Unprecedented strength in pure iron via high-pressure induced nanotwinned martensite

Martensitic transformation can easily induce a maximum hardness value of 800–900 HV (Vickers hardness) for steels with carbon contents of 0.6 wt.% and above. However, the occurrence of martensitic transformation in pure iron requires exceptionally high cooling rates (105–106°C/s), and the maximum achievable hardness is only about 150 HV. Here we report an extreme hardness of 830 HV in pure iron obtained through high pressure induced martensitic transformation at a rather slow cooling rate of just 10°C/s. This unprecedented strength originates from the formation of twin-related martensitic laths with an average thickness of 3.8 nm.