Processing and characterization of functionally graded titanium/titanium boride/titanium diboride composites by combustion synthesis/compaction and microwaves

The use of functionally graded materials (FGMs) in aircraft, armor, medical and electronics industries is becoming more common as their performances exceed the capabilities of homogeneous materials. The concept of functionally graded materials (FGMs) was initially proposed to minimize problems such as poor mechanical integrity and interfacial adhesion associated with the presence of abrupt interfaces in metal-ceramic bonded structures.; In this thesis, the combustion synthesis (CS) method was explored in fabricating FGMs in the Ti-B binary system. Among other methods CS has shown potential in terms of process economics and simplicity. Compositionally graded trilayered and five layered composites were produced by using three combustion methods: conventional combustion synthesis, combustion synthesis/compaction and microwave activated combustion synthesis. The porosity formation during CS has precluded the widespread use of this method. Therefore, a strategy was proposed to reduce the porosity, first, through control of vigorous combustion reactions and second, by applying pressure on the ignited samples. The control of the reactions was done by selecting compositions away from the stoichiometry without resorting to adding a third element into the system. These initiatives resulted in net shaped graded composites with improved density and hardness. As another processing method, microwave activated combustion synthesis was utilized by using SiC as a susceptor. When compared to conventional CS, microwave processed FGMs exhibited better microstructural homogeneity. All three methods resulted in Ti-TiB-TiB₂ graded composite materials with continuous and crack free interfaces. Moiré Interferometry tests were conducted to compare the deformation behavior of conventional combustion synthesized FGMs with that of microwave activated FGMs. The comparison was based on their in-plane displacements under compression loading. It was found that microwave produced FGMs exhibit a more compliant behavior. Under loads as low as 0.37 kN, the composites strained plastically. Also higher boron content layers exhibited a more compliant behavior compared to pure titanium layers.