On the Mechanisms of Different Work-Hardening Stages in Twinning-Induced Plasticity Steels

The detailed work-hardening behaviors of two twinning-induced plasticity (TWIP) steels with and without Al addition are investigated. The work-hardening rate curves of both TWIP steels can be divided into three stages. The dominant work-hardening mechanism is different at different stages. Dynamic strain aging (DSA) is responsible for the high work-hardening rate at the very beginning of the first stage for the TWIP steel without Al, but the DSA’s contribution is not significant in the TWIP steel with Al. However, DSA may only play a dominant role at the early plastic deformation. For the strain higher than 3 pct in the first stage, the difference of work-hardening rate between the two TWIP steels becomes smaller. This suggests that the main work-hardening mechanism in the first stage changes to the multiplication of dislocations at strains higher than 3 pct in TWIP steel without Al. The increase of work-hardening rate in the second stage is mainly due to the formation of deformation twins in both TWIP steels. Nevertheless, comparing to the TWIP steel without Al, TWIP steel with Al shows a lower work-hardening rate at the second stage. This is due to the fact that the addition of Al increases the critical twinning stress, resulting in a lower twinning capability. Deformation twin plays a more and more important role on the work-hardening with the increase of strain in the second stage due to the increase of twin volume fraction with strain. It is found that, except being obstacles to the dislocation glide, deformation twins can also act as a new source of the emission of partial dislocations. Furthermore, it is also found that dislocations can transmit across the twin boundary and be stored in the twins, implying that deformation twins can also accommodate local strains.