Optimizing the microstructure and synchronous improving the strength and toughness of Nb-Si based alloys via carbon nanotubes (CNTs) addition through laser melting deposition

Abstract

CNTs were first employed in Nb-Si based alloys fabricated by laser melting deposition (LMD) to enhance mechanical properties, particularly fracture toughness. The results showed that CNTs underwent complete decomposition in alloys fabricated at LP = 2100 W, and the breakdown of CNTs provides carbon atoms to the molten pool, facilitating the microstructure transformation from a hypoeutectic structure consisting of Nbss and Nb₃Si to a near-eutectic structure consisting of Nbss and γ-Nb₅Si₃, accompanied by a refinement of the microstructure. The fracture toughness of 2100 W-3CNTs alloy reached 15.2 MPa·m^1/2, 1.62 times higher than the alloy without CNTs addition. By optimizing process parameters, CNTs-reinforced Nb-Si based alloys were successfully fabricated at LP = 1800 W, achieving simultaneous improvement in fracture toughness and compressive strength. The fracture toughness of 1800 W-3CNTs alloy increased to 16.7 MPa·m^1/2, which was attributed to the content and size of ductile Nbss increased, as well as the debonding and pull-out of residual CNTs during the fracture process. These factors lead to crack deflection, bridging, and branching, thereby increasing the energy required for crack propagation. The Orowan strengthening caused by the array-like distribution of CNTs in Nbss significantly enhanced the compressive strength of the 1800 W-3CNTs alloy, making its compressive strength superior to other alloys, reaching the maximum value of 2190 MPa among all LMDed alloys.