Abstract
It was revealed that heterogeneous microstructures enhance the strain aging capability of bake-hardenable steels. The improved strain aging capability was validated through a quantitative comparison using a single material with identical chemical composition, grain size, and initial dislocation density. Beyond well-known improvements by hetero-deformation induced (HDI) strengthening and hardening, heterostructured low-carbon steel was found to have improved strain aging ability compared to its homogeneous counterpart. The mechanisms underlying enhanced strain aging capability are as follows: The strength enhancement mechanism of heterostructured materials, defined as HDI effect, produces additional geometrically necessary dislocations (GNDs) during plastic deformation. The increased dislocation density in the heterostructured low-carbon steel compared to its homogeneous counterpart (~ 1.90 × 1014 m 2 at 5 % plastic pre-strain) provides additional carbon segregation sites, forming Cottrell atmospheres during subsequent low-temperature heat treatment (200 °C for 20 min). The segregated carbon atoms function as pinning dislocations, and a greater number of pinned dislocations in the heterostructured low-carbon steel necessitates higher stresses for the initiation of plastic deformation. Consequently, the heterostructured low-carbon steel, subjected to the strain aging process with 5 % plastic pre-strain, achieved an improved yield strength over the homogeneous low-carbon steel in the same state, primarily attributed to the contributions of the HDI effect (~17.14 MPa) and an improved bake hardening response (~19.37 MPa). These findings uncover additional functionalities of heterostructured low-carbon steels, offering substantial implications that span from fundamental research to industrial applications, particularly due to their enhanced mechanical properties.
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