Abstract
Martensitic steels are widely used structural materials with outstanding mechanical properties. Their high strength is provided by the non-diffusional phase transformation of fcc γ 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.
Highlights
Lath-martensite steel is a low-alloy high-strength structural material with wide applications [1,2,3].The strength relates to the hierarchical fine sub-grain size of lath-packets and martensite laths and to the high dislocation densities [4,5]
(1) We have shown that in plastically deformed lath-martensite steel when the active Burgers vectors are within the plane of the lath lamellae these packets either soften or at least do not work harden
The active Burgers vectors are across the boundaries of the lath martensite lamellae the packets work harden substantially
Summary
Lath-martensite steel is a low-alloy high-strength structural material with wide applications [1,2,3]. The anisotropic stress–strain response of differently oriented lath-packets was shown in micro tensile experiments in a martensitic steel [16]. In sample #1 with long mean free paths of dislocations the flow stress was about 890 MPa, whereas in samples #2 and #3 with short mean free paths of dislocations the flow stress was about 1200 MPa [16] These experiments indicate that the local microscopic flow stress of lath martensite depends on the relative orientation between the habit plane direction of laths and the direction of the active Burgers vectors. We investigate a high-strength low-alloy lath-martensitic steel containing a negligible remaining austenitic phase, less than 1% in vol pct. We suggest that the deformation invoked composite structure provides the relatively good ductility and toughness of the high strength lath-martensite steel. The specimens were chemically etched in an electrolyte (10% perchloric acid, 70% ethanol, 20% glycerol) in order to remove any surface layer damaged by the sample preparation procedure
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
