Analysis on phase transformation and lamellar formation of Ti-47.5Al-3Nb-1.5/3.5Cr alloys by confocal laser scanning in-situ observation

gamma-TiAl alloy is a promising lightweight high temperature structural material basically composed of lamellar structure which is closely related to mechanical properties of gamma-TiAl alloys. In this work, the Ti-47.5Al-3Nb-1.5/ 3.5Cr alloys are investigated to understand the phase transformation and lamellar structure formation of these two alloys using the high-temperature confocal laser scanning microscope (HT-CLSM). The results of in-situ observation clearly demonstrate the dependences of the lamellar precipitation temperature and interlamellar spacing of the alloy on the cooling rate. Besides, greater undercooling is required to drive the lamellar formation in the case of higher the cooling rate. Therefore, the lamellar precipitation temperature decreases with the in-crease of the cooling rate. In addition, it is found that the lamellae usually precipitate firstly from the boundary of the alpha and gamma phases. The sizes of alpha and gamma phases decrease with the increase of cooling rate, which can refine the lamellae in the subsequent solid phase transformation. Furthermore, Since the Cr composition of 3.5Cr alloy is higher, more Cr atoms can diffuse, thus, the gamma lamellae bulges from the sample surface and forms steps between alpha and gamma lamellae at the cooling rate of 6 degrees C/min. The lamellar gamma continues to grow through the movement of steps, resulting in lamellar coarsening of 3.5Cr alloy, while the lamellar coarsening cannot be observed in 1.5Cr alloy.