Mechanical properties and biological responses of ultrafine-grained pure titanium fabricated by multi-directional forging

https://doi.org/10.1016/j.mseb.2019.05.002Get rights and content

Highlights

  • Ultra grain refinement by means of multi-directional forging (MDFing) was applied to pure titanium.

  • MDFing increased the tensile strength, Vickerd hardness, and decreased elastic modulus compared to pure titanium.

  • Cell proliferation on MDFed titanium was significantly promoted than on conventional titanium by acid treatment.

Abstract

We aimed to investigate strengthening of pure titanium by ultra-grain refinement using multi-directional forging and to examine the biocompatibility after sulfuric acid treatment. Commercial titanium grade 2 was multi-directional forged to large cumulative strain to obtain homogeneous ultrafine-grained structure. The evolved microstructure of the obtained material was equiaxed and ultrafine-grained with high dislocation density. Average grain size was evaluated to be approximately 100 nm. The ultrafine-grained titanium showed higher tensile strength, Vickers hardness and a lower elastic modulus compared to conventional pure titanium grade 2. The surface features of the multi-directional forged titanium after acid treatments were characterized by a roughness of 1 μm composed of equiaxed fine dimples with fractal structure and a hydrophilicity of 15. Cell proliferation on multi-directional forged titanium was significantly promoted than on conventional titanium. The above results demonstrated the high potential of multi-directional forged titanium as a superior biocompatible implant material.

Introduction

Pure titanium (Ti) and Ti alloys are now widely used as orthopedic or dental implant materials due to their excellent mechanical properties, biocompatibility and corrosion resistance etc [1], [2], [3]. Pure Ti has lower strength than Ti alloys and caused restriction of its applicable areas. Thus, Ti-6Al-4 V or Ti-6Al-7Nb is now mainly used as biomedical Ti alloys due to their superior mechanical properties [1], [2], [3], [4]. However, they have some disadvantages such as toxic alloying elements, large difference in the elastic moduli between born and Ti alloys, hard machinability etc [5]. Vanadium is a cytotoxic and known as a carcinogenic substance and aluminum is reported to be associated with the induction of neurotoxicity and neurodegenerative disease such as Alzheimers’s disease [6], [7]. The elastic modulus of cortical bone is reported to be approximately 10–30 GPa and that of Ti alloy such as Ti-6Al-4 V is approximately 110 GPa [8], [9]. These differences can cause uneven transmission of applied stress onto implant materials, called as stress shielding and this uneven loading sometimes results in the loosening and/or fracture of implants [10], [11]. Sumitomo et al. [12] fixed three different bone plates, made of Stainless Steel Wire SUS316L, Ti-6Al-4V and Ti-29Nb-13Ta-4.6Zr (TNTZ) beta titanium alloy plate in rabbit tibia. Elastic modulus of SUS316L is the highest among three metals and that of TNTZ is almost the half of that of Ti-6Al-4V. Better bone response was obtained by TNTZ and it is confirmed that elastic modules of the bone plate will naturally influence bone tissue reaction. TNTZ has no toxic elements in the composition, but the ultimate tensile strength of TNTZ was less than Ti-6Al-4V. Therefore, development of new implant materials of pure Ti with higher tensile strength and the equivalent elastic modulus to bone is highly desirable.

Recently, development of ultrafine-grained (UFGed) metals and alloys, where the grain size is less than 1 m, is paid attention. Strength of metallic materials increases with decreasing grain size, which is well known as Hall-Petch relation [13], [14]. Grain refinement can induce strengthening without any addition of alloying elements and would have potential to achieve strength desired [15]. The severe plastic deformation (SPD) is known as new method to attain UFGed structure and numerous number of researches on SPD and UFGed structures have been carried out [16]. Accumulative roll bonding [17], [18], [19], [20], high pressure torsion [21], [22], [23], [24], [25], [26], equal channel angular pressing (ECAP) [27], [28], [29], [30], [31], [32] and multi-directional forging (MDF) [33], [34], [35], [36] are commonly used for SPDs. Particularly, MDF is known as one of the most suitable methods for large sample preparation. Miura et al have carried out MDFing of hard-plastically deformable Ti and magnesium alloys having hexagonal-close-packed crystal structure and achieved UFGed structures and strengthening [34], [35], [36]. In case of Ti grade 2, which grade was determined by Japanese Industrial Standards, ultimate tensile strength over 1 GPa could be attained [35], [36].

On the contrary, various surface modification techniques for titanium dental implants, such as blasting, acid or alkaline treatment, UV irradiation and hydroxyapatite coating, are currently being investigated in order to attain tight bond bonding to titanium [37], [38], [39], [40], [41], [42], [43], [44], [45]. Especially a combination of sandblasting with large grits and acid etching has been commonly used surface modification technique [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. The surface modification technique for sandblasting with large grits and acid etching is called as SLA treatment. SLA treatment process produced macro-rough surface with blasting and micropits on the rough-blasted surface by acid etching. Etching of titanium using concentrated acid is also attractive method for preparing roughened surface [47]. The effectiveness of the combination of sulfuric acid etching and UV irradiation has been reported [48].

It is well known that the adhesion and proliferation of osteoblasts cell to titanium are influenced by the surface morphologies of titanium [49]. Generally, cells on rougher surfaces tended to exhibited more differentiated behaviors than those on smoother surface. However, the issues regarding optimal surface roughness and morphologies are still controversial and need to be clarified [50]. UFGed metallic materials are known to exhibit various specific properties notably different from coarse grained conventional ones [15], [16]. In the present study, the mechanical properties and in vitro biocompatibility of UFGed pure Ti fabricated by MDFing was examined. Efficacy of sulfuric acid treatment to Ti surface was investigated by osteoblast-like cell assay. The potential of the UFGed Ti as a dental implant material was evaluated with comparison of the properties of conventional Ti.

Section snippets

Materials

Two types of Ti grade 2 samples were employed in the present study, i) conventional coarse-grained Ti with grain size of about 20 m (CCG-Ti, Toho Titanium, Kanagawa, Japan) and ii) UFGed Ti fabricated by MDFing (UFG-Ti) which was supplied from Kawamoto Heavy Industries, Japan. The detail of MDF method is precisely described elsewhere [34], [35], [36]. The evolved macrostructures of both specimens were assessed by optical microscopy (PME 3, Olympus, Tokyo, Japan). Detailed microstructural

Microstructures of CCG-Ti and UFG-Ti

Typical photographs of macrostructure of CCG-Ti and UFG-Ti observed by optical microscopy are shown in Fig. 1. It appears that the coarse initial grains in CCG-Ti (Fig. 1a) were fragmented by mechanical twinning during MDFing to have much finer grained structure in UFG-Ti (Fig. 1b). Detailed microstructural observation of UFG-Ti by TEM was carried out and it is displayed with a corresponding selected-area-diffraction pattern in Fig. 2. Because the diameter of a diaphragm used for the analysis

Conclusion

Commercially pure Ti grade 2 was multi-directional forged to large cumulative strain to obtain homogeneous UFGed structure. The evolved microstructure of the UFGed-Ti was equiaxed and ultrafine-grained with high dislocation density. The average grain size of UFGed-Ti was less than 100 nm. The UFGed Ti has a higher tensile strength (approximately 995 MPa) and Vickers hardness (approximately 300 Hv) and and a lower elastic modulus (approximately 50 GPa) compared to CCG-Ti. Newly developed

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