Initiation mechanism of micro-defects in polycrystalline iron under high-temperature stress coupling effect

To investigate the failure mechanism of high-temperature fracture in polycrystalline iron, this study conducted in-situ observation experiments to study the dynamic damage evolution process of iron under tensile loading at high temperatures. Meanwhile, through molecular dynamics simulation, the micro-defect initiation mechanism of polycrystalline iron under the coupling effect of high temperature and stress was revealed at the microscopic scale. The results indicate that during the high-temperature deformation process of polycrystalline iron at 1400 K, grain boundary slip is the main deformation mechanism. When the grain boundary slip is obstructed, stress concentration can easily occur at the grain boundaries (especially the triple junctions), leading to crack initiation. Furthermore, molecular dynamics simulations have shown that in the initial stage of deformation, with increasing strain, both HCP phase transformation atoms and dislocation length exhibit a trend of initially increasing followed by decreasing, peaking at around 5 % strain. Afterward, the system undergoes dynamic recrystallization, with dislocation movement and grain boundary migration leading to dislocation elimination. When the strain reaches 12.3 %, microvoids initiate at the triple junctions and grow up in an O shape. At a strain level of 12.45 %, there is a sudden drop in system stress, indicating fracture initiation.