The utilization of titanium (Ti) alloys in the realm of medical and dental applications stems from their exceptional mechanical attributes, robust corrosion resistance, and biocompatibility [1,2]. Among the array of Ti alloys, the Ti6Al4V alloy has gained widespread prominence in the domain of medical implants. Nevertheless, this alloy exhibits inherent deficiencies such as low shear strength, limited wear resistance, and the potential bio-toxicity associated with the release of aluminum (Al) and vanadium (V) ions into the human body during biomedical applications. An additional significant concern pertains to the disparity in Young's modulus between Ti6Al4V (105-110 GPa) and bone (10-40 GPa), which poses a substantial challenge for effective bone healing and remodeling [3]. To ameliorate the biological and mechanical attributes of Ti alloys, strategic modifications to material composition and meticulous control over microstructure become pivotal.

Ti alloys are categorized into α, α+β, and β structures based on their microstructure at room temperature  [4]. The α structure boasts an elastic modulus of approximately 100-120 GPa, while the β structure exhibits an elastic modulus of approximately 60-80 GPa [3]. Evidently, the β structure features a lower elastic modulus that more closely aligns with that of bone. However, the β structure typically leads to compromised strength due to its inherently lower elastic modulus. Generally, the fortification of the β structure is achieved through alloying and the reduction of grain size [1]. It is essential to note that the process of alloying can also augment the elastic modulus due to changes in the atom-bond configuration within the β phase. Consequently, reducing grain size emerges as a more favorable approach for enhancing the β-structure's strength. Notably, when the grain size is reduced to the nanometer scale, a remarkable increase in strength ensues [5]. Moreover, the reduction in grain size leads to a proportionate increase in the volume fraction of grain boundaries. As a consequence of the heightened presence of dense defects (including voids and dislocations), the elastic modulus of grain boundaries is anticipated to be lower than that of the grains. Consequently, the grain boundaries contribute to the overall reduction in elastic modulus [1]. Based on this discourse, the adoption of an ultrafine β-Ti structure is projected to yield heightened strength accompanied by a diminished elastic modulus.

Copper (Cu) serves as a β-stabilizing element, and the fusion temperature of an alloy decreases with escalating copper content, thereby facilitating casting procedures [6]. The TiCu phase diagram denotes the occurrence of α-Ti and the precipitation of Ti₂Cu intermetallic compounds at a Cu concentration of 7.0 wt.% [7]. Additionally, the incorporation of Cu into Ti alloys is reported to confer acceptable corrosion resistance [8] and biocompatibility [9]. Numerous researchers have concentrated on the application of TiCu alloys in industrial contexts. Kikuchi et al. [6,10] emphasize that the presence of Ti₂Cu intermetallic compounds augments tensile strength while decreasing ductility in TiCu alloys when compared to commercially pure Ti (CP-Ti). Employing thin foil electron microscopy and XRD techniques, Williams et al [11]. scrutinized the process of Ti₂Cu precipitation during the aging of rapidly quenched near-eutectoid TiCu alloys and identified compounds such as Ti₂Cu. Sun et al [12]. demonstrated that the strength of the Ti2.5Cu alloy arises from the formation of Ti₂Cu intermetallic particles subsequent to the decomposition of the β phase into α-Ti and Ti₂Cu. Yao et al [13]. underscored that acicular Ti₂Cu particles contribute more significantly to strengthening in comparison to spherical counterparts in the Ti2.5Cu alloy. Souza et al [14]. found that the microstructure of the TiCu eutectoid alloy encompasses α'-martensitic phases when subjected to cooling rates exceeding 9°C/s, while slower cooling rates yield higher modulus values. Takada et al [15]. proposed that TiCu alloys composed of α-Ti or Ti₂Cu intermetallic compounds evidently exhibit adequate corrosion resistance. Osorio et al [16]. observed an escalation in the corrosion rate of TiCu alloys with increasing copper content. Taso [17] delved into the microstructure and electrochemical properties of Ti7Cu alloys, revealing the presence of finer α'-martensitic structures intermingled with α-Ti and Ti₂Cu intermetallic compounds in as-cast alloys. Notably, the heat-treated sample exhibited a notably nobler corrosion potential, while other characteristics displayed marginal disparities. Tin (Sn) finds extensive application in biomedical Ti alloys. Its propensity to react with Ti and Cu in the liquid state engenders a heightened nucleation rate and curtailed grain growth upon solidification [18]. This characteristic underscores the potential to generate ultrafine or nanostructured configurations during solidification when incorporating Sn into TiCu alloys. Moreover, Sn is recognized as a safe alloying element for Ti, having been determined to be non-toxic and non-allergenic [2]. A bimodal microstructure thus created offers an innovative fusion of heightened strength and diminished Young's modulus, making it a compelling candidate for biomaterial applications [19].

Furthermore, numerous investigations substantiate the potency of Sn in reinforcing Ti alloys. Tsao et al [20]. delineate that augmenting Sn content (ranging from 1 to 5 wt.%) in Ti7CuXSn alloys engenders enhanced solubility of Cu in the eutectoid microstructure (α-Ti + Ti₂Cu), prompting a transformation into ultrafine α'-martensitic microstructures and bolstering the volume fraction of Ti₂Cu phase. Han et al [21]. Note that elevating Sn content (1~3 at.%) in Ti alloys precipitates a reduction in lamellar spacing within the eutectic matrix, thereby enhancing both plasticity and strength. Hsu et al [22]. Establish a correlation between higher Sn content in TiSn alloys and heightened hardness values. Ho et al [23,24]. Elaborate on how the machinability of TiSn alloys escalates proportionately with the concentration of Sn.

In recent years, there has been a growing interest in the development and utilization of Ti alloys as biomaterials for various medical applications. These alloys offer exceptional mechanical properties and biocompatibility, making them an attractive choice for implants and prosthetic devices [3,25-27]. To further enhance their biocompatibility and osteointegration, researchers have explored the integration of bioceramic coatings, such as hydroxyapatite (HA), with Ti alloys. Incorporating elements like Sr into these coatings has emerged as a promising avenue for improving the bioactive properties of these materials [28-32]. HA, a well-known ceramic biomaterial, closely mimics the mineral component of natural bone. When deposited onto the surface of Ti alloys, it can promote osseointegration and enhance the long-term stability of implants within the human body [33-36]. However, despite its excellent biocompatibility, pure HA lacks certain characteristics that are crucial for optimal bone healing. This has led to the exploration of modified HA coatings with the incorporation of Sr [37-40]. Sr, an alkaline earth metal, has gained attention in recent years for its potential to improve bone health. Studies have shown that Sr can stimulate bone formation while inhibiting bone resorption, making it a valuable addition to biomaterials intended for orthopedic and dental applications [41-43]. When Sr is incorporated into HA coatings on Ti alloys, it can potentially enhance the osteogenic properties of the material, leading to improved implant success rates and reduced risk of implant-related complications [44-46]. The incorporation of Sr into HA coatings on Ti alloys represents a novel approach to designing biomaterials with enhanced biocompatibility and osteointegration capabilities. This synergy between the mechanical strength of Ti alloys and the biological properties of Sr-modified HA holds great promise for the development of advanced medical implants and prosthetic devices [28,45,47-49].

In this study, we embark on a comprehensive investigation involving varied Sn concentrations within the Ti7CuXSn alloy. The aim is to elucidate the influence of Sn on microstructure, microhardness, and corrosion behavior, thus contributing to the collective understanding of enhancing the properties of Ti-based alloys for biomedical applications. We also aim to provide a comprehensive overview of the research efforts focused on the development and characterization of titanium alloy substrates coated with Sr-modified HA. It will explore the fabrication techniques, physicochemical properties, in vitro and in vivo performance, and potential clinical applications of these innovative biomaterials. By highlighting the advantages and challenges associated with this approach, we hope to contribute to the advancement of materials science and the translation of these materials into clinical practice, ultimately benefiting patients in need of orthopedic and dental implants.

Utilizing commercially pure metals, namely Ti with a purity of 99.8 wt%, Cu with a purity of 99.99 wt%, and Sn with a purity of 99.99 wt%, the present study aimed to fabricate a series of Ti7CuXSn alloys, where X represents varying weight percentages (2, 7, and 15 wt.%), closely adhering to the eutectoid nature as delineated by the TiCu equilibrium phase diagram [24,50]. The preparation of the as-cast Ti7CuXSn alloys involved the utilization of an arc-melting technique, wherein the elemental constituents were meticulously fused together under a controlled environment. This amalgamation was achieved through the application of a consistent 300 A current, creating a molten alloy mixture placed atop a water-cooled Cu hearth enveloped within a pristine Argon (Ar) gas atmosphere. Ensuring the formation of a uniformly alloyed structure, the as-cast Ti7CuXSn alloys underwent multiple cycles of melting and solidification, accompanied by the periodic rotation of the solidified ingots. Following the as-casting process, subsequent heat treatments were performed to achieve optimal microstructural and compositional homogeneity. The as-cast Ti7CuXSn alloy samples were subjected to a homogenization step at 950°C, lasting for 2 hours, and promptly quenched in water to arrest any potential phase transformations. Comprehensive microstructural investigations, phase identification analyses, material property assessments, and corrosion evaluations were conducted on both the as-cast and heat treated alloys.

Phase composition determination was realized through XRD analysis, employing a D/max 2500 V/PC instrument. Prior to analysis, all specimens were meticulously prepared, involving grinding with progressively finer silicon carbide papers up to a 2000-mesh grade. Subsequently, specimens were subjected to polishing and etching with Keller's etchant, comprising 1 mL of Hydrofluoric Acid (HF), 2.5 mL of Nitric Acid (HNO₃), 1.5 mL of Hydrochloric Acid (HCl), and 95 mL of deionized water, to unveil the intricate microstructural features.

Optical microscopy (OM) provided insight into the alloy's microstructure, while scanning electron microscopy (SEM) facilitated a more detailed examination, further supplemented by energy dispersive spectroscopy (EDS) to unravel the elemental composition. The microscopy analyses were carried out utilizing sophisticated instruments including the S-3000N from HITACHI, Japan, and the JSM-7500F from JEOL, Japan. Microhardness, a crucial mechanical property, was evaluated using a Matuszawa Seiki MV-1 hardness testing machine. The tests were conducted with a load of 200 gm applied for 15 seconds. Three separate samples were utilized for each alloy composition, and each sample was subjected to a minimum of five random indentations, yielding an averaged microhardness value.

Electrochemical investigations were undertaken to assess the corrosion behavior of the alloys. Potentiodynamic polarization experiments were conducted in a 3.5 wt.% sodium chloride (NaCl) solution. The test specimens were meticulously prepared by grinding with 1200 grit Silicon Carbide (SiC) paper and subsequent cleaning with acetone. Employing a scan rate of 1 mV/s, potential steps were executed within the range of -1500 to +2000 mV (Ag/AgCl), while a saturated calomel reference electrode (Ag/AgCl) and a platinum (Pt) counter-electrode were employed. The ensuing electrochemical measurements and the resulting surface morphology of the specimens were scrutinized using the JSM-7500F SEM, while EDS analysis facilitated the characterization of alloy composition and the formation of passive films post-potentiodynamic polarization. 

The as-cast Ti7Cu15Sn substrates underwent shaping into specimens via wire electrical discharge machining (wire-EDM) with dimensions of 2 mm in length, 2 mm in width, and 20 mm in height. Subsequently, the specimens were meticulously polished using SiC abrasive papers with grit sizes of #100, #600, #1,000, #1,600, and #2,000, following a specific sequence. To eliminate contaminants and slight oxidation, the substrates were immersed successively in acetone, ethanol, and distilled water, undergoing ultrasonic cleaning. In the process of generating Sr-HA coatings on these substrates using the Growing Integration Layer (GIL) method [51,52] an electrolyte containing 0.15 M (26.427 g/L) calcium acetate (CA), 0.06 M (7.199 g/L) sodium dihydrogen phosphate (SDP), and 0.03 M (7.97 g/L) strontium hydroxide (Sr(OH)₂·8H₂O) was employed. The pH of the mixed electrolyte was determined to be 6.34. For the coating process, the substrates were partially covered with Teflon tape, leaving only 0.84 cm² of the surface area exposed. Five specimens were prepared with varying applied voltages. These substrates were connected to a DC power supply with the substrates serving as the anode, while a stainless steel vessel filled with the electrolyte mentioned earlier served as the cathode. To prevent overly rapid Sr-HA coating, the temperature of the electrolyte was carefully maintained below 20°C using a water cooling system. The substrates were subjected to different applied current densities (1 A/cm², 2 A/cm², and 3 A/cm²) with a 350 V. The Sr-HA coatings were allowed to grow on the sample surfaces over a period of 30 minutes.

The Microstructure of Ti7CuXSn

The OM micrographs in Figure 1 provide insight into the microstructural characteristics of both as-cast (AS) and heat-treated (HT) Ti7CuXSn alloys. In Figures 1(a), 1(c), and 1(e), the presence of a martensitic structure is evident, comprising plate-like α-Ti(Sn,Cu) phases intermingled with β grain boundaries. The movement of these boundaries, concomitant with the precipitate growth, facilitates the formation of the supersaturated σ-Ti(Sn,Cu) phase along the grain boundaries due to the eutectoid reaction during rapid solidification [20,53,54].

This phenomenon of plate-like α-Ti(Sn,Cu) martensitic structure, as observed, is a result of the complete decomposition of the β phase, commonly encountered in Ti alloys. On Figures 1(b), 1(d), and 1(f), a similar martensitic structure and pre-existing β-grains are depicted. A comparison between Figures 1(a) and 1(b) reveals that, upon heat treatment, the pre-existing β grain boundaries and α-Ti(Sn,Cu) martensitic structures coarsen. This tendency is observed across all specimens. Notably, the addition of Sn is instrumental in modifying the microstructure of the Ti7CuXSn alloys (X=2, 7, and 15 wt.%), as illustrated in Figure 1. Of significance, the grain size exhibits a decreasing trend with an elevated Sn content in the Ti7CuXSn alloy. This phenomenon can be attributed to the ability of Sn to precipitate (Ti,Sn)₂Cu intermetallics, effectively restraining the growth of grain size. Consequently, limited grain size martensitic structures and a substantial volume fraction of (Ti,Sn)₂Cu intermetallics manifest under high cooling rates.

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