Development of titanium based in situ composites by powder metallurgical route: microstructure evolution and mechanical properties

Abstract

Metal matrix composites (MMCs) combines ductility of a metal and strength of a reinforcement which makes it an excellent candidate material for advanced engineering applications. An example of such a matrix material is titanium which by itself has a unique combination of high specific strength (tensile-strength-to-density ratio) at both room and moderately elevated temperatures, and excellent corrosion resistance. This thesis studies the feasibility of producing a high strength titanium matrix composite using a conventional press and sinter route. The effects of different reinforcements (TiB2 and Si3N4) on the microstructural and mechanical properties of the resulting composites were studied in detail. Extensive studies of in situ TiB/Ti and Ti-Si3N4 composites were carried out in this thesis, which is divided into two parts dealing with the two composite systems separately. Phase identification, mechanical and tribological properties have been extensively studied along with microstructural analyses to evaluate the technological significance of these composites in industrial applications. The novelty of this work is to study the optimal concentration of the reinforcement for the development of titanium composites for industrial application with simultaneously studying the reaction mechanism by in situ neutron diffraction. In situ TiB/Ti composites In this study, the optimal concentration of titanium diboride (TiB2) particles was ascertained for the development of (Ti-TiB) metal matrix composites, prepared by a conventional powder metallurgy route. The effects of concentration were studied by reinforcing titanium diboride (TiB2) powder in different weight fractions (2, 5, 10 and 20 wt. %) into pure Ti powder during the fabrication process. The composites were sintered at higher temperatures under vacuum. The transmission electron microscopy (TEM) results revealed the formation of needle-shaped TiB whiskers, indicating that in situ reaction occurred during vacuum sintering of the powder compacts. All the composite samples had a high sintered density, and the hardness of the composites increased with increasing the weight fraction of reinforcement. The mechanical and tribological properties, such as tensile and flexural strength, impact and wear properties, were studied and found to be dependent on the weight fraction of the reinforcement. The recorded tensile strength for pure Ti was ~ 591.27 MPa, which was increased to ~ 654.72 and ~ 684.95 MPa for Ti-2 wt. % TiB2 and Ti-5 wt. % TiB2 sample, respectively. This corresponds to an increase of 10.73 % and 15.84 %, for Ti-2 wt. % TiB2 and Ti-5 wt. % TiB2 sample respectively as compared to pure titanium. For the first time, the conversion of phases from TiB2 to TiB was studied using the neutron diffraction technique. The formation of different phases during the heating and cooling sequences of vacuum sintering is reported in detail in this work. In situ neutron diffraction revealed that during phase transformation from TiB2 to TiB, no metastable phases were observed in the case of pure titanium. The TiB2 phase completely transformed to TiB without forming any undesirable phases. Ti-Si3N4 composites The use of silicon nitride (Si3N4) particles as a potential reinforcement for titanium metal matrix composites (Ti-MMCs) was evaluated. Si3N4/Ti composite samples were fabricated also by a conventional press and sinter route. The effects of sintering temperature and the concentration of Si3N4 in Ti matrix were investigated with respect to phase constituents, microstructure and tribology properties. Si3N4 weight fraction inside the Ti matrix was found to be the key parameter determining phase constituents and overall properties even at lower concentrations. Because of its instability in Ti at higher temperatures, in situ reactions between the reinforcing particles and matrix led to the formation of in situ phases. These phases were studied in detail using the neutron diffraction technique. The in situ neutron diffraction results revealed that the lower concentrations of Si3N4 in the Ti matrix resulted in a homogeneous distribution of silicon and nitrogen in the Ti matrix. The existence of metastable TiSi2 was observed in all of the samples. As the concentration was increased, new in situ phases were formed such as Ti3Si and Ti5Si3 phases, which were also seen during the ex situ characterisation tests. The formation of homogeneous Ti5Si3 phase resulted in excellent wear properties at the expense of ductility. The wear resistance was directly proportional to the weight fraction of Si3N4. The highest hardness value of 949 HV1 was recorded for Ti-5wt. % Si3N4 composite when sintered at 1300 °C for 3 hrs. This work would help understand these composites as a prospective wear resistant material in industrial applications. In summary, the effect of incorporating different reinforcements-different in nature, morphology and properties in Ti matrix were studied. However, it is difficult, if not impossible, to develop these composites by the simple press and sinter technique for achieving high density and excellent mechanical and tribological properties. Nonetheless, the composites developed in this study have ample potential and can be used for industrial applications. This project opens further questions for future investigations as well.