A modified heat treatment process designated quenching–partitioning–tempering (Q–P–T) process is developed based on the quenching and partitioning process proposed by J.G. Speer et al. [Acta Mater.51, 2611 (2003)] and D.K. Matlock et al. [Mater. Sci. Forum426–432, 1089 (2003)]. A Fe–0.485C–1.195Mn–1.185Si–0.98Ni–0.21Nb steel after Q–P–T process satisfies the designed requirement of tensile strength over 2000 MPa and elongation over 10%. The microstructure characterization indicates that this ultrahigh-strength steel consists of nanomicrostructures including lath martensite, filmlike retained austenite, and dispersive Nb-containing carbides. The effect of tempering time on the mechanical properties is analyzed based on microstructures.

The failure of superhard materials is often associated with stress-induced amorphization. However, the underlying mechanisms of the structural evolution remain largely unknown. Here, we report the experimental measurements of the onset of shear amorphization in single-crystal boron carbide by nanoindentation and transmission electron microscopy. We verified that rate-dependent loading discontinuity, i.e., pop-in, in nanoindentation load-displacement curves results from the formation of nanosized amorphous bands via shear amorphization. Stochastic analysis of the pop-in events reveals an exceptionally small activation volume, slow nucleation rate, and lower activation energy of the shear amorphization, suggesting that the high-pressure structural transition is activated and initiated by dislocation nucleation. This dislocation-mediated amorphization has important implications in understanding the failure mechanisms of superhard materials at stresses far below their theoretical strengths.

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