Rate-dependent ductile-brittle transition in a Medium Mn steel

Medium Mn steels (MMS) have received significant attention owing to their excellent tensile properties and rich deformation mechanisms. This work reports a strain rate-dependent ductile-brittle transition phenomenon in strain rate regime of 10^–6 s^-1 to 10¹ s^-1, where uniform elongation dramatically increases from ∼10 % at 10^–6 s^-1 to ∼50 % at 10⁰ s^-1. In other words, this MMS exhibits brittleness at a low strain rate but becomes ductile at a high strain rate. To unveil the origin of this unusual phenomenon, comprehensive characterizations focusing on microstructural evolution, fracture behavior, and strain heterogeneity at various strain rates have been carried out. Despite similar microstructural evolutions, the fracture morphology transits from brittle fracture featuring quasi-cleavage and intergranular cracks at low strain rate to ductile fracture featuring dimples at high strain rate. Strong but brittle fresh martensite and soft austenite matrix lead to higher strain heterogeneity at low strain rate, while strain incompatibility is largely alleviated due to plastic deformation of fresh martensite at high strain rate. Further tensile tests on the same MMS with pre-introduced fresh martensite and tempered martensite highlight the critical role of interstitial carbon solutes in governing the rate-dependent ductile-brittle transition. As identified by atom probe tomography, the difference in carbon clusters in fresh martensite at different strain rates indicates the activation of carbon drag effect on dislocations. The carbon drag effect is inversely proportional to strain rate, which is believed to affect the deformability of fresh martensite, leading to the strain rate-dependent ductile-brittle transition phenomenon.