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

采用反应重熔法制备了Ti_2AlC均匀弥散分布于γ-TiAl基复合材料。虽然Ti-Al二元系的γ-TiAl基体的平均直径在70 ~ 250 μm之间，但Ti-Al-C三元系的基体中存在尺寸小于50μm的较小晶粒。这导致了反应电弧熔炼过程中生成的Ti_2AlC的存在。这些Ti_2AlC（比TiAl合金的熔化温度高）在TiAl合金的凝固过程中起着重要的形核作用。这增加了TiAl的形核数量，并使基体的晶粒尺寸变小。Ti_2AlC虽然熔化温度很高，但熔化后会凝固，用常规的电弧熔炼似乎很困难。而普通电弧熔炼与燃烧反应相结合的反应电弧熔炼则可以获得高熔点材料的熔融状态。其次，研究了α_2-Ti_3Al的微观组织变化。根据Ti-Al二元相图，当成分为50at%Ti和50%Al时，不会出现α_2-Ti_3Al相，而在铸态γ-TiAl合金中，通常观察到由γ-TiAl和α_2-Ti_3Al组成的片层组织。根据Ti-Al-C三元相图，C在α_2-Ti_3A中的固溶量远大于γ-TiAl。因此，在达到平衡相的均匀化退火过程中，可以得到γ-TiAl单相，而α_2-Ti_3Al相则被湮灭。透射电镜观察表明，小尺寸的Ti_2AlC代替了α_2-Ti_3Al相析出。因此，新析出的Ti_2AlC不仅可以强化新概念复合材料，而且可以提高其断裂韧性。

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.