Nanosized precipitates activating ultrahigh strength of an ultrafine-grained ferritic steel during dynamic deformation

Although mechanical behaviors of ultrafine grained steel containing nanosized particles at low strain rates was extensively studied, its mechanical performance at high strain rates was rarely reported. In this study, the tensile deformation of the UFG ferritic steel containing nanosized Fe3C and VC particles were carried out at wide strain rate spans of 0.017 s(-1), 520 s(-1), 770 s(- 1), 920 s(- 1), 1280 s(-1) and 1400 s(-1) by using a rotating disk Hopkinson tensile system, and the corresponding mechanical behaviors and microstructural evolutions were systematically investigated. The results show that the peak stress noticeably increases from 1310 MPa to 2880 MPa when the strain rate increases from 0.17 s(-1) to 770 s(-1), and the highest peak stress of 2880 MPa is three times as much as the typical UFG ferritic steel deformed at the identical strain rate. Key strengthening factor is associated with the existence of numerous nanosized Fe3C and VC particles in the ferritic matrix, which can effectively and strongly block dislocation movement hence contributes to peak stress. Meanwhile, high strain rate also accelerates premature formation of micro-voids at interface between the nanosized Fe3C particle and the UFG ferrite matrix, resulting in the decrease of peak stress and the occurrence of softening behavior in plastic deformation when the strain rate exceeds 770 s(-1), and the key reason is that the dislocation accumulations around the nanosized Fe3C particles is excessively intensified by increasing strain rate. Thus, flow stress and elongation to fracture are reduced. This study provides new insights into the micro-mechanisms concerning the strengthening and damage mechanism of steels containing numerous nanosized particles, especially deformed at high strain rates.

Automated summary

This study investigates the mechanical behavior and microstructural evolution of ultrafine grained steel containing nanosized Fe3C and VC particles under different strain rates through tensile tests and microstructural analysis. The results show that the presence of nanosized particles enhances the peak stress of the steel, but at high strain rates, it may lead to softening behavior and reduced elongation to fracture.