Introduction Chapter Introduction 1.1 The challenges and motivation of this project Magnesium, one of the lightest structural metals, has a density of 1.74 g cm-3 which is respectively 35.6% and 61.3% lower than that of Al and Ti [1]. Weight saving possibilities from the application of Mg in structural parts has prompted intensive research, especially in the automotive and aerospace industries. Due to its low density, high strength-to-weight ratio and high specific stiffness at both ambient and elevated temperatures, Mg and Mg alloys are attractive choices among these three light weight metals. Good processing, machining and recycling possibilities are the additional advantages of Mg alloys [2]. In some applications, Mg shows poor corrosion resistance and low strength and poor creep resistance at elevated temperature. Sand cast unalloyed 13 mm diameter Mg at 20˚C shows yield strength of 90 MPa and 2-6% of elongation. [3]. This limitation shrinks the range of applications of Mg. Therefore, the enhancement in mechanical properties through proper control and manipulation of the structures is critical to broaden its applications. Mg is usually alloyed with other metals such as Al, Zn, Mn, Li, Y and rare earth elements or used as composites [4]. It was found that Mg alloy with wt% Al provides the optimal ductility and that with wt% Al produces the optimum combination of strength and ductility [3]. However, the solubility of alloying elements in Mg is limited, restricting the possibility of improving mechanical properties and chemical behavior. Enhanced mechanical properties have been reported in Mg composite reinforced with nanoscaled ceramic particles such as SiC [5], Al2O3 Introduction [6] and AlN [7]. The demand for greater performance of Mg alloys or composites leads to the development of Mg-based metal matrix composite (MMC) with nanostructure reinforced by in-situ ceramics [8]. Composites not only have the combined properties of their constituents, they also can be tailored to offer improved properties to meet different engineering requirements. Ductility is determined by the number of operating slip systems. The main deformation mode in magnesium and magnesium alloys is basal slip, i.e. slip on the (0001) plane with a 11 0 Burgers vector and secondary slip which is the prismatic slip on {10 0} in the 112 0 direction. This limits the inherent ductility of Mg at low temperatures. At elevated temperatures, the pyramidal slips {10 0} 1120 have also been observed, but their critical resolved shear stress at room temperature is roughly a 100-fold greater than for basal slip [9]. The limited number of operating slip systems makes Mg extremely orientation dependent and low ductility. As the production of Mg die-castings for automotive applications increases and environmentally approved disposal costs rise [10], the recycling of in-house scrap and post consumer scrap plays an increasingly important role in the supply of magnesium in the future. Mg chips and scraps have been recycled by consolidation followed by hot extrusion [11,12]. It was found that MM can provide an alternative cheaper way of recycling the Mg scraps as nc powders can be produced directly from the solid state without going through the melting process which is very expensive and environmentally harmful. MM is one of the most impressive methods for reducing grain size and producing nc powders [13,14]. It is also well documented that the Introduction mechanical properties of Mg alloys, especially yield strength and ductility, strongly depend on the grain size due to the large Taylor factor of Mg [15]. Bulk nc materials are normally produced from MMed fine grained powders through secondary processes, for example, conventional PM techniques consisting of mechanical consolidation, sintering and extrusion. Mechanical testing however often exhibits controversial results and the understanding on deformation behavior of fine grained/nc materials consolidated from MMed powders has yet to be fully comprehended. An increase in strength of the MMed materials is usually and inherently accompanied with the sacrifice of ductility. Although mechanical alloying (MA)/MM processes are effective in grain refinement, defects are introduced inevitably during processing such as contaminants, oxide particles and residual porosity which are detrimental to the mechanical properties. The major sources of contamination are (i) the milling media (balls and vial), (ii) process control agent (PCA) and (iii) atmosphere during processing. Residual porosity may result from poor interparticle bonding during consolidation and sintering. Mechanical properties are influenced by microstructural features including grain size, shape, pores and their distribution, flaws, surface condition, impurity level, second phase/dopants, stress, duration of its application and temperature, and crystal defects. To minimize and control the level of defects and contaminants without sacrificing material properties is the major challenge in this MM process. Introduction Unique properties of the nc materials with grain size less than 100 nm have attracted intensive research in recent years. Both types of the nc materials which comply with Hall-Patch relationship and those deviate from this relationship (“inverse Hall Petch”) have been reported in the literatures. A number of investigators have suggested that the inverse Hall–Petch relation can be attributed to the increased grain-boundary activity due to grain-boundary sliding and/or diffusional mass transfer via grain-boundary diffusion. As such, it is essential to understand and investigate the mechanical behaviors in fine grained materials consolidated from MMed or alloyed powders. Unfortunately, only limited fundamental information is available in the literature regarding the microstructural evolution and deformation behaviors of Mg composite consolidated from MMed powders. A fundamental understanding on bulk Mg composites via MM at different milling durations will provide the opportunity to understand how these materials are sensitive to processing parameters and grain size effect on their deformation behaviors. Contamination during milling and grain growth during secondary process of conventional pressureless sintering are the main concerns in this synthesis process. Normally contamination is more severe with longer milling duration but it can be controlled to maintain at an acceptable level by shortening the milling duration. It is important to optimize the processing temperature to avoid excessive grain growth without sacrificing the good bonding of the particles which in turn affects the mechanical properties. It is one of the objectives of the present project to find out the optimum primary and secondary processing parameters which produce the optimum combination of strength and ductility of the nc Mg composite. Introduction Grain refinement, reinforcement particles, dislocation and solid solution contribute to the strengthening of Mg composite consolidated from MMed powders. In nanoscale grain size region, different deformation behaviors have been reported when the grain size is reduced below a critical size. Experimental investigation will be carried out to validate the intrinsic relation between structural evolution after various milling duration and mechanical properties. Mechanical properties are expected to be enhanced in this nc Mg composite due to high density of grain boundary with different atomic structures. Microstructural evolution will be examined using different types of microscopes such as optical microscope, field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). Physical properties such as thermal properties, electrical resistivity, coefficient of thermal expansion and mechanical properties such as tensile tests at different strain rates, creep tests at various temperatures will be included in this study. 1.2 Objectives The main objectives of the present study are: 1. Synthesis of AlN composite powder by mechanochemical milling. 2. Synthesis of Mg-5wt%Al reinforced with 1wt% of in-situ formed AlN via mechanical milling and power metallurgy techniques. 3. Study of microstructures and mechanical properties of Mg MMCs at different ball milling durations. 4. Investigation of the deformation behavior of Mg-5wt%Al-1wt%AlN systems. Introduction 1.3 Scope of the thesis This thesis is composed of seven chapters including this chapter of introduction and the remaining chapters are organized as follows: Chapter reviews the unique mechanical properties of nc materials relative to their coarse grained counterparts. Nc materials processed via consolidation of MMed powders, their mechanical properties and deformation behaviors will be the focus in the literature review. Chapter presents the synthesis of AlN powder by mechanochemical process using Al and pyrazine as the starting materials. The mass structure, morphologies and thermal properties of the as-milled powder samples were examined after several milling durations ranging from 20 to 100 hours. Details of AlN morphologies for as-milled and annealed powders are discussed with the general explanation of growth mechanism. Chapter reports the processing of the pure nc Mg and nc Mg composite. Comparative studies on tensile, thermal, electrical properties between the nc pure Mg and the nc Mg composites at room temperature are presented in this chapter. Chapter presents the experimental investigation of strain rate effect on nc pure Mg and nc Mg composites in tension. True stress versus true stain rate curves and TEM micrographs were used to support the detailed discussion of strain rate sensitivity, activation volume and strain hardening behaviors. Introduction Chapter presents the experimental investigation on creep behaviors of the bulk nc Mg composites at various temperatures to further examine the possible deformation mechanism. In the last chapter, Chapter 7, conclusions from present study are presented and recommendations for further study of deformation mechanism for Mg MMCs are proposed. 1.4 References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. FH Froes, YW Kim, S Krishnamurthy, Mat Sci Eng A 117 (1989) 19-32. B Wolf, C Fleck, D Eifler, Int J Fatigue 26 (2004) 1357-1363. M Avedesian, H Baker (ed.), Magnesium and magnesium alloys, ASM International, Materials Park, OH, USA, 1999, pp. 14. BL Mordike, KU Kainer, Magnesium alloys and their applications, WerkstoffInformation-sgesellschaft, Frankfurt, Germany, 1998. H Ferkel, BL Mordike, Mater Sci Eng A 298 (2001) 193-199. SF Hassan, M Gupta, Mater Sci Eng A 392 (2005) 163-168. AT Maung, L Lu, MO Lai, Compos Struct 75 (2006) 206-212. SC Tjong, ZY Ma, Mater Sci Eng R 29 (2000) 49-113. EW Kelley, WF Hosford, Trans AIME 242 (1968) 5-13. http://www.magnesium-elektron.co.uk/data/downloads/ENGLISH%20Flux%20 ART2003b.pdf. ML Hu, ZS Ji, XY Chen, ZK Zhang, Mater Charact 59 (2008) 385-389. H Watanabe, K Moriwaki, T Mukai, K Ishikawa, M Kohzu, K Higashi, J Mater Sci 36 (2001) 5007-5011. JS Benjamin, T Volin, Metall Mater Trans B (1974) 1929-1934. C Suryanarayana, Prog Mater Sci 46 (2001) 1-184. K. Kubota, M. Mabuchi, K. Higashi, J Mater Sci 34 (1999) 2255-2262. . present study are: 1. Synthesis of AlN composite powder by mechanochemical milling. 2. Synthesis of Mg- 5wt%Al reinforced with 1wt% of in- situ formed AlN via mechanical milling and power metallurgy. Study of microstructures and mechanical properties of Mg MMCs at different ball milling durations. 4. Investigation of the deformation behavior of Mg- 5wt%Al-1wt %AlN systems. Introduction. and ductility [3]. However, the solubility of alloying elements in Mg is limited, restricting the possibility of improving mechanical properties and chemical behavior. Enhanced mechanical properties