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Vol. 8. Issue 5.
Pages 3696-3704 (September - October 2019)
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Vol. 8. Issue 5.
Pages 3696-3704 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.06.021
Open Access
Fabrication and properties of newly developed Ti35Zr28Nb scaffolds fabricated by powder metallurgy for bone-tissue engineering
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Wei Xua, Xin Lua,
Corresponding author
luxin@ustb.edu.cn

Corresponding author.
, Muhammad Dilawer Hayatb, Jingjing Tianc, Chao Huanga, Miao Chena, Xuanhui Qua, Cuie Wend
a Beijing Advanced Innovation Center for Materials Genome Engineering, Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
b Department of Chemical and Materials Engineering, University of Auckland, Auckland 1142, New Zealand
c Central Laboratory, Peking Union Medical College Hospital, Peking Union Medical College and Chinese Academy of Medical Sciences, Beijing 100730, China
d School of Engineering, RMIT University, Melbourne 3001, Australia
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Table 1. Corrosion parameters of specimens of solid Ti35Zr28Nb, porous Ti35Zr28Nb with different porosities, and CP-Ti with 50.5% porosity in naturally aerated PBS solution at 37 ± 0.5 °C.
Table 2. Pore characteristics, mechanical properties, and corrosion resistance of specimens of solid Ti35Zr28Nb, porous Ti35Zr28Nb with different porosities, and CP-Ti with 50.5% porosity in naturally aerated PBS solution at 37 ± 0.5 °C.
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Abstract

A newly developed porous Ti35Zr28Nb scaffold was manufactured via powder metallurgy (PM) using space-holder (NH4HCO3) sintering from pre-alloyed powder. The pore features, compression and anti-corrosion properties of the manufactured porous scaffolds were systematically studied. The results show that all the manufactured porous Ti35Zr28Nb scaffolds consisted of a single β phase. The porosity of the porous Ti35Zr28Nb scaffolds increased from 50% to 65% with in increasing the NH4HCO3 from 63% to 79% (volume ratio), and the average pore size (d50) was in the range of 230–430 μm. The yield strength in compression and the elastic modulus of the porous Ti35Zr28Nb scaffolds both decreased gradually with an increase in porosity, varying from 230.5 MPa to 79.7 MPa and 6.9 GPa to 1.8 GPa, respectively. The corrosion rate of the porous Ti35Zr28Nb scaffolds in the phosphate buffer saline (PBS) solution increased from 0.91 × 10−3 mm/yr to 4.18 × 10−3 mm/yr with an increase in porosity from 51.4% to 64.9%. In comparison with unalloyed Ti scaffolds with almost the same porosity, the porous Ti35Zr28Nb scaffolds showed a significantly higher compressive yield strength and a lower corrosion rate. These results suggest that porous Ti35Zr28Nb scaffolds fabricated by PM have substantial potential for hard-tissue engineering applications. Additionally, the relationship of porosity, mechanical and anti-corrosion properties of these Ti35Zr28Nb scaffolds has now been confirmed, and could be used in the process of material selection for their specific applications.

Keywords:
Corrosion resistance
Mechanical property
Porous Ti35Zr28Nb scaffold
Powder metallurgy
Pore characterization
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1Introduction

Because of acceptable mechanical properties, superior corrosion resistance, and excellent biocompatibility, titanium (Ti) and its alloys have been extensively applied in the fields of orthopedic and dental implants [1–6]. However, the release of toxic aluminum (Al) or vanadium (V) ions after long-term implantation for commonly used Ti alloys, such as Ti-6Al-4V (Ti64, wt.% hereafter), Ti-5Al-2.5Fe, and Ti-6Al-7Nb, can cause some diseases, such as Alzheimer’s disease and mental disorder [7]. Additionally, elastic modulus of these alloys is higher than that of natural bone (0.01–3 GPa and 3–30 GPa for trabecular and cortical bone, respectively), which can cause stress shielding and hence eventually lead to failure of implants [2,3]. Therefore, developing new Ti alloys with a lower elastic modulus and enhanced biocompatibility is an urgent demand for clinical application.

Recently, TiNbZr base alloys with different levels of niobium (Nb) and zirconium (Zr) added have attracted a number of interest. Because the addition of Nb and Zr not only improves the mechanical properties but also enhances the corrosion and wear resistance [8–15]. Hence, a novel β-type Ti35Zr28Nb alloy has been designed by Ozan et al. [16] based on some design theories, e.g., d-electron, molybdenum equivalence, and electron-to-atom ratio methods. The authors demonstrated that the designed bulk Ti35Zr28Nb alloy fabricated by cold-crucible levitation melting exhibited excellent biocompatibility and superior mechanical properties, with a tensile property of 755 ± 28.3 MPa, elastic modulus of 64 ± 4.5 GPa, and elongation of 11.3 ± 1.5% [17]. Nevertheless, the elastic modulus of the bulk Ti35Zr28Nb alloy was still at least two times higher than that of natural bone. To decrease the elastic modulus further, which can prevent stress shielding at the bone-implant interface and improve osteointegration, a porous structure can be introduced [17]. By introducing a porous structure, not only strong bonding between the implants and natural bone could be achieved, but also cellular activity, e.g. migration and proliferation of osteoblasts cells, will be facilitated [18–20]. In addition, the connected porous structure also can transport the nutrients and oxygen, which is essential for vascularization.

So far, a number of techniques have been used to manufacture the porous scaffolds, such as powder metallurgy (PM) space-holder technique, foaming [21], and additive manufacturing (AM) [22,23]. Among these, PM space-holder technique seems an ideal processing route due to high production efficiency, low cost, and the ability to customize material compositions, mechanical properties, and shapes [24]. Sodium chloride [25], carbamide [26], sugar pellets [27], tapioca [28], saccharose [29], magnesium powder [30], and ammonium hydrogen carbonate (NH4HCO3) [31–36] are commonly temporary space-holder materials. Among them, NH4HCO3 is considered a good potential due to low cost and easy removal that results in good shape retention. However, no work has been done to fabricate the porous Ti35Zr28Nb scaffolds by PM space-holder technique, and hence the corresponding porosity, mechanical and anti-corrosion properties of this fabricated scaffolds have not been investigated thoroughly, which is crucial to the development of low-cost porous biomaterials for bone-tissue engineering.

In this study, Ti35Zr28Nb scaffolds were manufactured by PM employing NH4HCO3 as the space-holder material. In addition to manufacture biomimetic porous scaffolds with high interconnectivity and appropriate mechanical properties for tissue-engineering applications, it also aims to establish a relationship among the porosity, mechanical and anti-corrosion properties of the newly developed Ti35Zr28Nb alloy.

2Materials and methods2.1Specimen’s preparation

Atomized Ti28Nb35.4 Zr powders (purity ≥ 99.9%, 75 ≤ particle size ≤ 150 μm) was supplied by Wen group (RMIT University, Australia) who fabricated the powders using continuous inert gas atomization method. The impurity contents for Ti28Nb35.4 Zr powders are as follows: <0.005 N, <0.01 C, 0.09 O, <0.002 H, 0.02 Fe, 0.03 Al, <0.01 V, 0.01 Ni, 27.7 Nb, 35.4 Zr, and Bal. Ti. Due to it is difficulty to press into blocks, this powder was milled by ball milling machine for 30 min ((HSVM, Nanjing Chishun Science and Technology Co., Ltd., Nanjing, China). The frequency of vibration ball milling machine and the ball-to-powder weight ratio were 1400 r/min and 3:1, respectively. The details of the milling process are available from reference [37,38]. The as-milled Ti35Zr28Nb powder was blended with space-holder particles (NH4HCO3) for 3 h. The size of the NH4HCO3 particles was in the range 180–250 μm and the volume ratios were set at five points between 63–79%. After mixing, the mixture powders were cold-pressed into blocks at 260 MPa. The sintering process was carried out in four steps. The first step was carried out at 80 °C for 2 h to burn out the space holder particles. Then, the compacts were heated up to 175 °C for 1 h to remove excess water. Thirdly, the blocks were heated to 1000 °C at 5 °C/min for 2 h. Finally, the blocks were heated to 1550 °C for 2 h at 2 °C/min, and then the specimens were obtained when the temperature of the furnace cooled to room temperature. This information has been added in the revised manuscript.

2.2Materials characterization

Phase constituents were performed a by Japanese neo-science (Rigaku) Dmax-RB 12KW rotating anode X-ray diffractometer (λ = 0.15406 nm) using Cu radiation. The voltage and electric current for X-ray diffractometer are a 40 kV and 50 mA, respectively. The scanning speed is 10°/min, and scanning angle is in the range of 20°–90°. 7 min for each sample was run. The pore morphology was observed via Japanese LEXT OLS4000 optical microscope. The average pore size (d50) of the specimens was obtained from optical images using Image-Pro-Plus6 software. The porosity was measured based on the Archimedes method in accordance with the procedure listed in GB/T 5163-2006 [39], as follows:

where ρ and ρ1 is the density of the alloy and water (1 g/cm3), respectively; ρ3 is the theoretical density of the alloy (6.36 g/cm3); m1, m2, and m3 is the mass of the alloy in the air (g), the alloy covered with paraffin in air (g), and the alloy covered with paraffin in water (g), respectively; P is the porosity, %. Compression specimens were cut by electric discharge machining (EDM). The size of each specimen is φ 3 mm × 5 mm. Mechanical properties were tested on a compression tester (Instron), and the strain rate of the machine was 2 × 10−3 s–1. Elastic modulus was determined from the engineering stress-strain curves. Three specimens were meatured and the average values were obtained.

2.3Corrosion tests

The corrosion resistance of the PM-fabricated Ti35Zr28Nb alloy was tested by America Princeton VersaSTAT MC electrochemical workstation according to ASTM G 59-97 (2014) [40]. As proved by Hoar and Mears [41], the anti-corrosion properties of Ti and its alloys exhibited similar results between the human body and sodium chloride-containing inorganic solutions. Hence, it can study the anti-corrosion properties of Ti and its alloys in sodium chloride-containing inorganic solutions to simulate the anti-corrosion properties in human body fluid. Accordingly, PBS (pH = 7.2) was used to assess the corrosion resistance of PM-fabricated Ti35Zr28Nb alloy at 37 ± 0.5 °C. The composition of the PBS solution was as follows: NaCl 8 g L−1, KCl 0.2 g L−1, KH2PO4 0.2 g L−1, and Na2HPO4 1.15 g L−1. For purposes of comparison, a porous CP-Ti (50.5% porosity) and a solid Ti35Zr28Nb alloy, both fabricated by PM, were studied simultaneously. The specimens were prepared as follows: (1) A copper (Cu) wire was laser connected to each sample; (2) Each welded specimen was set in an epoxy resin with an exposed working surface of 1 cm2; (3) All specimens were ground with abrasive paper to 2000 grit; (4) All samples were cleaned with ethanol and distilled water ultrasonically for 10 min, respectively, and then dried in vacuum for further electrochemical testing.

The corrosion resistance of each specimens was studied by three-electrode system.

Specimen, saturated calomel electrode (SCE), and platinum foil were the working electrode, reference electrode, and counter electrode, respectively. To stabilize the potential, each specimen was immersed into the solution for 2 h, and the variation of corrosion potential with time was recorded simultaneously. The potentiodynamic (PD) polarization curves were tested. The scan rate and scan range were 0.5 mV/s and 0.3–2.0 V vs. OCP, respectively. Corrosion potential (Ecorr), corrosion current density (Icorr), and film breakdown potential (Eb) were worked out from the PD polarization curves. The corrosion rate was calculated according to ASTM 102-89 (2015) using Faraday’s equation [42], as follows:

where CR is the corrosion rate (mm/yr), K is 3.27 × 10−3 (mm g/μA cm yr), ρ is the density of the alloy, g/cm3, and EW is 17.7 for Ti35Zr28Nb and 11.9 for CP-Ti, which is the equivalent weight. All experiments were repeated five times to verify the reproducibility.

2.4Statistical analysis

In the present study, the experimental results are expressed as means ± standard deviations. In addition, the differences between groups were observed by one-way analysis of variance (SPSS 14.0 for Windows, SPSS Inc., Chicago, IL) to determine the statistical significance. P < 0.05 was considered statistically significant.

3Results and discussion3.1Phase identification and pore characteristics

The phase of the fabricated porous Ti35Zr28Nb scaffolds with different NH4HCO3 added was identified by XRD. As Fig. 1 shown, no significant differences in the XRD patterns of each specimen was observed, indicating that the content of NH4HCO3 had no effect on the phase of porous Ti35Zr28Nb scaffolds. The diffraction peaks of the porous Ti35Zr28Nb scaffolds confirmed that all the samples consisted of a single β phase.

Fig. 1.

XRD patterns of Ti35Zr28Nb alloys with different volume ratios of NH4HCO3 added.

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Optical micrographs of the porous Ti35Zr28Nb alloy specimens sintered at 1550 ℃ with different the NH4HCO3 added are shown in Fig. 2. It can be seen from Fig. 2(a–e), with an increase in NH4HCO3 ratio from 63 vol.% to 79 vol.%, the pore size increased gradually. Furthermore, the pores of the porous structures were uniformly distributed and had distinctive irregular shapes of two different types: macro pores, marked A; and micro pores, marked B (Fig. 2(a)). The micro pores were several micrometers in size and presumably resulted from volumetric shrinkage during sintering, while the macro pores exhibited sizes of several hundred micrometers and were created by the vaporization of NH4HCO3. In increasing the NH4HCO3 content, the macro pores increased significantly, with little to no change in the micro pores. It is commonly believed that the combination of micro pores and macro pores can provide more places for new bone tissue to grow and paths for nutrient and body fluid to transport, which can promote new bone regeneration and hence enhance the time of osteointegration [43,44].

Fig. 2.

Micrographs of Ti35Z28Nbr alloys with different volume ratios of NH4HCO3 added: (a) 63%; (b) 67%; (c) 71%; (d) 75%; (e) 79%.

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Fig. 3 shows the distribution of pore size of Ti35Zr28Nb alloy with different NH4HCO3 added. Wherein d10, d50, and d90 represent pore diameters of 10%, 50%, and 90% at cumulative pore sizes (by frequency), respectively, and d50 is usually utilized to express the average pore size. It can be seen from Fig. 3 that the NH4HCO3 had a significant effect on the pore size. In increasing in NH4HCO3 content from 63% to 79%, d50 of the Ti35Zr28Nb scaffolds increased from 229.9 μm to 427.4 μm. The pore size distribution was in the ranges of 100–400 μm, 150–450 μm, 200–500 μm, 250–550 μm, and 300–600 μm, respectively, and it tended to stabilize at about 200 μm.

Fig. 3.

Distribution of pore size of Ti35Zr28Nb alloys with different volume ratios of NH4HCO3 added: (a) 63%; (b) 67%; (c) 71%; (d) 75%; (e) 79%.

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3.2Compression properties at room-temperature

Engineering stress–strain curves (a), and compressive yield strength and elastic modulus (b) of the Ti35Zr28Nb scaffolds, CP-Ti scaffold, and solid Ti35Zr28Nb are shown in Fig. 4, and no significant differences (P > 0.05) were confirmed in each group for three tests. As can be seen in Fig. 4(a) shown, the porous Ti35Zr28Nb, porous CP-Ti, and solid Ti35Zr28Nb show similar stress–strain behaviors. There is no fracturing occurred during the compression process, which indicates that these Ti alloys have excellent elastic–plastic deformation capability. As can be seen in Fig. 4(b), the elastic modulus and the yield strength in compression of the as-fabricated porous alloys decreased with an increase in porosity, and ranges from 230.5 MPa to 79.7 MPa and 6.9 GPa to 1.8 GPa, respectively. Compared with unalloyed CP-Ti with similar porosity (about 50%) (199.5 ± 14.8 MPa and 6.5 ± 1.1 GPa), the Ti35Zr28Nb scaffolds showed a higher yield compression strength and a similar elastic modulus.

Fig. 4.

(a) Engineering stress–strain curves; and (b) compression yield strength and elastic modulus, of specimens of solid Ti35Zr28Nb, porous Ti35Zr28Nb with different porosities, and CP-Ti alloys with 50.5% porosity.

(0.38MB).

Implant materials for orthopedic and dental application need to have sufficient strength and similar elastic modulus to the natural bone. In this study, with 63%–79% NH4HCO3 added, the strength of the Ti35Zr28Nb scaffolds (79.7–230.5 MPa) fabricated in this study satisfied the compressive strength requirement of human trabecular bone (2–80 MPa) [45], and the corresponding elastic modulus (1.8–6.9 GPa) was also close to that of trabecular bone [45]. These satisfactory mechanical properties and the suitable average pore size make the Ti35Zr28Nb scaffolds (with 54.1–61.1% porosity) a promising candidate as implant alloy for hard-tissue engineering applications.

3.3Corrosion behaviors

Open-circuit potential (Eocp) curves for the solid Ti35Zr28Nb, porous Ti35Zr28Nb with different porosities, and CP-Ti with 50.5% porosity are shown in Fig. 5. All curves are similar in PBS at 37 ± 0.5 °C. Following immersion of the specimens, Eocp abruptly increased in a positive direction. Then, Eocp increased slowly and reached quasi-stationary position at the end of the test. With increasing porosity, the stable potential of the porous Ti35Zr28Nb scaffolds decreased gradually. After the stabilization of OCP, compared with the unalloyed CP-Ti, the porous Ti35Zr28Nb scaffolds with similar porosity (about 50%) showed higher OCP values, indicating that the Ti35Zr28Nb alloy with this porosity was potentially more anti-corrosion and had a greater self-passivating ability.

Fig. 5.

Open-circuit potential as a function of time for specimens of solid Ti35Zr28Nb, porous Ti35Zr28Nb with different porosities, and CP-Ti with 50.5% porosity in naturally aerated PBS solution at 37 ± 0.5 °C.

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Fig. 6 plots PD polarization curves for each specimen. No significant difference was observed among the cathodic polarization curves of the different specimens. This result indicats that similar cathodic reaction was happened on their surfaces, but at a different rate. As for the anodic curves for each specimen, taking solid Ti35Zr28Nb alloy as an example, there were three characteristic potential domains, as marked domain 1, domain 2, and domain 3 in Fig. 6. In the domain 1, with the potential increased, the current density raised continuously. This is because the spontaneously formed oxide film is replaced by a less protective oxide layer [46]. In the domain 2, which is called the passivation plateau, the current density kept unchanged with increasing of potential, indicating that the alloy turned into a stable passivation region. In the domain 3, with the potential increased, the current density increased again. This is mainly because high overpotential, which leads to the oxide films breakdown. As for the anodic polarization curves of Ti35Zr28Nb scaffolds with different porosity and CP-Ti with 50.5% porosity, similar behaviors were observed, although Ecorr, Icorr, and Eb varied.

Fig. 6.

Potentiodynamic polarization curves for specimens of solid Ti35Zr28Nb, porous Ti35Zr28Nb with different porosities, and CP-Ti with 50.5% porosity during exposure to naturally aerated PBS solution at 37 ± 0.5 °C.

(0.3MB).

The accuracy corrosion parameters, including Ecorr, Icorr, and Eb were obtained form PD polarization curves, and the corrosion rate was calculated based on equation (4). These results are listed in Table 1. There is no significant differences (P > 0.05) were confirmed in each group for five tests. It is well known that there is a dissolution reaction before passivation reaction in the anodic polarization curves, which will lead to a loosely-defined anodic Tafel region [47]. Hence, Icorr was determined from cathodic polarization curve by Tafel extrapolation method, as shown in Fig. 6. It can be seen from Table 1 that the Ecorr and Eb of the Ti35Zr28Nb alloy specimens decreased gradually as porosity increased from 50% to 67%, while Icorr and CR increased, which indicates a decline in corrosion resistance with increasing porosity. Compared with CP-Ti with 50.5% porosity, the Ti35Zr28Nb alloy in the same porosity (51.4%) exhibited lower CR and higher Eb, indicating that the porous Ti35Zr28Nb alloy possessed higher corrosion resistance.

Table 1.

Corrosion parameters of specimens of solid Ti35Zr28Nb, porous Ti35Zr28Nb with different porosities, and CP-Ti with 50.5% porosity in naturally aerated PBS solution at 37 ± 0.5 °C.

Alloys  Porosity (%)  Ecorr (mV vs. SCE)  Icorr × 10−8 (A/cm2Eb (V)  CR × 10−3 (mm /yr) 
Ti35Zr28Nb1.9 ± 0.3  −46.7 ± 1.1  1.9 ± 0.3  1.61 ± 0.11  0.17 ± 0.01 
51.4 ± 1.1  −118.1 ± 9.5  4.9 ± 0.5  1.15 ± 0.06  0.91 ± 0.05 
55.9 ± 1.2  −122.5 ± 8.7  7.4 ± 0.6  1.08 ± 0.05  1.52 ± 0.07 
58.8 ± 1.4  −161.1 ± 3.3  9.7 ± 1.1  0.99 ± 0.03  2.14 ± 0.08 
61.1 ± 1.5  −191.1 ± 3.8  12.4 ± 1.5  0.94 ± 0.04  2.91 ± 0.07 
64.9 ± 1.6  −216.1 ± 9.8  16.1 ± 1.7  0.87 ± 0.02  4.18 ± 0.11 
CP Ti  50.5 ± 1.2  −123.5 ± 5.1  10.1 ± 1.8  1.07 ± 0.05  1.77 ± 0.12 
3.4Relationship among porosity, mechanical and anti-corrosion resistance

Recently, porous metal materials have attracted interests because of its excellent biological performance [48]. However, porous structure inevitably decreases the mechanical and anti-corrosion properties, as discussed above. Hence, establishment of relationship among the porosity, mechanical and anti-corrosion properties of the porous Ti35Zr28Nb scaffold is essential for their specific applications. The porosity, mechanical and anti-corrosion properties of the PM-fabricated Ti35Zr28Nb scaffolds in this study are summarized in Table 2 as well as the porous CP-Ti with 50.5% porosity. It can be seen that the d50 and porosity of Ti35Zr28Nb scaffolds fabricated in this study can be controlled in the ranges of 210–430 µm and 50–65% respectively. This is believed to satisfy the implantation requirements, because based on the previous studies, average pore sizes with 210–430 µm and porosity of 20–50 vol.% are beneficial for osseointegration [2,49]. The elastic modulus and yield strength of the porous Ti35Zr28Nb scaffolds can be controlled in the ranges of 230.5–79.7 MPa and 6.9–1.8 GPa, respectively. It is believed that the matching mechanical between the implants and the living bone tissue can reduce stress-shielding effect, and hence enhance the ability of osseointegration. The porous Ti35Zr28Nb scaffolds obtained in this study show many mechanical properties close to those of human trabecular or cancellous bone (yield stress 2–80 MPa and elastic modulus 0.01–3 GPa), which indicates that these fabricated alloys have good mechanical compatibility. In addition, with increasing porosity, the corrosion rates of the porous Ti35Zr28Nb scaffolds increased gradually. Such as, the corrosion rate of the Ti35Zr28Nb scaffold with 51.4% porosity was about 2.5 times than that of the solid Ti35Zr28Nb alloy. Nevertheless, this is only 3/5 of that of CP-Ti with similar porosity, indicating a higher corrosion resistance of the Ti35Zr28Nb alloy. These properties, together with its low-cost manufacturability, make the Ti35Zr28Nb alloy an attractive new implant material for bone-tissue engineering. Also, the relationship among porosity, mechanical and anti-corrosion properties of Ti35Zr28Nb scaffolds has been summarized, and can be used as a design guidance for the material-selection process for their specific applications.

Table 2.

Pore characteristics, mechanical properties, and corrosion resistance of specimens of solid Ti35Zr28Nb, porous Ti35Zr28Nb with different porosities, and CP-Ti with 50.5% porosity in naturally aerated PBS solution at 37 ± 0.5 °C.

Alloys  Porosity (%)  Average pore size (d50, μm)  Compression yield strength (MPa)  Elastic modulus (GPa)  Corrosion rate × 10−3 (mm /yr) 
Ti35Zr28Nb1.9 ± 0.3  –  1058.1 ± 35.1  50.8 ± 3.9  0.17 ± 0.01 
51.4 ± 1.1  229.9 ± 5.6  230.5 ± 7.6  6.9 ± 0.6  0.91 ± 0.05 
55.9 ± 1.2  276.2 ± 3.7  186.3 ± 7.1  5.1 ± 0.5  1.52 ± 0.07 
58.8 ± 1.4  341.5 ± 5.7  161.6 ± 4.9  3.9 ± 0.3  2.14 ± 0.08 
61.1 ± 1.5  379.1 ± 7.8  132.5 ± 3.5  2.9 ± 0.4  2.91 ± 0.07 
64.9 ± 1.2  427.4 ± 10.1  79.7 ± 3.5  1.8 ± 0.5  4.18 ± 0.11 
CP Ti  50.5 ± 1.0  201.8 ± 4.1  199.5 ± 14.8  6.5 ± 1.1  1.77 ± 0.12 
4Conclusions

Porous Ti35Zr28Nb scaffolds were fabricated via PM through adding NH4HCO3. The fabricated porous scaffolds were characterized by single β phase. By adjusting the volume ratio of the NH4HCO3, the porosity and average pore size of the fabricated porous scaffolds could be controlled in the range of 50–65% and 230–430 μm, respectively. The elastic modulus and yield strength of the porous Ti35Zr28Nb scaffolds are in the ranges of 230.5–79.7 MPa and 6.9–1.8 GPa, respectively. Compared with porous unalloyed Ti scaffolds with the same porosity (about 50%), the porous Ti35Zr28Nb scaffolds exhibited higher strength and similar elastic modulus. With increasing porosity, the corrosion potential and breakdown potential of the porous Ti35Zr28Nb scaffolds shifted in negative directions, while the corrosion current density increased gradually. The corrosion rate of the porous Ti35Zr28Nb scaffolds increased with increasing porosity and was only 3/5 of that of porous unalloyed Ti scaffolds with the same porosity. Excellent properties and in vitro corrosion resistance generated using the PM manufacturing route make the porous Ti35Zr28Nb scaffolds an attract hard-tissue engineering alloy. Additionally, the association among the porosity, mechanical and anti-corrosion properties of Ti35Zr28Nb has been established, so it can be used as a design guidance for the material-selection process for specific applications.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

This research work is supported by the National Natural Science Foundation of China (51874037), 13th Five-Year Weapons Innovation Foundation of China (6141B012807) and State Key Lab of Advanced Metals and Materials, University of Science and Technology Beijing (2019-Z14). CW acknowledges the financial support for this research by the National Health and Medical Research Council (NHMRC), Australia through project grant (GNT1087290).

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