Synthesis of Transition Metal Aluminides from Elemental Powder Mixtures

Sammanfattning: Popular Abstract in English Structural components such as those used in gas turbines for energy conversion applications, are subjected to high loads at elevated temperatures. In order to ensure that the components survive under these conditions for a sufficiently long time, it is imperative to select materials with good high temperature properties like strength and corrosion resistance. Such materials are usually alloys with a microstructure consisting of several phases where one or more phases have a strengthening effect. These phases are generally hard and are compounds formed from two or more metals. Current trends show an increasing interest in the use of such intermetallic compounds. The vast potential of intermetallic compounds like aluminides emerges from a combination of their attractive characteristics such as high melting point, high-temperature strength and excellent oxidation resistance. Transition metal aluminides fall into this category and the formation of these compounds from elemental powder mixtures is the focus of this thesis. Thermal effects (release and absorption of heat) have been studied during the heating of powder compacts containing aluminum and a transitional element (Ti, Fe, Nb and Ta), using a differential scanning calorimeter (DSC). The reaction behavior in samples containing two transitional elements (Ni and Ti), have also been studied. The effects of changes in sample composition, particle size and heating rate on the onset temperatures were investigated and the products obtained during various stages of reaction were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results from the present study show that high melting transition metal aluminides can be produced using the powder metallurgical route at relatively low temperatures. However, the synthesis of pure compounds requires careful control over critical process parameters like particle sizes of the reactants, composition, temperature and time. The knowledge obtained from studies on the evolution of phases in various powder mixtures is useful in optimizing process parameters. Studies on Al-Ni-Ti compacts have revealed the formation of ternary phases during reactive sintering, which have not been observed during conventional production.