Mechanism of grain refinement and mechanical property enhancement of Ti-45Al-8Nb alloy by directed laser deposition

Abstract

Directed laser deposition (DLD) as an important additive manufacturing process, has a good application prospect for the fabricating of complex high Nb-TiAl alloy parts in aerospace and energy industries. The mechanical properties of parts manufactured by DLD are affected by the interlamellar spacing of precipitated phases. In this paper, Ti-45Al-8Nb alloy specimens were manufactured by DLD, and the interactions between the semi-coherent interfaces, deformation twins, and dislocations were analyzed by transmission electron microscopy (TEM), it revealed the strengthening mechanism of interlamellar spacing on mechanical properties of Ti-45Al-8Nb alloy. The results indicate that the laser power is one of the important factors affecting the interlamellar spacing, and the microstructures of the specimens are composed of α2 phase, γ phase, and B2 phase. By the laser power adjusting, the interlamellar spacing λ decreases by 40.1%, the microhardness of the alloy was increased by 15.9%, and the tensile strength increased by 14.8%. Also, the relationship between the tensile strength σb and the interlamellar spacing λ satisfies the Hall-Petch relationship: σb=161.4+3204λ−1/2. The obstruction and absorption of dislocation motion by the lamellar interface and deformation twins are the main reasons for the improved mechanical properties of the alloy.