Development of potantial intermetallic materials produced by ECAS

In this thesis, it is aimed to produce metallic-intermetallic, metallic-intermetallic-ceramic composites to achieve the synergic effects of combining the high strength and stiffness of intermetallic (Ti-Al) materials, high toughness of metallic material and hardness properties of the ceramics. Contrary to conventional techniques, recently developed electric current activated/assisted sintering (ECAS) technique has been used in this study. This technique enables the cold-formed compact obtained from uni-axial compression to be inserted into a container which is heated by passing electric current for a short time. Direct current resistive sintering technique has been applied to the Ti-Al powder mixture. Using the calculated stoichiometric ratio according to Ti-Al phase diagram, not only intermetallic (brittle) TiAl3 phase, but also metallic (ductile) phase was obtained in the final composite sample. The formation of intermetallics was investigated 2000 A and a voltage range of 1,5-2 V was applied for 90 s holding time. TiAl3 was selected among the Ti-Al system owing to its excellent properties i.e, low density (3,3 g/cm3), high elastic modulus (216 GPa) and superior oxidation resistance at elevated temperatures (value of oxidation at 1000 ºC). Firstly, using Ti and Al starting elementel powders, Ti-TiAl3 in situ composite was obtained then some of the elements in various weight percentages such as; %5-10 Nb, %2,5-5 B, %5-10 TiB2 were added for improving the properties of potential intermetallic materials. Alternatively commercial TiAl3 powders were used to compare the results but they were not of sufficient quality because of oxidation problem in the open air furnace conditions. In metallographic observations, quite dense, homogenously distributed microstructures have been observed, and weight percentages of the phases were detected via SEM-EDS analyses. This type of analyses has a limitation for detecting B elements because of its low atomic number. From this aspect, XRD analysis is important to identify the constituent phases like (TiB). Density, hardness, fracture toughness, elastic modulus (Nano indentation) and oxidation properties of the potential intermetallic composites have been investigated. It is clear from the results that; the density of sintered samples were slightly decreased with the raising percentage of element. For the fracture toughness values it has been gained from 1,6 to ile 5,23 MPa.1/2 increment with wt.%10 Nb reinforcement. The maximum hardeness value has been obtained with wt.%5B as 965 HV. The elastic modulus of samples have been measured via nano indentation test such as; 100, 190, 107, 350 GPa for Ti, Nb, TiB, TiAl3 phases, respectively. Nb, B and TiB2 additions have been provided improvement for oxidation resistance, besides this activation energy value have been increased to 129,98 from 70,76 kj/mol. with wt.%5 TiB2 reinforcement.