Rapid directionally solidified microstructure characteristic and fracture behaviour of laser melting deposited Nb–Si–Ti alloy

Abstract

Aiming at achieving fine and directionally-solidified microstructure of Nb–Si based alloy, Nb, Si and Ti powder particles were utilized as the raw materials, and laser melting deposition (LMD) experiments were conducted with 1500 ​W and 2000 W laser power, respectively. The microstructure characteristic, micro-hardness, and indentation fracture toughness were examined by scanning electron microscope (SEM), X-ray energy dispersive spectrometer (XEDS) and X-ray diffraction (XRD). The results showed that the two kinds of as-deposited Nb–17Si–23Ti alloy samples were mainly composed of NbSS, (Nb, Ti)3Si and Ti-rich NbSS, and the microstructure presents rapid directionally-solidified characteristic, in which the <100> crystallographic direction of NbSS and <110> crystallographic direction of (Nb, Ti)3Si tend to be parallel to the LMD building direction. With the laser power increasing from 1500 ​W to 2000 W, the microstructure became more oriented and homogeneous. For the sample at 2000 W, the dendrite spacing was only about 1–2 ​μm, and NbSS and (Nb, Ti)3Si couples grow alternately and they are parallel to the building direction strictly. In the meantime the micro-hardness and indentation fracture toughness reached 1075 HV and 20.3 ​MPa·m1/2, respectively, indicating significant improvement by the directional growth of fine NbSS and (Nb, Ti)3Si phases, and the dimple fracture mode of the NbSS dendrites.