Study on the intrinsic factors determining impact toughness of TC21 alloy

Highlights: • The crack initiation energy accounts for most of the total impact energy. • The crack initiation energy is proportional to the degree of plastic deformation of the microstructures. • The degree of plastic deformation in crack propagation zone is less than that in crack initiation zone. • The ability of microstructure to deflect crack is the crucial factor to improve crack propagation resistance. Ti-6Al-3Mo-2Sn-2Zr-2Nb-1.5Cr-0.1Si (TC21) alloy exhibits high specific strength and superior fracture toughness, making it attractive in the aerospace field. However, its fracture resistance under impact loading was not fully understood. Therefore, the impact toughness of TC21 alloy with various microstructures was studied using instrumented Charpy impact testing. Combining microstructure characterization, crystallographic orientation analysis, nanoindentation test, fractography observations with analysis of kernel average misorientation (KAM) distribution near crack path, the heterogeneity of deformation at small length scales, as well as crack initiation and propagation mechanisms were revealed. Analysis of load-displacement curves indicated that the crack initiation energy accounts for most of the total impact energy. The crack initiation energy is proportional to the degree of plastic deformation of the microstructures in the crack initiation region. The thick lamellar structure either in bimodal or fully lamellar structure can generate large plastic deformation, thus promote the crack initiation energy. In contrast, the KAM value in crack propagation region was much lower than that in crack initiation region, suggesting that impact energy consumed by plastic deformation during crack propagation was lower than during crack initiation. The crack propagation energy increased with the increasing degree of the tortuosity of propagation path. Especially in fully lamellar microstructure, the high angle α colony boundaries deflected the crack propagation direction and thus increased the crack propagation energy