Deformation Induced Soft and Hard Lath Packets Enhance Ductility in Martensitic Steels

Martensitic steels are widely used structural materials with outstanding mechanical properties. Their high strength is provided by the non-diffusional phase transformation of fcc g into thin lamellar bcc plates during fast cooling. Coherency strains between the fcc and bcc lamellae induce large dislocation densities in the range of 1016 m−2, well above the densities attainable by conventional plastic deformation. Using high resolution X-ray line profile analysis, scanning electron microscopy, and hardness tests we show that during tensile deformation when the active Burgers vectors are within the lath plane the lath-packets work soften. On the contrary, when the active Burgers vectors are oblique to the lath-plane the lath-packets work harden. The softening and hardening processes in the differently oriented lath-packets produce a composite of hard and soft components on the length scale of lath-packet size. The stress–strain response of the alloy is discussed in terms of the different mean free paths and the different annihilation lengths of dislocations in the softened and hardened lath-packets. The relatively good ductility is shown to be produced by the composite microstructure induced by plastic strain.