Microstructure And Mechanical Properties Of In Situ TiB Reinforced Titanium Matrix Compositre Prepared By SPS

The in situ reaction synthesized TiB/Ti composite was fabricated by spark plasmasintering(SPS). Densification process of the spark plasma sintering for TiB/Ti compositewas investigated. The effects of the SPS parameters on the densification, microstructure andmechanical properties have been studied. The crystal structure model of TiB was obtainedand the formation process of stacking fault in TiB crystals was analysed.Four typical sintering stages are divided according to the shrinkage features of thecompact and microstructure evolution in the SPS process. The characteristic for these fourstages are particles activation and rearrangement, Ti particle inteneration and deformation,in situ reaction, and sintering pressure applying, respectively. The fine product will beproduced when these four stages are carried on in order.SPS process for TiB/Ti composite follows the mechanism of sintering neck formationand growth. In the early period of sintering, due to the discharge effect, the local area withhigh-temperature emerged from where the surface of the Ti particle become melting andthen evaporation. Then the metal vapor deposited nearby to form the sintering neck. At last,due to the growth of the sintering necks and the applying of sintering pressure, theshrinkage of the compact carried on rapidly.The in situ synthesised TiB have a B27crystal structure in form of a whisker-likeshape. It exhibits typical crystallographic planes of (100),(101) and (101), and a zone axisalong [010] direction. A number of stack faults which paralleled (100) plane exists insideTiB crysal. The typical morphology of TiB includes whisker, cluster, bunch and tube.The orientation relationship between TiB and-Ti was observed by HREM as follows:(001)TiB∥(0001)-Ti，(100)TiB∥(010)-Ti，[010]∥[2110]-TiBut the sintering temperature is higher than that of-β phase transition of Ti, and theorientation relationships between TiB and β-Ti can be determined as follows:(001)TiB∥(110)β-Ti，[010]TiB∥[11]β-TiBased on HREM observation results and the crystal structure model of TiB, theformation mechanism of the stacking fault in TiB was proposed: the stacking fault of TiBwhich along (100) plane, impenetrates through the whole crystal grain; and Ti atoms whichinside the stacking fault deviates a distance of±0.508c from the normal arrangement. Apartial dislocation appears between the adjacent growth steps with different stacking orders. The tensile strength of TiB/Ti composite first increases and then decreases with risingTiB content; the best tensile strength of1022MPa was obtained from3vol%TiB at thesintering temperature of950℃. The yield strength of TiB/Ti composites increases with anincrease in TiB content. Compressive strength of TiB/Ti composites also increases withincreasing TiB content. However, the plasticity of TiB/Ti composites decreases.The strength of TiB/Ti composite increases with an increase in TiB content at a highstrain rate, but the elongation first increases and then decreases;3%TiB/Ti composite hasthe best combination of strength and toughness. The strain rate has great influence in flowstress of TiB/Ti composite, and the strain rate sensitivity of TiB/Ti composite is determinedby the aspect ratio of TiB whiskers and TiB content. Generally, the strain rate sensitivitydecreases with increasing TiB content.