Gradient microstructure and fatigue properties of TC21 titanium alloy processed by laser shock peening

The gradient microstructure evolution, microhardness, residual stress, and fatigue properties of TC21 titanium alloy subjected to laser shock peening (LSP) were investigated. In this study, the different effects of LSP on α-Ti and β-Ti were researched in detail. Results show that LSP could induce a large number of dislocation structures, among which dislocation lines (DLs), dislocation walls (DWs), and dislocation networks (DNs) were mainly distributed in α-Ti, while dislocation tangles (DTs) were mainly distributed in β-Ti and acicular α'. Sub-grain boundaries were formed by these DWs, DNs and DTs. It is worth noting that a large number of parallel twin structures also formed in α-Ti, and the formation of these twin structures was the result of the transformation of hexagonal close-packed Ti (HCP-Ti) to face-centered cubic Ti (FCC-Ti). The orientation relationship between the HCP-Ti and FCC-Ti can be described as (0002)HCP//(1̅11̅)FCC and [21̅1̅0]HCP//[1̅1̅0]FCC, which is consistent with the classic Shoji-Nishiyama relationship and HCP/FCC transformations. The coarse grains were divided by the sub-grain boundaries and twins, the microstructure was improved. As the depth from the peened surface increased, the dislocation density decreased and the twin structures gradually disappeared, the microstructures presented a gradient distribution. As for fatigue performance, the fatigue life of the TC21 was increased from 1.8 × 105 cycles to 4.7 × 105 cycles at 750 MPa stress level after LSP treatment, an increase of about 161%. Under the combined action of the microstructure and the compressive residual stress, the crack initiation and crack growth rate of the TC21 titanium alloy were delayed, and the fatigue life was finally improved.