Effect of Hydrogen Content and Microstructure Inhomogeneity on Fatigue Crack Growth Behavior of Ti-0.3Mo-0.8Ni Alloy Argon-Arc Welded Joints

The effects of hydrogen content and microstructure inhomogeneity on the fatigue crack growth (FCG) behavior of Ti-0.3Mo-0.8Ni alloy argon-arc welded joints were elucidated in detail. Sheets with a thickness of 10 mm were welded using gas tungsten arc welding (GTAW). Samples with varying hydrogen content were prepared using a hydrogen charging technique. The crack growth path and fracture morphology were examined using optical and scanning electron microscopies. FCG rate and fracture toughness tests were conducted to analyze the FCG behavior of welded joints. The results indicated that the hydrogen content and microstructure inhomogeneity of the welded joint had limited effects on the FCG rate in the Paris zone. However, as the hydrogen content increased, the FCG rate in the instability zone increased, and the fracture toughness (KQ) value decreased. The microstructure had a greater effect on the FCG rate in the instability zone; the fluctuation amplitude of the FCG path increased significantly, and brittle fracture intensified. Lamellar alpha in the weld zone (WZ) provided an ideal interface for plastic slip, whereas the hydride accelerated separation from the matrix and formed a diffusion channel, significantly increasing the FCG rate in the instability zone. Cracks in the base metal (BM) primarily propagated along the grain boundary of equiaxed alpha. In the WZ, cracks predominantly propagated along the alpha/beta phase boundary or the hydride and cut through the hydride or alternating lamellar alpha/needle beta grains. The fracture morphology in the fatigue source and Paris zones featured a petal-like pattern with partial fatigue bands and cleavage-like river patterns with local brittle fracture and fatigue bands, respectively. In the instability zone, the morphology was characterized by ductile tearing with brittle fracture or local ductile tearing.