Development and Performance of Low-Cost Beta-Type Ti-Based Alloys for Biomedical Applications Using Mn Additions

The microstructures, mechanical properties, and biocompatibility of various low-cost β-type Ti-Mn alloys fabricated by both cold crucible levitation melting (CCLM) and metal injection molding (MIM) were investigated after solution treatment. Mn was chosen as a potential low-cost alloying element for Ti. Among the alloys fabricated by both methods, Ti-9Mn shows the best combination of tensile strength and elongation, and their performances are mostly comparable to or superior to those of Ti-6Al-4V (Ti-64) ELI. However, alloys fabricated by MIM show a higher O and C content, along with precipitated Ti carbides and pores, which all cause the ductility of the alloys fabricated by MIM to be lower than that of the alloys fabricated by CCLM. Furthermore, the cell viability and metallic ion release ratios of the alloys fabricated by CCLM are comparable to those of commercially pure Ti, making this alloy promising for biomedical applications. The Young’s modulus of the alloys is also lower than that of Ti-64 ELI (which is of approximately 110 GPa), which can possibly reduce the stress shielding effect in implanted patients.

levitation melting (CCLM) showed a performance comparable or superior to that of the Ti-64 ELI for biomedical applications [ 15 ]. Furthermore, Ti-Mn alloys containing up to 13 mass% Mn showed cytotoxicity levels comparable to that of CP-Ti, but the alloy containing 18 mass% Mn showed cytotoxicity [ 15 ]. Thereafter, further cost reduction was considered by fabricating Ti-Mn alloys using a metal injection molding (MIM), a powder metallurgy near-net shape fabrication method [ 16 ]. The alloys fabricated by MIM showed some mechanical properties comparable to those of the alloys fabricated by CCLM, but because of both high porosity and high amount of interstitial impurities -inherent to the fabrication method -there was a drastic decrease in ductility compared to the alloys fabricated using CCLM [ 16 ]. Therefore, it is necessary to improve the ductility of the alloys.

Ingots Preparation
For the fi rst part of this study, ingots of Ti- (6,9,13, and 18 mass%)Mn were fabricated by CCLM. The ingots were initially subjected to a homogenization treatment for 21.6 ks at 1,273 K followed by ice water quenching and then to hot forging and hot rolling, both at 1,173 K. Finally, in order to retain the β phase in the alloys, the plates were subjected to solution treatment for 3.6 ks at 1,173 K in vacuum, followed by ice water quenching. Hereafter, the alloys fabricated by CCLM will be identifi ed by the subscript "LM," as in Ti-Mn LM .
For the second part of this study, ingots of Ti- (8,9,12,13,15, and 17 mass%)Mn were fabricated by MIM. Sintering was conducted in vacuum at 1,373 K for 28.8 ks. Finally, in order to retain the β phase in the alloys, the plates were subjected to solution treatment for 3.6 ks at 1,173 K in vacuum, followed by ice water quenching. Further details on the specimen preparation methods are described elsewhere [ 16 ]. Hereafter, the alloys fabricated by MIM will be identifi ed by the subscript "MIM," as in Ti-Mn MIM .

Microstructure Characterization
For all parts of this study, the material characterization was performed by chemical composition analysis, optical microscopy (OM), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). Furthermore, electron probe microanalysis (EPMA) was carried out on the alloys fabricated by MIM. The experimental conditions and other details are described elsewhere [ 15 , 16 ].

Mechanical Properties Evaluation
For all parts of this study, the mechanical property investigation was conducted by means of Vickers hardness tests, Young's modulus measurements, and tensile tests. Furthermore, compressive tests were carried out on the alloys fabricated by MIM. The experimental conditions and other details are described elsewhere [ 15 , 16 ].

Biocompatibility Evaluation
The biocompatibility of the alloys fabricated by CCLM was evaluated by means of immersion tests in simulated body fl uids (SBF), followed by X-ray photoelectron spectroscopy (XPS) of the surface of SBF-immersed samples, and also cytotoxicity tests using MC3T3-E1 cells. The experimental conditions and other details are described elsewhere [ 15 ].  [ 17 ]. Mn loss of up to 25 % was expected [ 17 ]. However, the observed difference (2-10 %) is considerably smaller in the Ti-Mn LM , thanks to the shorter melting time required by this method. Figures 19.1 and 19.2 show the optical micrographs and XRD profi les of the Ti-(6-18)Mn LM , respectively. As shown in Fig. 19.2 , only diffraction peaks attributed to β planes are detected in the XRD profi les of Ti-(9-18)Mn LM . However, diffraction peaks that can be attributed to ω and α or α ′ phases are also detected in the XRD profi le of Ti-6Mn LM . A concentration of approximately 6.3 mass% Mn is required to fully retain the β phase upon quenching [ 4 ]. However, the Mn content of Ti-6Mn LM used for this study is 5.60 mass%. Furthermore, the presence of the athermal ω phase has been confi rmed by the TEM observations. Figure 19.3a-d shows   [ 15 , 16 , 18 ]. On the other hand, the decrease in the volume fraction of the athermal ω phase with increasing Mn leads to a decrease in both Vickers hardness and Young's modulus [ 4 , 19 ]. Figure 19.6 shows the tensile properties of Ti-(9-18)Mn LM along with that of Ti-64 ELI obtained from literature [ 5 ]. The tensile properties of Ti-6Mn were not obtained because the specimens suffered premature fracture due to the high amount of athermal ω phase. Ti-9Mn LM and Ti-13Mn LM show ultimate tensile strength (UTS, σ B ), 0.2 % proof stress ( σ 0.2 ), and elongation comparable to those reported for Ti-64 ELI ( σ B : 965 MPa, σ 0.2 : 869 MPa, elongation: 15 %) [ 5 ]. However, the results shown by Ti-9Mn LM ( σ B : 1,048 MPa, σ 0.2 : 1,023 MPa, and elongation: 19 %) make the use of this alloy in biomedical applications more promising than either Ti-13Mn LM or Ti-64 ELI.   [ 20 , 21 ]. The Ti-Mn LM alloys immersed in a 1 % lactic acid solution, which is a solution recommended for accelerated immersion tests [ 22 ], showed a maximum amount of released Mn ions of approximately 0.9 μg/cm 2 (Ti-18Mn) over a period of 7 days. Because the amount released is orders of magnitude lower than the amount administered during parenteral nutrition, it is believed that the amount of released Mn ions will not reach the levels known to cause symptoms related to Mn intoxication by itself, even in an implant with considerable surface area. Furthermore, XPS analysis [ 15 ] revealed that the immersion tests using 1 % lactic acid caused the passive oxide layer of the alloys to become thinner, but the oxide layer regenerated. Figure 19.8 shows the results obtained for the cytotoxicity test. After counting the living cells following a 72 h incubation period, signifi cant differences are observed between Ti-18Mn LM , CP-Mn, and the other materials. The cytotoxicity of Ti-(6-13)Mn LM is comparable to that of CP-Ti. The cell proliferations of Ti-18Mn LM and CP-Mn are lower than those of the other alloys. The incubation time of the 72 h test leads to a higher amount of released Mn ions from the alloy. It is supposed that the amount of released Mn ions from Ti-18Mn LM can inhibit cell proliferation [ 15 ].  The C content of the alloys is of approximately 0.06 mass%. This is likely due to C pickup during the debinding process [ 23 ]. Furthermore, Mn evaporation is also observed to occur during fabrication by MIM, as the Mn content of the alloys is lower than the nominal content [ 16 ].  Ti-8Mn MIM (Fig. 19.11a ). The DF image of the ω spots (Fig. 19.11g ) [ 24 ]. Figure 19.12 shows the results of the EPMA analysis of the precipitates. The precipitates are low in Mn and high in C. The precipitates are thus identifi ed as Ti carbides. The volume fraction and average diameter of both pores and carbides in the alloys have been estimated from the optical micrographs using an image analysis software [ 16 ]. There is no signifi cant variation of either the volume fraction and average diameter of both pores and precipitates among Ti-(8-17)Mn MIM . However, the shapes of the pores in Ti-(8-17)Mn MIM are irregular, although there are no signifi cant differences among the pore morphologies of the alloys. The irregular pore shape is most likely due to the fact that the Mn powder has a more irregular shape compared to the Ti powder [ 16 ]. The high C levels facilitate the formation of carbides. The morphology of the carbides does not particularly vary among Ti-(8-17)Mn MIM . Figures 19.13 and 19.14 show the Vickers hardness and Young's modulus of the Ti-(8-17)Mn MIM alloys, respectively. The hardness does not signifi cantly vary among the alloys, and they are comparable to that of Ti-64 ELI, indicated by the dashed line in Fig. 19.13 [ 5 ]. The Young's moduli of the alloys are all lower than that for annealed Ti-64 ELI, indicated by the dashed line in Fig. 19.14 [ 5 ]. Besides the parameters already discussed for the alloys fabricated by CCLM, such as the amount of athermal ω phase and the effects of increasing Mn, which can affect both hardness and Young's modulus measurements, other parameters must be considered for the alloys fabricated by MIM [ 15 , 16 ]. The presence of the pores, which cause localized stress concentration, tends to decrease the hardness of the alloys [ 23 , 25 ]. Conversely, the presence of carbides, which can cause precipitation strengthening, tends to increase the hardness [ 23 , 25 ]. However, because there is little variation of the volume fraction and average diameter of both pores and carbides, these parameters do not cause signifi cant variation of the measured hardness and Young's modulus among the alloys. Furthermore, higher O contents increase the solid solution hardening effect, which in turn increases the hardness [ 25 -27 ]. These opposing effects balance each other so that there is little hardness variation among the alloys.  Figure 19.15 shows the tensile properties of Ti-(8-17)Mn MIM along with those of Ti-64 ELI [ 5 ]. Both σ B and σ 0.2 values for Ti-(8-12)Mn MIM are higher than the corresponding values for Ti-64 ELI [ 5 ]. However, the elongation of the Ti-Mn MIM alloys is lower than that of annealed Ti-64 ELI. It is noted that the elongation of Ti-Mn MIM , which contains higher O content, pores, and carbides, is lower than that of Ti-Mn LM , which do not contain pores and carbides and have lower O content. Thus, the lower elongation of Ti-Mn MIM can be attributed to the combined effects of a higher O content primarily, along with the presence of pores and carbides, which are inherent to the MIM process [ 23 , 28 , 29 ]. Figure 19.16 shows the compressive properties of Ti-(8-17)Mn MIM . The compressive 0.2 % proof stress ( σ c0.2 ) and compressive strain ( ε c ) values for Ti- (8)(9)(10)(11)(12)(13)(14)(15)(16)(17) Mn MIM are higher than those for Ti-64 [ 4 ], also shown in Fig. 19.16 . The two main parameters that affect the compressive properties of an alloy are the Mn content (which causes solid solution strengthening) and the amount of the ω phase (which causes precipitation strengthening). The higher volume fraction of athermal ω phase in Ti-8Mn MIM and Ti-9Mn MIM causes their σ c0.2 values to be higher than those of Ti-(12-17)Mn MIM . As the volume fraction of the athermal ω phase decreases in Ti-(12-15)Mn MIM , the solid solution strengthening effect of the Mn balances the effect of the decreasing ω phase. The increase in compressive strain with increasing Mn content is also due to the decrease in the volume fraction of the athermal ω phase with increasing Mn content.

Conclusions
Mn was selected as a low-cost β -stabilizer alloying element to fabricate Ti alloys of various compositions. The alloys were fabricated by both CCLM and MIM. Microstructural observations, mechanical performance tests, and biocompatibility tests were used to evaluate the mechanical properties and biocompatibility of the fabricated Ti-Mn alloys. Regarding the use of Mn as an alloying element with Ti and the competitive applicability of Ti-Mn alloys, it was possible to conclude that: 1. The alloys are primarily composed of equiaxed β grains. Some of the alloys with lower Mn content also contain the athermal ω phase. The β phase stability increases, thus decreasing the volume fraction of the athermal ω phase, with increasing Mn content. The alloys fabricated by MIM contain high amounts of O and C, which are due to the powders and organic binder used, respectively. Because of the high C content, there is the precipitation of Ti carbide in the alloys fabricated by MIM. Furthermore, the alloys fabricated by MIM also contain pores. 2. The hardness, Young's modulus, and tensile strength of Ti-(9-13)Mn LM are comparable or superior to those of Ti-64 ELI. The hardness, Young's modulus, tensile strength, and compressive properties of Ti-(8-13)Mn MIM are comparable or superior to those of Ti-64 ELI. However, the ductility of the alloys fabricated by MIM is adversely affected by the high oxygen content and the presence of carbides and pores. 3. Among both alloys fabricated by CCLM and those fabricated by MIM, Ti-9Mn shows the best balance between tensile strength and ductility. In particular, the Ti-9Mn LM shows the largest elongation among all the Ti-Mn alloys. Conversely, Ti-9Mn MIM shows the lowest compressive properties among these alloys. 4. Every Ti-Mn LM alloy shows ion release rates consistent with their chemical composition when immersed in SBF. Ti-(6-13)Mn LM shows good cell viability ratios, which are comparable to those of CP-Ti. However, higher Mn concentrations should be avoided because of risks of Mn intoxication.
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