Origin of sustained work hardening capacity in an ultrahigh-strength Q&P1500 steel

Ultrahigh-strength steels often suffer from limited resistance to plastic instability, restricting lightweight structural applications. Here, a 1.5 GPa quenching and partitioning (Q&P) steel containing 10 vol.% austenite exhibits appreciable ductility and sustained work hardening. Combining in-situ neutron diffraction with microscale strain mapping, we track the dynamic evolution of local stress and strain during deformation. The initially soft austenite hardens rapidly, and its phase stress eventually exceeds that of the martensite matrix. This strength reversal continuously redistributes local strain, promotes strain delocalization and delays plastic instability, offering a new strategy to design ultrastrong ductile alloys through dynamic stress/strain partitioning.