Gradient microstructure response in different phases of Ti2AlNb alloy with laser shock peening

Abstract

Ti2AlNb alloy is a new type of lightweight and heat-resistant structural material that has been an active research topic with multi-phases due to its improved mechanical properties. However, the phase response under severe plastic deformation (SPD) at high strain rates is unclear, which limits the understanding of its strengthening mechanism. Here, laser shock peening (LSP) was employed to investigate the microstructure response of α2, O, and B2/β phases in Ti2AlNb alloy. After LSP, the highest density dislocations were generated in the B2/β phases, followed by the O phases, and the least dislocations were observed in the α2 phases. In the B2/β phases, only dislocation tangles (DTs) were observed. Dislocation lines (DLs) and DTs were mainly distributed in the O phases. As for the α2 phases, DLs were generated while a small number of DTs were piled up at the grain boundaries. In addition, parallel dislocations (PDs) with a spacing of about 135 nm can be observed on the surface of the α2 phases, and the dislocations interspersed between PDs were composed of screw dislocations with long segments. In addition to dislocation structures, stacking faults (SFs) were observed in both α2 phases and O phases, and mechanical twins were formed in the O phases, which were generated to coordinate the SPD induced on the surface. These dislocation structures formed dislocation cells (DCs) and sub-grain boundaries which further developed into high-angle grain boundaries (HAGBs) together with mechanical twins, dividing the coarse grains and refining the microstructure. As the depth increased, the responses of the three kinds of phases decreased. The PDs in the α2 phases have widened spacing and could not penetrate the grains. The mechanical twins were gradually reduced until no obvious twin structure could be observed. Finally, dislocations, grain size, microhardness, and residual stress presented a gradient distribution. The improved microhardness (maximum 470 HV) and compressive residual stress (maximum −600 MPa) which were consistent with the refined grains and high-density dislocations gradually decreased to that of the base metal (BM).