Ti-48.5at%Al-1.5at%Mo-(0-2)at%C (or N) intermetallics prepared by Vacuum Arc Remelting were investigated. There are two heat treatment steps; L: heat treatmemt at 1390℃, A: aging at 900℃. The specimens are treated in three ways; 1) only L step (L), 2) step L followed by A (LA) and 3) step A followed by step L (AL). The microstructures of TiAl intermetallics containing various C or N content have been investigated using optical microscope (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Then, tensile test was performed at a strain rate of $2\times10^{-4}s^{-1}$ in air at room temperatre and 800℃.
The additon of C or N, especially above 1.0at%, was found to be effective in refining lamellar grains. In all specimens containing N, precipitates below 1㎛ were formed at the lamellar interface. And in the specimens containing above 1.0at%N, large precipitates over 1㎛ were formed within the lamellar matrix. TEM observations revealed that the former are $Ti_3AlN$ and the latter are $Ti_2AlN$. The distributions and positions of $Ti_3AlN$ and $Ti_2AlN$ precipitates were dependent on each heat treatment, L, LA and AL.
The room temperature elongation of specimens containing above 1.0at%C or N is similar to that of Ti-48.5Al-1.5Mo. Till the amount of C or N is increased to 1 at%, the yield strength and fracture stress increase. The tensile test at 800℃ revealed that the addition of above 1.0% improve the high temperature yield strength, relatively. The improvement of mechanical properties in room temperature and 800℃ is considered to be caused by grain refinement and composite-like fracture mechanism due to the precipitates.
