm or more [49,50]. The lattice symmetry of ω can be either non-compact hexagonal (space group: P6/mmm ) [32] or trigonal (space group: P3m1 ) [33], both with three atoms in each unit cell. Both hexagonal and trigonal structures can be obtained from β phase by displacing atoms that "collapse" pairs of adjacent and parallel {222} β planes in intermediate positions.
Between two pairs of the same type {222} β , an adjacent plane remains stationary and becomes the basal plane of ω phase [31]. The size of the structural "collapse" basically depends on the chemical composition. The trigonal ω ath phase, which is associated with an incomplete "collapse, " is observed for alloys that are rich in β-stabilizing elements [34,35]. On aging, trigonal ω ath gradually develops into hexagonal ω iso phase. This process happens through solute partitioning, i.e., diffusion, accompanied by the "collapse" of the {222} β planes in the solute-depleted regions. The shear formation of the ω ath phase from β has a dominant character of atomic repositioning (shuffle) with only marginal volume changes due to lattice deformation. For this reason, the β → ω ath transformation is not classified as a martensitic transformation despite its shear nature and the conservation of the parent phase composition.
Since chemical composition plays an important role in the phases volume fraction of Gum Metal, it is expected that oxygen, a fundamental requirement for these alloys, also plays an important role in this regard. The role of oxygen in the properties and deformation mechanisms of Gum Metal is a well-studied variable that still has several contradictions about it. The addition of oxygen, as well as any interstitial element, is responsible for an increase in mechanical strength, as it produces a solid solution hardening [10,14]. However, its influence is much more complex, and some authors suggest that the addition of oxygen hinders the formation of αʺ and ω phases, which play a fundamental role in the deformation mechanisms since these phases strongly influence the elastic behavior of Ti-β alloys [10,[36][37][38][39][40].
In addition, the plastic deformation mechanism of Gum Metals has drawn great attention from researchers around the world, since it presents atypical features, different from those usually found in metallic materials. But a change of paradigm regarding this topic occurred after more in-depth scientific research had been conducted.

Plastic deformation mechanisms-change of paradigm
Ti alloys can undergo deformation by two main mechanisms: dislocation slip and twinning [1,10]. Some β-Ti alloys may also present stress-induced martensite (SIM) as a deformation mechanism, which considerably increases the formability of these alloys [1]. This transformation produces the effect known as superelasticity, which in this case is related to the transformation of martensite αʺ from the parent β phase [9,11,41,42]. SIM transformation is quite sensitive to alloy composition, in which it is reported that the martensitic transformation start temperature ( M S ) decreases monotonically with the addition of β-stabilizing elements [1,7,13].
Despite this established knowledge about the plastic deformation mechanism of Ti alloys, the Gum Metals drew attention in the literature since they were introduced as alloys that are deformed without hardening, i.e., the plastic deformation occurs by a free-dislocation mechanism. Saito et al. [26] also point out in their pioneer work that the superelastic behavior presented by Gum Metal is not related to phase transformations, contrary to what was found for shape memory alloys, which also present superelastic properties [26,43]. It was suggested that the plastic deformation takes place due to novel mechanisms, such as local nanodisturbances, formation of giant faults in the lattice [ Fig. 2(a)], and transgranular crystal rotation [26-28, 36, 37, 43]. These atypical deformation mechanisms would be favored by the microstructure that is formed after the plastic deformation required for Gum Metal processing. This microstructure has a marble-like appearance and an elastic deformation energy field around itself. An example of a marble-like structure formed in a Ti-23Nb-0.7Ta-2Zr-1O (at.%) Gum Metal after plastic deformation is depicted in Fig. 3 [44].
These proposed new mechanisms do not promote hardening during plastic deformation, culminating in a stress plateau on the stress-strain curve as depicted in Fig. 2(b). It can also be seen in Fig. 2(b) that after the plastic deformation of about 90% that is required for the processing of the Gum Metal, a significant decrease in the elastic modulus occurs, which is very attractive for applications that require lower elastic modulus. This non-linear elastic behavior is typically found in INVAR materials [26].
Over the following years, from Saito's work, new studies were carried out with the same Gum Metal composition, as well as new compositions were elaborated using the suggested electronic parameters as previously mentioned in this review [27,28]. These new compositions will be addressed in the following topics. In these new studies, it was observed that Gum Metal undergoes plastic deformation by mechanisms other than those previously suggested by Saito et al. [26,29]. The presence of dislocation slip with subsequent generation of stationary loops was found ( Fig. 4) [28,36], in addition to the formation of αʺ stress-induced martensite, which was directly related to the superelastic behavior [29,[45][46][47]. In addition, the formation of twins was observed during the cold deformation process [48][49][50][51].
Furthermore, it has been reported in the literature that cold working can result in the formation of stress-induced ω and/or αʺ-phase precipitates [38,45,52], which play an important role in the mechanisms of deformation of Ti alloys.
Despite these new studies that disagree with some of Saito's conclusions, there are some current works that agree with the mechanisms presented by him. But these represent only a small fraction of the published studies in the last decade, and most of the articles consider that the Gum Metal deforms plastically by the conventional plastic mechanisms. However, this is still a complex topic since encompasses a combination of distinct plastic deformation mechanisms, which are directly related to the phase volume fractions and alloy chemical composition.
Aiming to predict both the mechanical plastic deformation mechanisms and phase distribution that are present in Gum Metal processed alloys, some design approaches were created based on the electronic parameters cited by Saito. These design approaches will be described in deep in the sequence.

Theoretical approaches for alloy design
A variety of approaches to materials design have been proposed to save both time and cost necessary for materials development. For example, a thermodynamic approach to the prediction of phase diagrams called CALPHAD has been widely used [53,54]. In addition, molecular dynamics and Monte Carlo simulations have recently made remarkable progress. The phase-field method has been found to be effective in predicting the microstructural evolution in alloys and oxides [55][56][57][58].
In recent times, first-principles calculations of electronic structures have been extensively performed over the world due to the progress in computer hardware as well as software. However, in multiple-components systems, including Ti alloys, the number of combinations of alloying species and compositions is infinitely large. Therefore, although the physical and chemical properties of metals and alloys are closely connected with their electronic state, it is complex and impracticable to calculate the electronic state accurately for such complex alloys. In this case, for rational alloy design, it is opportune to use alloying parameters that exhibit explicitly the character of each element in a mother metal. The DV-Xα cluster method has been used for this purpose.
The DV-Xα cluster method is a molecular orbital calculation method assuming the Hartree-Fock-Slater approximation [58][59][60][61][62][63], which simulates the local electronic structure around the alloying element. Depending on the crystal structure, appropriate cluster models are employed in this calculation. In Ti alloys, for body-centered cubic (BCC) β-Ti, the cluster model consists of a central Ti atom, 8 Ti atoms in the first nearest neighbor, and 6 Ti atoms in the second nearest neighbor [64]. An alloying element, M, is substituted for a central Ti atom. So, the BCC cluster model is expressed as MTi 14 .
In alloyed BCC Ti with 3d transition elements, for example, the energy levels that originated mainly from the Md orbitals appear above the Fermi energy level, E f [65]. This E f energy level is set at zero for pure BCC Ti using the Ti 15 cluster model (i.e., M = Ti) and used as a reference. In this case, since the d level of an isolated M atom is higher than that of an isolated X atom, the electrons move from M to X to reduce the energy. As the result of such a charge transfer, the effective charge becomes positive for M and negative for X (Fig. 5a) [64]. Thus, the energy level controls the direction for charge transfer, and hence it is related to electronegativity. Another important alloying parameter is the bond order (hereafter referred to Bo). The bond order due to the d-d covalent bond between atoms is calculated following the Mulliken population analysis [66]. When both atoms are transition metals, the d-d covalent bond is most dominant between them, resulting in a stronger chemical bond operating between M and X.
The determination of electronic parameters Bo and Md (eV) has been performed for various alloying elements in BCC Ti [64]. These parameters obtained for BCC Ti have been employed for the analysis of alloy properties because these parameters appear to be relatively insensitive to the crystal structure [64,67]. For an alloy, the average values of Md and Bo are defined simply by taking the compositional average and expressed as Md and Bo . In order to understand the alloying effect of each element M in Ti, the Bo − Md diagram is built for Ti-M binary alloys as shown in Fig. 5(b) [64]. It is important to note that for oxygen the value of these two parameters is not known, and therefore it is not considered in the calculation.
As shown in Fig. 6, the boundaries of the Bo − Md diagram, defined from several of the recently investigated alloys, correspond to transitions between phase fields and deformation mechanisms, as well as represents the martensitic transformation temperatures ( M s and M f ). The boundary for M s is presented by using a dotted curve, below which the α phase coexists with the β and/or ω phase in the alloy at room temperature. In turn, the boundary for M f is given by using a solid curve, below which the martensite phase exists predominantly in the alloy at room temperature. Since the plastic deformation mode changes from the twin to slip mechanism as the stability of the β phase increases, close to the β/β + ω-phase boundary the diagram is separated into either the slip or the twin dominant subregion. It is interesting to note that most practical alloys are located along this slip/twin boundary, including the first Gum Metals reported by Saito, namely at the position of Bo = 2.87 and Md = 2.45 (eV). This region is highlighted in Fig. 6. In addition, among the β-phase alloys, Young's modulus decreases with increasing Bo parameter, following an arrow direction shown in Fig. 6.
For most Ti alloys, superelasticity emerges in the least stable β-phase alloy (i.e., Ti-(40-42)Nb), along the β/β + ω-phase boundary, where the reversible stress tends to operate so as to induce the reversible transformation between the martensite and the β phase at room temperature. However, several studies [4,26,68,69] have shown that the addition of O, Al, Sn, and Zr shifts the β/β + ω-phase boundary to the further lower content of the β-stabilizing elements. Thus, superelastic behavior, for example, is more pronounced in the O-containing Ti-Nb alloys than in the O-free Ti-Nb alloys, as O suppresses the ω-phase formation [70]. This is also the case in the multi-components alloys, for example, Ti-5Zr-p Nb-30Ta-0.23O alloys ( p = 20, 25, 30, 35, and 40) [71].
Thus, since the knowledge of the phase stability presented on the Bo − Md diagram makes possible to predict various Gum Metal major properties such as mechanical strength, Young's modulus and superelasticity, the present molecular orbital approach has been proved to be an important tool to design novel Gum Metal compositions in a reasonable way without relying on many trial-and-error experiments.

Recent developments of Gum Metal Novel gum metal compositions
The first Gum Metal proposed in 2003 [Ti-12Ta-9Nb-3V-6Zr-1.5O and Ti-23Nb-0.7Ta-2Zr-1.2O (%mol)] [26] emerged as β-Ti alloys with the general composition of Ti-24 mol% (Ta, Nb, V)-(Zr, Hf)-O, ensuring that a compositional electron/atom ratio (e/a) of approximately 4.24 was satisfied. The origin of this requirement lies in the first-principles calculations of the elastic constants of binary BCC Ti-X alloys with Ta, Nb, or V, which predict that C 11 − C 12 falls to zero for e/a = 4.24, where C ij are components of the conventional stiffness tensor. The consequence is that the single-crystal Young's modulus becomes zero on the commonly observed slip systems for BCC metals [72].
The stiffness of polycrystalline specimens should be also below, though greater than zero. There are several recent reports of Ti-Zr-Nb-Ta quaternary alloys developed for biomedical applications, where low stiffness is an advantage, in which the e/a values are close to 4.24. Moreover, Luke et al. [73] concluded that the critical electron/atom (e/a) ratio, corresponding to the limit of BCC stability in Ti alloying with transition metals elements to its right in the periodic table, is ~ 4.15.
Subsequently, the studies were then directed to metastable β-Ti alloys located in the region delimited in the diagram, where the other "magic" numbers for Gum Metals are fully or partially satisfied, aiming to better understand the mechanism of strength and deformation in these alloys after a high level of cold work. A synthesis of the relationship between mechanical properties with the different oxygen content and processing condition for some of the Gum Metal is listed in Table 1.
More recently, Liu et al. [79] used synchrotron-based highenergy X-ray diffuse scattering (HE-XRDS), in combination with computer simulations, to explore in-situ the deformation behavior of a single-crystal gum-type Ti-24Nb-4Zr-8Sn-0.10O (wt%) alloy, providing new insight into the intrinsic deformation mechanism for the anomalous mechanical behaviors of the gum-type metallic materials. Upon loading, the existing α" embryos grow and the B2 and BCC regions are transformed into nanodomains of α″ and δ martensites, respectively. Both the number and size of the nanodomains of these two kinds of martensite increase under the applied load, leading to a two-step superelasticity and non-linear elastic behavior of the system, with an elastic strain of ~ 5% and maximum stress of ~ 600 MPa. Another investigation performed in metastable Ti-4Al-4Fe-0.25Si alloy revealed that nanodomains were also observed in relatively lean alloyed and metastable Ti alloys showing an α'-martensite transformation [80]. The nanodomains induced the twinned martensitic structure during deformation, which can greatly affect the mechanical properties of the alloy. Besides designing and producing new compositions, a great effort has also been applied to developing new processing routes,

New processing routes
Gum Metal generally requires cold working as one condition for achieving their exceptional mechanical properties, such as superelasticity, superplasticity, and low elastic modulus. Generally, cold work decreases their elastic modulus and increase the yield strength, as already mentioned in the previous topics. Considering these aspects, the processing of these materials usually involves a large amount of plastic deformation [26,81].
The earliest Gum Metals were mainly manufactured through a combination of powder metallurgy processes (such as powder mixing and sintering), solution treatment and plastic forming processes (such as forging and swaging) [26,28,29,43,82], as summarized in Table 2. As the pioneer in the field, Saito et al. [26] first produced the alloy using these mentioned techniques, giving rise to outstanding Gum Metal properties. This first study has influenced other authors to follow similar processing routes [28,29,43,82]. The original processing route begins with the use of high-purity elemental powders. Then, processes for consolidation, pore closing, and phase solubilization are carried out. Finally, the samples undergo a plastic forming processes to achieve the desired microstructure and properties.
Similarly, arc-melted samples have been processed with a combination of solution heat treatment and plastic forming [29,44,47,[83][84][85][86][87]. It is important to highlight that the first processing step, related to the fabrication of the alloy itself, has a minor influence on the final properties, as pointed out by Guo et al. [28]. This is mainly because the samples undergo a high level of plastic deformation in the final processing step, which has a major effect on the properties. Therefore, the choice of manufacturing the alloy by powder metallurgy or by arc melting is more related to the cost of the process or to the access to the referred processes instead of the properties. The groups of  Sintering, forging, solution treatment, and HPT [88] Sintering, forging, solution treatment, and cold rolling [29] Hot Isostatic Press and cold rolling [29,93] Arc Melting and plastic forming Arc melting, forging, and solution treatment [47] Arc melting, forging, solution treatment, and compression [83] Plasma melting, solution treatment, and hot extrusion [6] Arc melting, hot rolling, solution treatment, and cold rolling [84] Arc melting, hot swaging, and solution treatment [86] Arc melting, hot swaging, solution treatment, and HPT [85] Arc melting, hot extrusion, and ECAP [44,87] Additive manufacturing Selective laser melting and solution treatment [89] Selective laser melting [90][91][92] processes mentioned previously resulted in interesting microstructures and mechanical properties, such as non-linear elasticity, non-work hardening, low elastic modulus, anisotropy, presence of stress-induced αʺ-martensite, twinning, and presence of ω phase within the β-phase matrix [26,28,29,43,44,47,[82][83][84][85][86][87].
In this context, the original samples processed by swaging and/or rolling opened a window of opportunity for other severe plastic deformation processes, such as ECAP (Equal Channel Angular Pressing) [44,87] and HPT (High-Pressure Torsion) [85,88]. Besides the grain refinement, the main findings on the influence of ECAP in Gum Metal are related to the decrease in the strain hardening rate, presence of adiabatic shear bands, and a significant softening during dynamic loading [44]. The occurrence of the stress-induced martensitic transformation β → αʺ has been also reported after ECAP. Furuta et al. [88] reported some important results of a Gum Metal after HPT. They found that the yield and tensile strength increased significantly in comparison with the sample without HPT. The authors also reported the formation of fine grain sizes (in the range of 100-200 nm) created by a transgranular shear during HPT processing, which is unrelated with conventional mechanisms (i.e., dislocation glide and deformation twinning). Another important finding in samples processed by HPT has been found by Silva et al. [85].
The authors proposed a novel mechanism for the increase of β-phase volume fraction after HPT. Since the α" phase becomes unstable after HPT, it leads to a reverse martensitic transformation back to the β phase, which also helps to lower the elastic modulus of the alloy.
Following the trend of new metal manufacturing processes, Gum Metal can also be manufactured by additive manufacturing [89,90]. Batalha et al. successfully processed a β-TNTZ alloy by selective laser melting (SLM) with optimized processing parameters. In addition, biocompatible implants prototypes were built in the study (Fig. 7) [90]. The authors found a columnar grain structure aligned with the building direction, which can be related to anisotropy of the mechanical properties. The same research group has also successfully produced oligocrystalline structures using a combination of SLM and heat treatment [89].
Considering the relevance of additive manufacturing for the production of metallic biocompatible implants, Xu et al. [91] studied the scanning strategy and the corrosion properties in 3.5 wt% NaCl solution of Gum Metals produced by SLM. They reported a single β phase with (001) planes parallel to the horizontal planes of the samples, which is dependent on the scanning strategy. Since the corrosion behavior is dependent on the crystallographic orientation, parameters of the SLM can Reprinted from Batalha et al. [90]. Copyright (2020) by permission from Springer Nature. be adjusted to modify the orientation of the grains and the corrosion behavior. Nadammal et al. [92] studied the influence of the laser energy densities in an additively manufactured Gum Metal. It was reported that the higher values for energy densities resulted in the highest mechanical strengths, with a small decrease in ductility. Cellular to dendritic solidification morphology was observed for the processed samples. Considering the importance of this alloy for biocompatible implants, they have also performed a cytocompatibility in vitro test and the authors reported a proliferation of osteoblasts on the sample's surface, similar to the cell response of pure Ti.

Recent application
A great interest in Gum Metal by researchers and industry has been observed, since they present a unique set of properties as described previously. Up to date, the main Gum Metal applications are in the biomedical area, but other uses have been suggested in the literature, which will be described later. As presented in Fig. 1, the number of published articles in the last decade regarding β-Ti alloys is high and continues to increase each year, indicating a significant and continuous development in this field. This acquired knowledge about Gum Metal can be employed by industry to produce new products or even enhance the properties of the established ones.
In the biomedical field, Gum Metal can be used to produce various products such as implants, stents, and orthodontic wire. For each of these products, a specific set of properties is needed to fulfill the application's requirements. For example, for implants, it is important for the material to present low elastic modulus, high mechanical strength, and good biocompatibility. Nowadays, the most widely deployed Ti alloy for implant application is the Ti-6Al-4 V which has a microstructure composed of (α + β) phases [2,3,85,94]. Although this alloy presents a high fatigue endurance limit, high specific strength, biocompatibility, and good corrosion resistance, it presents some non-ideal properties concerning implant applications. [16,74,94]. Their elastic modulus (~ 110 GPa) is comparatively high relative to the human bone elastic modulus (~ 30 GPa) [74,85,95,96], which can cause stress shielding and lead to osteolysis and subsequent bone fracture [2,74,85]. Also, some chemical elements in its composition are controversial regarding cytotoxicity, such as Al, which is associated with neural diseases [85]. Gum Metal can replace the conventional Ti-6Al-4 V alloy in these types of application that require low elastic modulus and non-toxic alloying elements, and so, are a promising candidate to be used for implant manufacturing. [7,28,97,98]. Golasinski et al. [99] compared the performance of the Ti-36Nb-2Ta-3Zr-0.3O (wt%) Gum Metal to the Ti-6Al-4 V alloy based on mechanical, in vitro corrosion, and biocompatibility tests. The authors reported that the investigated Gum Metal presents higher mechanical strength (over 1000 MPa) and lower elastic modulus [99] than the Ti-6Al-4 V. In addition, the studied Gum Metal was significantly less susceptible to pitting corrosion compared to Ti-6Al-4 V, confirming its excellent corrosion behavior [99]. Also, a large reversible deformation was observed, which is very important for other biomedical applications such as stents, as will described in the sequence.
Gum Metal's superelasticity at room temperature combined with their good formability makes these alloys very suitable for cardiovascular applications, such as stents [26,36,75,100]. The stents are a very important biomedical apparatus since they can be used to avoid cardiovascular diseases. According to the World Health Organization (WHO), cardiovascular diseases (CVDs), including heart diseases and stroke, account for nearly one-third of deaths throughout the world [75,101]. Therefore, efforts are made to control and prevent these problems [75]. It is included in these efforts the development of micro-traumatic surgeries with the implantation of stents, which is acknowledged to be one of the most effective approaches for CVDs [75,102]. Conventional surgery, called coronary angioplasty, is a procedure used to open clogged heart arteries [103]. During the stent placement, the component is collapsed around a balloon at the tip of the catheter, so is guided through the artery to the blockage [103]. At the obstruction, the balloon is inflated and the stent expands, locking it into place inside the artery [103]. The stent stays in the artery (permanently or not, depending on the type of cardiovascular problem) to hold it open to improve blood flow to the patient's heart. Once the stent is in place, the balloon catheter is deflated and removed [103].
Traditional metallic materials stents (316L stainless steel, Co-Cr based alloys, and NiTi) may induce local immune response and inflammatory reactions by Ni and/or Cr ions release, which can lead to neurological diseases such as Alzheimer's [16,75,96]. A stent must present, in addition to high corrosion resistance and excellent biocompatibility, sufficient radial strength and axial flexibility, as well as magnetic resonance imaging visibility [75]. For self-expanding stents, large recoverable strain (superelastic behavior) is also required for both deployment and crush resistance. All these required properties are typical of Gum Metal, which makes them a promising candidate to be applied in stents manufacturing. Thus, Gum Metal can be used to produce self-expanding stents that remain inside the delivery catheter within the superelastic regime or can enter into the superelastic regime after deployment if sufficient deformation is applied (due to vessel interaction or external forces) [75].
The typical mechanical properties of Gum Metal that are interesting for stents applications can be improved by using specific thermomechanical heat treatments such as aging. After aging heat treatment of a Ti-29Nb-13Ta-4.6Zr-O Gum Metal, Plaine et al. [75] reported the precipitation of fine and dispersed second phases into the β matrix [104][105][106]. The presence of the α and ω precipitates enhanced the mechanical properties of these Gum Metal. Combined with the high elastic strain, the superelasticity, and the excellent biocompatibility, the employed aging treatment improves the Gum Metal suitability to be used as self-expansible stent material [75]. Other works in the literature have reported similar results after aging treatment of Gum Metal [18,51,74,106].
Another important application for Gum Metal in the biomedical field is as orthodontic wire. Chang et al. [107] have recommended the use of Gum Metal for this application since this family of β-Ti alloys presents the required properties and characteristics to produce orthodontic wire as follows: (i) low elastic modulus; (ii) superelasticity; (iii) high spring back without energy loss between loading and unloading in the stress-strain relation, which enables easy control of orthodontic force; (iv) ease to bend and to handle (good formability); (v) non-toxic alloying elements in its composition; (vi) lower work hardening, in order to avoid intraoral breakage; (vii) low coefficient of friction, becoming suitable for sliding mechanics of orthodontic tooth movement [107]. Since the Gum Metal fits this set of properties, the authors suggest that the use of Gum Metal in this application could reduce not only the overall orthodontic treatment time but also the number of needed orthodontist visits for wire changes [107]. Thus, the Gum Metals are highlighted as a promising candidate with high potential to improve and enhance the effectiveness of orthodontic treatment [107].
Gum Metal can also be employed to produce a spinal fixation rod that can be used to fix a plurality of spinal-fixing screws embedded and fixed in the vertebrae of a human body. This biomedical application is reported based on the following patent [108]. The component has a cylindrical shape with an adjusted diameter of 4 to 7 mm [108]. The inventors state that this device has an excellent fatigue strength and good mechanical properties, which are very important properties for this kind of application. This component is produced by high-temperature swagging followed by aging heat treatment in a temperature range around 600 to 800 K, for a period between 43.2 and 604.8 ks [108]. This invention was filed in 2011 and represents one of the few patents found for Gum Metals. Although there is a significant increase in the scientific research regarding Gum Metal, the amount of filed patents related to Gum Metal products is very small. This can be attributed to the recent discovery and development of these alloys, which are still being studied by scientific researchers. Most of the effort of scientific research in this field is condensed on improving the properties of these alloys, as well as better understanding their behavior during processing and use. However, these studies are mostly performed on a laboratory scale.
Nevertheless, not only in the biomedical field the Gum Metal can be applied. Max-Planck-Institut für Eisenforschung scientists [109,110] suggested an interesting application based on its unusual mechanical properties. Based on the Gum Metal's very low elastic stiffness and very high ductility they suggested that they can be used in the aerospace field as a device that works like a crash absorber. [109,110]. They argue that when an aircraft's turbine is damaged by hail or a bird strike, there is a risk that individual parts may shatter and damage the fuselage. If parts of the protective casing around a turbine were made of Gum Metal, the flying debris could be captured because the impact would not destroy but only deform them [109,110].
Lastly, Todd et al. [81] suggested other possible applications for Gum Metal as, for example, lightweight spring manufacturing, which makes use of the high yield strain of these alloys in a similar way that was described for orthodontic wires [107]. The use in satellite components was also pointed out by Todd et al. [81], once Gum Metal has Invar property and is able to retain its shape during thermal cycling experienced in orbit [26]. Also, Gum Metal can be used in automobile parts, since they present good resistance to permanent damage in minor collisions; and in sporting goods (like golf clubs), due to their unusual elastic properties [81].

Future perspectives and conclusion remarks
Since their discovery, the behavior of Gum Metal has been better understood in terms of processing, composition, and electronic parameters, enabling new compositions and processing routes to be proposed, as well as different new possible applications, suggested. For the next following years it is reasonable to expect an increase in research regarding Gum Metal processing and applications, or at least a stay in its current relevance, which is considerably high. Given the future developments, both in capacity and control of the manufacturing processes, as well as in the numerical computational methods, several new possibilities may favor the development of this alloy system.
Considering the processing routes there are still many possibilities for studies regarding the manufacturing of these alloys. For example, considering the severe plastic deformation (SPD) methods, an upscaling [111][112][113] could be performed to produce bigger products that could be designated for implant application. More studies related to the production of these alloys by SPD should be performed, with a focus on HPT and ECAP methods, which are the two most studied SPD methods [114]. Also, alternative SPD methods can be suggested and implemented, as high-pressure sliding (HPS) [111,115].
Considering the additive manufacturing methods there are still many possibilities to better tailor the properties of Gum Metal. A systematic investigation of different scanning strategies and optimization of input laser parameters could improve the mechanical properties, and even reduce the elastic modulus by choosing an adequate specific building direction. Phase stability and microstructure control, which are directly related with to plastic deformation mechanisms, could be better understood and samples could be produced by using optimized parameters, which could favor stent manufacturing. Besides that, there are only a few studies related to the use of this technique to produce this alloy system. With the popularization of this manufacturing method, and consequently the expected increase in the scientific production in this area, new studies should emerge reporting results of Gum Metal manufacturing for different additive manufacturing techniques, such as laser powder bed fusion (LPBF), directed energy deposition (DED), among others. Also, laser remelting and/or cladding techniques could be used to modify surface properties to improve osseointegration aiming implant applications [116].
Regarding Gum Metal compositions, two main approaches are expected for the development of new compositions in a future trend. The first approach is based on the theoretical composition design that uses the stability maps based on the electronic parameters suggested by Saito [26,67] (Fig. 6). New Gum Metal can be designed by small changes in chemical composition or by alloying different biocompatible elements. It is still important to fulfill the "magical numbers" suggested for Gum Metal, otherwise the alloys will not present their typical properties. In addition, a greater focus on the influence of interstitial alloying elements on the properties of Gum Metal should be evaluated, since these elements, such as O and N, exert a significant influence on Ti alloys' properties in general. Such studies, combined with the improvement of the various fabrication methods that were covered in this review, as well as the creation of new manufacturing methods, will allow the development of Gum Metal with a precise interstitial alloying control.
The second approach to the development of novel compositions is based on the recent advances in machine learning methods [117]. New compositions can be obtained from a The constrained prediction process used in the β low-assisted alloy design for low-modulus and low-β stabilizer β-Ti alloys. Reprinted from Wu et al. [118]. Copyright (2020) by permission from Elsevier. database of properties of β-Ti alloys, which, combined with mathematical and thermodynamic formulations, can lead to obtaining new alloy compositions with specific and optimized properties. An example of the power of such a tool is reported by Wu [118]. In this work, the authors used a neural network machine called "βlow" that enables a highthroughput recommendation for new β-Ti alloys with elastic modulus lower than 50 GPa. Figure 8 illustrates the working scheme of the machine learning-assisted approach for recommending unique "recipes" for Ti alloys with low elastic modulus [118]. An interesting point here is the fact that this research was performed by just using a database with small content (based on literature and experiments results). The unexplored space of the Gum Metal chemical compositions is still large and could be improved by using a database with a larger amount of data. In this work, a new Ti-12Nb-12Zr-12Sn (wt%) alloy was recommended by the βlow neural network machine, based on optimization for lower values of the martensitic start transformation temperature ( M S ) and elastic modulus. The recommended alloy was manufactured and characterized and the measured properties present good agreement with the predictions of the βlow neural network machine. As computer processing power increase in the coming years, and with the advent of Ti alloy databases with larger content, the possibilities of machine learning methods will be significantly increased, thus enabling that new studies provide novel insights on Gum Metal and β-Ti alloys.
Regarding future trends for applications, it was shown in the present paper that Gum Metal are still being studied by scientific researchers, and the direct use of them are not well stablished in industry. In the next decade, these alloys will probably increase their market share, mainly due to the advances in manufacturing methods, as well as due to an adequate understanding of the fundamental principles of this system, which has been obtained and refined over the last two decades. Based on that, it is expected that initially, biomedical applications should be implemented, like implants and stents, and further, it expands to other fields.
In the light of the present article, the authors conclude that very interesting and innovative research has been conducted on Gum Metal alloys. With the development of new technologies demanding new materials and functionalities, this class of Ti alloys will become an important candidate for different applications and many researchers should still address this topic in the coming future.

Data availability
All data generated or analyzed during this study are included in this published article.

Conflict of interest
The authors declare no conflict of interest.

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