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Breaking the limit of Young’s modulus in low-cost Ti–Nb–Zr alloy for biomedical implant applications
Journal Pre-proof Breaking the limit of Young's modulus in low-cost Ti–Nb–Zr alloy for biomedical implant applications Taekyung Lee, Sangwon Lee, In-Su Kim, Young Hoon Moon, Hyoung Seop Kim, Chan Hee Park PII:
S0925-8388(20)30764-7
DOI:
https://doi.org/10.1016/j.jallcom.2020.154401
Reference:
JALCOM 154401
To appear in:
Journal of Alloys and Compounds
Received Date: 24 December 2019 Revised Date:
14 February 2020
Accepted Date: 16 February 2020
Please cite this article as: T. Lee, S. Lee, I.-S. Kim, Y.H. Moon, H.S. Kim, C.H. Park, Breaking the limit of Young's modulus in low-cost Ti–Nb–Zr alloy for biomedical implant applications, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154401. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
T. Lee: Conceptualization, Methodology, Formal analysis, Writing – Original Draft, Writing – Review & Editing, Supervision S. Lee: Investigation, Visualization I.-S. Kim: Validation, Investigation, Visualization Y.H. Moon: Visualization, Resources H.S. Kim: Conceptualization, Methodology C.H. Park: Conceptualization, Methodology, Resources, Writing – Original Draft
Breaking the Limit of Young’s Modulus in Low-Cost Ti-Nb-Zr Alloy for Biomedical Implant Applications
Taekyung Lee1,*, Sangwon Lee2, In-Su Kim1,2, Young Hoon Moon1, Hyoung Seop Kim3, Chan Hee Park2
1
School of Mechanical Engineering, Pusan National University, Busan 46241, Republic of Korea
2
Advanced Metals Division, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea 3
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
* Corresponding author: Prof. T. Lee ([email protected], +82-51-510-2985)
Abstract Biomedical implant material simultaneously requires high yield strength and low Young’s modulus. The lower boundary for the YM of Ti-13Nb-13Zr alloys is regarded to be approximately 65 GPa, which limits further improvement in their mechanical compatibility. This study proposes a novel process of cold caliber rolling to break this limit and decrease YM to 47 GPa, which is the bare minimum value since the development of this alloying system. As a result, the developed alloy exhibits a mechanical compatibility similar to those of most advanced β-Ti alloys but with a significantly lower amount of high-cost alloying elements. This study attributed the mechanical improvement to three mechanisms based on
microstructural characterizations. Manufacturing advantages of the suggested method were discussed as well.
Keywords: titanium; elastic modulus; martensite; tensile properties; low cost
1. Introduction Ti alloys have attracted much attention for biomedical and healthcare applications due to their ‘good mechanical properties’, high corrosion resistance, and biocompatibility [1][2]. The term ‘good mechanical properties’ used above alludes to two key factors: high yield strength (YS) and low Young’s modulus (YM). High YS enables the reduction of a material’s volume to accommodate complex stress states, thus expanding the range of potential product designs and the indication of implant surgery. It also increases resistance to high-cycle fatigue fracture by inhibiting the initiation of fatigue cracking [3,4]. Meanwhile, the large difference in YM between metal and human bone (10-30 GPa) instigates a loading concentration on the former, thereby weakening the bone tissue around an implant and potentially causing bone absorption [5]. A low YM is important to avoid this phenomenon, which is known as the stress shielding effect. Ti alloys are broadly classified into three types on the basis of a predominant phase constituent. α-Ti alloys possess good corrosion resistance but low strength, while (α + β) dual-phase Ti alloys exhibit higher strength, toughness, and fatigue resistance. Commercially pure Ti (CP-Ti) and Ti-6Al-4V are representative alloys for each classification, respectively. However, these alloys exhibit YM over 100 GPa that is extremely higher than that of human bone. As an alternative, β-Ti alloys have been actively developed for decades based on various alloying systems including Ti-Nb-Ta-Zr [6–12], Ti-Nb-Ta-Sn [8], Ti-Nb-Zr-Sn [13], Ti-Nb-Mo-Sn [14,15], Ti-Nb-Sn [16,17], and Ti-Nb-Zr [18][19][20]. This type of alloys
possesses much lower YM as well as superior strength-to-weight ratio and corrosion resistance. However, they accompany significant production expenses because they require a considerable amount of high-cost alloying elements, such as Nb, Ta, Mo, and Hf. The Ti-13Nb-13Zr (TNZ) alloy was developed at the early 90s, and is currently registered in the ASTM standard for implant applications [21]. The ASTM standard proposes two processing methods for this alloy: solution treatment (ST) and capability aging (CA). STTNZ is fabricated by a heat treatment in the β domain (i.e., above the β-transus temperature) followed by rapid cooling. CA-TNZ is produced by applying a subsequent heat treatment to ST-TNZ in the (α + β) domain. TNZ alloy is generally regarded as a β-Ti alloy [22], but more specifically, it is considered as a ‘near β’ or ‘β-rich (α + β)’ Ti alloy [1,23]. Such classifications imply a lower amount of β-stabilizing alloying elements in this alloy compared with typical β-Ti alloys. Various manufacturing processes have been developed to enhance the mechanical performance of this alloy beyond that obtained by the ASTM standard [23–28] to replace β-Ti alloys for decreasing the material costs. Sections of these studies succeeded in exceeding a mechanical strength over 1 GPa [27,28]. Nevertheless, it was extremely difficult to decrease YM of TNZ alloy. No one has achieved a YM lower than that of ST-TNZ (64-70 GPa) for decades; as a result, these values have been considered as the lower-bound or unbreakable YM limit for TNZ alloys. Such a limit has been successfully broken in this study through a novel technique called a cold caliber rolling (CCR). This process also improves mechanical strength to the highest level ever and has applicability in the manufacturing industry. Therefore, this paper explains the mechanical improvement by the CCR process and elucidates the related mechanisms in terms of microstructural evolution through this novel process.
2. Experimental procedures Ti-13.8Nb-14.0Zr-0.06Fe-0.07O-0.01C-0.007N (numbers are mass percentages) rods with a diameter of 15 mm and length of 50 mm were used in this study. ST-TNZ was fabricated by solution treatment at 1073 K for 1 h and subsequent quenching in a water bath. Sections of ST-TNZ were aged at 773 K for 6 h, followed by air-cooling to fabricate CATNZ. Other sections were used for the fabrication of CCR-TNZ; they were machined to a diameter of 12.5 mm and then subjected to the CCR process, consisting of 12-pass deformation with an area reduction of 72% performed at room temperature (~295 K). The YS and YM of ST-, CA-, and CCR-TNZ were evaluated through a tensile test and ultrasonic measurement at room temperature. The first two alloys, whose mechanical properties are well known, were used as references to ensure the reliability of the experiments. The tensile tests were conducted at a strain rate of 10−3 s−1 with a contact-type extensometer for precise measurement. Tensile specimens were machined to 2.5 mm in gage diameter and 10 mm in gage length. The YS was determined as the 0.2%-offset strength. Disc samples were prepared with a diameter of 6 mm and thickness of 3 mm to measure YM through the ultrasonic method. An ultrasonic detector with two types of transducers measured wave velocities along the longitudinal and transverse directions. Microstructure of CCR-TNZ was investigated using X-ray diffraction (XRD) method, electron backscatter diffraction (EBSD) analysis, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XRD samples were mechanically polished using #400- to #2400-grit abrasive paper. XRD analysis was carried out in 2θ range from 30° to 80° at a scan rate of 0.2°·min−1. EBSD samples were mechanically polished in the same procedures, followed by electro-polishing at 25 V in a solution of 5% perchloric acid and 95% methanol. EBSD data were obtained at a step size of 50 nm and then processed using TSL OIM Software Ver. 7. The data with low confidence index (< 0.1) were excluded from the
analysis to ensure data reliability. SEM samples were prepared in the same manner and then chemically etched in an aqueous solution of 2% nitric acid and 2% hydrofluoric acid. TEM samples were prepared by the focused ion beam technique.
3. Results Figure 1 demonstrates the variation in YS and YM of TNZ alloys measured in this work and in the literature [5,20,22–28]. All variants of TNZ alloys possess lower Young’s moduli than those of traditional implant alloys, such as Co-Cr-Mo (200-230 GPa), stainless steel (190 GPa), CP-Ti (105 GPa), and Ti-6Al-4V (110 GPa) [5,22,29]. In particular, ST-TNZ exhibited the lowest YM (61-64 GPa) among the conventional TNZ alloys and low YS (545 MPa), whose value matches the reported YM limit. Although CA-TNZ showed a 42 % increase in YS (772 MPa), it also accompanied an undesirable increase in YM (77 GPa). This is also confirmed by the markedly higher slope of CA-TNZ in the elastic regime, as shown in Figure 1a.
Figure 1. Mechanical properties of the investigated TNZ alloys: (a) engineering stress vs strain curves and (b) YS and YM compared with the data reported in the literature. “#1” and “#2” indicate the data obtained from the two separate batches of CCR-TNZ.
Previous studies on TNZ alloys can be classified into two categories based on their
manufacturing strategy, except for the ASTM standard. One category attempted to tailor mechanical properties through a process optimization (PO), including the control of ST temperature, rolling temperature, and cooling rate [25]. All these trials always led to high YM (69-91 GPa) above the limit with a wide variety of YS. The other category exploited the dynamic globularization (DG) phenomenon through either warm caliber rolling [26,27] or warm sheet rolling [28]. The DG approach successfully enhanced YS to ~1 GPa. Nevertheless, the measured YM values (78-81 GPa) were still higher than the limit and similar to that of CA-TNZ, suggestive of failure in decreasing the YM through these conventional manufacturing strategies. In contrast, the novel CCR-TNZ successfully broke the reported YM limit for TNZ alloys. It is worthwhile noting that the measured YM values of CCR-TNZ were at the bare minimum among those of TNZ alloys reported ever. As such values were extraordinarily low, they were measured thoroughly using different methods and batches of alloy. First, YM was determined by reading the slope of elastic regime in the tensile flow curves (i.e., tangent modulus). Two separate batches of CCR-TNZ were manufactured to ensure data reproducibility, which yielded YM values of 47 GPa and 54 GPa. Secondly, the ultrasonic method was employed to measure YM, yielding a value of 51 GPa. Similar results obtained from the different batches and methods ensured the reliability of the YM measurement for CCR-TNZ. Thirdly, the reliability of measurement is further guaranteed by the consistency of the mechanical properties of ST- and CA-TNZ found in this study and those in the literature. The measured data were within the range of the reported data for both YS and YM, as shown in Figure 1b. It is also noted that YS values of CCR-TNZ (963 MPa and 1042 MPa) are comparable to the highest value among the conventional TNZ alloys. High YS and low YM are both important for implant application. To evaluate the comprehensive performance considering both factors, researchers have employed the term
mechanical compatibility, which is defined as YS/YM [22,28]. The mechanical compatibilities of the developed CCR-TNZ and other Ti-based biomaterials from the literature are plotted in Figure 2a [5–11,13–20,22–28,30–32]. CP-Ti exhibited the lowest mechanical compatibility (4.4 × 10−3) owing to its low YS and high YM. Ti-6Al-4V alloy had an increased value (7.8 × 10−3) due to its higher YS. Conventional TNZ alloys were distributed vertically in the diagram, as marked by the red circle in Figure 2a. In other words, the YS of TNZ alloys can vary widely depending on the processing technique, whereas YM is strictly limited to a narrow range over the YM limit. Among these conventional TNZ alloys, DG-TNZ exhibited the highest mechanical compatibilities (up to 12.9 × 10−3) due to the significant strengthening to ~1 GPa with preserved YM as compared with CA-TNZ.
Figure 2. (a) Contour map of mechanical compatibility of various Ti-based biomaterials and (b) relative material cost with respect to the mechanical compatibility. “A”, “B”, and “C” correspond to Ti-23Nb-0.7Ta-2Zr-1.2O [31,32], Ti-30Nb-1Mo-4Sn [15], and Ti-33Nb-4Sn [17], respectively, nominated for further discussion. “#1” and “#2” are the data obtained from the two separate batches of CCR-TNZ.
CCR-TNZ exhibited a remarkable improvement in mechanical compatibility; the values for two CCR batches (20.4 × 10−3 and 19.4 × 10−3) nearly doubled the best mechanical compatibility of conventional TNZ alloys. The improvement mainly stemmed from breaking the YM limit as CCR-TNZ exhibited similar YS values as compared with those of DG-TNZ. Accordingly, CCR-TNZ matched most advanced β-Ti alloys in terms of mechanical compatibility. This is a huge accomplishment because those alloys possess a significantly higher amount of expensive β-stabilizing elements. For example, the YS and YM of CCRTNZ are comparable to those reported in Ti-23Nb-0.7Ta-2Zr-1.3O alloy or ‘Gum Metal’ (marked as “A” in Figure 2) [31,32] and Ti-30Nb-1Mo-4Sn alloy (marked as “B”) [15]. It also exhibited similar mechanical compatibility to Ti-33Nb-4Sn alloy (marked as “C”) [17]. These alloys contain 2-3 times more Nb than CCR-TNZ, resulting in a radical increase in material cost. The relative material costs were estimated based on unit price of each alloying element, as presented in Figure 2b. Although this plot is not able to consider the total expenses, it still demonstrates the meaningfully low material cost of CCR-TNZ with high mechanical compatibility. Figure 3 demonstrates XRD line profiles of ST-, CA-, and CCR-TNZ. α variants (i.e., α, α′, and α″ phases) are not distinguishable through the XRD analysis because the 2θ gap among α variants is too small to provide a clear difference. Accordingly, this issue is discussed later with other experimental results. ST- and CCR-TNZ showed an absence of β
peaks, whereas CA-TNZ exhibited diffraction peaks corresponding to β phase. The difference between the two groups was particularly confirmed by the diffraction peaks for (110)β, as marked by the arrow in the inset of Figure 3. The XRD line profile of CCR-TNZ showed a stronger intensity for the (100) peak than (002) peak on the plane normal to the rolling direction (RD). The similar XRD peaks were observed in TNZ alloys subjected to warm caliber rolling [33], implying that the texture development of hcp phase was mainly attributable to the caliber rolling itself rather than the rolling temperature.
Figure 3. XRD line profiles of ST-, CA-, and CCR-TNZ. The observed plane of the CCRTNZ is the transverse section normal to the RD. The inset is a magnified image of the 2θ range from 36° to 42°.
In Figure 4, SEM and EBSD micrographs depict the trace of fine plates aligned with the RD. The fraction of β phase is significantly low (0.6%), as shown in Figure 4c, indicating the inhibited decomposition of α′ martensite into α and β phases during the CCR process. Hence, the fine plates aligned with the RD are considered as α′ martensite. Figure 4d demonstrates a kernel average misorientation (KAM) map to estimate strain energy stored in
CCR-TNZ, which yielded an average KAM value of 1.22. As KAM value is considered as a measure of local stress, the high KAM value of CCR-TNZ indicates a large amount of energy stored in the alloy through severe plastic deformation [34]. As shown in Figure 4e, the {002} pole figures exhibited a strong basal texture, where the basal plane was located parallel to the RD. The basal poles were distributed in the transverse section of sample while maintaining perpendicularity to the RD. The {100} pole figures indicated the strong <100>//RD texture of CCR-TNZ. These characteristics are in good agreement with the conclusions drawn from the XRD analysis, i.e., suppressed β transformation and strong <100>//RD basal texture.
Figure 4. Microstructures of CCR-TNZ observed on the longitudinal section, where the RD is aligned with the vertical axis: (a) SEM micrograph, (b) EBSD inverse pole figure map, (c) phase map, (d) KAM map, and (e) (002) and (100) pole figures for α′ phase.
The low-magnification TEM micrograph in Figure 5a showed the formation of an ultrafine-grained structure comprising fragmented plates with thicknesses of 100-200 nm. A high frequency of dislocation tangles was also observed, which is in good agreement with the high KAM value of CCR-TNZ. The high-magnification TEM micrograph and corresponding selected area diffraction (SAD) pattern in Figure 5b revealed not only hexagonal closepacked α′ but also orthorhombic α″ martensites in CCR-TNZ. β grains were rarely confirmed,
which is consistent with the results from the XRD, SEM, and EBSD analyses. Various orientation relationships were observed between the two martensites, of which, the (100)[021]α′//(010)[−201]α″ orientation is discussed below.
Figure 5. Bright-field TEM micrograph of CCR-TNZ with (a) low magnification and (b) high magnification with a corresponding SAD pattern.
4. Discussion In β-Ti alloys, the strategy for enhancing mechanical compatibility is related to the control of β-stabilizing elements. A lack of them induces α″ martensitic transformation and resultant double yielding, which markedly degrades the YS of the material [17]. An excessive amount gives rise to an increased YM [35]. However, such a strategy is not applicable for TNZ alloys due to their significantly low amount of β-stabilizing elements. This is quantitatively supported by a remarkably lower Mo equivalency (1.4) than those of typical βTi alloys (≥ 10.0) [1]. Such an alloying system results in two differences in microstructural evolution between TNZ alloys and β-Ti alloys. First, α phase is precipitated in the former,
whereas it attracts negligible attention in the latter. Secondly, both types of martensitic transformation (i.e., β → α′ and β → α″) are available in the former [26,36]. Consequently, the strategy for TNZ alloys should be established around the following three factors: suppression of α precipitation, control of two different martensites, and utilization of defects (i.e., grain boundaries and dislocations). The YM values reported for TNZ alloys can indeed be interpreted in terms of α phase. α phase possesses high strength, hardness, and YM [15,37]. The decomposition of α′ martensite into α and β phases occurred in CA-, PO-, and DG-TNZ fabricated below the βtransus temperature. The decomposition, or α precipitation, leads to increased YM of these alloys compared to α-free ST-TNZ [36]. In contrast, the CCR process performed at room temperature prevented the decomposition of α′ martensite. The ‘multi-pass’ characteristic of the CCR process also assisted in sustaining the low processing temperature by cooling the material between rolling passes. As a result, CCR-TNZ possessed a unique microstructure with a high fraction of α′ and α″ martensites, thereby avoiding the increased YM seen in other TNZ alloys. CCR-TNZ with the (α′ + α″) microstructure exhibited a lower YM than ST-TNZ with the full α′ microstructure [36]. Based on the present results, α″ martensite is more beneficial to a low YM than α and α′ phases in TNZ alloy. Such a deduction is theoretically supported by the diagram of YM vs critical electron/atom (e/a) ratio for Ti-TM binary alloys where the “TM” stands for transition metal [35]. This diagram demonstrates two valleys, indicating the optimized e/a values for the lowest YM: 4.09 and 4.24. The alloying design for β-Ti alloys aims at the second valley to suppress metastable ω phase. In contrast, the e/a values of TNZ alloys (4.08) lie in the range of the first valley, where the YM reduction is attained by α″ martensites. Meanwhile, a recent study of Ozan et al. [18] provided good insight into YM of β and αʹʹ phases. The study investigated Ti-Nb-Zr alloys subjected to various amount of cold
rolling, in which the lowest YM was confirmed at an area reduction of 20%. Further increase in cold rolling induced α″ → β reverse transformation, and thus increased YM. Consequently, α″ martensitic transformation induced by the CCR process contributed to breaking the YM limit. The <100>α′//RD basal texture contributed to an additional YM reduction. The relationship of (100)[021]α′//(010)[−201]α″ indicates that the b-axis of α″ martensite is aligned with the RD. The b-axis of orthorhombic α″ crystal has a higher lattice constant (5.05-5.09 Å) than the a-axis (2.97-3.05 Å) and c-axis (4.76-4.77 Å) [36]. The crystal rotation towards the b-axis aligned with the RD resulted in an increased distance between atoms along the loading direction. Such a reorientation of atoms reduced YM as this property is a measure of atomic bonding [38]. Indeed, the similar type of YM reduction was confirmed in Ti-35Nb-4Sn alloy [16] and nanocrystalline Fe [39]. Multi-pass caliber rolling imposes a significant amount of deformation by high redundant strain and complex stress state, inducing strong grain refinement and increased dislocation density [40]. The low processing temperature adopted in this work further intensified the generation of these defects by suppressing the recrystallization and recovery of the microstructure. The selection of ST-TNZ as the starting material also contributed to the effective grain refinement because the fine martensitic plates in ST-TNZ were fragmented into fine grains with a lower aspect ratio during the CCR process (i.e., DG mechanism) [41]. The radical increase in such defects provided an increasing frequency of ‘barriers’ against the movement of mobile dislocations, resulting in the improved YS of CCR-TNZ. It is also noted that the increasing number of defects barely increases YM [9,38]. This feature is distinguished from the case of α precipitates that increase both YS and YM, leaving a grainboundary strengthening superior to precipitation hardening in terms of mechanical compatibility.
Finally, it is worth summarizing the advantages of the proposed method from the perspective of the manufacturing industry. The caliber-rolling process is able to fabricate a bulk product with a length of 1 m or longer [40]. In addition, the process is basically a rod rolling, thereby possessing the manufacturing benefits of conventional rolling process, such as mass production and continuous fabrication. These characteristics assist the CCR process to be applicable in the industry, which is contrasted by most of severe plastic deformation processes. Meanwhile, the TNZ alloying system has been verified by the ASTM standard for decades [21], which could reduce the expenses of biomedical certification for CCR-TNZ.
5. Conclusions The developed CCR process broke the reported YM limit for TNZ alloys and decreased its value to the bare minimum among the data reported ever. It also enhanced YS of a material to ~1 GPa. The remarkable YM reduction and mechanical strengthening gave rise to the significant mechanical compatibility of CCR-TNZ. Such achievements were attributed to three factors. First, the CCR process suppressed the decomposition of αʹ martensite into (α + β) phase, avoiding the undesirable increase in YM due to hard α phase. Secondly, α″ martensites contributed to the YM reduction, which is theoretically supported by the e/a ratio of TNZ alloys. Sections of these martensites possessed an orientation relationship beneficial to YM reduction, which increased the atomic distance along the loading direction. Thirdly, a high frequency of defects caused an effective grain-boundary strengthening that made up for the absence of α-precipitation hardening. The mechanical improvement, low-cost benefits, and manufacturing advantages make this alloy a promising candidate for biomedical implant applications.
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2018R1C1B6002068). The authors gratefully acknowledge the assistance of Bando Titanium Co. Ltd. for the material costs information presented in Figure 2b.
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* Cold caliber rolling of Ti-13Nb-13Zr achieved markedly low Young's modulus. * Such a low Young's modulus is attributed to the formation of the α″ martensite phase. * Cold caliber rolling provides benefits in cost-saving and manufacturing advantages.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30764-7
DOI:
https://doi.org/10.1016/j.jallcom.2020.154401
Reference:
JALCOM 154401
To appear in:
Journal of Alloys and Compounds
Received Date: 24 December 2019 Revised Date:
14 February 2020
Accepted Date: 16 February 2020
Please cite this article as: T. Lee, S. Lee, I.-S. Kim, Y.H. Moon, H.S. Kim, C.H. Park, Breaking the limit of Young's modulus in low-cost Ti–Nb–Zr alloy for biomedical implant applications, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154401. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
T. Lee: Conceptualization, Methodology, Formal analysis, Writing – Original Draft, Writing – Review & Editing, Supervision S. Lee: Investigation, Visualization I.-S. Kim: Validation, Investigation, Visualization Y.H. Moon: Visualization, Resources H.S. Kim: Conceptualization, Methodology C.H. Park: Conceptualization, Methodology, Resources, Writing – Original Draft
Breaking the Limit of Young’s Modulus in Low-Cost Ti-Nb-Zr Alloy for Biomedical Implant Applications
Taekyung Lee1,*, Sangwon Lee2, In-Su Kim1,2, Young Hoon Moon1, Hyoung Seop Kim3, Chan Hee Park2
1
School of Mechanical Engineering, Pusan National University, Busan 46241, Republic of Korea
2
Advanced Metals Division, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea 3
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
* Corresponding author: Prof. T. Lee ([email protected], +82-51-510-2985)
Abstract Biomedical implant material simultaneously requires high yield strength and low Young’s modulus. The lower boundary for the YM of Ti-13Nb-13Zr alloys is regarded to be approximately 65 GPa, which limits further improvement in their mechanical compatibility. This study proposes a novel process of cold caliber rolling to break this limit and decrease YM to 47 GPa, which is the bare minimum value since the development of this alloying system. As a result, the developed alloy exhibits a mechanical compatibility similar to those of most advanced β-Ti alloys but with a significantly lower amount of high-cost alloying elements. This study attributed the mechanical improvement to three mechanisms based on
microstructural characterizations. Manufacturing advantages of the suggested method were discussed as well.
Keywords: titanium; elastic modulus; martensite; tensile properties; low cost
1. Introduction Ti alloys have attracted much attention for biomedical and healthcare applications due to their ‘good mechanical properties’, high corrosion resistance, and biocompatibility [1][2]. The term ‘good mechanical properties’ used above alludes to two key factors: high yield strength (YS) and low Young’s modulus (YM). High YS enables the reduction of a material’s volume to accommodate complex stress states, thus expanding the range of potential product designs and the indication of implant surgery. It also increases resistance to high-cycle fatigue fracture by inhibiting the initiation of fatigue cracking [3,4]. Meanwhile, the large difference in YM between metal and human bone (10-30 GPa) instigates a loading concentration on the former, thereby weakening the bone tissue around an implant and potentially causing bone absorption [5]. A low YM is important to avoid this phenomenon, which is known as the stress shielding effect. Ti alloys are broadly classified into three types on the basis of a predominant phase constituent. α-Ti alloys possess good corrosion resistance but low strength, while (α + β) dual-phase Ti alloys exhibit higher strength, toughness, and fatigue resistance. Commercially pure Ti (CP-Ti) and Ti-6Al-4V are representative alloys for each classification, respectively. However, these alloys exhibit YM over 100 GPa that is extremely higher than that of human bone. As an alternative, β-Ti alloys have been actively developed for decades based on various alloying systems including Ti-Nb-Ta-Zr [6–12], Ti-Nb-Ta-Sn [8], Ti-Nb-Zr-Sn [13], Ti-Nb-Mo-Sn [14,15], Ti-Nb-Sn [16,17], and Ti-Nb-Zr [18][19][20]. This type of alloys
possesses much lower YM as well as superior strength-to-weight ratio and corrosion resistance. However, they accompany significant production expenses because they require a considerable amount of high-cost alloying elements, such as Nb, Ta, Mo, and Hf. The Ti-13Nb-13Zr (TNZ) alloy was developed at the early 90s, and is currently registered in the ASTM standard for implant applications [21]. The ASTM standard proposes two processing methods for this alloy: solution treatment (ST) and capability aging (CA). STTNZ is fabricated by a heat treatment in the β domain (i.e., above the β-transus temperature) followed by rapid cooling. CA-TNZ is produced by applying a subsequent heat treatment to ST-TNZ in the (α + β) domain. TNZ alloy is generally regarded as a β-Ti alloy [22], but more specifically, it is considered as a ‘near β’ or ‘β-rich (α + β)’ Ti alloy [1,23]. Such classifications imply a lower amount of β-stabilizing alloying elements in this alloy compared with typical β-Ti alloys. Various manufacturing processes have been developed to enhance the mechanical performance of this alloy beyond that obtained by the ASTM standard [23–28] to replace β-Ti alloys for decreasing the material costs. Sections of these studies succeeded in exceeding a mechanical strength over 1 GPa [27,28]. Nevertheless, it was extremely difficult to decrease YM of TNZ alloy. No one has achieved a YM lower than that of ST-TNZ (64-70 GPa) for decades; as a result, these values have been considered as the lower-bound or unbreakable YM limit for TNZ alloys. Such a limit has been successfully broken in this study through a novel technique called a cold caliber rolling (CCR). This process also improves mechanical strength to the highest level ever and has applicability in the manufacturing industry. Therefore, this paper explains the mechanical improvement by the CCR process and elucidates the related mechanisms in terms of microstructural evolution through this novel process.
2. Experimental procedures Ti-13.8Nb-14.0Zr-0.06Fe-0.07O-0.01C-0.007N (numbers are mass percentages) rods with a diameter of 15 mm and length of 50 mm were used in this study. ST-TNZ was fabricated by solution treatment at 1073 K for 1 h and subsequent quenching in a water bath. Sections of ST-TNZ were aged at 773 K for 6 h, followed by air-cooling to fabricate CATNZ. Other sections were used for the fabrication of CCR-TNZ; they were machined to a diameter of 12.5 mm and then subjected to the CCR process, consisting of 12-pass deformation with an area reduction of 72% performed at room temperature (~295 K). The YS and YM of ST-, CA-, and CCR-TNZ were evaluated through a tensile test and ultrasonic measurement at room temperature. The first two alloys, whose mechanical properties are well known, were used as references to ensure the reliability of the experiments. The tensile tests were conducted at a strain rate of 10−3 s−1 with a contact-type extensometer for precise measurement. Tensile specimens were machined to 2.5 mm in gage diameter and 10 mm in gage length. The YS was determined as the 0.2%-offset strength. Disc samples were prepared with a diameter of 6 mm and thickness of 3 mm to measure YM through the ultrasonic method. An ultrasonic detector with two types of transducers measured wave velocities along the longitudinal and transverse directions. Microstructure of CCR-TNZ was investigated using X-ray diffraction (XRD) method, electron backscatter diffraction (EBSD) analysis, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XRD samples were mechanically polished using #400- to #2400-grit abrasive paper. XRD analysis was carried out in 2θ range from 30° to 80° at a scan rate of 0.2°·min−1. EBSD samples were mechanically polished in the same procedures, followed by electro-polishing at 25 V in a solution of 5% perchloric acid and 95% methanol. EBSD data were obtained at a step size of 50 nm and then processed using TSL OIM Software Ver. 7. The data with low confidence index (< 0.1) were excluded from the
analysis to ensure data reliability. SEM samples were prepared in the same manner and then chemically etched in an aqueous solution of 2% nitric acid and 2% hydrofluoric acid. TEM samples were prepared by the focused ion beam technique.
3. Results Figure 1 demonstrates the variation in YS and YM of TNZ alloys measured in this work and in the literature [5,20,22–28]. All variants of TNZ alloys possess lower Young’s moduli than those of traditional implant alloys, such as Co-Cr-Mo (200-230 GPa), stainless steel (190 GPa), CP-Ti (105 GPa), and Ti-6Al-4V (110 GPa) [5,22,29]. In particular, ST-TNZ exhibited the lowest YM (61-64 GPa) among the conventional TNZ alloys and low YS (545 MPa), whose value matches the reported YM limit. Although CA-TNZ showed a 42 % increase in YS (772 MPa), it also accompanied an undesirable increase in YM (77 GPa). This is also confirmed by the markedly higher slope of CA-TNZ in the elastic regime, as shown in Figure 1a.
Figure 1. Mechanical properties of the investigated TNZ alloys: (a) engineering stress vs strain curves and (b) YS and YM compared with the data reported in the literature. “#1” and “#2” indicate the data obtained from the two separate batches of CCR-TNZ.
Previous studies on TNZ alloys can be classified into two categories based on their
manufacturing strategy, except for the ASTM standard. One category attempted to tailor mechanical properties through a process optimization (PO), including the control of ST temperature, rolling temperature, and cooling rate [25]. All these trials always led to high YM (69-91 GPa) above the limit with a wide variety of YS. The other category exploited the dynamic globularization (DG) phenomenon through either warm caliber rolling [26,27] or warm sheet rolling [28]. The DG approach successfully enhanced YS to ~1 GPa. Nevertheless, the measured YM values (78-81 GPa) were still higher than the limit and similar to that of CA-TNZ, suggestive of failure in decreasing the YM through these conventional manufacturing strategies. In contrast, the novel CCR-TNZ successfully broke the reported YM limit for TNZ alloys. It is worthwhile noting that the measured YM values of CCR-TNZ were at the bare minimum among those of TNZ alloys reported ever. As such values were extraordinarily low, they were measured thoroughly using different methods and batches of alloy. First, YM was determined by reading the slope of elastic regime in the tensile flow curves (i.e., tangent modulus). Two separate batches of CCR-TNZ were manufactured to ensure data reproducibility, which yielded YM values of 47 GPa and 54 GPa. Secondly, the ultrasonic method was employed to measure YM, yielding a value of 51 GPa. Similar results obtained from the different batches and methods ensured the reliability of the YM measurement for CCR-TNZ. Thirdly, the reliability of measurement is further guaranteed by the consistency of the mechanical properties of ST- and CA-TNZ found in this study and those in the literature. The measured data were within the range of the reported data for both YS and YM, as shown in Figure 1b. It is also noted that YS values of CCR-TNZ (963 MPa and 1042 MPa) are comparable to the highest value among the conventional TNZ alloys. High YS and low YM are both important for implant application. To evaluate the comprehensive performance considering both factors, researchers have employed the term
mechanical compatibility, which is defined as YS/YM [22,28]. The mechanical compatibilities of the developed CCR-TNZ and other Ti-based biomaterials from the literature are plotted in Figure 2a [5–11,13–20,22–28,30–32]. CP-Ti exhibited the lowest mechanical compatibility (4.4 × 10−3) owing to its low YS and high YM. Ti-6Al-4V alloy had an increased value (7.8 × 10−3) due to its higher YS. Conventional TNZ alloys were distributed vertically in the diagram, as marked by the red circle in Figure 2a. In other words, the YS of TNZ alloys can vary widely depending on the processing technique, whereas YM is strictly limited to a narrow range over the YM limit. Among these conventional TNZ alloys, DG-TNZ exhibited the highest mechanical compatibilities (up to 12.9 × 10−3) due to the significant strengthening to ~1 GPa with preserved YM as compared with CA-TNZ.
Figure 2. (a) Contour map of mechanical compatibility of various Ti-based biomaterials and (b) relative material cost with respect to the mechanical compatibility. “A”, “B”, and “C” correspond to Ti-23Nb-0.7Ta-2Zr-1.2O [31,32], Ti-30Nb-1Mo-4Sn [15], and Ti-33Nb-4Sn [17], respectively, nominated for further discussion. “#1” and “#2” are the data obtained from the two separate batches of CCR-TNZ.
CCR-TNZ exhibited a remarkable improvement in mechanical compatibility; the values for two CCR batches (20.4 × 10−3 and 19.4 × 10−3) nearly doubled the best mechanical compatibility of conventional TNZ alloys. The improvement mainly stemmed from breaking the YM limit as CCR-TNZ exhibited similar YS values as compared with those of DG-TNZ. Accordingly, CCR-TNZ matched most advanced β-Ti alloys in terms of mechanical compatibility. This is a huge accomplishment because those alloys possess a significantly higher amount of expensive β-stabilizing elements. For example, the YS and YM of CCRTNZ are comparable to those reported in Ti-23Nb-0.7Ta-2Zr-1.3O alloy or ‘Gum Metal’ (marked as “A” in Figure 2) [31,32] and Ti-30Nb-1Mo-4Sn alloy (marked as “B”) [15]. It also exhibited similar mechanical compatibility to Ti-33Nb-4Sn alloy (marked as “C”) [17]. These alloys contain 2-3 times more Nb than CCR-TNZ, resulting in a radical increase in material cost. The relative material costs were estimated based on unit price of each alloying element, as presented in Figure 2b. Although this plot is not able to consider the total expenses, it still demonstrates the meaningfully low material cost of CCR-TNZ with high mechanical compatibility. Figure 3 demonstrates XRD line profiles of ST-, CA-, and CCR-TNZ. α variants (i.e., α, α′, and α″ phases) are not distinguishable through the XRD analysis because the 2θ gap among α variants is too small to provide a clear difference. Accordingly, this issue is discussed later with other experimental results. ST- and CCR-TNZ showed an absence of β
peaks, whereas CA-TNZ exhibited diffraction peaks corresponding to β phase. The difference between the two groups was particularly confirmed by the diffraction peaks for (110)β, as marked by the arrow in the inset of Figure 3. The XRD line profile of CCR-TNZ showed a stronger intensity for the (100) peak than (002) peak on the plane normal to the rolling direction (RD). The similar XRD peaks were observed in TNZ alloys subjected to warm caliber rolling [33], implying that the texture development of hcp phase was mainly attributable to the caliber rolling itself rather than the rolling temperature.
Figure 3. XRD line profiles of ST-, CA-, and CCR-TNZ. The observed plane of the CCRTNZ is the transverse section normal to the RD. The inset is a magnified image of the 2θ range from 36° to 42°.
In Figure 4, SEM and EBSD micrographs depict the trace of fine plates aligned with the RD. The fraction of β phase is significantly low (0.6%), as shown in Figure 4c, indicating the inhibited decomposition of α′ martensite into α and β phases during the CCR process. Hence, the fine plates aligned with the RD are considered as α′ martensite. Figure 4d demonstrates a kernel average misorientation (KAM) map to estimate strain energy stored in
CCR-TNZ, which yielded an average KAM value of 1.22. As KAM value is considered as a measure of local stress, the high KAM value of CCR-TNZ indicates a large amount of energy stored in the alloy through severe plastic deformation [34]. As shown in Figure 4e, the {002} pole figures exhibited a strong basal texture, where the basal plane was located parallel to the RD. The basal poles were distributed in the transverse section of sample while maintaining perpendicularity to the RD. The {100} pole figures indicated the strong <100>//RD texture of CCR-TNZ. These characteristics are in good agreement with the conclusions drawn from the XRD analysis, i.e., suppressed β transformation and strong <100>//RD basal texture.
Figure 4. Microstructures of CCR-TNZ observed on the longitudinal section, where the RD is aligned with the vertical axis: (a) SEM micrograph, (b) EBSD inverse pole figure map, (c) phase map, (d) KAM map, and (e) (002) and (100) pole figures for α′ phase.
The low-magnification TEM micrograph in Figure 5a showed the formation of an ultrafine-grained structure comprising fragmented plates with thicknesses of 100-200 nm. A high frequency of dislocation tangles was also observed, which is in good agreement with the high KAM value of CCR-TNZ. The high-magnification TEM micrograph and corresponding selected area diffraction (SAD) pattern in Figure 5b revealed not only hexagonal closepacked α′ but also orthorhombic α″ martensites in CCR-TNZ. β grains were rarely confirmed,
which is consistent with the results from the XRD, SEM, and EBSD analyses. Various orientation relationships were observed between the two martensites, of which, the (100)[021]α′//(010)[−201]α″ orientation is discussed below.
Figure 5. Bright-field TEM micrograph of CCR-TNZ with (a) low magnification and (b) high magnification with a corresponding SAD pattern.
4. Discussion In β-Ti alloys, the strategy for enhancing mechanical compatibility is related to the control of β-stabilizing elements. A lack of them induces α″ martensitic transformation and resultant double yielding, which markedly degrades the YS of the material [17]. An excessive amount gives rise to an increased YM [35]. However, such a strategy is not applicable for TNZ alloys due to their significantly low amount of β-stabilizing elements. This is quantitatively supported by a remarkably lower Mo equivalency (1.4) than those of typical βTi alloys (≥ 10.0) [1]. Such an alloying system results in two differences in microstructural evolution between TNZ alloys and β-Ti alloys. First, α phase is precipitated in the former,
whereas it attracts negligible attention in the latter. Secondly, both types of martensitic transformation (i.e., β → α′ and β → α″) are available in the former [26,36]. Consequently, the strategy for TNZ alloys should be established around the following three factors: suppression of α precipitation, control of two different martensites, and utilization of defects (i.e., grain boundaries and dislocations). The YM values reported for TNZ alloys can indeed be interpreted in terms of α phase. α phase possesses high strength, hardness, and YM [15,37]. The decomposition of α′ martensite into α and β phases occurred in CA-, PO-, and DG-TNZ fabricated below the βtransus temperature. The decomposition, or α precipitation, leads to increased YM of these alloys compared to α-free ST-TNZ [36]. In contrast, the CCR process performed at room temperature prevented the decomposition of α′ martensite. The ‘multi-pass’ characteristic of the CCR process also assisted in sustaining the low processing temperature by cooling the material between rolling passes. As a result, CCR-TNZ possessed a unique microstructure with a high fraction of α′ and α″ martensites, thereby avoiding the increased YM seen in other TNZ alloys. CCR-TNZ with the (α′ + α″) microstructure exhibited a lower YM than ST-TNZ with the full α′ microstructure [36]. Based on the present results, α″ martensite is more beneficial to a low YM than α and α′ phases in TNZ alloy. Such a deduction is theoretically supported by the diagram of YM vs critical electron/atom (e/a) ratio for Ti-TM binary alloys where the “TM” stands for transition metal [35]. This diagram demonstrates two valleys, indicating the optimized e/a values for the lowest YM: 4.09 and 4.24. The alloying design for β-Ti alloys aims at the second valley to suppress metastable ω phase. In contrast, the e/a values of TNZ alloys (4.08) lie in the range of the first valley, where the YM reduction is attained by α″ martensites. Meanwhile, a recent study of Ozan et al. [18] provided good insight into YM of β and αʹʹ phases. The study investigated Ti-Nb-Zr alloys subjected to various amount of cold
rolling, in which the lowest YM was confirmed at an area reduction of 20%. Further increase in cold rolling induced α″ → β reverse transformation, and thus increased YM. Consequently, α″ martensitic transformation induced by the CCR process contributed to breaking the YM limit. The <100>α′//RD basal texture contributed to an additional YM reduction. The relationship of (100)[021]α′//(010)[−201]α″ indicates that the b-axis of α″ martensite is aligned with the RD. The b-axis of orthorhombic α″ crystal has a higher lattice constant (5.05-5.09 Å) than the a-axis (2.97-3.05 Å) and c-axis (4.76-4.77 Å) [36]. The crystal rotation towards the b-axis aligned with the RD resulted in an increased distance between atoms along the loading direction. Such a reorientation of atoms reduced YM as this property is a measure of atomic bonding [38]. Indeed, the similar type of YM reduction was confirmed in Ti-35Nb-4Sn alloy [16] and nanocrystalline Fe [39]. Multi-pass caliber rolling imposes a significant amount of deformation by high redundant strain and complex stress state, inducing strong grain refinement and increased dislocation density [40]. The low processing temperature adopted in this work further intensified the generation of these defects by suppressing the recrystallization and recovery of the microstructure. The selection of ST-TNZ as the starting material also contributed to the effective grain refinement because the fine martensitic plates in ST-TNZ were fragmented into fine grains with a lower aspect ratio during the CCR process (i.e., DG mechanism) [41]. The radical increase in such defects provided an increasing frequency of ‘barriers’ against the movement of mobile dislocations, resulting in the improved YS of CCR-TNZ. It is also noted that the increasing number of defects barely increases YM [9,38]. This feature is distinguished from the case of α precipitates that increase both YS and YM, leaving a grainboundary strengthening superior to precipitation hardening in terms of mechanical compatibility.
Finally, it is worth summarizing the advantages of the proposed method from the perspective of the manufacturing industry. The caliber-rolling process is able to fabricate a bulk product with a length of 1 m or longer [40]. In addition, the process is basically a rod rolling, thereby possessing the manufacturing benefits of conventional rolling process, such as mass production and continuous fabrication. These characteristics assist the CCR process to be applicable in the industry, which is contrasted by most of severe plastic deformation processes. Meanwhile, the TNZ alloying system has been verified by the ASTM standard for decades [21], which could reduce the expenses of biomedical certification for CCR-TNZ.
5. Conclusions The developed CCR process broke the reported YM limit for TNZ alloys and decreased its value to the bare minimum among the data reported ever. It also enhanced YS of a material to ~1 GPa. The remarkable YM reduction and mechanical strengthening gave rise to the significant mechanical compatibility of CCR-TNZ. Such achievements were attributed to three factors. First, the CCR process suppressed the decomposition of αʹ martensite into (α + β) phase, avoiding the undesirable increase in YM due to hard α phase. Secondly, α″ martensites contributed to the YM reduction, which is theoretically supported by the e/a ratio of TNZ alloys. Sections of these martensites possessed an orientation relationship beneficial to YM reduction, which increased the atomic distance along the loading direction. Thirdly, a high frequency of defects caused an effective grain-boundary strengthening that made up for the absence of α-precipitation hardening. The mechanical improvement, low-cost benefits, and manufacturing advantages make this alloy a promising candidate for biomedical implant applications.
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2018R1C1B6002068). The authors gratefully acknowledge the assistance of Bando Titanium Co. Ltd. for the material costs information presented in Figure 2b.
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* Cold caliber rolling of Ti-13Nb-13Zr achieved markedly low Young's modulus. * Such a low Young's modulus is attributed to the formation of the α″ martensite phase. * Cold caliber rolling provides benefits in cost-saving and manufacturing advantages.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: