Microstructure Evolution and its Effect on High-Temperature Compressive Properties of Directionally Solidified Ti-44.5al-3nb-0.6si-0.2c Alloy by Electromagnetic Confinement after Heat Treatment

Abstract

To regulate the microstructure of directionally solidified TiAl alloy and improve its service temperature and high-temperature mechanical properties， the large-size single crystal with full lamellar structure of Ti-44.5Al-3Nb-0.6Si-0.2C prepared by electromagnetic confinement directional solidification was heat-treated in the α single-phase region. The results show that this alloy has high stability and no recrystallization under holding at 1340°C for 30 minutes. The (α2+γ) lamellar structure reformed during the subsequent cooling process is consistent with the orientation of the (α2+γ) lamellar structure in the original as-cast state, and the refinement effect is very significant. The thickness of the α2 phase and the γ phase reached 83nm and 64nm, which is 1/3 and 1/10 of the original structure, respectively. The dislocation density and the number of α2/γ phase boundaries in the refined lamellar structure increase significantly after heat treatment, dislocation strengthening and interface strengthening are enhanced, resulting in a substantial rise in compressive peak strength at 1000°C. The compressive peak strength at 1000°C of the refined lamellar structure achieved through the HT1 heat treatment process is 2.6 times higher than that of the as-cast structure, reaching 709 MPa. This exceeds almost all TiAl alloys under the same conditions reported so far. In addition, the compressive peak strength and refined lamellar thickness at 1000°C follow the Hall-Petch relationship.

Keywords: Titanium aluminides Directional solidification Heat treatment Lamellar structure High-temperature compressive property