Effects of grain size and interlamellar spacing on the strength and ductility of fully lamellar TiAl-Mn polycrystals

초록

The strength of polycrystalline TiAl based intermetallics having a fully lamellar structure has been studied by some investigations, and it has been demonstrated that both the grain size and lamellar morphology play an important role in the yield strength. Very little information, however, is available on the influence of grain size and lamellar morphology on the ductility of polycrystalline TiAl based intermetallics having a fully lamellar structure. In this study, fully lamellar Ti-(43.5~47)mol%Al-(1.6~1.7)mol%Mn polycrystals added with (0~2)vol% TiB2 particulates were fabricated via reactive sintering process in order to investigate the effects of grain size and interlamellar spacing on the strength and ductility at ambient and elevated temperature. For the reactive sintering, elemental Ti and Al-Mn powders were mixed with TiB2 particulates, and consolidated by extrusion. Then, fully lamellar titanium aluminide polycrystals were formed from the extruded mixtures by an exothermic reaction between Ti and Al under a Vacuum Hot Pressing. Microstructural observation revealed that lamellar grains are refined by the addition of TiB2 particulates and lamellar spacing tends to increase with increasing Al content and, in some case, with increasing TiB2 content. From the result of tensile tests at room temperature and 1073K, it was found that the elongation is improved with an increase in lamellar spacing at a fine-grained condition. At room temperature, a distinct inverse relationship between ductility/yield strength and lamellar spacing was revealed at a fine-grained condition.