Characterization investigations of W-xTi alloys and W-xTi/2 WT% LaB6 composites fabricated by mechanical alloying and sintered using pressureless sintering or spark plasma sintering techniques

Composites are materials composed of two phases: first phase is matrix and second phase is reinforcement. First phase is classified in itself into metal matrix, ceramic matrix, and polymer matrix. Metal matrix composites are categorized as high technology materials. These composites are generally used for high temperature applications. Various particulate reinforcements such as oxides, borides, and nitrides are used in this group of composites. Particulate reinforcement powders (second phase) used in metal matrix composites (MMC) can be produced easily. Meantime, powder metallurgy has various advantages. Because of these advantages, MMCs reinforced by particulate material is usually categorized as a specific group. High elastic modulus, low density, high electrical and thermal conductivity, and etc. are some of main specifications of these composites. Particulate reinforced metal matrix composites (PRMMC's) have both metal and ceramic characteristics. Elements are homogeneously dispersed in PRMMC's. These composites have lower density and outstanding mechanical properties like higher wear resistance, hardness, or etc. compared to unreinforced matrixes. Generally, powders used in PRMMC's are produced by mechanical alloying (MA). By this technique, very fine sized (even nano sized) and brittle powders with homogeneous compositions can be fabricated. Based on MA, some solid solutions which are not possible to be produced thermodynamically in equilibrium condition can also be obtained. Tungsten (W), being the densest (19.25 g/cm3) and having one of the highest melting point (3422˚C) among other elements, is used as matrix material. W is a transition element located in VI group. W and its alloys have high melting temperature and high elastic modulus. These materials are used for developing matrix materials utilized at high temperatures due to their superior properties. Tungsten (W) and tungsten alloys have high melting temperatures, high thermal shock resistances, low thermal expansion coefficients which make them suitable for high temperature applications. However, production processes of W and its alloys are rather challenging, due to their high melting temperature and low ductility. Production of W powders with high density by utilizing powder metallurgy (PM) and mechanical alloying (MA) needs high temperature conditions (1650˚C-1770˚C). Sintering temperature of these elements are decreased with addition of small amounts of transition metals such as Pd, Pt, Ni, Co, and Ti into matrix (1300˚C ~1500˚C). Oxides, nitrides, borides and carbides like ZrC, TiN, Y2O3, LaB6 powders are used in aid of developing high temperature characteristics of W powders. These reinforcements are located in grain boundaries therefore prevent grain growth and dislocation movements, ensuring material properties to increase at high temperatures. W and its alloys are used in turbines, automobile production and electronic industry. Furthermore, these products are utilized in heavy materials, balance weigth, and some parts of motors as heavy alloys. Other applications of these products are electrode scrapers and electrodes of arc furnaces. The combination of W with various second phase particles of different ratios expands the appropriate function range of these composites. According to the mentioned points, this PhD thesis is divided into two main parts. First part is based on MA, powder processing and powder characterization of W-Ti-LaB6 powder composite, and second part consists of sintering produced composite powders. In powder processing part, MA is applied in room temperature via high energy milling. Based on this method, W nano composite powders are produced. In second part, sintering were achieved by two different methods: pressureless sintering (PLS) and spark plasma sintering (SPS). The main aim of this study is to fabricate W-xTi-2.0LaB6 composite with the highest density, high hardness, high strength and high thermal shock resistance at lowest sintering temperature. Ti was used as activator and LaB6 as reinforcement element. This proposed research shows that mechanical alloyed tungsten (W) based matrix systems have nanostructure at room temperature by means of production via high-energy milling. The reason of using milling processes in general is to synthesize composite structures via mechanical alloying and to provide hybridization of ductile W-Ti matrixes in particular by reinforcing them with brittle and hard secondary particles (LaB6). In spite of their superior properties such as high hardness, high thermal and wear resistance and good thermal values, there is no universal literature on the utilization of Ti as transition metal and LaB6 as reinforcement in the W matrix. During powder characterization examinations, Ti solution in the W structure in non-equilibrium condition was measured by Vegard law, amount of Ti and effect of MA time on crystallite size and lattice internal strain were studied by W-H and Lorentzian methods. Tungsten based hybrid composites were manufactured by pressureless sintering (PLS) and spark plasma sintering (SPS) and characterized. Final composite material consists of elements which are W as matrix, Ti as activator, and LaB6 as reinforcement. Nano composite powders were produced by mechanical milling using different durations and a high-energy mill at room temperature which is one of the original approaches of this research. Investigations and resultant publications regarding the synthesis and characterization of W-Ti hybrid composites manufactured via these conditions will be the pioneering examples in the scientific literature. Furthermore, performing two different sintering methods (PLS and SPS) is the next priority of this thesis study. Microstructural and phase characterization investigations of the synthesized powders and sintered products were carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM) whereas thermal characterization investigations were conducted by using differential thermal analysis (DTA) and differential scanning calorimetry (DSC). In the light of data obtained from microhardness measurements mechanical tests (wear resistance and microhardness) were also performed.