Correlation Between Strain Gradient and Fracture Damage Mechanism of TA18 Alloy Based on In Situ SEM/EBSD Analysis

TA18 alloy is an ideal material for piping systems in the aerospace field, the low notch sensitivity and high damage tolerance enhance its engineering application value. However, the complex failure mechanism influenced by strain gradient necessitates further improvement of the service stability for TA18 alloy in practical engineering application. This study combines the advantages of high-precision coupling between dynamic stress and microstructure evolution in in situ SEM/EBSD testing to reveal the fracture damage mechanism during the tensile deformation process of extruded TA18 alloy (denoted as TA18-J). The fracture mechanism of TA18-J alloy is dominated by the α phase, with the β phase only coordinating local plastic deformation. The incompatible deformation of alloy and grain size effect of initial microstructure are the primary factors for the occurrence of strain gradient. The strain gradient leads to multiple concurrent ways of crack initiation and propagation during the tensile deformation of TA18-J alloy, further promoting the multi-scale dislocation activity mode and morphology distribution. The grain boundary energy and gradient-distributed strain energy are the critical driving force for crack nucleation and propagation. The TA18-J alloy undergoes a severe plastic deformation stage with an elongation of approximately 20% after necking, and the tolerance scale of notch insensitivity for TA18 alloy was quantified. Macroscopic and microscopic fracture characterizations indicated that the failure mechanism of the TA18-J alloy was a mixed fracture mode dominated by ductile fracture.