Developing and characterization of niobium boride reinforced Al5Si hybride powders and composites with mechanical alloying and pressureless sintering methods

During the past few years, materials design has continued pursuing light weight, environmentally friendly, low cost, high-quality and high-performance material properties. Accordingly, there has been growing interest in metal-matrix composites (MMCs) with superior physical and mechanical properties. The need for advanced engineering materials in the areas of aerospace, automotive and the other structural applications led to a rapid development of metal matrix composites (MMCs). At this point, aluminum and aluminum-based alloys became the most preferred base metal and alloys as the matrix materials. These materials stand out among the other metal matrix materials due to low density, corrosion resistance, low melting point, extremely high thermal conductivity, heat treatment capacity, flexible processability and low cost properties. MMCs are generally produced by homogeneous distribution of hard and stiff secondary phases into the ductile and tough base metal or alloy matrix. The secondary phases are often chosen as a ceramic or refractory material. Combining metallic properties (ductility and toughness) with ceramic properties (high strength and modulus) in this way, allows the production of materials which have superior performances in comparison with the monolithic metals or alloys. These are frequently summarized as higher tensile and compressive strength, wear resistance, low thermal expansion, high thermal and electrical conductivity, resistance to aggressive environments, good impact and erosion resistance, good fatigue and fracture strength and high temperature performances. There are usually two types of reinforcement as fibers or particles distributed through the metal matrix. High costs and technical difficulties in production of fiber-reinforced composites limits their much practical use. Recently, particulate reinforced MMCs are of great interest and constitutes a more suitable alternative thanks to production facilities, relatively low cost and characteristic isotropic properties compared to fiber reinforced composites. In addition, particulate MMCs materials, has remarkable use for a variety of automotive applications. These materials typically include SiC and TiC particle reinforced aluminum matrixes with specific modulus and strength which make them suitable for disc brake drum, connecting rod, cylinder liners and other high temperature applications. Many industrial processes have been developed for the production of metal matrix composites from their components. The most common types of these production methods consist of a solid phase production technique known as powder metallurgy (PM) and conventional casting which is a liquid phase production process. The problems associated with liquid phase production techniques of composite materials are the difficulties in controlling the distribution of reinforcement and obtaining a homogeneous matrix microstructure. Also, at elevated temperatures relevant to liquid phase production it is possible for unfavorable interfacial reactions between matrix and reinforcement to take place and the process of these chemical reactions may have an adverse effect on the mechanical properties of the composite. Technical difficulties associated with conventional casting methods such as reinforcement segregation and aglomeration, chemical interfacial reaction, highly local residual porosity, poor interface bonding restrict the usefulness of this production route, thus solid phase production techniques may be preferred. The most common solid phase production is based on powder metallurgy techniques. Metal matrix composites in all alloy compositions can be produced by powder metallurgy techniques without typical segregation occurence during casting processes. However, one of the main challenges unique to this method is to obtain a uniform distribution of reinforcement through the metal matrix. Homogeneous distribution of the reinforcement phase is the first condition required for the outstanding performance of a composite material and agglomeration of reinforcement particles impairs material properties. Differences in size, density, shape, flow or electric charge development of particles constitute all factors that contribute to agglomeration. Hence, in order to improve the particle distribution, high-energy ball milling method is used. This technique developed by Benjamin to produce hardened nickel super alloys by oxide dispersion is still commonly known as mechanical alloying or mechanical milling. Mechanical alloying is a process based on grinding the powder mixture together and material diffusion via repetitive plastic deformation- welding-fracture mechanisms in order to obtain a homogeneous alloy. MA is an appropriate technique for the development of metal matrix composite materials with better properties than that are obtained using other methods. Besides inhibition of the agglomeration of reinforcement particles and maintenance of uniform reinforcement distribution in the matrix, it also eliminates possible defects in the reinforcement structure which lead to fracture of the composite material before the envisaged time with the help of continuous collisions during MA. Within the scope of this thesis, experiments regarding the effects of various boride powder particles on the physical, microstructural and mechanical properties of the Al-5 wt.% Si matrix were conducted. Two different types of boride powders were used. Difference between in these two boride powder particles was based on production methods. One was mechanochemically synthesized and leached in ITU Particulate Materials Laboratories (NbB-NbB2-Nb3B4) while the other one was commercial (NbB2). Hence, according to experimental results different boride powders were used as matrix reinforcement. The main objective of this work is to get a hybrid composite structure with advanced features by combining the soft, ductile and lightweight main matrix metal aluminum with hard, stiff alloying element silicon and the addition of boride powders as the stable ceramic reinfocements into this master alloy. Experimental studies, began with the mechanochemical synthesis of the various boride powders (NbB-NbB2-Nb3B4, VB2-VB-V3B4, LaB6) that will be used as reinforcements for the alloy matrix. These experiments were performed at ITU PML. Commercial quality boride reinforcements (NbB2, VB, LaB6) were available within the laboratory facilities. Al-5 wt.% Si master alloy matrix was reinforced with commercial quality and laboratory synthesized boride powders of 2% weight percentage. As reinforcements, niobium boride powders were used beforehand, vanadium boride and lanthanum boride powders were studied following the previous results. MA durations were chosen as 1, 2, 3, 4, 6 and 8 hours for master alloy matrix composition and 1, 4 and 8 hours for niobium reinforced hybrid composite compositions. MA durations for vanadium boride and lanthanum boride reinforced hybrid composite compositions, were determined as 4 hours. Powder mixtures without mechanical alloying (MA), as-blended powders, were also used for the same compositions. All experiments were conducted in a Spex™ Mixer/Mill with a rate of 1200 rpm using hardened steel vial and balls. For phase and microstructure analysis, X-ray diffractometry (XRD) and scanning electron microscopy (SEM) experiments were performed on as-blended and mechanically alloyed powders. Also, crystallite size and lattice deformation values were determined via TOPAS V3.0 in-built software. Following phase characterization, particle size and BET surface area measurements were carried out while sintering conditions were determined by differential thermal analysis. Hydraulic press with a uniaxial pressure at 450 MPa was used to compact MA'd and as-blended powders into a cylindrical shape. Then, green compacts were sintered at 570°C for 2 h under Ar gas atmosphere. X-ray diffractometry (XRD), optical microscopy (OM) and scanning electron microscopy (SEM) techniques were applied to carry out phase and microstructural characterization investigations of the sintered composites. Densities of sintered compacts were determined by Archimedes' Principle in ethanol medium. Vickers microhardness and sliding wear tests were also conducted on the sintered samples. By means of characterization examinations, both MA and boride reinforcement were found to enhance the physical and mechanical properties of the material. Thus, physical and mechanical properties of as-blended systems were the weakest ones. Commercial quality vanadium boride reinforced hybrid composites MA'd for 4 h had the highest density value of 99,41%, while laboratory synthesized vanadium boride reinforced hybrid composites MA'd for 4 h had the highest microhardness value of 139.1±12.98 HV0.1 and the lowest total wear loss (0.072 mm3).