Production and characterization of functional graded materials by combustion synthesis

In this study, it is aimed to produce and characterize nickel aluminide (NiAl) functional graded materials (FGM) having layers with pores in controlled proportions and sizes, by volume combustion synthesis (VCS) by using Ni+Al reactants and urea (or NaCl) space holder (SH). Each layer, aimed to form a layer in FGM structures, was initially produced and examined as a separate piece. For this purpose, in separate chapters within the scope of the thesis, the effects of type and ratio of thermal diluent in VCS, cold press pressure before VCS, pore size and ratio in the produced parts, space holder type, using NiAl and Ni1,2Al and adding preformed and in-situ TiB2 to NiAl were investigated. Commercial TiB2 (TiB2-H) and NiAl powder (NiAl-H) which was produced in this study were used as thermal diluent. Adiabatic temperatures were calculated for each system. It was determined that the appropriate thermal diluent ratios in the Ni+Al mixture were 30% in order to preserve the size and shape of the manufactured parts and their pores. The strength of the parts produced with NiAl-H was higher than those produced with TiB2-H. The suitable cold press pressure was determined as 100 MPa. Urea and NaCl particles in the size ranges of 150-300, 300-500, 500-800 microns were used as SH in the parts at a rate in of 20-75% range. It was observed that some of the BO particles could not be removed in water due to the thick and closed pores of the cell walls in the samples containing less than 20% BO particles. When the amount of BO was over 75%, the samples could not maintain their integrity in the water. Higher compressive yield strength was obtained with urea SH. Highest strength was 181.8±1.6 Mpa. It was seen to decrease with increasing size and amount of (Microhardness ~240 HV 0.2). Numerical correlations between strength and pore size and ratio were determined. For this purpose, Bal'shin, Hall-Petch, Bhattacharjee and Gibson-Ashby equations were employed. Parts in Ni1,2Al composition showed lower strength but better thermal insulation coefficient than NiAl. It was understood by XRD analysis that TiB2 particles in different proportions were formed in-situ in the porous NiAl structure. It was seen by SEM examinations that the TiB2 particles formed were smaller than 1 micron in size. The highest microhardness values were obtained in these parts (430-600 HV0.2). By making use of the parameters determined in the experiments with single-layered parts, all FGM parts having layers with controlled proportions (% 20, 40, 60 or % 40, 60, 75) and controlled sizes of porosity, could be formed with a single VCS reaction. The strength values of FGMs were limited to the layer with the lowest strength. It was determined that the thermal insulation index, which was measured according to the standard, increased with the increasing ratio and decreasing size of the pores in FGM parts.