Microstructure characteristics of a γ single-phase Ti-55Al-7.5Nb alloy fabricated via additive manufacturing

Developing a new ordered single-phase TiAl alloy, with no solid-state transformation operated at a long-term high temperature, is important in the aerospace industry. Here, the preparation of newly Ti-55Al-7.5Nb alloy via direct laser deposition (DLD) is proposed and studied. The oriented grain growth in Ti-55Al-7.5Nb alloy is successfully achieved by using laser additive manufacturing (AM) without interlayer pause during the laser scaning back and forth. AM features a rapid cooling rate while Ti-55Al-7.5Nb alloy owns a narrow L → α and α → γ phase transformation temperature window, and then enabling the TiAl liquid phase directly transform into a γ single-phase, which is confirmed by the combination of experiments and thermodynamic calculations. The heat treatment in the AM thermal cycles induces a thermal gradient, promoting the new recrystallized grains growing base on the preferred growth of [111]γ along the heat flow during the DLD solidification stage. The AM Ti-55Al-7.5Nb alloy exhibits continuous γ single-phase columnar grains, meanwhile, accompanied by the twins growing along the building direction.