Research PaperMechanical behavior and microstructure of compressed Ti foams synthesized via freeze casting
Graphical abstract
Introduction
Metallic materials have significant advantages over ceramic and polymer materials as orthopedic implants owing to their excellent properties such as superior strength, fracture toughness and ductility (Geetha et al., 2009, Ryan et al., 2006). Particularly, significant attention has been paid to Ti and Ti-based alloys owing to their relatively low modulus, high strength, superior corrosion resistance, and excellent biocompatibility (through the formation of an oxide layer upon contact with air) (Yamamoto et al., 2012, Long and Rack, 1998, Choi et al., 2014).
Despite the increasing reputation of Ti and Ti-based alloys as orthopedic implants, they still suffer from stress-shielding effect when used in the bulk form because they have significantly greater elastic moduli than that of bone, thus eventually resulting in osteoporosis and loosening of the implant (elastic modulus for natural bone: 3–20 GPa; elastic modulus for Ti and Ti-based alloys: 55–117 GPa) (Geetha et al., 2009, Li et al., 2004, Krishna et al., 2007). Therefore, their porous counterparts with lower elastic moduli values are often preferred for orthopedic implant applications. Moreover, the porous structure of the implant plays an important role in bone integration, i.e., the porous surface facilitates strong interlocking with the bone tissue around the implant, resulting in high resistance to fatigue loading and biomechanical compatibility (Geetha et al., 2009, Sauer et al., 1974, Niinomi, 2008).
Several manufacturing methods have already been proposed for porous Ti. One of the most common methods is the space-holder technique. In the space-holder method, Ti powder is mixed with organic solvents and carbamide as space-holder, which is removed later by heat-treatment to leave hollow spaces (Niu et al., 2009). A gel-casting is somewhat similar to the space-holder method. A mixture of Ti powder and some solvents forms a porous structure through casting and gelation, subsequently followed by drying and sintering (Erk et al., 2008). Another interesting process is the printing processing in which Ti powder and solvent are mixed to produce Ti ink and fabricate a 3-dimensional porous structure of Ti foam through sintering (Hong et al., 2011). Finally, Ibrahim et al. also demonstrated the processing of porous Ti using spark plasma sintering (Ibrahim et al., 2011). In this study, we selected a freeze-casting method to produce Ti and Ti–5%W alloy foams, because this processing method can control the pore morphology, pore size, and porosity of Ti foams fairly reasonably. Therefore, the freeze-casting method can allow relatively easy scale-up and commercialization.
Along with the elastic modulus, we must also investigate the plastic properties and fundamental deformation mechanisms for the porous implant materials because those properties are critical for their successful use as load-bearing implant. For example, the femur bone is normally expected to support approximately 30 times the weight of a typical adult female body (Magee, 2008, D’Angeli et al., 2013). However, only a few studies have focused on comprehensive analysis of mechanical properties of porous Ti and Ti-based alloys from the perspective of their potential use in biomedical applications. For instance, the hardness, compressive strength, and stiffness (Young׳s modulus) were analyzed through compressive tests on porous Ti and Ti-based alloys (Taniguchi et al., 2016, Hong et al., 2011, Muñoz et al., 2015). Additionally, a fatigue test was performed on strain accumulated Ti–6Al–4V foam with the corresponding images and modeling results analyzed on the tested samples (Zhao et al., 2016). Despite these studies on compressive strength, fatigue, and fracture, a systematic analysis is still required particularly on the variations in microstructural and physical properties such as variations in pore morphology, deformation mechanism, and elastic modulus during compression of porous Ti and Ti-based alloys.
Therefore, in the present study, we synthesized Ti and Ti–5%W alloy foams via freeze casting for a systematic analysis on the microstructural evolution of the Ti foams during compression. Tungsten (W) was added to improve the strength and wear resistance through a solid-solution strengthening effect of W in Ti grains (Choi et al., 2014, Frary et al., 2003). Moreover, we analyzed the microstructural evolution, elastic moduli, compressive strengths and deformation mechanisms of the freeze-cast porous Ti and Ti–5%W alloy foams using compression test, electron backscatter diffraction (EBSD) and X-ray line profile analysis (XLPA). In particular, we investigated the subgrain boundaries and the dislocation densities of the compressed samples using EBSD and XLPA. We also examined the effect of porosity and dislocations on the mechanical properties (e.g., elastic modulus, flow stress) of Ti foams. This systematic study using a range of analytical test methods are expected to provide valuable insights on the mechanical and deformation behavior of porous Ti and Ti-based alloy foams and other applicable porous implants under complex stress and strain states for their potential use as biomedical materials.
Section snippets
Preparation of Ti foams via freeze casting
Pure Ti foam was synthesized from commercially pure Ti powder (Alfa Aesar, MA, USA) with a mesh value of 325 (particle size is smaller than 44 µm). The concentration of metallic impurities was less than 0.2%. Among the nonmetallic elements, oxygen and nitrogen have the highest concentrations with values of 0.694% and 0.3%, respectively, according to the manufacturer׳s analysis. Ti–W alloy foam was also synthesized. In this case, 5 wt% W powder with an average particle size of 1 µm was added to the
Microstructures of initial foams
The initial microstructures of the pure Ti and Ti–W foams are shown in Fig. 1, Fig. 2a, respectively. The black areas correspond to the pores, while the colored areas indicate the orientations of the grains. The volume fraction of pores was determined as the fraction of black areas in the SEM images. According to quantitative metallography, these two fractions are equal within error range. The pore volume fraction values for the pure Ti and Ti–W foams were 33% and 32%, respectively, thus
Discussion
The measured elastic moduli of Ti and Ti–W foams were ~23 GPa and ~22 GPa, respectively, which are much lower than that of bulk Ti (approximately 114 GPa) owing to the uniformly distributed porosity. The relationship between the elastic moduli of the foam (E) and bulk Ti (Eb), and the porosity (P) can be expressed as follows (Luo and Stevens, 1999):where the value of parameter k depends on the material. For the foams used in this study, k=0.05. In order to confirm the validity of Eq.
Conclusions
In this study, we analyzed the mechanical properties and microstructural evolution during compression of Ti and Ti–W foams with the porosity of 32–33% synthesized via freeze casting. The following conclusions were drawn:
- 1.
The Young׳s moduli for both Ti and Ti–W foams were ~23 GPa, which is in accordance with the predicted value from the porosity and elastic modulus values of bulk Ti. On the other hand, the yield strength was significantly affected by W content. The yield strength values for Ti and
Acknowledgements
This work was supported by the Hungarian Scientific Research Fund, OTKA, Grant No. K-109021. The authors are grateful to Mr. Gábor Varga for the EBSD investigations and Dr. György Krállics for providing the compressive stress-strain data for bulk Ti. Choi and Choe also would like to acknowledge supports from the National Research Foundation (NRF) of Korea (2015R1D1A1A01060773; 2009-0093814; 2014R1A2A1A11052513; 2013K1A3A1A39074064).
References (33)
- et al.
Effect of tungsten additions on the mechanical properties of Ti-6Al-4V
Mater. Sci. Eng. A
(2005) - et al.
Tribological properties of biocompatible Ti–10W and Ti–7.5TiC–7.5W
J. Mech. Behav. Biomed. Mater.
(2014) - et al.
Load along the femur shaft during activities of daily living
J. Biomech.
(2013) - et al.
Microstructure and mechanical properties of Ti/W and Ti-6Al-4V/W composites fabricated by powder-metallurgy
Mater. Sci. Eng. A
(2003) - et al.
Ti based biomaterials, the ultimate choice for orthopedic implants – a review
Prog. Mater. Sci.
(2009) - et al.
Processing of porous Ti and Ti5Mn foams by spark plasma sintering
Mater. Des.
(2011) - et al.
Low stiffness porous Ti structures for load-bearing implants
Acta Biomater.
(2007) - et al.
An evaluation of yield criteria and flow rules for aluminium alloys
Int. J. Plast.
(1999) - et al.
Gradient ultrafine-grained titanium: Computational study of mechanical and damage behaviour
Acta Mater.
(2014) - et al.
Titanium alloys in total joint replacement—a materials science perspective
Biomaterials
(1998)
Porosity-dependence of elastic moduli and hardness of 3Y-TZP ceramics
Ceram. Int.
The evolution of non-basal dislocations as a function of deformation temperature in pure magnesium determined by X-ray diffraction
Acta Mater.
On the influence of space holder in the development of porous titanium implants: mechanical, computational and biological evaluation
Mater. Charact.
Mechanical biocompatibilities of titanium alloy for biomedical applications
J. Mech. Behav. Biomed. Mater.
Processing and properties of porous titanium using space holder technique
Mater. Sci. Eng. A
Deformation mechanism map for titanium
Scr. Metall.
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