Development of High temperature Mo-Si-B alloys – Additive Manufacturing

In this project we work on the fabrication and the thermodynamic and metallurgical basics associated with the additive manufacturing of dense Mo-Si-B-based alloys.

Mo-Si-B alloys are potential creep resistant materials for accessing harsh loading scenarios beyond Ni based superalloys due to their excellent mechanical performance at ultra-high temperatures (> 1200°C). Here, we report on the fabrication through laser additive manufacturing of a Mo rich Mo-Si-B alloy with and without dispersion of oxide (La₂O₃) particles. The major phase in the solidified material is dendritic α-Mo. The inter-dendritic regions contain a mixture of the Mo₅Si₃ (T1) + Mo₅SiB₂ (T2) phases, and not the expected equilibrium Mo₃Si + Mo₅SiB₂ (T2) phases. This combination of phases is shown to yield improved high temperature creep resistance but was only accessible through by addition of Nb, W or Ti that substitute Mo in the intermetallic phases.  Whereas here it is attributed to the large undercooling in the small melt pool produced during laser processing. We show that this phase mixture is stable upon annealing at 1200°C for 200h. We also demonstrate successful dispersion of oxide particles mainly in the inter-dendritic regions leading to a high indentation fracture toughness of ~18 MPa√m at room temperature. Toughening originates from crack trapping in the ductile α-Mo and the formation of micro-cracks and crack deflection in the vicinity of oxide particles.