Full length articleA more defective substrate leads to a less defective passive layer: Enhancing the mechanical strength, corrosion resistance and anti-inflammatory response of the low-modulus Ti-45Nb alloy by grain refinement
Graphical abstract
Introduction
Titanium (Ti) and its alloys are widely used metallic biomaterials for hard tissue replacement due to the high specific strength, good corrosion resistance, and adequate biocompatibility [1]. However, the elastic modulus of the conventional α-type pure Ti and α + β-type Ti-6Al-4V alloy is 104-114 GPa, which is larger than that of natural bones (10-30 GPa) [2,3]. As a consequence, the implant will bear the majority of the mechanical load leading to the stress shielding effect after implantation, resulting in possible implant loosening and failure. Moreover, the alloying elements in the Ti-6Al-4V alloy raise clinical concerns about the biosafety and inflammatory reactions [4].
Recently, β-type Ti alloys with low elastic moduli have been fabricated with biologically friendly alloying elements such as niobium (Nb), molybdenum (Mo), tantalum (Ta), and zirconium (Zr) [5]. Among the different types of binary β-type titanium alloys with a body-centered cube (bcc) lattice, the Ti-45Nb alloy with a single β-phase [2,3,6] is especially attractive on account of the small elastic modulus of 60-70 GPa [7] and favorable osteogenic properties of Nb [8]. However, low-modulus Ti alloys usually have insufficient mechanical strength [2,9]. Traditional heat treatment can improve the strength of β-type Ti alloys via precipitation hardening but unfortunately, the elastic modulus increases at the same time [10,11] due to unavoidable changes in the crystalline structure. Moreover, the hardening precipitates may reduce the corrosion resistance of β-type Ti alloys [12,13]. Alternatively, grain boundary hardening at room temperature provides the possibility to increase the strength based on the Hall-Petch relationship [14] without changing the elastic modulus [6]. Severe plastic deformation (SPD) is commonly adopted to fabricate ultra-fine-grained (UFG) materials by introducing extremely high plastic strain to the metals [15]. In fact, SPD techniques such as rolling and folding (R&F) [6], high-pressure torsion (HPT) [1,3,9], and hydrostatic extrusion (HE) [16] have been shown to enhance the strength of Ti-45Nb alloys by producing a ultra-fine-grained (UFG) structure with unchanged elastic moduli. However, orthopedic and dental biomaterials must fulfill multiple requirements besides the mechanical properties. Under in vivo conditions, the metallic implants are frequently in contact with an aggressive medium and significant leaching of metallic ions from the implants is possible [17]. Therefore, sufficient corrosion resistance and biocompatibility are also critical to metallic implants.
The effects of grain refinement on the corrosion resistance and biological response of Ti-45Nb alloys are not well understood as there are multiple factors in addition to conflicting results [18]. Therefore, it is necessary to investigate systematically the influence of grain boundaries on the surface passivity. A native passive oxide film forms on Ti and its alloys upon exposure to air [12] and the properties of the passive oxide layer are mainly determined by the stoichiometry and defects [19], both of which are affected by the grain size of the materials. The passive layer on the Ti-45Nb alloy is mainly composed of TiO2, Nb2O5, and their sub-oxides [14] and TiO2 is more corrosion resistant when doped with Nb2O5 [14,20,21]. Hence, the influence of grain size on the chemical composition of the passive layer (TiO2, Ti2O3, TiO, Nb2O5, NbO2, and NbO) should be assessed quantitatively. Moreover, defects also change the properties of the passive layer [22]. Nanoscale grain refinement has been reported to improve the anti-corrosion behavior of α-type pure Ti [23] as well as β-type alloys such as Ti-29Nb-13Ta-4.6Zr [24] and Ti–24Nb–4Zr–8Sn [25] by reducing the number of defects and increasing the thickness of the passive layer. In addition, the properties of the passive films are crucial to the biological response in vivo [4,14]. If the passive layer is damaged, exfoliated, or dissolved, the corrosion products and released ions may cause detrimental side effects and eventual implant failure [26,27]. In general, metallic ions released from implants can activate the immune system [28] to cause inflammation and produce cytotoxicity to the peri-implant macrophages. Therefore, the desirable metallic implants should be optimized by tailoring the surface passive film [4,29] in conjunction with mitigation of metallic ions release.
In this study, the effects of grain refinement on the Ti-45Nb alloy are studied in details in order to elucidate the relationship between the grain size, microstructure, and properties of the surface passive layer. The biomedical Ti-45Nb alloy is subjected to HPT processing and the influence of grain refinement on the corrosion resistance are investigated systematically by a variety of electrochemical measurements as well as in-depth examination on the microstructure of the passive layer. As a potential biomaterial for orthopedics and dentistry, the inflammatory response of the UFG Ti-45Nb alloy is evaluated both in vitro and in vivo.
Section snippets
HPT processing
The elemental composition of Ti-45Nb bars obtained from Ningxia Orient Tantalum Industry Co. Ltd. was determined by inductively-coupled plasma optical emission spectrometry (ICP-OES) on a Perkin Elmer OPTIMA 7000DV spectrometer (Table S1, Supporting Information). Ti-45Nb disks with a thickness of 0.8 mm and diameter of 10 mm were cut and then processed by HPT at room temperature under an applied pressure of 3.0 GPa at a rotational speed of 1 revolution per minute (rpm) for 0.25-10 revolutions
Grain size and mechanical properties
As shown by the OM image in Fig. 1a, the CG sample shows a mean grain size of 77 ± 12 µm. After 10-turn HPT processing, the curvy and not well-defined grain boundaries in the TEM images (Fig. 1b-c) indicate the typical non-equilibrium feature of grain boundaries generated by SPD processing [33]. Those grains with poorly defined boundaries are different from grain boundaries in most conventionally processed materials [34]. The grain size of the Ti-45Nb alloy is refined homogeneously to 88 ± 35
Influence of HPT processing on the mechanical properties
As illustrated in Fig. 2, HPT processing increases the mechanical strength and microhardness of the Ti-45Nb alloy. The microhardness increases gradually with increasing accumulative strain and the most pronounced hardening effect is observed for ε of 3-3.5. HPT processing has been reported to lead to inhomogeneous grain refinement due to the different strain at various distances from the center [37]. However, in this study, HPT processing for 10 turns (ε = 5.5) produces fairly homogeneous
Conclusion
HPT processing enhances the mechanical properties of the β-type Ti-45Nb alloy via grain boundaries strengthening. The ultimate tensile strength increases from 370 MPa to 658 MPa but the elastic modulus of the Ti-45Nb alloy remains at the low level of 61-72 GPa because the single-phase bcc β-structure is preserved during HPT processing. The Ti-45Nb sample after grain refinement has better anti-corrosion properties, because the smaller surface work function after HPT processing facilitates the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge financial support from the National Natural Science Foundation of China (no. 31922040), Shenzhen Science and Technology Research Funding (nos. SGLH20180625144002074 and JCYJ20180507182637685), Youth Innovation Promotion Association of Chinese Academy of Sciences (nos. 2017416 and 2020353), Natural Science Foundation of Guangdong Province (no. 2018A030313873), Shenzhen – Hong Kong Innovative Collaborative Research and Development Program (no. 9240014), Guangdong – Hong
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