2016 International Conference on Sustainable Energy, Environment and Information Engineering (SEEIE 2016) ISBN:

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1 2016 International Conference on Sustainable Energy, Environment and Information Engineering (SEEIE 2016) ISBN: Strengthening Effect of N-addition on Ti-Si-N Ternary Alloys Fabricated by Spark Plasma Sintering P. KHEMGLAD 1, J. KAJORNCHAIYAKUL 2, K. KONDOH 3 and A. KHANTACHAWANA 4,* 1 Department of Mechanical Engineering, Faculty of Engineering, King Mongkut s University of Technology Thonburi, 126 Pracha Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand 2 National Metal and Materials Technology Center, Pathumthani 12120,Thailand 3 Joining and Welding Research Institute, Osaka University, Osaka, Japan 4 Department of Mechanical Engineering and Biological Engineering Program, Faculty of Engineering, King Mongkut s University of Technology Thonburi, 126 Pracha Uthit Rd., Bang Mod, Thung Khru, Bangkok 10140, Thailand *Corresponding author Keywords: Ti-Si-N ternary alloys, Spark plasma sintering, Substrate powders. Abstract. In the present work, the strengthen mechanism of Ti-Si-N ternary alloys prepared by Spark Plasma Sintering (SPS) is investigated. Ti, Si and TiN substrate powders were prepared in order to obtain nominal composition of Ti-0.35Si and Ti-0.35Si-1TiN (wt %). Homogenization was performed before extrusion. Microstructure was analyzed by Optical Microscope (OM) and X-ray diffraction (XRD). In order to evaluate the mechanical properties of extruded specimens, micro hardness test and tensile test were carried out. It is found that the abnormal phase with high N- content was observed in matrix phase of Ti-0.35Si-1TiN alloy. It is also obvious that addition of 1TiN into Ti-0.35Si (wt %) can increase yield stress and ultimate tensile stress from 859 to 1006 MPa and 914 to 1089 MPa, respectively. Hence, the strengthening mechanism of this alloy is not a solid solution mechanism, but it is due to the abnormal phase with high N content. Introduction Now, Titanium (Ti) alloys is popularly used in engineering work due to their high strength per weight ratio. Especially, it is used as implant material due to high corrosion resistance and good biocompatibility [1-3]. However, inadequate strength can cause to fracture the implant [1] so, the bone replacement material have to further development in strength of material. Silicon (Si) and Nitrogen (N) are biocompatibility element which can strengthen to Ti alloys [4, 6] by difference solid solution mechanism. Si can strengthen Ti alloys by substitutional solid solution mechanism but N can strengthen Ti alloys by interstitial solid solution mechanism. Now, there are many study on binary alloys system of Ti-Si and Ti-N which Si and N can narrowly solid solution strengthen to Ti binary alloys. From difference of solid solution mechanism, it s interesting to study effect of additive these two element into Ti alloys simultaneously. However, there is still no publication investigated the effect of these two element in Ti alloys. Spark Plasma Sintering (SPS) is a process where the substrate is in the form of powder. This process applies pressure and heat to metal powder for welding between particles. All of the processes, the powders is in solid state [5] because the temperature used in SPS is lower than the melting point, so it possible to limit the growth of grain which affect to mechanical properties of the materials. This research interested to investigate the strengthening mechanism of Ti-Si-N ternary alloys with adding of TiN fabricated by SPS. 562

2 Experimental Pure elemental powders of Ti (99.5%, average size 22 μm), Si (99.9%, average size 5 μm) and intermetallic compound powder of TiN (average size 1-2 μm) were mixed to obtain nominal compositions, Ti-0.35Si and Ti-0.35Si-1TiN by Table Milling with 120 rpm rolling speed for an hour followed by Rocky Milling with 60 Hz shaking for 3 hour. The total mass of each powder sample was about 150 g. The raw material powders were then put into plastic bottle with Zirconium dioxide (ZrO 2 ) balls (10 mm in diameter). The balls to samples weight ratio was 5:1. After mixing, the mixed powders was separated from ZrO 2 balls. The mixed powders was consolidated to cylindrical shape (42 mm in diameter) in Spark Plasma Sintering (SPS) machine by applied pressure of 30 MPa at 1000 C for an hour. The SPS samples was heat treatment at 1000 C for 3 hour in vacuum (less than 50 Pa). The pre-heating was performed at 1100 C for 5 minute in Argon (Ar) flowed before extrusion to 12 mm diameter of rod. Microstructure was observed by Optical Microscope (OM) and X-ray diffraction (XRD) after each processes. The mechanical properties of extruded sample was evaluated by Vickers Micro Hardness test and tensile test. Micro hardness test was performed 20 times per a sample then values was averaged. Tensile test using crosshead speed of 0.3 mm/min was performed 3 samples per a condition. Then 0.2% offset yield stress and ultimate tensile stress values was averaged. Results and Discussion Microstructure Figure 1 shows XRD profiles of mixed powders after Rocky Milling. The peaks are basically similar to those of pure Ti, however, small peaks of Si can be observed in both Ti-0.35Si and Ti- 0.35Si-1TiN specimens and small peaks of TiN can be observed in Ti-0.35Si-1TiN specimen. This can be confirmed the existence of Si and TiN particles. Figure 1. X-Ray Diffraction Patterns of Rocky Milled Ti-0.35Si and Ti-0.35Si-1TiN Powders. Figure 2 shows XRD profiles of Ti-0.35Si and Ti-0.35Si-1TiN specimens after SPS, heattreatment and extrusion, respectively. It is seen that there is no peak from Si and TiN particle after SPS process. It is able to said that Si and N dissolve into -Ti matrix of Ti-0.35Si and Ti-0.35Si- 1TiN alloys by SPS process which is relatively low temperature. Figure 2. X-Ray Diffraction of Ti-0.35Si and Ti-0.35Si-1TiN samples after Spark Plasma Sintering, Heat Treatment and Extrusion. 563

3 Moreover, XRD results of specimens after heat-treatment and extrusion processes did not show any peaks from intermetallic compounds or precipitations except only from Ti peak. This is confirmed that no reaction taken place among Ti, Si and N after these two processes. In this study, microstructure was observed by Optical Microscope after each process. Figure 3 shows optical micrograph from Ti-0.35Si and Ti-0.35Si-1TiN specimens after SPS, heat-treatment and extrusion, respectively. Ti-0.35Si specimen shows needle-liked structure in laminar grain boundary after SPS process. This phenomenon can be explained by the phase change of -Ti structure to -Ti structure at relatively low temperature during SPS process because of effect of Si content, which is -stabilizer to Ti alloys [7]. On the other hand, Ti-0.35Si-1TiN specimen shows equiaxed grain structure affected by N content which is - stabilizer to Ti alloys. Moreover, there are some abnormal areas observed in grain boundary of Ti-0.35Si-1TiN specimens under this area. This is caused from high N content dissolved in this area more than another area [6]. After heat-treatment, Ti-0.35Si-1TiN alloy shows more uniform structure compared with those of before heat-treatment. However small size of abnormal area can be detected near the grain boundary (Figure 3d). This abnormal area could be detected more after extrusion process. (Figure 3f) Figure 3. Microstructure of Ti-0.35Si and Ti-0.35Si-1TiN alloys after (a, b) Spark Plasma Sintering, (c, d) Heat Treatment and (e, f) Extrusion. Mechanical Properties Mechanical properties were evaluated by Vickers micro hardness test and tensile test. Figure 4 shows the micro hardness of Ti-0.35Si and Ti-0.35Si-1TiN specimens after SPS process. It is obvious that with adding 1TiN into Ti-0.35Si alloy, hardness increases from hardness from to Hv. Figure 5 shows 0.2%-offset yield strength, ultimate tensile strength and percentage of elongation of Ti-0.35Si and Ti-0.35Si-1TiN specimens after SPS process. This is also clearly seen that yield stress and ultimate tensile stress increase from 859 to 1006 MPa and 914 to 1089 MPa, respectively with adding 1TiN into Ti-0.35Si (wt %). On the other hand, elongation decreases from 27.7 to 8.8% with adding 1TiN into Ti-0.35Si (wt %). This can be mentioned that specimen become brittle after adding TiN, resulting in a second phase which contain high N content and strengthen the material. 564

4 Figure 4. Vickers Micro Hardness of Ti-0.35Si and Ti-0.35Si-1TiN. Figure 5. Tensile Test of Ti-0.35Si and Ti-0.35Si-1TiN. Conclusions Strengthening mechanism of Ti-0.35Si (wt %) powder alloy by adding 1TiN was investigated and clarified. It is found that Si can completely dissolve into -Ti uniformly and TiN particle can perfectly decomposed after applying pressure of 30 MPa at 1000 C during SPS process. However, N could not diffuse uniformly and completely since abnormal phase with high N-content was detected after heat-treatment. It is also be able to summarized that with adding 1TiN into Ti-0.35Si alloy, hardness increases from hardness from to Hv while that yield stress and ultimate tensile stress increase from 859 to 1006 MPa and 914 to 1089 MPa, respectively. Acknowledgments This work is supported by the Thailand Graduate Institute of Science and Technology (TGIST) National Science and Technology Development Agency contract number TGIST and Research and Researcher for Industry (RRI) Fund, and also Joining and Welding Research Institute, Osaka University, Osaka, Japan. References [1] M.B. Nasab and M.R. Hassan, Metallic Biomaterials of Knee and Hip - A Review, Department of Mechanical and Manufacturing Engineering, Engineering Faculty University Putra Malaysia (2010). [2] M. Long and H.J. Rack, Titanium alloys in total joint replacement a materials Science perspective, School of Chemical and Materials Engineering, Clemson University, Clemson, SC 29634, USA. 565

5 [3] M. Niinomi, Recent research and development in titanium alloys for biomedical applications and healthcare goods, Department of Production Systems Engineering, Toyohashi University of Technology, 1-1, Hibarigaoka, Tempaku-cho, Toyohashi , Japan. [4] H.C. Hsu, S.C. Wu, S.K. Hsu and et al., Structure and mechanical properties of as-cast Ti-Si alloys, Department of Dental Technology and Materials Science, Central Taiwan University of Science and Technology, Taichung, Taiwan, ROC (2014). [5] S.L. Kang, Sintering Densification, Grain Growth & Microstructure, 1st ed., Elsevier Butterworth-Heinemann, (London, 2005) pp [6] T. Mimoto, J. Umeda and K. Kondoh, Titanium Powders via Gas-Solid Direct Reaction Process and Mechanical Properties of Their Extruded Materials, Department of Mechanical Engineering, Graduate School of Engineering, Osaka University, (2015). [7] B. Predel, Phase Equilibria, Crystallographic and Thermodynamic Data of Binary Alloys, Vol.5J, Landolt-Börnstein - Group IV Physical Chemistry, (1998) pp