Effect of Ultra‐High Lanthanum Content on Martensite Packet Size and Fracture Behavior of Low Alloy Wear‐Resistant Steel: In Situ Tensile Electron Backscatter Diffraction Analysis

Low alloy wear‐resistant steels with different La concentrations (0, 25, 40, 96, 320, 620, 1500, and 4800 ppm) were prepared. The effect of La content on the martensite packet size was systematically investigated after forging at 1200°C and quenching at 890°C. With increasing La content, the average martensite packet size first decreased and then increased, reaching a minimum of 5.43 μm at 96 ppm La, which represented a 13.8% reduction compared with the La‐free steel. When the La content reached 320 ppm and above, the martensite packet size started to coarsen and became larger than that of the La‐free sample. Four samples with different martensite packet sizes are selected for in situ tensile electron backscatter diffraction analysis. The results demonstrate that small‐sized martensite packets possess superior cooperative deformation capability during tensile loading, which facilitates greater plastic deformation at the crack tip and effectively delays crack initiation and propagation. In contrast, when the steel contains 4800 ppm La, the martensite packets become significantly coarsened, resulting in reduced cooperative deformation capacity during plastic deformation. The weakened plastic deformability, introduction of inclusions, and increased fraction of Σ3 coincidence site lattice grain boundaries collectively induce pronounced brittle fracture in the steel at a La content of 4800 ppm.