Influence of Niobium or Molybdenum in Titanium Alloy for Permanent Implant Application Yuswono Marsumi 1, a and Andika Widya Pramono 1,b

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1 Advanced Materials Research Vol. 900 (2014) pp (2014) Trans Tech Publications, Switzerland doi: / Influence of Niobium or Molybdenum in Titanium Alloy for Permanent Implant Application Yuswono Marsumi 1, a and Andika Widya Pramono 1,b 1 Research Centre for Metallurgy Indonesian Institute of Sciences Kawasan PUSPIPTEK Building 470, Serpong Indonesia a yuswono_2005@yahoo.com, b andika_pram@yahoo.com Keywords: Ti-6%Al-6%V alloy, Ti-6%Al-6%Mo/Nb alloy, phase phase stabilization Abstract:Titanium (Ti) alloy metal has been used for permanent implant in the human body. Its high strength and hardness take place due to the phase formation at room temperature. Ti-6%Al- 6%V alloy is most popular, of which vanadium (V) content is used as the phase stabilizer. However V can induce allergic reaction from the body. V can be substituted by other element, such as Mo and Nb. Microstructure observations for Ti-6%Al-6%Mo and Ti-6%Al-6%Nb alloys show that phase exists as matrix having good workability at room temperature. After preheat at 1000 o C, no cracking failure occurs during forging and rolling treatment. Thermal spray method is used for CaPO 4 surface treatment. A CaPO 4 layer on the alloy substrate forms after the molten CaPO 4 is hot sprayed on to the alloy substrate surface. Corrosion test results indicate that the increase in Mo or Nb content up to 6% leads to the increase in corrosion resistance. Introduction Metals or alloys have been used for implant materials in the human body because of their optimum mechanical properties, such as good ductility and high strength. Those properties are not entirely owned by polymers and ceramics. Ceramics materials possess high strength but exhibit significant brittleness, whereas polymers are ductile but low in strength. Nevertheless not all metal alloys can be used as implants permanently, considering that the blood plasma is corrosive and in direct contact to the metal surface. Stainless steel 316 L, for instance, may not be used for a permanent metallic implant since it corrodes in the blood plasma. There are two types of metal alloys used for permanent implant, i.e. titanium (Ti) and cobalt (Co) alloys. They can be accepted by the body for a long period of time, and even the hard or soft tissue can grow on their surfaces. Ti alloys have been more utilized for implant than Co alloys and non-metallic materials do. Besides their mechanical properties, the price of Ti metal as a raw material is lower than that of Co metal. Therefore, Ti-alloy-based implants dominate about 60% of the implant materials. The most popular Ti alloy is Ti-6% Al-6%V. Aluminum (Al) content of 6% is used as reinforcing element, due to precipitation of intermetallic phases TiAl 3 [1]. Vanadium (V) content is used for forming the phase. However this alloy is not good when it is used for implant components, because of the content of V. Allergic reaction of the body occurs due to the presence of V [2]. On the other hand the formation of phase by addition of V in the microstructure of Ti alloys can strengthen the alloys up to three times higher than the pure Ti metal does. To overcome this problem, the function of V should be substituted with other types of metal elements. The purpose of this research is to observe the influence of molybdenum (Mo) and niobium (Nb) in substituting V for forming or stbilizing the phase. Further, Mo or Nb contents in Ti alloys may not cause allergies in the body. In addition, the influence of Mo or Nb contents can increase strength and hardness, formabilities (roll and forging treatment), as well as corrosion resistance. In order to stimulate tissue growth on the Ti metal alloys, CaPO 4 has been applied for surface treatment. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /02/14,09:14:38)

2 54 Advanced Materials and Processing Technologies: IFMPT 2014 Background Theory a. Crystal Structure of Titanium Titanium as a pure metal has hcp crystal structure (hexagonal closed packed, phase) and bcc crystal structure (body center cubic, phase). Figure 1 shows an illustration of the crystal structures of titanium. The phase formed in pure Ti is unstable at room temperature, except in the Ti alloys such as alloy Ti-6% Al-6%V. Because of V content, such phase becomes stable at room temperature. Fig. 1: Description of the crystal structure of titanium (a) (hcp), (b) (bcc) [3,4]. The present of phase and phase in the Ti alloy can be used as a reference for the production of titanium alloys. The classification of Ti alloys has been divided into: 1. Ti alloy whose matrix is phase; and 2. Ti alloy, of which two phases ( and ) co-exist. Transformation phase (hcp) to phase (bcc) as matrix occurs during heating. It means that Ti alloys have good hot workabilities (rolling and forging), because of one-way direction of slip motion on a cubic crystal structure during deformation. Nevertheless this situation may rise the risk of failure due to cracking [5]. b. Titanium Alloys Ti alloy Srengthening of Ti alloy is caused by solid solution of Al, Ni, and Cu content in matrix [4]. Ti alloys with hcp crystal phase matrix are less ductile and more difficult to hot work treatment, due to limited slip movement in the hcp crystal structure during deformation [5]. The increase in the mechanical properties is hardly obtained through heat treatment. Ti alloy Ti alloy consists of and phases at room temperature. The formation of phase happens due to the presence of element that acts as phase stabilizer. In Ti-6%Al-4%V alloy, for example, V content is as stabilizer at room temperature. Ti alloys are high in strength and corrosion resistance. With the presence of phase in Ti alloys (bcc crystal structure), Ti alloys become easy to fabricate through hot forming treatment [4]. Hot forming treatment principle of Ti-6%Al-V alloys can be seen in the phase diagram (Figure 2). There are areas that limit the co-formation of and phases.

3 Advanced Materials Research Vol Fig. 2: Influence of V content in the alloy Ti- 6%Al to the formation of and phase Experimentation c. Design of Composition The initial composition is Ti-6%Al. To determine the effect of Mo and Nb, the amount of Mo or Nb added is varied, i.e.: 2%, 4%, and 6%. After melting and casting, the sub-sequent processes are homogenizing and hot work treatment (rolling and forging). After the hot work treatment, the obtained specimen is observed for microstructure, mechanical properties, corrosion resistance, as well as surface coating. Raw Materials Titanium (Ti) is used as basic metal, whereas aluminum (Al), molybdenum (Mo) and niobium (Nb) as alloying elements. Purity of each element is 99% Ti, 98% Al, 99.9% Mo, and 99.9% Nb. The prepared raw materials are shown in Figure 3a. Figure 3b is CaPO 4 powder used for surface coating. (A) (B) Fig. 3: a) Raw material for making specimens of Ti alloys, b) CaPO 4 powder for surface coating. d. Making of Ti-6%Al-Mo/Nb Alloy Specimens Ti has a high melting point of 1812 o C. Mo and Nb metals have also high melting points, i.e.: 2623 o C and 2477 o C respectively. Since the base metal and alloying elements have relatively

4 56 Advanced Materials and Processing Technologies: IFMPT 2014 high melting point, then the electric arc furnace is used for melting process. Ti metal along with the alloying elements (Mo, and Nb) has a high affinity for oxygen. The liquid phase of these metals is very easily oxidized when in contact with oxygen in the air. Therefore the melting can only be done within the chamber enclosed in an inert environment, i.e. argon gas environment. The crucible for molten metal utilized is pure copper with water-cooling circulation. The weight of specimen after the arc-melting is about 6 gram. e. Specimens Characterization Testing of specimens is conducted as follows: o Microhardness test and tensile test; o Corrosion test, of which corrosion reaction is accelerated in the Hank solution [6,7,8] using potentiostate equipment followed by EDAX/SEM observations on the alloy surface. Forging and rolling treatment are conducted to observe hot formabilities or workabilities. Preheating temperature of Ti-6%Al-Mo/Nb refers to the phase diagram of Ti-6% Al-V (Figure 2). By heating the specimens at 1000 o C, phase (hcp) is expected to transform into phase (cubic crystal). The underlying reason is that cracking during rolling and forging can be avoided if the cubic crystal dominates. Optical microscope is used to observe microstructures, of which and phase formed at room temperature. CaPO 4 is embedded onto the surface of specimens using plasma thermal spray method, followed by XRD analysis. Results And Discussion a. Arc-Melt Specimens and Chemical Composition Analysis The as-cast specimens of Ti-6% Al-6% Mo and Ti-6% Al-% Nb alloys are shown in Figure 4. The specimens have been homogenized at a temperature of 1100 o C. The corresponding chemical composition analysis is shown in Table 1. Ti-6%Al-6%Mo Ti-6%Al-6%Nb Fig. 4: Specimens of Ti-6% Al-6%Mo (left) and Ti-6% Al-6%Nb (right) after melting with the arc furnace in the inert argon gas environment.

5 Advanced Materials Research Vol Tabel 1. Chemical Composition Analysis of Ti-Al-Mo dan Ti-Al-Nb alloys Alloying elements Al (%) Mo (%) Nb (%) Ti (%) (% weight) Ti-Al bal Ti-Al-Mo bal ,93 - bal bal Ti-Al-Nb 5, bal 5, bal 5, bal b. Hot Forging and Roll No crack failure has been observed during roll and forging treatment for all specimens after preheating at 1000 o C, indicating that Ti-6%Al-Nb/Mo have sufficiently good formabilities (Figures 5 and 6). This situation happens apparently due to phase transformation to phase, of which the latter one has a cubic crystal lattice at temperature of 1000 o C. Fig. 5: Ti-6%Al-6%Mo (left) and Ti-6%Al- 6%Nb (right) plate specimens with 1 mm thickness after forging by 80% reduction. Fig. 6: Ti-6%Al-6%Mo (left) and Ti-6%Al- 6%Nb (right) plate specimens with 1 mm thickness after rolling by 80% reduction. c. Microstructures of Ti-6%Al and Ti-6%Al-Mo/Nb Alloys Microstructure Observation after Hot Working at 1000 C Microstructure of Ti-6%Al after forging previously heat treated at 1000 C is shown in Figure 7. Its matrix is phase, where precipitation of intermetallic TiAl 3 appears as black spots or circles indicated by arrows. TiAl 3 precipitation in the matrix acts as reinforcement to increase hardness and strength of Ti alloys. Their hardness (356 HV) and strength (702 MPa) are higher than those of pure Ti.

6 58 Advanced Materials and Processing Technologies: IFMPT 2014 Fig. 7: Microstructure of Ti-6%Al after forging previously heat treated at 1000 o C. The main matrix is phase. Etching: solution of 3% HF. Microstructure of Ti-6%Al-Mo alloys after forging previously heat treated at 1000 C is shown in Figure 8. At 2%Mo, the matrix is phase (bright areas in Figure 8a). At 4%Mo, its matrix consists of (bright) and (dark) phases (Figure 8b). 6%Mo stabilizes the phase at room temperature (Figure 8c). When phase is formed at temperature of 1000 o C, it can partially transformed to phase during cooling. grains are much smaller (Figure 8c) compared with those of 2%Mo and 4%Mo. The formation of phase at room temperature increases hardness and srength of Ti alloys. Microstructure of Ti-6%Al-Nb alloys is shown in Figure 9. At 2%Nb, large-grain phase matrix is formed (Figure 9a). Increasing Nb content up to 4% results in more elongated and thinner grains of phase (Figure 9b). At 6%Nb, its matrix is still phase with significantly small elongated grains (Figure 9c). This situation indicates that the addition of Nb does not significantly affect the formation of phase.

7 Advanced Materials Research Vol (A) (B) Fig. 8: Microstructure Ti-6% Al-Mo alloys after forging previously heat treated at 1000 o C: (a) Ti-6%Al-2%Mo, (b) Ti-6%Al- 4%Mo, (c) Ti-6%Al-6%Mo. Its structure consists of phase (bright) and phase (dark). Etching: solution of 3% HF. (C) (A) (B) Fig. 9: Microstructure of Ti-6%Al-Nb alloys after forging previously heat treated at 1000 o C: (a) Ti-6%Al-2%Nb, (b) Ti-6%Al- 4%Nb, (c) Ti-6% Al6%-6%Nb. Its structure consists of phase grains. Etching: a solution of 3% HF. (C)

8 60 Advanced Materials and Processing Technologies: IFMPT 2014 Microstructure of Ti-6%Al-Nb after Heat Treatment at 1100 C To further observe the influence of Nb on the phase formation, specimens are heated at 1100 o C followed by air-cooling. Figure 10 shows the change in matrix morphology. At 2%Nb, the matrix is phase (Figure 10a). At 4%Nb, the matrix is still phase in majority (Figure 10b). The phase is significantly formed at 6%Nb content (Figure 10c). It is then clear that Nb can act as phase stabilizer during heat treatment at 1100 o C. (A) (B) Fig. 10: Microstructure of Ti-6% Al-Nb alloys after heat treatment at 1100 o C followed by air cooling: (a) Ti-6%Al-1%Nb, (b) Ti-6%Al-4%Nb, (c) Ti-6%Al-6%Nb. Its structure consists of phase (bright) and phase (dark). Etching: a solution of 3% HF. (C) d. Microhardness Measurement Hardness Measurement after Hot Forging Hardness measurement is conducted after forging treatment preceded by heat treatment at 1000 o C. Increasing Mo and Nb contents up to 6% in Ti-6%Al leads to an increase in hardness (Figure 11). This means that Mo and Nb contents affect the hardness of Ti-Al- Mo/Nb alloys. In Figure 11, the increase in hardness is associated with the formation of phase (Figures 8a, 8b, 10a, and 10b). At 6% Mo, the hardness reaches its highest value of 456 HV with the strength of 1021 MPa. According to Figure 9c, the addition of Nb up to 6% does not significantly induce the formation of phase. Its hardness is even lower than that of the 6% Mo content. Hardness Measurement after Heat Treatment The role of Nb content predominates after reheating at 1100 o C followed by air-cooling. The hardness and strength significantly increase up to 642 HV (Figure 12) and 1400 MPa respectively. It suggests that the formation of phase at room temperature affects the increased hardness and strength of Ti alloys. By air cooling from 1100 o C, the majority of phase stays untransformed, so that phase dominates matrix at room temperature (Figure 10c).

9 Advanced Materials Research Vol Fig. 11. Influence of Mo and Nb contents in Ti-6%Al alloy on the increase in hardness after hot forging previously heat treated at 1000 o C. Fig. 12. Influence of Nb content in Ti-6% Al on the increase in hardness after: 1) hot forging previously heat treated at 1000 o C (lower curve), and 2) heat treatment at 1100 C followed by air cooling (upper curve). e. CaPO 4 Surface Treatment for Ti-6% Al-Mo Alloy CaPO 4 powder was coated on to the surface of Ti alloy using thermal spray method. Strong bonding between the layer of CaPO 4 and the alloy substrate occurs due to the very high-speed and hot spray of CaPO 4 onto the alloy surface. CaPO 4 embeds in to the surface Ti alloys, as shown in Figure 13. Result of XRD test (Figure 14) shows that high speed liquid CaPO 4 has ability to absorb water in the air to form hydrate compound. Apex angle 2 curve between 30 o -32 o indicates the hydrate compound of hydroxy apatite Ca 5 (PO 4 ) 3 OH. Figure 13. CaPO 4 layer on the surface of Ti alloy after thermal spray.

10 62 Advanced Materials and Processing Technologies: IFMPT 2014 Fig.14: XRD analysis of CaPO 4 layer on the surface of Ti alloy after thermal spray. Curve apex angle 2 between 30 o -32 o is compound of Ca 5 (PO 4 ) 3 OH. f. Corrosion Test Influence of Mo and Nb content on the corrosion rate is shown in Figure 15. The increase in Mo and Nb contents up to 6% reduces the corrosion rate. Corrosion rate of 0.05 mpy was accordance with the standards of Co alloys [7,8]. Fig. 15. The effect of Mo and Nb content on the corrosion speed measured using polarization test equipment. Conclusion o Vanadium in the Ti-6%Al-6%V alloys that acts as phase stabilizer can be substituted with Mo and Nb elements o 6% Mo and 6%Nb contents in the alloy Ti-6%Al act effectively as phase stabilizers.

11 Advanced Materials Research Vol o Ti-6%Al-6%Mo and Ti-6%Al-6%Nb have good hot workability with no crack presence during forging and roll processes. o The increase in hardness and strength is affected by Mo or Nb which act as the phasestabilizer. o Ti alloys can be coated with a non-metallic material using thermal spray method. Through this method, the molten hydroxy apatite (Ca 5 (PO 4 ) 3 OH) is embedded on to the surface of Ti alloy creating a strong bond between the coating material and the alloy substrate. o 6%Mo and 6%Nb contents increase the corrosion resistance of Ti alloy. Rererences [1] Hashimoto, K.; Kimura, M.; and Mizuhara, Y.: Alloy design of gamma titanium alumunides based on phase diagrams, Intermetallics, Vol. 6, Issues 7-8, pp , [2] Niinomi, M.: Recent metallic materials for biomedical applications, Metallurgical and Materials Transactions A. Vol. 33A, pp , March [3] ASM Handbook: Titanium a technical guide, Donachie, Matthew J. Editor, ASM International [4] Metals Handbook: Properties and selection: nonferrous alloys and special-purpose materials, Davis, Joseph R. Editor, Vol. 2, 10th edition ASM International [5] Dieter, G. E.: Mechanical metallurgy, International Student Edition, Second Edition, McGraw-Hill Series in Material Sceince Engineering. Tokyo, Auckland, copyright [6] ASM Handbook: Materials for medical devices, Vol. 23, Narayan, Roger J. editor, ASM International, [7] Gurappa, I.: Characterization of Different Materials for Corrosion Resistance under Simulated Body Fluid Conditions, Materials Characterization 49, pp , [8] Hiromoto S.; Onodera, E.; Chiba, A.; Asami, K.; and Hanawa, T.: Microstructure and corrosion behaviour in biological environments of the newforged low-ni Co-Cr-Mo alloys, Biomaterials, Vol. 26, Elsevier - Science Direct, pp , 2005.