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

采用反应熔融法制备了分布均匀的Ti_2AlC增强γ-TiAl基复合材料。在Ti-Al二元体系中，γ-TiAl基体的平均直径在70 ~ 250 μm之间，而在Ti-Al- c三元体系中，基体中存在较小的~ 50μm晶粒。这是由于反应电弧熔炼过程中形成的Ti_2AlC的存在。在冷却过程中，Ti_2AlC合金（比TiAl合金具有更高的熔化温度）在熔融TiAl凝固过程中起着重要的成核作用。这增加了TiAl形核的数量，使基体的晶粒尺寸变小。还注意到Ti_2AlC虽然具有极高的熔化温度，但熔化后凝固；使用传统的电弧熔炼似乎很困难。而反应电弧熔炼将普通电弧熔炼与燃烧反应相结合，可以得到高熔点材料的熔融状态。其次，研究了α_2-Ti_3Al的显微组织变化。由Ti- al二元图可知，当组合物为50at%Ti和50%Al时，不会出现α_2-Ti_3Al相。然而，众所周知，在铸态γ-TiAl合金样品中通常观察到由γ-TiAl和α_2-Ti_3Al组成的片层结构。此外，从Ti-Al-C三元图可以看出，C在α_2-Ti_3A中比γ-TiAl更容易溶解。因此，在达到平衡相的均匀退火过程中，可以得到γ-TiAl单相，因此α_2-Ti_3Al应该被湮灭。透射电镜观察清楚地表明，在α_2-Ti_3Al的位置有细小的Ti_2AlC析出。因此，新析出的Ti_2AlC不仅可以增强复合材料的强度，而且可以提高复合材料的断裂韧性。

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.