Microstructure and phase structure of laser solid forming Ti₂AlNb-based alloy

Ti₂AlNb-based alloys of two different compositions were deposited using a 5 kW CO₂ laser. Optical microscope (OM), scanning electron microscope (SEM), energy-dispersive spectrometer (EDS), X-ray diffraction (XRD) and microhardness testing method, were used to investigate the typical microstructure evolution, phase transformation and the corresponding hardness evolution. Experimental results show that for Ti-20%Al-27%Nb (atomic fraction) (A₁), the phase transformation occurrs: B2→B2+O→B2 along the laser deposition direction. While Ti-22%Al-27%Nb (A₂) is mainly composed of β(TiNb) dendritic and B2 + O in the interdendritic. The primary dendrite trunk spacing changes from 25 μm to 80 μm along the deposition direction. For the two different compositions Ti₂AlNb-based alloys, the microhardness variation of A₁ alloy along the deposition direction is low→high→low, which agrees well with the corresponding microstructure and phase transformation. The microhardness of A₂ alloy distributes uniformly except the local region of the dendritic.