Unravelling the effect of diverse microstructural features on nano-mechanical properties and multiple pop-in behaviours in a multiphase lightweight steel

In the current investigation, nano-indentation tests have been performed in Ni-containing Fe–Al–Mn–C steel in homogenized (undeformed) and forged (deformed) conditions in order to comprehend the nanoscale mechanical properties and multiple pop-in behaviours. The nano-hardness is observed to be higher in both BCC (~ 10.3 ± 0.9 GPa) and FCC (~ 9.9 ± 0.9 GPa) phases of undeformed specimen in comparison with the deformed one, which predominantly indicates the influence of nano-sized precipitates on the nano-hardness. Conversely, the grain boundary (GB) and the interphase region in deformed specimen show a higher nano-hardness of ~ 13 ± 0.8 GPa and ~ 10 ± 1.3 GPa in BCC and FCC, respectively, than the undeformed specimen due to the presence of coarse precipitates along GB and interphase. Further, the elastic modulus (\({E}_{s}\)) of the individual phases is calculated for both the specimens, and its dependency on the microstructural features is explored. In addition, the differences in multiple pop-in behaviours (i.e. the variation in pop-in length) in the P–h curve is explored and correlated with various underlying mechanisms. The multiple pop-in behaviour is observed to be associated with the factors such as the precipitate–dislocation interactions, the presence of pre-existing dislocations and the strain field generated due to dislocations.

Graphical abstract