Low lattice mismatched nano-precipitates in die steels

• Novel DNA die steel exhibits super hard (57.3 HRC) and tough (353.5 J). • Coherent B2-NiAl nanoprecipitate with lattice coherency coefficient (LCC) lower than 0.31% strengthen dislocation shearing and cross-slip. • Coherency and order strengthening contribute 530 MPa of yield strength increment. • Tempering at 620 °C triggers discontinuous precipitation, Laves phase. • Transition from shear to Orowan bypass mechanism correlates well with LCC. Conventional hot work dies steels experience a deterioration in toughness and hardness above 550 °C, owing to the coarsening of carbides and elemental segregation as well as degradation of sorbite. To address this limitation, we conceptualized and designed a DNA steel. By tempering at 580 ℃, DNA steel achieved a peak hardness of 57.3 HRC together with an unnotched impact energy of 353.5 J. Ultimate tensile strengths of DNA steels at 25 ℃ and 580 ℃ were 1936 MPa and 1572 MPa, respectively. Both were 428 MPa and 506 MPa higher than those of 8418 die steels at corresponding temperature. Observation by high resolution TEM testified that the increase in strengths at ambient and elevated temperatures stems from the fully coherent regime of predominant continuous precipitation (CP) of ordered B2 type NiAl precipitates (average 12 nm in size) with lattice coherency coefficient (LCC) of 0.31%. Coherency and order strengthening by CP covered nearly 85% contribution of strength increment. In contrast, after tempering at 620 °C, discontinuous precipitation (DP) became dominant, accompanied by precipitate coarsening (LCC≈0.83%, size > 30 nm) and formation of Laves phases. This resulted in reduction of dislocation storage capacity and concentration of localized strain (impact energy drops from 354 J to 98 J).