Synthesis and evaluation of new concept composite materials prepared by reactive arc-melting

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We successfully fabricated γ-TiAl matrix composites reinforced with Ti_2AlC dispersed uniformly in the matrix by reactive are-melting. Although mean diameter of the γ-TiAl matrix is widely ranging from 70 to 250 μm for Ti-Al binary system, smaller grains with sizes of 〜50μm exist in the matrix for Ti-Al-C ternary system. This caused to the presence of Ti_2AlC formed during reactive arc-melting. Those Ti_2AlC (having higher melting temperature than TiAl alloys) play an important role as nucleation sites when the melting TiAl solidify in the cooling sequence. This enhances the number of the nucleation of TiAl, and makes the grain sizes of the matrix small. It is also noted that even though Ti_2AlC has extremely high melting temperature, it is solidified after melting ; it seems to be difficult by using conventional arc-melting. However, reactive arc-melting combined common arc melting with combustion reaction enables to get the molten state of high melting materials.Secondly we focused on the microstructural changes of α_2-Ti_3Al. According to the Ti-Al binary diagram, no α_2-Ti_3Al phase will be appeared, if the compositions used are 50at%Ti and 50%Al. It is, however, well known that lamella structure consist of γ-TiAl and α_2-Ti_3Al are usually observed in the samples as cast state γ-TiAl alloys. Furthermore, based on the Ti-Al-C ternary diagram, C can be dissolved much more in α_2-Ti_3A than γ-TiAl. Therefore, during homogeneous annealing to be achieved at equilibrium phase, γ-TiAl single phase can be obtained, consequently α_2-Ti_3Al should be annihilated. Moreover, under TEM observation, it is clearly shown that small sized Ti_2AlC were precipitated in the place of α_2-Ti_3Al. Consequently, newly precipitated Ti_2AlC not only can strengthen, but also can enhance fracture toughness of the new concept composite materials.