Crack formation mechanisms in laser-directed energy deposited high-Nb TiAl alloys

Abstract

In the additive manufacturing (AM) of TiAl alloys, cracking is an important bottleneck restricting the development of TiAl alloys. This study systematically investigates crack nucleation locations and formation mechanisms in monolayer deposition samples through comprehensive characterization using Electron Back-Scattered Diffraction (EBSD) and Scanning Electron Microscopy (SEM). The laser direct energy deposition (LDED)-fabricated Ti-47.5Al-6.8Nb-0.2W alloy exhibits two distinct types of cracks: macrocracks and microcracks. Macrocracks primarily originate from the brittleness of the α₂ phase and the residual stresses induced by rapid heating and cooling during the printing process, showing no significant correlation with the microstructure of the TiAl deposited layer. These macrocracks nucleate at the diffusion layer between the Ti6Al4V (TC4) substrate and the TiAl deposited layer, subsequently propagating through the entire deposited layer. In contrast, microcracks are closely associated with phase transformation effects. Specifically, the α₂→β₀ phase transformation generates transformation strain, leading to localized stress concentration and thereby promoting microcrack initiation. Furthermore, the inherent brittleness of the α₂ phase facilitates microcrack propagation. These findings provide critical insights into the cracking mechanisms in additively manufactured TiAl alloys, offering valuable guidance for process optimization and crack suppression strategies.