SYNTHESIS OF NANOSTRUCTURED TITANIUM BASED INTERMETALLICS BY MECHANICAL ALLOYING ZHANG FAN B. Eng., Shanghai Jiao Tong Univ., M. Eng., Central-South University of Technology A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2003 Acknowledgements I would like to take this opportunity to express my heartiest gratitude to the following people and organizations for their invaluable helps during my research works in the Department of Mechanical Engineering and Production, National University of Singapore. First of all I am most grateful to my advisors, Associate Professor Lu Li and Associate Professor Lai Man On, for giving me the opportunity to study in NUS as a research scholar for my degree, for their continuous and great support, guidance and encouragement throughout this project. My most sincere thanks go to Mr. Ho Yan Chee, a technician in NUS Work Shop 2, for his great helps in sealing of the encapsulation tubes for HIP samples. Without his help, it is unlikely that this thesis would have reached completion. Many heartfelt thanks are due to staff members in Materials Science Lab, Mr. Thomas Tan, Ng Hong Wei, Maung Aye Thein, Abdul Khalim Bin Abdul, Juraimi Bin Madon, Boon Hong, and Mdm. Zhong XiangLi, for their technical support and helps on various types of experimental equipment such as XRD, DTA, SEM, Planetary Ball Mill, Argon Chamber, HIP, Cold Presser, etc. I wish to thank Dr. Ma Qian for many helpful discussions when he worked in Materials Science Division, NUS. I also wish to thank Prof. Xie Shui Sheng and Dr. Liu Zhi Guo at Institute of NonFerrous Metals, China, for their helps in HIPing of the MAed powders. i Dr. W.A. Kaczmarek, and Australian National University were acknowledged for providing me a chance to a joint project on mechanochemical synthesizing of materials. During the course of this project, I am glad to have opportunities to collaborate with many of my classmates, Xue Wenbin, Zheng Qi, Li Rui, Liu Hui Liang, Sun Manlong, Kek Joon Kee, Tham Leung Mun, etc. I am really grateful to them for exchanging ideas and their helps in this project. I wish to express my gratitude to the National University of Singapore for offering me the Research Scholarship, which makes this work possible. There are still many other people not specified in the above list, but to whom I owe considerable gratitude, for their assistance in this project. Finally, I would like to express my heartfelt thanks to my wife, Weihua, for her great support, patience, encouragement, and tolerance of my busy work schedules over the years, and to my daughter, Wanyi, and my parents for their spiritual supports. This thesis is dedicated to them. ii Table of Contents Title Page i Acknowledgements iii Table of contents viii Summary Publications xi List of Figures xii List of Tables xx Introduction 1.1 History of mechanical alloying 1.2 Mechanical alloying process 1.2.1 Ductile/ductile system 1.2.2 Ductile/brittle systems 1.2.3 Brittle/brittle systems Mechanism of mechanical alloying 1.3.1 Diffusion 1.3.2 Chapter 1.3 1.4 1.5 1.6 Activation energy Milling Devices 10 1.4.1 Planetary ball mill 11 1.4.2 Horizontal ball mill 13 1.4.3 Shaker ball mill 15 1.4.5 Attritor 16 Milling conditions 17 1.5.1 Milling mode 18 1.5.2 Milling time 20 1.5.3 Milling atmosphere 21 1.5.4 Ball to powder weight ratio 23 1.5.5 Process control agent 24 Intermetallic compound: titanium aluminides 25 1.6.1 Ti3Al based alloys prepared by MA 27 iii 1.6.2 Chapter 2.1 2.2 2.3 Chapter 3.1 γ-TiAl based alloys prepared by mechanical alloying 30 1.6.3 TiAl3 prepared by mechanical alloying 34 Experimental Techniques 37 X-ray Diffraction (XRD) 37 2.1.1 Bragg’s Law 38 2.1.2 Structural characterization 39 2.1.3 40 Estimation of crystallite sizes and lattice strains Thermal analysis 42 2.2.1 Differential thermal analysis (DTA) 44 2.2.2 Reaction kinetics study using DTA 44 2.2.3 Thermogravimetric analysis (TGA) 46 Hot isostatic pressing (HIP) 46 Experimental procedures 49 Powder preparation 49 3.1.1 Al3Ti and γ-TiAl systems 49 3.1.2 TiN systems 50 3.1.3 Al-TiN composite 50 3.2 Powder characterization using XRD 51 3.3 Thermal analyses 52 3.4 Consolidation of Al3Ti powders 52 Formation of Al3Ti Intermetallic Compound via Mechanical Chapter 56 Alloying 4.1 Introduction 56 4.2 Experimental results 60 4.2.1 Contamination in the milled powders 60 4.2.2 Evolution of powder morphology 62 4.2.3 Evolution of powder mixing 71 4.2.4 Structural evolution 81 4.2.5 Thermal stability of MAed Ti-75Al powders 89 4.2.5.1 Group 1: samples after and hrs of milling 89 4.2.5.2 Group 2: samples after 10, 15 and 20 hrs of 94 iv milling 4.2.6 4.3 4.2.5.3 Group 3: samples after 30 and 40 hrs of milling 4.2.5.4 Reaction kinetics subsequent annealing 104 4.2.6.1 Evolution of grain size during ball milling 106 4.2.6.2 Evolution of grain size during thermal treatment 112 119 Reaction kinetics of ball-milled 75Al-25Ti powders during dynamic heating 4.3.1.1 4.3.1.2 133 Reaction kinetics in group (after 30 and 40 hour of milling) Ti-75Al powders 141 Mechanisms of grain growth prohibition in ball-milled 75Al-25Ti powders 4.4 Conclusions Chapter 119 Reaction kinetics in group (after 10, 15 and 20 hours of milling) Ti-75Al powders 4.3.1.3 119 Reaction kinetics in group (after and hours of milling) Ti-75Al powders 4.3.2 102 Evolution of grain size during ball milling and Discussions 4.3.1 99 141 148 Fabrication of Nanocrystalline TiAl via Mechanical Alloying 151 5.1 Introduction 151 5.2 Experimental results 155 5.2.1 Contamination during ball milling 155 5.2.2 Morphology and particle size of ball-milled Ti-58Al powders 5.2.3 5.2.4 157 Microstructures of ball-milled Ti-58Al and Ti-Al-2Mn2Nb powders 160 5.2.3.1 Ti-58Al powder mixtures 160 5.2.3.2 Ti-Al-2Mn-2Nb powder mixtures 164 Structural evolution of Ti-58Al and Ti-Al-2Mn-2Nb powder mixtures during ball milling v 169 5.2.5 5.2.6 5.2.7 5.3 5.2.4.1 Ti-58Al alloying system 169 5.2.4.2 Ti-Al-2Mn-2Nb alloying system 176 Evolution of grain size during milling 183 5.2.5.1 Ti-58Al system 183 5.2.5.2 Ti-48Al-2Mn-2Nb system 188 Phase evolution of the ball milled powders during thermal treatment 193 5.2.6.1 Ti-58Al binary system 193 5.2.6.2 Ti-48Al-2Mn-2Nb quaternary system 200 Grain growth during the heat treatment 204 5.2.7.1 204 Ti-58Al binary system Discussions 207 5.3.1 Formation of amorphous phase in Ti-Al system 207 5.3.2 Thermodynamic consideration of amorphization of Ti-Al 5.3.3 system 209 Amorphization of Ti-48Al-2Mn-2Nb quaternary system 212 5.4 Conclusions Chapter 215 Fabricating of TiN and Al-TiN composite via mechanochemical route 6.1 Introduction 216 6.2 Experimental results and discussion 219 6.2.1 Formation of TiN 219 6.2.1.1 Structural evolution during ball milling process 219 6.2.1.2 Thermal treatment of milled samples 225 Formation of Al-TiN metal-matrix-composite 229 6.2.2.1 Two step formation of Al-TiN 230 6.2.2.2 One step formation of Al-TiN 232 6.2.2 6.3 Conclusions Chapter 216 237 Mechanical properties of HIPed Al3Ti fabricated using 238 Mechanical Alloying 7.1 Introduction 238 vi 7.2 7.3 7.4 Chapter 8.1 Experimental results 240 7.2.1 Microstructure after HIPing 240 7.2.2 Mechanical properties of HIPed Al3Ti 246 7.2.2.1 Indentation fracture features 246 7.2.2.2 Mechanical properties 250 Discussions 251 7.3.1 MAed versus cast samples 251 7.3.2 Comparison between samples milled for and 30 hours 253 Conclusions 254 Conclusions and recommendations for future works 256 Conclusions 256 8.1.1 Synthesis of nanocrystalline Al3Ti via MA 256 8.1.2 Synthesis of nanocrystalline TiAl via MA 258 8.1.3 Synthesis of TiN and Al-TiN composite via mechanochemical route 8.1.4 Mechanical properties of HIPed Al3Ti fabricated using Mechanical Alloying 8.2 258 259 Recommendations for the future works 259 8.2.1 Atmospheric protections during MA processing 259 8.2.1.1 During ball milling process 259 8.2.1.2 After ball milling 260 8.2.1.3 During the powder sealing for HIPing 261 8.2.2 More detailed study in the Ti-75Al multilayer films 261 prepared by MA 8.2.3 More studies in mechanical properties of Al3Ti 262 Appendix 263 References 265 vii Summary In this project, the formation of nanocrystalline Al3Ti, TiAl, TiN, and Al-TiN composites via mechanical alloying were investigated. In Ti-75Al system, the Phase formation during mechanical alloying of Ti-75Al powder mixtures has been studied. An Al (Ti) solid solution by the diffusion of Ti into Al was formed at the early stage of the milling. The upper solubility of Ti in Al was determined as 6.7 wt.% in the present study. Further milling led to the formation of L12 ordered Al3Ti with a lattice parameter of 3.983 Å. It was observed that the formation of new phase occurred after the layer thickness of Ti/Al multilayer was refined to less than 40 to 100 nm (after 30 hours of ball milling). It was found that the interfacial reactions in the thin film Ti/Al stacks with the help of continuos supply of mechanical energy during ball milling was similar to that of sputtered Ti/Al films during heat-treatment. The kinetics of the formation of new phase was depended on the thickness of Ti/Al layer thickness during milling. Thermal stability of the mechanically alloyed Ti-75Al powders was systematically studied using differential thermal analyzer. The phase transformation during heat-treatment was detailed studied for powders after different duration of ball milling. The grain sizes and lattice strains during mechanical alloying of Ti-75 at.% Al powder mixtures were studied using X-ray diffraction methods. Nanocrystalline L12Al3Ti was obtained after a certain time period of ball milling. Minimum grain sizes of 17 nm for Al and 28 nm for Ti have been determined using XRD. During subsequent thermal annealing processing, an obvious recrystallization resulting in significant reduction of grain size was observed. The recrystallization in nanocrystalline Al3Ti viii was affected by both the temperature and the degree of order. The incubation period for recrystallization at 400°C was about hours while those at 510 and 700°C were about hours. The completion time of recrystallization in Al3Ti at 400 and 700°C was about 15 hours and hours at 510°C. It is clear that the recrystallization at 700°C was retarded as a result of the higher degree of order structure which limited the mobility of the boundaries. Phase transformation occurring within the recrystallization temperature range was observed to have little influence on the recrystallization itself. However, transformation products have significant effects on it, which is originated from the degree of order in the products. The recrystallization in this alloy system provides an excellent means to maintain the nanocrystalline microstructure during the necessary consolidation thermal cycle by decreasing the processing temperature and increasing the hold time considerably. The Ti-75Al powders milled for 30 hours were consolidated into bulk material using hot iso-static pressing (HIPing). The fracture toughness of such materials was estimated using Indentation method. The fracture toughness obtained from the MAed and HIPed powder compact samples was lower than that of the cast and homogenized bulk sample, regardless of the final grain size obtained. This could be due to the intrinsic brittle nature of Al3Ti, or due to the presence of flaws resulted from material processing which affected the test results. Compared with the 0-hour-milled sample, the 30-hour-milled sample exhibited significantly higher strength (~35% higher) and fracture toughness (~24% higher). This could be due either to the sample having fewer flaws after HIPing or the effect of grain size achieved after MA and HIPing. Binary Ti-58Al and quaternary Ti-48Al-2Mn-2Nb (at.%) alloys have been prepared by mechanical alloying. The effects of addition of alloying elements (Mn and Nb) on both the structural evolutions during mechanical alloying and thermal stability ix Appendix: Raw data of mechanical properties of HIPed samples Mechanical properties of 30-hour-milled and HIPed samples Points HV1.0 (kg/mm2) * L ( µm) K1C (MPa m1/2) 546.5 102.4 2.16 591.3 122.5 2.06 587.1 127.0 2.01 575.8 125.2 2.01 540.9 134.8 1.88 557.9 149.8 1.81 560.8 129.8 1.95 569.7 122.9 2.02 573.7 137.1 1.92 10 577.8 118.5 2.07 11 644.2 129.0 2.09 12 635.9 133.3 2.05 13 724.2 128.4 2.23 14 684.4 142.4 2.05 15 643.0 134.9 2.05 16 637.1 158.1 1.88 Average 603.1 131.01 2.01 STDEV 52.57 12.82 0.11 * L is the total length of the cracks at four corners of the indentations 263 Mechanical properties of 0-hour-milled and HIPed samples Series HV1.0 * L ( µm) K1C (MPa m1/2) 447.1 148.1 1.63 454.8 174 1.51 459.8 67 2.45 441.6 167.4 1.52 499.1 156.1 1.68 446.4** 96.2** 1.11 454.1** 60.9** 1.40 409.4 152.1 1.54 418.0 124.7 1.72 Average 447.8 127.4 1.62 STDEV 25.66 42.84 0.36 * L is the total length of the cracks at four corners of the indentations ** Indentation load is 300 gram, while the rest of the points, 1.0 kg 264 References Benjamin J.S. 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Mech., vol.28, pp.529-538. 1987. 278 [...]... micrograph of Ti-75Al powder after 1 hour of milling Fig 4.3 (c) SEM micrograph of Ti-75Al powder after 5 hour of milling Fig 4.3 (d) SEM micrograph of Ti-75Al powder after 10 hour of milling Fig 4.3 (e) SEM micrograph of Ti-75Al powder after 15 hour of milling Fig 4.3 (f) SEM micrograph of Ti-75Al powder after 20 hour of milling Fig 4.3 (g) SEM micrograph of Ti-75Al powder after 30 hour of milling... powders after 50 h of milling Fig 5.3 (a) SEM micrograph of cross-section of Ti-58Al powder mixture after 1 h of milling Fig 5.3 (b) SEM micrograph of cross-section of Ti-58Al powder mixture after 5 h of milling, at low magnification Fig 5.3 (c) SEM micrograph of cross-section of Ti-58Al powder mixture after 5 h of milling, at high magnification Fig 5.3 (d) SEM micrograph of cross-section of Ti-58Al powder... 7.5 h of milling, at low magnification Fig 5.3 (e) SEM micrograph of cross-section of Ti-58Al powder mixture after 7.5 h of milling, at high magnification Fig 5.3 (f) SEM micrograph of cross-section of Ti-58Al powder mixture after 15 h of milling, at high magnification Fig 5.4 (a) SEM micrograph of cross-section of Ti-Al-2Mn-2Nb powder mixture after 1 h of milling Fig 5.4 (b) SEM micrograph of cross-section... heat-treatment conditions Table 7.1 Occurrence of indentation crack with change in indentation load Table 7.2 Comparison of mechanical properties of different types of Al3Ti xxi Chapter 1 Introduction 1.1 History of mechanical alloying Mechanical alloying (MA) is a high energy ball milling process in which elemental powder mixture is milled to achieve true alloying at the atomic level In addition to elemental... stability of mechanically alloyed Ti-Al powders with additions of Mn and Nb”, Key Engineering Materials, vols.230-2: pp130-135, 2002 8 F Zhang, L Lu, M.O Lai, and F.H (Sam) Froes, “Grain Growth and Recrystallization of Nanocrystalline Al3Ti Prepared by Mechanical alloying Journal of Materials Science, vol.38, no.3, pp.613-619, 2003 9 F Zhang, L Lu and M.O Lai, “Structural changes and thermal stability of mechanically... micrograph of cross-section of Ti-Al-2Mn-2Nb powder mixture after 5 h of milling, at low magnification Fig 5.4 (c) SEM micrograph of cross-section of Ti-Al-2Mn-2Nb powder mixture after 5 h of milling, at high magnification Fig 5.4 (d) SEM micrograph of cross-section of Ti-Al-2Mn-2Nb powder mixture after 10 h of milling, at low magnification Fig 5.4 (e) SEM micrograph of cross-section of Ti-Al-2Mn-2Nb powder... mixture after 10 h of milling, at high magnification Fig 5.4 (f) SEM micrograph of cross-section of Ti-Al-2Mn-2Nb powder mixture after 15 h of milling, at high magnification Fig 5.4 (g) SEM micrograph of cross-section of Ti-Al-2Mn-2Nb powder mixture after 30 h of milling, at low magnification xvi Fig 5.4 (h) SEM micrograph of cross-section of Ti-Al-2Mn-2Nb powder mixture after 30 h of milling, at high... Ti-AL-Mn-Nb powders” (Accepted by Materials Research Bulletin, 2003) 10 F Zhang, L Lu and M.O Lai, “Interfacial reactions in multilayered Ti/75Al thin films prepared by mechanical alloying and its mechanical properties after consolidated by HIPing”, (to be submitted to Journal of Materials Research) xi List of Figures Fig 1.1 Chronological evolution of MA process Fig 1.2 Schematic view of events occurring during... SEM micrograph of Ti-75Al powder after 15 hour of milling, at high magnification Fig 4.9 (a) SEM micrograph of Ti-75Al powder after 20hour of milling, at low magnification Fig 4.9 (b) SEM micrograph of Ti-75Al powder after 20 hour of milling, at high magnification Fig 4.10 (a) SEM micrograph of Ti-75Al powder after 40 hour of milling, at low magnification Fig 4.10 (b) SEM micrograph of Ti-75Al powder... powder after 5hour of milling, at low magnification Fig 4.6 (b) SEM micrograph of Ti-75Al powder after 5 hour of milling, at high magnification Fig 4.7 (a) SEM micrograph of Ti-75Al powder after 10 hour of milling, at low magnification Fig 4.7 (b) SEM micrograph of Ti-75Al powder after 10 hour of milling, at high magnification Fig 4.8 (a) SEM micrograph of Ti-75Al powder after 15 hour of milling, at low . SYNTHESIS OF NANOSTRUCTURED TITANIUM BASED INTERMETALLICS BY MECHANICAL ALLOYING ZHANG FAN B. Eng., Shanghai Jiao Tong Univ., M. Eng., Central-South University of. compound: titanium aluminides 25 1.6.1 Ti 3 Al based alloys prepared by MA 27 iv 1.6.2 γ-TiAl based alloys prepared by mechanical alloying 30 1.6.3 TiAl 3 prepared by mechanical alloying. i Table of contents iii Summary viii Publications xi List of Figures xii List of Tables xx Chapter 1 Introduction 1 1.1 History of mechanical alloying 1 1.2 Mechanical alloying