Dendritic Growth and Microstructure Evolution with Different Cooling Rates in Ti48Al2Cr2Nb Alloy

The influence of cooling rates on the dendritic growth and microstructure evolution of Ti48Al2Cr2Nb alloy is studied by electromagnetic levitation combined with copper mold casting. The different cooling rates of the conical as-cast sample with diameters from 4.7 to 0.8 mm were calculated by ANSYS software. The results show that primary dendrite arm spacing decreases with increase in cooling rate. Peritectic transformation (L + β → α) and the transformation of α → (α₂ + γ) are restrained at cooling rate of 2.3 × 10⁴ K s⁻¹. With further increase in cooling rate (2.6 × 10⁴ K s⁻¹), a fine and homogeneous microstructure can be observed in the conical casting sample with the diameter of 0.8 mm. It consists of a large amount of massive γ phase, lath-like γ phase, and only few lamellar structures (α₂ + γ). The formation of the microstructure in the alloy is attributed to the strong chilling, giving rise to the high undercooling and the high dislocation density during rapid solidification.