Revealing dislocation activity modes during yielding and uniform deformation of low-temperature tempered steel by acoustic emission

The distinctive distribution of acoustic emission (AE) characteristic parameters generated during tensile testing of low-temperature tempered AISI 4140 steel was investigated. Two clusters of acoustic emission signals were distinguished using power-law distribution fitting and k-means clustering methods. These clusters were identified as resulting from dislocation motion during yielding and dislocation entanglement during uniform plastic deformation. The conclusion is further confirmed by transmission electron microscopy images at different strains. In particular, the unique "arch-shaped" distribution of the acoustic emission energy during yielding implies a change in unusual dislocation motion modes. The effect of carbide precipitation was qualitatively discussed as not considering the primary cause of the formation of this arch-shaped distribution. The evolution of dislocation motion modes during yielding of low-temperature tempered martensite was elucidated by comparing the significant difference in cumulative energy values during yielding of annealed and low-temperature tempered specimens. Dislocations emit from Frank–Read or grain boundary sources and slip along short free paths, contributing to the initial increase in AE signals energy. Subsequently, the primary source of acoustic emission energy “arch-shaped” peak during yielding was generated by the avalanche behavior of accumulated dislocations, leading to the accelerated dislocation motion.