Development and properties of dental Ti–Zr binary alloys

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Abstract

In this study, two medium Zr-containing Ti-based alloys with commercially pure titanium as control were systematically investigated to assess their potential biomedical application. After samples subjected to TMP and CR, it was found that the Zr addition significantly affected the microstructure, phase constitutions, mechanical properties and cytocompatibility. The microstructural results showed that increasing Zr concentrations resulted in more refined grains. Furthermore, Zr changed the phase constitution: CR Ti–20Zr was formed by the single α-phase while CR Ti–30Zr alloy was formed by the coexistence of α and deformation-induced FCC phases. The P-type FCC phase was dominant and more prone to occur than the B-type one. The mechanical tests demonstrated that the increasing Zr content led to a simultaneous increase in micro-hardness, strength and plasticity of CR samples due to the combined effects of solution strengthening, work hardening and the FCC phase. The SEM fractography indicated that the brittle fracture of CR Ti–20Zr due to deformation twins and ductile fracture of CR Ti–30Zr because of FCC phase. Furthermore, Ti–Zr alloys presented comparable cytocompatibility to the CP-Ti control based on cell viability, proliferation and intracellular O2− content of MSCs. Specifically, alkaline phosphatase activity in BMSCs were significantly higher for grain refined CR Ti–30Zr. Considering all these results, CR Ti–30Zr alloy exhibited the optimal comprehensive performance to be potential dental materials.

Introduction

Commercially pure titanium (CP–Ti) has been the most appealing alternative options for biomedical application. However, standard diameter titanium implant placement may inappropriate for narrow edentulous ridges or reduced interdental spaces without complex bone augmentation procedures (Grandin et al., 2012). Thus, small diameter implants (SDIs ≤3.5 mm) are preferred. Unfortunately, Ti-SDIs have been associated with an increased risk of fatigue fracture due to insufficient mechanical strength (Grandin et al., 2012). As a result, there has been a drive to develop SDIs from new Ti alloys with advanced tensile strength and friendly biocompatibility, which can be achieved by the addition of substitutional and interstitial elements, thermomechanical processing (TMP) (Kuroda et al., 2016), and other techniques such as powder sintering (Yilmaz et al., 2018), powder injection molding (PIM) (Yılmaz et al., 2017a), and severe plastic deformation (SPD) (Langdon, 2013).

In order to improve the functional properties, not only new Ti alloys have been fabricated, such as Ti–Ga (Qiu et al., 2014), Ti–Bi (Qiu et al., 2015), Ti–In (Wang et al., 2013) binary alloys and Ti–Nb–Sn ternary alloys (Yılmaz et al., 2017b), but also multilayer coatings have been developed, such as nano-sized Ti/TiN (Sobol et al., 2012) and TiN/Ag systems (Wan et al., 2020). Nevertheless, it turns out that most of these Ti alloys are improper to be utilized as SIDs with the requirement of high tensile strength. Ti–6Al–4V has been an ideal replacement for CP-Ti due to its improved mechanical properties, but the biocompatibility of Ti–6Al–4V has been called into question since the release of toxic elements ions can cause the long-term ignition and carcinogenic effects (Yılmaz et al., 2017a).

Recently, Ti–Zr alloys among various novel Ti alloys have captured great attention as promising metallic materials for biomedical applications (Barter et al., 2012; Chelariu et al., 2013; Cordeiro et al., 2017; Correa et al., 2014; Gottlow et al., 2012; Han et al., 2014; Ho et al., 2008, 2012; Hsu et al., 2009; Li et al., 2011; Sista et al., 2011; Zhang et al., 2009, 2018; Zhao et al., 2011b). Zirconium (Zr) belongs to the group IV B in the periodic table, and is known to have similar chemical structure and properties to those of Ti thereby Zr acts as an ideal solid-solution strengthening component when alloyed with Ti. Furthermore, the Zr can enhance the strength by grain refinement and solid-solution strengthening (Yılmaz et al., 2017a). So far, the Ti–Zr binary alloys have demonstrated enhanced corrosion resistance (Cordeiro et al., 2017; Han et al., 2014; Zhang et al., 2009, 2018), improved micro-hardness (Cordeiro et al., 2017; Correa et al., 2014; Han et al., 2014; Ho et al., 2008, 2012; Kuroda et al., 2016), reduced Young's modulus compared with CP-Ti/Ti–6Al–4V (Cordeiro et al., 2017; Correa et al., 2014; Kuroda et al., 2016; Zhao et al., 2011b) and comparable biocompatibility to CP-Ti (Grandin et al., 2012) (Liu et al., 2017b). Though these Ti–Zr alloys exhibit superiorities over than those of CP-Ti and Ti–6Al–4V, there is a fatal drawback that limits further biomedical applications. Concretely, they are most used in as-cast condition (Cordeiro et al., 2017; Correa et al., 2014; Han et al., 2014; Ho et al., 2008; Hsu et al., 2009), which results in two major problems: (i) The casting process can easily lead to segregation and defects of the microstructure (Wang et al., 2019), which has an adverse impact on the mechanical properties and biocompatibility of the alloys; (ii)The casting process leads to the formation of α′-phase, i.e. martensite. Though, the martensitic structure has a strengthening effect, it harms the toughness and biocompatibility significantly. For example, Zhao et al. found that the strength was improved but the toughness was significantly reduced after the formation of martensite in Ti–30Zr based alloys (Zhao et al., 2011a, 2011c). Correa et al. (2014) and Han et al. (2014) investigated the low Zr-containing α′-Ti alloys, it turned out that the micro-hardness increased at first due to the solid-solution strengthening, however, then it decreased owing to the coarsening of martensite. Similarly, the same tendency can be found in the strength evolution of α′-TiZr alloys investigated by Hsu et al. (2009). Moreover, the biocompatibility of α′-TiZr alloy decreased compared with that of CP-Ti (Correa et al., 2014), which can be ascribed to the stress corrosion caused by the martensite. The biocompatibility and mechanical properties such as strength and toughness are of the significance to develop SDIs, and these properties highly depend on the microstructure and crystalline structure (Correa et al., 2014). Thus, the martensitic structure that damages these performance properties is a significant issue to be solved immediately.

To our best knowledge, previous researches concerning with Ti–Zr binary biomaterials are mainly focused on the mechanical properties, corrosion property, and biocompatibility of the low concentration of Zr (below 20 wt% Zr) (Barter et al., 2012; Cordeiro et al., 2017; Correa et al., 2014; Gottlow et al., 2012; Han et al., 2014; Ho et al., 2009, 2012; Zhang et al., 2009, 2018) and high Zr concentration (above 40 wt% Zr) (Chelariu et al., 2013; Hsu et al., 2009; Sista et al., 2011), and the medium Zr concentration into Ti (20–30 wt% Zr) is rarely reported (Zhao et al., 2011b). Based on the mentioned above, the Ti–Zr alloys are expected to undergo the TMP that can improve the homogeneity of microstructure to enhance their performance properties. Moreover, the information about the microstructure, mechanical properties and biocompatibility of Ti–Zr system are limited and non-systematic. Therefore, the purpose of this work is to investigate the comprehensive properties of a novel Ti–Zr binary system and broaden its biomedical applications. In doing so, two medium Zr-containing Ti alloys were fabricated and developed by TMP and cold rolling (CR). The microstructure, mechanical properties and in vitro biological viability were analyzed systematically.

Section snippets

Materials and methods

The hot-swaged CP-Ti, Ti–20Zr and Ti–30Zr were subjected to hot rolled at 950 °C from the initial 12 to 4 mm in thickness with a strain rate of 3 s−1. The sheets were reheated to the target temperature and holding for 1 min to stabilize the rolling temperature between passes. Then the HR CP-Ti, Ti–20Zr were annealed at 700 °C, and Ti–30Zr was annealed at 650 °C for 16h, respectively. Small bars with a dimension of 30 mm × 12 mm × 4 mm were cut from the annealed materials, and then were rolled

XRD analysis

The obtained XRD patterns of CP-Ti and Ti–Zr alloys are shown in Fig. 1. On one hand, compared with the PDF card 44–1294, only α-phase diffraction peaks are visible with the absence of diffraction peak of β and ω phase of all the samples except for the CR Ti–30Zr sample. It is noted that the XRD patterns of CR Ti–30Zr alloy exhibit the appearance of new diffraction peaks at 2θ = 44.5° and 2θ = 64.5° (Fig. 1b), which is consistent with the results in ref (Shahmir and Langdon, 2017). According to

Conclusions

From the results shown in this investigation, it is possible to conclude that the martensitic structure that harms properties of Ti–Zr alloys was eliminated fully and a uniform microstructure that demonstrates improved performance properties was achieved successfully. The phase constitution, microstructure, mechanical properties and biocompatibility highly depended on the Zr addition.

  • (1)

    The microstructural results showed that increasing Zr concentrations resulted in more refined grains.

Data availability statement

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Author statement

Jie Jiang: Writing- Original draft preparation and revision, Validation.

Chuan Zhou: Writing- Original draft preparation and revision, Validation.

Yanwei Zhao: Methodology, Sample preparation and fabrication.

Xiaoxiang Wang: Conceptualization, Review & Editing.

Fuming He: Conceptualization, Review & Editing, Funding acquisition.

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

This work was supported by the National Natural Science Foundation of China (No. 31670970) and Zhejiang Provincial Key Development Project (2019C03081). There are no conflicts of interest to declare.

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